The Galaxys Gamma-Ray Glow

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					FOCUS                                                                                                                                                                                                                                                                                                                                                                                                             GALACTIC HIGHLIGHTS

                                                                  The Galaxy’s Gamma-Ray Glow
                       Visible light makes up only a small part of the full spectrum of electromagnetic radiation. When        astronomers use antennae or satellite-borne telescopes to look at the universe at those other wave-
                       lengths, they unmask strange landscapes and physical processes that remain concealed from               optical telescopes. A team of researchers led by ROLAND DIEHL at the MAX PLANCK INSTITUTE                                                                                                                                                                                                               FOR

                       EXTRATERRESTRIAL PHYSICS                 in Garching focuses on gamma-ray light, which has given them   deep insights into the Milky Way – and even resorts to radioactivity as a source of these gamma rays.

                                                                                                                                                                                                                                            rom time immemorial, human
                                                                                                                                                                                                                                       F    beings have looked up at the
                                                                                                                                                                                                                                       diffusely gleaming band of light that
                                                                                                                                                                                                                                                                                                                                                                                       of sources of thermal radiation: ex-
                                                                                                                                                                                                                                                                                                                                                                                       ploding stars (called supernovae),
                                                                                                                                                                                                                                                                                                                                                                                       planets and even interstellar dust,
                                                                                                                                                                                                                                       stretches across the sky on clear                                                                                                               which emits light at temperatures
                                                                                                                                                                                                                                       nights. For millennia, the mysterious                                                                                                           close to absolute zero, or minus
                                                                                                                                                                                                                                       glow remained largely a theme of                                                                                                                273.15 degrees Celsius.
                                                                                                                                                                                                                                       mythology. In the 5th century BC,                                                                                                                  Whereas cold bodies emit infrared
                                                                                                                                                                                                                                       Greek philosopher Democritus ex-                                                                                                                light at long wavelengths, hot super-
                                                                                                                                                                                                                                       pressed his belief that this “Milky                                                                                                             novae emit a considerable part of
                                                                                                                                                                                                                                       Way” consisted of countless faint                                                                                                               their energy as X-rays, thus reaching
                                                                                                                                                                                                                                       stars – a revolutionary claim at the
                                                                                                                                                                                                                                       time, and it was not actually con-

                                                                                                                                                                                                                                                                                 PHOTOS: MELLINGER/COLUMBIA UNIVERSITY/IPAC/MPI FOR RADIO ASTRONOMY/MPI FOR EXTRATERRESTRIAL PHYSICS
                                                                                                                                                                                                                                       firmed until 2,000 years later.
                                                                                                                                                                                                                                          “My third observation concerns
                                                                                                                                                                                                                                       the nature of the Milky Way (...) No
                                                                                                                                                                                                                                       matter which part of it one targets
                                                                                                                                                                                                                                       with the telescope, one finds a huge
                                                                                                                                                                                                                                       number of stars, several of which are
                                                                                                                                                                                                                                       quite large and very striking; yet, the
                                                                                                                                                                                                                                       number of small stars is absolutely
                                                                                                                                                                                                                                       unfathomable.” The man who wrote
                                                                                                                                                                                                                                       this in March 1610 was using a self-
                                                                                                                                                                                                                                       constructed telescope to advance in-
                                                                                                                                                                                                                                       to unknown regions, for which he
                                                                                                                                                                                                                                       would go down in history: Galileo
                                                                                                                                                                                                                                       Galilei. He discovered what was lit-
                                                                                                                                                                                                                                       erally out of this world – hence the
                                                                                                                                                                                                                                       book title “SIDEREUS NUNCIUS” (“The
                                                                                                                                                                                                                                       Starry Messenger”). In it, the Italian
                                                                                                                                                                                                                                       mathematician and astronomer de-
                                                                                                                                                                                                                                       scribed his observations of Jupiter’s
                                                                                                                                                                                                                                       satellites, the Earth’s moon and the
                                                                                                                                                                                                                                       Milky Way (box, page 23).
                                                                                                                                                                                                                                          Examining this object, known as

                                                                                                                                                                               PHOTO: NASA/ESA/JEFF HESTER, ARIZONA STATE UNIVERSITY
                                                                                                                                            In 1054, Chinese astronomers                                                               the galaxy, with the naked eye re-                                                                                                                    Cygnus
                                                                                                                                             observed a supernova flaring                                                                                                                                                                                                                    (swan)
                                                                                                                                                                                                                                       vealed the light of innumerable stars                                                                                                                                                               Vela
                                                                                                                                                 up in the constellation of                                                                                                                                                                                                                                                                (sail)
                                                                                                                                                                                                                                       – suns similar to our own that gen-                                                                                                                                Central area of the galaxy
                                                                                                                                              Taurus. Since then, the light
                                                                                                                                              from the remains of the ex-                                                              erate their energy from the processes
                                                                                                                                              ploded sun has been shining                                                              of nuclear fusion. The hot surfaces of
                                                                                                                                                on the Earth’s firmament.                                                              these globes of gas radiate light like                                                                                                          Our Milky Way as seen through various astronomical
                                                                                                                                               Stellar corpses, such as this                                                                                                                                                                                                           windows – that is, different spectral ranges: Visible light,
                                                                                                                                                                                                                                       a burning candle. But stars are not
                                                                                                                                            Crab Nebula, provide gamma-                                                                                                                                                                                                                radio emission from carbon monoxide, infrared light,
                                                                                                                                                                                                                                       the only bodies in the universe that                                                                                                            the radio continuum range, gamma-ray continuum
                                                                                                                                                    ray astronomers a rich
                                                                                                                                                 opportunity for research.                                                             generate thermal radiation. As-                                                                                                                 emission, and gamma rays with a specific energy that
                                                                                                                                                                                                                                       tronomers are familiar with a variety                                                                                                           originates from radioactive decay (from top).

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                                                                                                                                                                                                                                                                                         A Fried Egg in the Universe
                                                                                                                                                                                                                                                           In the early 17th century, Galileo Galilei discovered that the Milky                                                     the H1 areas to glow in the light of the 21-centimeter-line. This
                                                                                                                                                                                                                                                           Way consisted of individual stars. Almost 150 years passed before                                                        radiation penetrates the dust curtains almost unobstructed and can
                                                                                                                                                                                                                                                           a scientist again concerned himself with this celestial structure.                                                       be picked up by radio telescopes. The Doppler effect from the
                                   Inverse Compton effect        ® Bremsstrahlung.         This occurs           magnesium in sparklers fills the                                                                                                          Thomas Wright of Durham believed that the stars were arranged as                                                         galactic rotation makes it possible to determine, from the mea-
                                                                    when a charged particle, such as             room with an almost white light. As-                                                                                                      a flat object similar to a grindstone that stretched across the en-                                                      sured wavelength, the distance of the source of radiation along the
  Bremsstrahlung                                                                                                                                                                                                                                           tire sky, and that the Milky Way was nothing but the projection of                                                       line of sight. This new window on the universe allowed astronomers
                                                                    an electron traveling at almost the          trophysicists identify such character-
                                                                                                                                                                                                                                                           this grindstone.                                                                                                         to discover the spiral structure of the Milky Way.
                                                                    speed of light, is diverted in the           istic coloring as the fingerprints of
                                                                                                                                                                                                                                                           German philosopher Immanuel Kant seized on this theory – and                                                             However, in the 1970s, researchers found that hydrogen alone was
                                                                    electric field of an atomic nucleus,         the chemical elements. The closer
                                                                                                                                                                                                                                                           came very close to the truth. In his General Natural History and                                                         not sufficient as an indicator for the galaxy’s morphology because,
                                                                    converting part of its energy into           one looks, the more clearly the spec-                                                                                                     Theory of the Heavens, published in 1755, he explained the Milky                                                         for example, it is less concentrated in the spiral arms than expect-
                                                                    radiation.                                   tral lines stand out. Incidentally, in                                                                                                    Way as an extended and very diluted layer of stars. The Sun, the                                                         ed. The search began anew.
           Radioactive                  Annihilation of
           decay                     matter and antimatter       ® Inverse Compton effect. A high-               the optical spectra of the Sun or oth-                                                                                                    Earth and all the other                                                                                                  The most important indicator turned out to be clouds of interstel-

                                                                                                                                                                                                                                                                                                                                   ILLUSTRATION: ROBERT HURT/SSC/CALTECH/JPL/NASA
                                                                    energy electron transfers part of            er stars, they appear as dark absorp-                                                                                                     planets were part of this                                                                                                lar molecules; they emit radiation in the characteristic light of
There are basically four different processes                                                                                                                                                                                                               layer, but not at its center.
                                                                    its energy to a particle of light,           tion lines, because the elements in                                                                                                                                                                                                                                carbon monoxide (CO) at a wavelength of 2.6 millimeters. Now it
that produce cosmic gamma rays (description                                                                                                                                                                                                                Depending on the line of
                                                                    similar to a collision of billiard           the cooler stellar gaseous atmos-                                                                                                                                                                                                                                  was gradually becoming possible to refine the portrait of the Milky
in the running text to the right).                                                                                                                                                                                                                         sight, along the plane of                                                                                                Way. Accordingly, the galaxy (from the Greek word gala: milk) is a
                                                                    balls.                                       pheres absorb light that originates in                                                                                                    the layer or vertically out                                                                                              slightly bent wheel, 100,000 light years in diameter and 5,000
                                                                 ® Annihilation of matter and anti-              the underlying interiors of the star at                                                                                                   of it, we would see differ-                                                                                              light years in thickness. The center with its supermassive black hole
             into the shortwave end of the ther-                    matter. An electron finds its an-            exactly those wavelengths at which                                                                                                        ent numbers of stars.                                                                                                    is surrounded by a spherical bulge of stars with an embedded,
             mal spectrum. But thermal processes                    tiparticle (positron) and both end           they would emit light by themselves.                                                                                                      But how were the as-                                                                                                     elongated, cigar-shaped structure – a kind of bar – approximately
             can hardly generate any light at                       up completely in gamma-ray light                                                                                                                                                       tronomers to find out                                                                                                    15,000 to 25,000 light-years in length.
                                                                                                                 NEWS FROM                                                                                                                                 whether the apparent view         Sun
             shorter wavelengths – nevertheless,                    – in the form of two photons.                                                                                                                                                                                                                                                                                   A torus consisting mainly of dense dust and gas clouds with em-
                                                                                                                 THE ATOMIC NUCLEUS                                                                                                                        of the Milky Way in the                                                                                                  bedded stars surrounds the center at a distance of approximately
             the universe is full of such emis-                  ® Radioactive decay. An elemen-                                                                                                                                                           sky reflected its actual                                                                                                 15,000 light-years. Several spiral arms make up the characteristic
             sions. So where does this very high-                   tary constituent of the atomic nu-           All these common features notwith-                                                                                                        spatial structure? Stellar     If we were to look at the                                                                 feature of the galaxy. Most of them bear the names of the heav-
             energy gamma radiation originate?                      cleus undergoes a spontaneous                standing, there is a crucial difference                                                                                                   statistics devised at the      galaxy from well outside of its                                                           enly constellations in which we observe them: the Sagittarius and
                                                                    transformation. The modified nu-             between the optical and the gamma                                                                                                         end of the 18th century by plane, it would look like a                                                                   Perseus Arms, the Norma and Scutum-Crux Arms, the 3-Kiloparsec
             EXOTIC MEANS                                                                                                                                                                                                                                  Friedrich Wilhelm Herschel Catherine wheel. Our sun is just
                                                                    cleus then adopts a more stable              range: spectral lines in the visible                                                                                                                                                                                                                               Arms and the Cygnus Arm. Our solar system is located in the Orion
             BEYOND HOT                                                                                                                                                                                                                                    promised a solution: Her-      one of about 150 billion stars.                                                           Arm, 26,000 light-years from the galactic center and 50 light-
                                                                    configuration, thereby emitting              part of the spectrum bear witness to                                                                                                      schel recorded the coordi-                                                                                               years north of the main galactic plane. The system, which contains
             It arises in exotic cosmic regions                     gamma rays.                                                                                                                                                                            nates and brightness of all stars visible through his telescope. The                                                     around 150 billion suns, is surrounded by a spherical halo contain-
             through “non-thermal” processes –                   To distinguish between                                                                                                                                                                    undertaking failed: apart from the uncertainty of these measure-                                                         ing thousands of globular star clusters and consisting of a thin hy-
             non-thermal because the concept of                  these source processes,                                                 126 Super-heavy
                                                                                                                                                                                                                                                           ments – for example, although it was possible to determine the                                                           drogen plasma.
                                                                                                                                                  nuclei                                                                                                   apparent brightness of the stars, it was impossible to determine
             temperature becomes devoid of all                   researchers such as                             Known nuclei                                                                                                                                                                                                                                                       The entire galaxy rotates; objects closer to the center rotate faster,
                                                                                                                                                                                                                                                           their absolute luminosity and hence their distance – there was also
             meaning here. Roland Diehl from                     Roland Diehl do what                                         82                                                                                                                           a fundamental problem: the Milky Way is filled with gas and dust                                                         and those further from the center, more slowly. The curve of this
                                                                                               NUMBER OF PROTONS

             the Max Planck Institute for Extra-                 Joseph von Fraunhofer                                                                                                                                                                     clouds that absorb the light from the stars. This obscures the view,                                                     differential rotation with a galactocentric radius becomes constant
             terrestrial Physics in Garching thus                first did with sunlight               Stable                                                                                                                                              particularly toward the central region, and prevents the overarch-                                                       at relatively small galactocentric distances; this cannot be ex-
                                                                                                       nuclei                                                                                                                                              ing structure from being recognized.                                                                                     plained by visible mass alone, and is the main argument for the
             describes these regions as “beyond                  200 years ago: they                          50
                                                                                                                                                                                                                                                                                                                                                                                    existence of invisible dark matter.
             hot.” Researchers do not measure                    split incoming radiation                                   82                                                                                                                             Therefore, the stellar census will never be complete, except for the
                                                                                                                                                                                                                                                           region around the Sun up to a radius of about 10,000 light-years.                                                        And the astronomers face yet another problem: despite the rota-
             temperatures, but rather energies of                into a spectrum and                 28
                                                                                                   20             50                 Unknown nuclei                                                                                                        The breakthrough did not come until the middle of the 20th cen-                                                          tion, the spiral arms do not unwind, but maintain their shape for

                                                                                                                                                           ILLUSTRATIONS: ROHRER, BASED ON ORIGINALS FROM ROLAND DIEHL, MPI FOR EXTRATERRESTRIAL PHYSICS
             radiation, which can measure as                     sort it by wavelength.                                                                                                                                                                                                                                                                                             billions of years. One explanation for this is shockwaves that com-
                                                                                                 8                                                                                                                                                         tury, when astronomers had learned to look at the sky with differ-
             much as 1020 electron volts – far                   This results in a rain-                  28
                                                                                                2      20          “Magic” nucleon numbers                                                                                                                 ent eyes: with radio telescopes.                                                                                         pact the matter in the spiral arms and that propagate throughout
             more than can be generated by par-                  bow, the colors of               2 8 NUMBER OF NEUTRONS                                                                                                                                                                                                                                                            the whole system like a traffic jam on the highway. Researchers
                                                                                                                                                                                                                                                           Hydrogen is the most common element in the universe. As part of
             ticle accelerators on Earth. “If we                 which represent the dif-                                                                                                                                                                                                                                                                                           are still puzzling over what causes these density waves.
                                                                                               Characteristic gamma rays provide access to the                                                                                                             interstellar matter, neutral hydrogen (H1) fills the space between
             had gamma-ray eyes, we would see                    ferent kinds of energy.                                                                                                                                                                   the stars, and thus fills the Milky Way. This means that the distrib-

                                                                                                                                                                                                                                                                                                                                            ILLUSTRATION: ROHRER
                                                                                               world of atomic nuclei – both in the universe                                                                                                                                                                                                                                                                             Bulge
             the Milky Way in the light of                       “We use the color infor- and in particle accelerator laboratories on Earth.                                                                                                               ution of clouds of hydrogen gas trace the shape of the whole sys-
             processes that are familiar to us from              mation in the spectrum                                                                                                                                                                    tem, similar to the way in which bones shape the human body. But
                                                                                                                                                                                                                                                           how can these cosmic “bones” be made visible? The nanouniverse                                                                   Galactic disk
             high-energy physics in particle ac-                 to learn about the underlying                   events in atomic shells; lines in
                                                                                                                                                                                                                                                           provides the answer: in the ground state of hydrogen, the spin di-
             celerator laboratories,” says Diehl.                processes,” explains Diehl.                     gamma-ray light, however, reflect                                                                                                         rections of the atomic nucleus and the electron that orbits around
             Rather than normal stars, flashes                      This method might sound compli-              processes inside the atomic nucleus.                                                                                                      it are antiparallel. If two hydrogen atoms collide, the spin direc-
             from pulsars and searingly bright in-               cated, but its basics are familiar to           Furthermore, gamma radiation is                                                                                                           tions of the nucleus and the electron may be flipped to end up
             terstellar gas would determine the                  everyone of us from chemistry                   very energetic with extremely small                                                                                                       parallel to each other – and after a certain time, they return to
                                                                                                                                                                                                                                                           the antiparallel ground state.
             overall picture of the galaxy, and                  lessons at school: sprinkling sodium            wavelengths – between one billionth
             this Milky Way band would show up                   into the flame of a Bunsen burner               and ten trillionths of a millimeter.                                                                                                      This process releases energy, which is radiated as an electromag-                                                                                                         Globular star clusters
                                                                                                                                                                                                                                                           netic wave. Its wavelength is 21.049 centimeters (frequency:
             prominently in the sky.                             turns the flame yellow and the spec-            Spectral lines are characterized by
                                                                                                                                                                                                                                                           1420.4 MHz), and therefore lies in the radio range of the electro-                                                       From this perspective, the Milky Way looks like a fried egg
                Astrophysicists distinguish be-                  trum exhibits an intense line at a              their energy, measured in electron                                                                                                        magnetic spectrum. Despite the extremely low density of interstel-                                                       in a pan – just 5,000 light-years thick at the center, flattening
             tween essentially four sources of                   wavelength of 589.2 nanometers                  volts, as stated above. Despite their                                                                                                     lar matter, atoms will always collide frequently enough to cause                                                         toward the outside, and 100,000 light-years in diameter.
             cosmic gamma-ray quanta:                            (millionths of a millimeter); and               penetrating power – and fortunately

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                                                                                                                                                                                                                                                                                          Shell structure of   Gravitational collapse   Collapsing star shell   Supernovae as
                                                                                                                                                                                                                                                                                            a massive star      resulting from the        and supernova         retorts: When massive
                                                                                                                                                                                                                                                                                                                   end of central            shockwave
                                                                                                                                                                                                                                                                                                                  nucleus burning                               stars explode at the
                                                                                                                                                                                                                                                                                                                                                                end of their lives, they
                                                                                                                                                                                                                                                                                                                                                                create large quantities
                                                                                                                                                                                                                                                                                                                                                                of elements heavier
         for life on Earth – the Earth’s atmos-             print of the annihilation of electrons          life of about 700,000 years. In the                                                                                                                                                                                                                 than hydrogen and             strument was built by scientists led
         phere is too thick for cosmic gamma                and positrons was expected to be                1990s, researchers around Roland                                                                                                                                                                                                                    helium. During such           by Volker Schönfelder from the Max
                                                                                                                                                                                                                                                                                                                                                                events, complicated
         rays. That is why high-energy physi-               (“The Dark Side of the Milky Way,”              Diehl detected this relatively long-                                                                                                                                                                                                                processes take place
                                                                                                                                                                                                                                                                                                                                                                                              Planck Institute for Extraterrestrial
         cists must resort to satellites in space           page 40 ff.).                                   lived radioactivity over wide areas                                                                                                                                                                                                                 in the interior of            Physics, together with French col-
         – such as the European observatory                                                                 along the plane of the Milky Way.                                                                                                                                                                                                                   these bursting stars.         leagues at the Centre d’Etude Spatiale
                                                            RADIOACTIVITY IMPLIES                                                                                                                                                                                                                                                                               When it’s over, all
         Integral (INTErnational Gamma Ray                                                                  This was considered direct evidence                                                                                                                                                                                                                                               des Rayonnements in Toulouse and
                                                            PRODUCTIVITY                                                                                                                                                                                                                                                                                        that remains of these
         Astrophysics Laboratory). Scientists                                                               that radioactivity is omnipresent in                                                                                                                                                                                 Proto-                                                       several other European research
                                                                                                                                                                                                                                                                                                                                                                supernovae are com-
         used such observatories orbiting in                But gamma-ray emission from the                 the universe, and that new atomic                                                                                                                                           Shockfront region:                neutron star:
                                                                                                                                                                                                                                                                                                                                                                pact neutron stars or
                                                                                                                                                                                                                                                                                        nuclear fusion reactions          neutrino heating in
         space beyond the Earth’s atmosphere                Milky Way reveals even more. In                 nuclei are being produced in large                                                                                                                                          (explosive nucleosynthesis)       the shockfront region from within     exotic black holes.              SPI determines the energy of gam-
         to discover a well-defined spectral                1978, researchers found a line in the           quantities in our galaxy more or less                                                                                                                                                                                                                                             ma rays with a never before attained
         line at 511 kilo-electron volts (keV)              spectrum at 1808.65 keV. This is the            at the present time.                                                                                                                                                                                                                                                              precision. The germanium detectors

                                                                                                                                                                                       ILLUSTRATION: ROHRER, BASED ON AN ORIGINAL FROM ROLAND DIEHL, MPI FOR EXTRATERRESTRIAL PHYSICS
         in the gamma-ray spectrum of the                   energy physicists predicted for the                This finding by the Max Planck                                                                                                                                           text of another discovery: Traces                    still replenishing radioisotopes to-             in the instrument are cooled to 90
         galaxy: exactly where the finger-                  aluminum isotope 26Al with a half-              scientists was a surprise in the con-                                                                                                                                       from the decay of the same alu-                      day. Cassiopeia A, for example, ex-              degrees Kelvin (minus 183 degrees
                                                                                                                                                                                                                                                                                        minum isotope, 26Al, had been                        ploded just 350 years ago in the                 Celsius), and at regular intervals, a

                                                                                                                                                                ILLUSTRATION: ROHRER
                                                                                                                                                                                                                                                                                        shown as early as the end of the                     Milky Way. When astrophysicists                  sophisticated heating method repairs
                                                The Cycle of the Elements                                                                                                                                                                                                               1970s to have existed in meteorite                   study the gamma-ray lines of iso-                damage done to their delicate crystal
                                                                                                                                                                                                                                                                                        samples of the early solar system.                   topes such as 26Al, 56Co (cobalt) or             structures by the heavy bombard-
           The universe is like a chemist’s laboratory: nature has created 83                                                                                                                                                                                                           This radioactive energy obviously                    44Ti (titanium), they learn much                 ment of cosmic radiation. In this
           stable chemical elements with a total of 284 atomic nucleus con-                                                                                                                                                                                                             contributed to melting matter in the                 about the cycle of elements in the               way, the sensors maintain their spec-
                                                                                                  CONDENSATION              Dense
           figurations (isotopes). The two most abundant elements, hydrogen                                             molecule clouds
           and helium, originate from the Big Bang around 14 billion years                                                                 STAR BIRTH                                                                                                                                   embryonic solar system, and thus                     cosmos (see box, left).                          tral precision for years – and consid-
           ago, and still comprise approximately 98 percent of normal bary-                                                                                                                                                                                                             mediated the formation of rocks                                                                       ering the time-consuming measure-
                                                                                        Interstellar                                                                                                                                                                                                                                         A SUPERNOVA            AS
           onic matter in the universe today; back then, small quantities of              matter                                                                                                                                                                                        from which the planets formed                                                                         ment procedure, this is crucial: gam-
           lithium and beryllium were also created. As chemical evolution                                                                                                                                                                                                                                                                    A MIDWIFE?
                                                                                                                                                        Stars                                                                                                                           around 4.5 billion years ago. So, on                                                                  ma-ray photons from 26Al decay
           continued, fusion reactions in the interiors of stars then formed                                                                                                                                                                                                            the one hand, there was the gamma-                   But what does the birth of our solar             rarely hit the instrument, typically at
           the heavier elements, up through iron.
                                                                                                                 Dust         STAR WINDS                                                                                                                                                ray signal from 26Al from the                        system have to do with a supernova?              intervals of minutes, and many
           In this way, massive stars use nuclear fusion to create the energy                                                                                                                                                                                                           galaxy, marking the creation of this                 Some scientists suspect that a cosmic            months pass before the astrophysi-
           they need to prevent collapsing under the huge force of gravity
           until, finally, the interior of the star is composed of iron – the
                                                                                                                                           SUPERNOVAE                                                                                                                                   isotope in the very recent past, cos-                catastrophe of this nature was liter-            cists have gathered the thousands of
                                                                                       Chemical elements
           most stable element. Elements that are heavier than iron (such as           in free space                                                                                                                                                                                    mologically speaking; and on the                     ally the trigger for interstellar mater-         these gamma-ray photons required to
                                                                                       (for example 26Al)                 Supernova
           silver, gold, lead and platinum) are created by the successive cap-                                             remains                                                                                                                                                      other hand, there were indications                   ial to accumulate and form the                   create a useful spectrum.
           ture of neutrons. This process probably takes place in the interior                                                                                                                                                                                                          of considerable quantities of 26Al                   primeval solar nebula; “an almost                   Roland Diehl’s team recently pre-
           of many kinds of stars, presumably also in supernova explosions                                                                                                                                                                                                              that had played a role in forming                    anthropocentric view,” in Diehl’s                sented its latest results in Nature. The
           and in the firework-like merging of two neutron stars.                  Coming and going in the cosmos: This simplified illustration
                                                                                   shows the cycle of the elements. Stars are born out of the                                                                                                                                           the environment of the early solar                   opinion, “then a very nearby source              scientists used SPI to search for vari-
           The nuclear reactor of a low mass star, such as our Sun, converts       interstellar matter, and after widely varying periods of evolution,                                                                                                                                  system.                                              of nucleosynthesis would have been               ations in this gamma radiation along
           hydrogen to helium over billions of years. If the supply of fuel runs   return their “ashes” to the medium. Elements such as 26Al
           low, the interior of the star will consist only of helium, the “ash”
                                                                                                                                                                                                                                                                                          The isotope is apparently ubiqui-                  an extra bonus for us.” The Max
                                                                                   drift about for only a relatively short time in the interstellar
           from hydrogen burning. At this stage, the globe of gas expands to                                                                                                                                                                                                            tous. “We believe that 26Al is largely               Planck researcher links this with
                                                                                   gas clouds that exist over periods of 100 million years.
           become a Red Giant. In stars that are more massive than the Sun,                                                                                                                                                                                                             a by-product of cosmic nucleosyn-                    specific questions: Could 26Al input
           helium may ignite and burn to produce carbon and oxygen. Stars in       elements such as germanium, lead or platinum as the iron nuclei                                                                                                                                      thesis processes,” says Roland Diehl.                from such special regions distort our

                                                                                                                                                                                                                                                                                                                                                                                                                                 PHOTOS: CESR
           this late stage of evolution develop strong winds: light from the       successively capture neutrons. Astrophysicists consider the shock-
                                                                                                                                                                                                                                                                                        In other words, the astrophysicists                  gamma-ray view of the Milky Way
           interior becomes so intense that it overcomes gravity and carries       wave that travels through the exploding star at high speed and
           the outer envelope away. After some time, only the more compact         with great kinetic energy, pulling a kind of “explosive nuclear fu-                                                                                                                                  measure radioactivity wherever new                   as a whole? And: How much ra-
           central part of the giant star remains, called a White Dwarf. In this   sion flame” in its wake, to be another point at which elements are                                                                                                                                   atomic nuclei are produced in large                  dioactivity does the galaxy contain
           way, chemical elements produced inside the star may escape and          created.                                                                                                                                                                                             quantities. Thus, aluminum, which                    in total?
           mix with the interstellar matter.                                       Whether the cause is a more or less gentle stellar wind or a violent                                                                                                                                 also originates from the interiors of                  Diehl and his colleagues are hop-
           Stars evolve more intensely and rapidly the more massive they are.      bang, the element-enriched debris from the star mixes with inter-                                                                                                                                    massive stars, is released in stellar                ing that the answer will be found
           Nuclear reactions in their cores produce increasingly heavy ele-        stellar matter, which is how 26Al is spread in space. The gamma                                                                                                                                      explosions and leaves its fingerprints               with a spectrometer called SPI on
           ments. This ordered nucleosynthesis proceeds faster with subse-         rays from this isotope can be measured over millions of years – so
                                                                                                                                                                                                                                                                                        in the gamma radiation of supernova                  board the Integral satellite. This in-
           quent stages, creating magnesium or aluminum, and then finally          there is much time available between release from their source and
           iron. Because the iron nucleus has the maximum nuclear binding          their radioactive decay. At some point, much later, the well-mixed                                                                                                                                   remnants, for example in the spectra
           energy, production of even heavier elements requires great              gas composite fragments into dense interstellar clouds, from which                                                                                                                                   of SN 1987 A or Cassiopeia A.
                                                                                                                                                                                                                                                                                                                                                          The SPI gamma spectrometer on the Integral satellite (top left).
           amounts of energy. Therefore, a dynamic environment is needed           spheres form, in which new fusion reactors light up, and the cycle                                                                                                                                     This explains galactic radioactivity
                                                                                                                                                                                                                                                                                                                                                    Unlike light, gamma rays cannot be bundled. An image is achieved by
           with occasionally high neutron density – conditions such as those       is complete. The Sun belongs to one of the later star generations,                                                                                                                                   rather well. Nucleosynthesis un-                                   casting a shadow with the pattern of holes of a tungsten mask
           that prevail in a supernova. According to theory, the r-process         formed from enriched gas. Our human bodies consist of atoms that
                                                                                                                                                                                                                                                                                        doubtedly took place in massive                                (top right). A type of camera made of 19 semiconductor detectors
           takes place seconds after the beginning of the explosion and forms      were once inside stars: we are children of the cosmos.
                                                                                                                                                                                                                                                                                        stars billions of years ago, but it is                     (bottom right) transforms gamma-ray interactions into voltage pulses.

    24    M   A X   P   L A N C K   R   E S E A R C H   1/2006                                                                                                                                                                                                                                                                                                                         1/2006      M   A X   P   L A N C K   R      E S E A R C H   25

                                                                                                GRAPHIC: MPI FOR EXTRATERRESTRIAL PHYSICS

                                                                                                                                            galaxy, we see stars or gas clouds                                                 fairly familiar with the geometric
                                                                                                                                            that move either toward or away                                                    structure of the galaxy, we can as-
                                                                                                                                            from us. This is where the Doppler                                                 sign the intensity to the relevant
                                                                                                                                            effect comes into play, which anyone                                               distances of the sources, and thus
                                                                                                                                            watching a Formula 1 race on televi-                                               estimate the total quantity of ra-
                                                                                                                                            sion experiences: when the race car                                                dioactive aluminum-26.”
                                                                                                                                            speeds past the microphone, the                                                       The result is surprising: the astro-
                                                             Energy                                                                         sound of the engine changes from                                                   physicists in Garching found three
                The SPI gamma spectrometer has allowed scientists
                                                                                                                                            high to low. In the optical equiva-                                                solar masses of aluminum-26 in the
            at the Max Planck Institute for Extraterrestrial Physics                                                                        lent, light waves from a source are                                                Milky Way. “This isotope is extreme-
                  to measure the gamma-ray line from radioactive                                                                            compressed (higher frequency) as                                                   ly rare in the universe. For example,
            aluminum-26 at such high precision that, for the first                                                                          they approach a stationary observer                                                the initial quantity in the young Sun
             time, they can derive conclusions about astrophysical                                                                          and stretched as they recede from                                                  was just fifty parts per million of
                         processes based on variations of its shape.
                                                                                                                                            the observer (lower frequency). As                                                 that of the stable isotope, which is
                                                                                                                                            color depends on frequency and                                                     aluminum-27,” explains Diehl.
                       the plane of the Milky Way and thus                                                                                  hence wavelength, the light seems to
                                                                                                                                                                                                                               A LANDSCAPE BEHIND
                       pinpoint their place of origin. To this                                                                              shift into blue in the first case and
                                                                                                                                                                                                                               CLOUDS OF GAS
                       end, they used a natural physical                                                                                    into red in the second.
                       phenomenon and its consequences:                                                                                        The researchers did indeed find a                                               Apparently there are very efficient
                       the differential rotation of the Milky                                                                               shift of this kind in the color (ener-                                             26Al factories in the Milky Way. The-

                       Way and the Doppler effect.                                                                                          gy) of the gamma-ray line from                                                     orists have long suspected super-
                         The galaxy rotates around its cen-                                                                                 26Al. This fits well with the assump-                                              novae of being the main suppliers of
                       ter, not rigidly like a wheel, but at                                                                                tion that the radioactive sources re-                                              this isotope. Diehl and his colleagues
                       different speeds for different dis-                                                                                  ally are part of the inner regions of                                              have concluded from their measure-
                       tances from the “hub,” so that the                                                                                   the galaxy, rather than being locat-                                               ments that, every century, two mas-
                       “neighborhoods” along the radius                                                                                     ed in foreground regions; further-                                                 sive stars must explode as super-
                       change with time. At the location of                                                                                 more, they rotate at just the speeds                                               novae within the Milky Way to en-
                       our Sun, the orbiting speed is about                                                                                 that were found in other observa-                                                  sure the supply of observed 26Al. The
                       220 kilometers per second. Looking                                                                                   tions of the central regions of the                                                scientists therefore consider this to
                       from our point of observation to-                                                                                    Milky Way. Roland Diehl takes it                                                   confirm not only theory, but also
                       ward the central regions of the                                                                                      one step further: “Because we are                                                  findings in other galaxies similar to
                                                                                                                                                                                                                               our Milky Way, where astrophysi-
                                                                                                                                                                                                                               cists have observed just such a rate
                                                                                                                                                                                     PHOTO: MPI FOR EXTRATERRESTRIAL PHYSICS

                                                                                                                                                                                                                               of stellar explosions.
                                                                                                                                                                                                                                  Gamma radiation opens up a view
                                                                                                                                                                                                                               onto a landscape that lies concealed
                       Offset of the 26 Al line

                                                                                                                                                                                                                               behind dense interstellar gas clouds
                                                                                                                                                                                                                               in the optical window. In coming
                                                                                                                                                                                                                               years, the researchers want to con-
                                                                                                                                                                                                                               tinue their measurements with Inte-
                                                                                                                                                                                                                               gral to make them more precise.
                                                                                                                                                                                                                               Once it was the light of innumerable
                                                                                                                                                                                                                               stars that excited mankind; today,
                                                                                                                                                                                                                               researchers enthuse over the diffuse
                                                                                                                                                                                                                               glow of radioactivity. “Each isotope
                                                                                             Galactic longitude
                                                                                                                                                                                                                               contributes its own story,” says
                       Through the Doppler effect, offsets in the aluminum-26 gamma-ray line energy provide in-                                                                                                                Roland Diehl. That of aluminum-26
                       sight into the inner regions of the galaxy. The illustration shows the energy of the gamma-
                                                                                                                                                                                                                               is particularly illuminating, not only
                       ray line and thus the direction of motion in the central Milky Way, versus the viewing di-
                       rection in units of galactic longitude. Regions to the right of the central region (0 degrees)                                                                                                          for understanding the interior of our
                       are receding, while regions on the left are approaching the observer. In the first case, the                                                                                                            galaxy, but also the early history of
                       researchers measure a lower, in the second a higher, energy value for the aluminum line.                                                                                                                our solar system.        HELMUT HORNUNG

                  26                  M           A X   P   L A N C K   R   E S E A R C H   1/2006

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