Chapter 24 The Galaxy The Milky Way Galaxy = The Galaxy Phenomenon of the Milky Way in the Sky Milky Way = the band of faint white light that encircles the entire inside of the celestial sphere (it is brighter in the direction of the summer night sky) Significance of this observation about the structure of the Galaxy: we live in a flat disk shaped galaxy and we are not at the center of it Structure of the Galaxy I. Flat wheel shaped disk 100,000 LY in diameter with two parts o Thin disk: 1,000 LY o Thick disk: 3,000 LY in thickness In these disks lie the o Open star clusters o Young O and B associations o GMC’s o Cold H I regions o Hot H II regions II. Spherical concentric halo a few times that diameter, contains old globular star clusters III. Central bulge 24,000 LY in diameter, contains older stars spaced very close together Old planetary nebulas are found in all parts Sun’s Location in the Galaxy In the disk, about 26,000 LY (0.6 of the way out) from the center Harlow Shapley, using the RR Lyrae variable stars in the globular star clusters, first determined this Spiral Structure of the Galaxy’s Disk At optical wavelengths dust prevents a complete view of the disk, we can only “see” about 1,000 to 3,000 LY into the disk Optically we do see bright blue O and B spectral class stars along the spiral arms near us (just as they are seen along spiral arms of other galaxies) Must use radio observations of cold H I clouds in the disk to map out the spiral structure These clouds emit 21 cm radio waves Stellar Populations “Metallicity” – astronomers put the elements into only three groups: hydrogen, helium, and “everything else” “Metals” refers to “everything else” and includes some elements that are gases Population I – stars with relatively large percent of metals (and objects related to these kinds of stars). Our sun and other objects from spiral arms o Open clusters o Young stars o Cepheid variables o Associations Population II – stars with very low percent of metals (considered “1st generation stars”) o Globular star clusters o Dim, red main sequence stars o RR Lyrae variable stars Rotation of the Galaxy The motion of the Sun with respect to distant globular star clusters has been measured Doppler effect in their spectra gives a roughly circular orbit with a velocity of about 220 km/sec (toward Cygnus) The Sun’s galactic orbit has a circumference of 2(26,000 LY) To travel this distance at 220 km/sec requires about 225 million years Mass of the Galaxy – use of Newton’s revision to Kepler’s 3rd law (m1 + m2) p2 = a3 p = 225 million years and a = 26,000 LY (converted to AU), about 100 billion solar masses If the typical mass per star is ½ solar mass then the number of stars in the Galaxy is at least twice this: 200 billion stars (there may be more mass than this!) Dark Matter in (and around) the Galaxy The disk of the Galaxy contains lots of interstellar dust, but this is NOT dark matter Dark matter = material that is not emitting any kind of radiation, invisible at all wavelengths The presence of this dark matter has been deduced by studying the orbital motion of various objects in the Galaxy A “galactic rotation curve” shows that the velocity of objects does NOT fall off with increasing distance from center, but stays “flat” 90 percent of the mass of the Galaxy may be this “dark matter” Could be: o Black holes o Dim red stars o Brown dwarfs o Neutrinos or other exotic particles o Weakly Interacting Massive Particle (WIMPs) Real reason has not been discovered Nucleus of the Galaxy The strongest radio source in Sagittarius is called “Sagittarius A” It lies about 24,000 LY beyond the stars that make the pattern of Sagittarius The VLA = Very Large Array radio telescope system in New Mexico has resolved this into components as small as 3 AU across. The part known as “Sagittarius A*” lies at the center of the Galaxy and is believed to contain a super massive black hole. The diameter of the event horizon of a black hole with 1 million solar masses would be 6 million km (3.5 million miles) As matter gradually falls toward the SMBH an accretion disk has formed and is responsible for the strong radio signals The Origin and Evolution of the Galaxy The age of the universe is estimated to be about 13.7 billion years and the age of the Galaxy is somewhat less Soon after the formation of the universe, from the “Big Bang”, only energy existed. Later, matter began to condense from the energy After more than 400,000 years galaxies such as ours began to form from the huge collections of (mainly) hydrogen atoms The huge collections of gas began a gravitational contraction to form the parts of the Galaxy we know now: o Halo is the oldest part in the senses that stars and globular star clusters of stars are still around that formed early o Disk formed due to the rotation of the proto galaxy. Both young and old stars are found in the disk and it is the only part where stars are still forming Chapter 25 Galaxies The True Nature of the “Nebulae” Catalogs of nebulas: o Messier (French comet hunter) o New General Catalog (William Herschel) The Great Spiral Nebula in Andromeda: o M31 in the Messier Catalogue o NGC 224 in the NGC Hubble’s Distance to M31 He used Cepheid variable stars and the “period luminosity relationship” It is now the “the Great Spiral Galaxy in Andromeda”, 2,500,000 LY (770 kpc) away Classification of Galaxies Based on apparent shape seen in sky: Elliptical (E) o Range: circular (E0) to elongated (E7) o Contain little gas/dust, no current star formation Normal Spiral (S) o Range: large nucleus and tightly wound spiral arms to small nucleus and loosely wound arms (Sa > Sb > Sc) Barred Spiral (SB) o Range: SBa > SBc like normal spirals but arms emerge from a “bar” in the center Irregular (Irr) o No symmetry, have gas/dust, young/old stars Properties of Galaxies Amount of rotation of original proto galaxy may be the main reason for different galaxy types o Little or no rotation > elliptical o Rotation > spiral The irregular galaxies tend to be the smallest, the dimmest, and ones with fewer stars The spirals tend to be large, but the ellipticals are the largest of all The ellipticals also have the greatest range in size, brightness, and number of stars (can be giant or dwarf, nothing in between) The ellipticals have little or no observable gas or dust and thus no new star formation, unlike spirals and irregulars Most of the apparently bright galaxies are of the spiral type, but if we include faint dwarf ellipticals and faint irregulars, spirals are actually in the minority The most numerous class of the galaxy is the irregular Methods of Determining Distances Measurements of the properties of galaxies depends on knowledge of their distances Astronomers use a variety of methods that generally build upon one another The fundamental distance determination method is triangulation or surveying, which uses the parallax of nearby stars (d = 1/p) For greater distances other methods are used. One of these can be called the “Standard Light Bulb” method (formerly “Standard Candle”) Basically this means finding some way to estimate the true brightness of an object and then calculate how far away it has to be to look that dim Examples I. Variable stars (RR Lyrae, Cepheid) o These distance indicators are easily recognized by their colors, their periods, and amplitudes of light variation. RR Lyrae stars all have M = 0.5 while the Cepheids obey the Period-Luminosity relation. II. Novas, supernovas o These eruptive variables occur in our Galaxy and in nearby galaxies where we can estimate their maximum brightness. When one occurs in a more distant galaxy, we assume it has the same M as the ones closer to us The “Law of the Redshifts” Slipher discovered that 38 of 41 bright “spiral nebulas” (galaxies) had redshifts in their spectra (i.e. they are moving away from us) Edwin Hubble and Milton Humason continued this work by obtaining spectra of more galaxies They found that the fainter and presumably more distant galaxies always had greater redshifts in their spectra (“Law of the Redshifts”) Let z = “redshift” z = [WL(observed) – WL(rest)] WL(rest) Assuming z is due to the Doppler effect v = c x [WL(observed) – WL(rest)] WL(rest) v = c x z = cz (velocity in km/sec) The Hubble Law Hubble and Humason also measured the distances (using Cepheids) to the galaxies they got redshifts for and found a linear relationship by converting z to v and graphed vs. the distances they had measured. The slope of the line is called H (for Hubble and Humason) H = v / d = the Hubble “ratio”, Hubble Constant, measured in km/sec per Mpc (or km/sec per MLY) Thus v = H x d = Hd “Hubble Law” Assume H = 25 km/sec per MLY, and then solve for the distance, d d = v/H = cz / H = 300,000 z / 25 d = 12,000 z (in MLY) Example: find d if a galaxy has z = 0.05 d = (300,000 x 0.05) / 25 = 12,000 x 0.05 = 600 MLY Notes: use of the Hubble Law to measure distances is the biggest “meterstick” Can use this when no objects of any kind can be resolved in a galaxy, when only the spectrum can be obtained The discovery of the Hubble Law also means that the universe is expanding Chapter 26 Quasars and Active Galaxies Active galaxies = those that have bright nuclei associated with great energy production or energetic outflows of matter (most emit strong radio waves) (Radio galaxies = active galaxies with more radio output than light output) o Seyfert galaxies = spiral galaxies with bright, star-like nuclei, and strong emission lines in their spectra (NGC 1566) o Double lobe = 2 regions of strong radio emission occur on either side of a peculiar galaxy (peculiar means the galaxy does not it well into the Hubble classification scheme (Cygnus A, Centaurus A) o Jet = a stream of material projects from the center of a galaxy. The stream is emitting radio waves, sometimes light and other wavelengths, like x rays (elliptical M 87) The observations of these active galaxies imply that some unknown object exists at the center of an active galaxy The “Look Back” Principle The farther you look into space the earlier back in time you see The reason for this is the finite speed of light 300,000 km/sec (or 1 LY / year) We see the Sun not as it is now, but as it was about 8 minutes ago M 31, at a distance of 2.5 million LY, as it was about 2.5 million years ago We see a galaxy with z = 0.05 not as it is now, but as it was 600 million years ago Quasars A new kind of object was discovered in the early 1960’s based on radio emission Optical photos showed “star like” dots Since stars are not strong radio sources, these were called quasars (from quasi stellar radio sources) 3C 48 was the first quasar discovered Spectra taken to learn about them only added to the puzzle Later, M. Schmidt (1963) explained that these spectra had emission lines with large redshifts (the quasars are moving away very rapidly and are very far away) They must have a great luminosity to be seen at distances of billions of LY Some quasars very in times as short as a few days This implies that the region responsible for the great luminosity must be about a few light days across: only about 10 times the size of our solar system! To explain quasars we must use the “look back” principle: if the quasars are billions of LY from us, we must be seeing objects as they were billions of years ago Quasars must be the hyperactive centers of very distant, thus young galaxies The Power Source of Quasars Thousands of quasars have now been discovered, but only 1% are strong radio emitters (thus “QSO’s”) They are all very luminous and all show big redshifts in their spectra The accretion disks of supermassive black holes are believed to be the power source No radiation can be emitted from the black hole itself! The accretion disk funnels energy out along the two poles of the magnetic field, at right angles to the disk. This produces two strong jets of particles and energy If we see one of the jets nearly face on, it is perceived as a quasar If we see the jets at other angles, we perceive active galaxies with jets or double lobed radio emission Gravitational Lenses Because quasars produce very bright and very narrow beams of light, they can be used to test Einstein’s General Relativity. Space should be curbed in the vicinity of dense concentrations of matter Light from a distant quasar could be deflected as it passes by a massive galaxy or cluster of galaxies billions of LY away In 1979 the first “double quasar” was discovered. It is actually light from one quasar that has been split into two beams and deflected to appear as two The name is “0957+561” based on its position in the sky Einstein “rings”, “crosses”, and “arcs” have also been detected, confirming his prediction of the curvature of space in the vicinity of compact mass Chapter 27 Evolution and Distribution of Galaxies Observations of Distant Galaxies Spectra, colors, and shapes Spectra show “metals” in the most remote galaxies we can now access Thus galaxies formed within the first billion years (maybe within a few hundred million years) of the universe Do galaxies develop or evolve? Nearby galaxies tend not to except for collisions But they must have, early in the history of the universe. To see such galaxies we must look far back into space (and early back in time) We can now do this with the HST and big new ground-based telescopes like Keck I and II For distant galaxies we study their color (blue=young, red=old), and shape (spiral=young, elliptical=old) Long exposure (deep) images obtained through large telescopes show that distant galaxies do not fit into Hubble’s classification scheme They are also generally smaller than nearby, modern galaxies Thus early galaxies evolved or developed with time, primarily by interacting (colliding) with each other The Interaction of Modern Galaxies Currently, galaxies don’t seem to evolve much unless they collide When galaxies collide, stars don’t Reason: separation between galaxies is tens of times their diameters; separation between stars is 10,000,000 of times their diameters In merger two roughly equal galaxies combine. In cannibalism a larger galaxy devours a smaller one. The Distribution of Galaxies in Space Various studies of the distribution of galaxies in space indicate that the universe is isotropic and homogenous Thus, “the universe must look the same to any observer on very large scales”. This is known as the cosmological principle Congregation Levels of Galaxies Name Number of Galaxies Size (MLY) Group 10s 3 million LY Cluster 100s to 1,000s 10 – 20 million LY Supercluster 10,000s 150 – 300 million LY Examples: The “Local Group” (we are in it) The Virgo cluster d=50 MLY The Coma cluster d=250 – 300 MLY The “Local Supercluster” (we are in it) Note: “regular” clusters of galaxies are symmetrical and contain mostly ellipticals The “irregular” clusters of galaxies are not symmetrical and have some spirals “Rich” and “poor” refer to clusters with many and few galaxies, respectively Large Scale Structure of the Universe Detailed studies in certain directions reveal a “sponge – like” distribution of superclusters of galaxies The superclusters of galaxies are like the material of a sponge that surrounds empty pockets or bubbles The empty pockets in the universe are called voids Voids are typically 150 MLY across and superclusters can appear strung out to 600 MLY The Hubble Ultra – Deep Field Formation of the Structure in the Universe Forming voids and filaments The large scale structures are believed to result from processed that occurred when the universe was only a few hundred thousand years old Although the density of the early universe must have been very smooth, the slight irregularities led to the formation of proto-galaxies (pancakes of gas) These proto-galaxies joined to make small galaxies. These in turn congregated to make larger galaxies and clusters of galaxies A Universe of (Mostly) Dark Matter In the neighborhood Our flat galactic rotation curve implies the presence of dark matter Around galaxies Studies of rotation rates of other spirals lead to mass estimates. Flat rotation curves imply dark matter for most spirals, as in our galaxy In clusters of galaxies Galaxies should fly away from each other, but don’t. Must be mass we don’t see. Mass-to-light ratio In units of solar values, the Sun = 1, but for groups, clusters, and superclusters of galaxies the values are in the 100s indicating that most matter in them is not radiating light. Dark matter is NOT dust. What is the dark matter? Dark matter may be 10 times more abundant than luminous matter in most galaxies. We still don’t know what it is. The experiment to look for “ MACHOs” (massive astrophysical compact halo objects) in the halo of the galaxy by their gravitational lensing properties has found far too few to explain the dark matter Chapter 28 The Big Bang Cosmology = the study of the origin, structure, and evolution and fate of the universe (if any) “The universe” (everything that there is!) includes stars, comets, planets, galaxies, etc. and space itself Note: “string theory” postulates the existence of additional dimensions and other possible universes but we shall consider only what we can currently see Observational Cosmology Olber’s paradox: if the universe is infinite, with stars uniformly distributed within it and is static, “why is the night sky dark?” The expanding universe One profound implication of Hubble’s result is that the universe is observed to be expanding In an expanding universe, the velocity at which a galaxy recedes from us increases with increasing distance Are the more-distant galaxies flying through space at ever-greater speeds? No! Space itself is expanding, carrying clusters and superclusters of galaxies in it. The galaxies themselves do not get larger. Important implications of an expanding universe: I. There is no center All observers in the universe see the same result! All other galaxies seem to recede from any given galaxy. Thus there is NOT a center to the universe (it is a meaningless concept… or is it?) II. There is no edge (whether finite or not) III. Gravity tries to slow down the expansion Studies of distant Type Ia supernovas indicate they are fainter than expected This can be explained by an acceleration of the expansion rate Since the cause of such acceleration is unknown, the generic term “dark energy” has been coined for the name IV. There must have been a beginning for time and space! (= the “Big Bang”) We must also realize that it is not appropriate to use the doppler effect to describe redshift Superclusters are not moving through space, rather space expands and separates the superclusters This is the cosmological redshift = space expands. Fortunately this can still be described by an equation like the doppler equation The “Big Bang” Era Time Temp (K) Remarks ? 0 ? “Big Bang” initiated expansion Supergravity 10-43 s 1032 Planck time only radiation existed Inflation 10-32 s 1027 Rapid expansion, 1050 x bigger 10 Particle 1s 10 Protons, quarks, electrons, radiation Nucleo 180-300 s 108 Temp low enough for fusion, H and He Radiation/matter 4x105 years 3000 Universe transparent to radiation Matter 2x108 years 100 Galaxies formed Current 13.7 BY 2.725 Now Observations Supporting the “Big Bang” I. Quasar census: there are no quasars locally, now; but many far away, long ago. This means the universe is changing as the “Big Bang” theory predicts II. Cosmic microwave background radiation = “CBR” was discovered in 1965 by Penzias and Wilson using a (microwave) radio telescope. This radiation comes to us from all directions in space equally and all the time. The “CBR” is very strong evidence of the “Big Bang” event o Point a radio telescope in a random direction o Look out into space as far as possible o The earliest back in time that we can see is when the universe cooled to about 3000 K at age = 400,000 o The peak emission from that spot in the universe was then in the visible o BUT the Hubble Law states that if radiation comes from great distances, it will be redshifted to infrared and radio (microwave) wavelengths The COBE (cosmic background explorer) satellite in 1990 measured the CBR and showed a perfect fit to the theory (T = 2.725 K) Results from the WMAP (Wilkinson microwave anisotropy probe) indicate that the age of the universe if 13.7 BY (plus/minus 0.2 BY). The time at which the universe became transparent to radiation has been pinned down to 379,000 years (plus/minus 8,000 years) Theoretical Cosmology Cosmological principle = universe must look the same to any observer on a very large scale Perfect cosmological principle = universe must look the same to an observer on a very large scale for all time This led in the 1950s to the steady state theory = the universe has always existed and will always exist Evidence of evolution of the universe and the discovery of CBR are inconsistent with this theory “Big Bang” Theory (Plus Inflation) This is the current best explanation that astronomers have developed for the origin and evolution of the universe Inflation had to be added to the BB to explain the “flatness” problem If the universe if 13.7 BY old, then two superclusters in opposite directions are 27.5 BLY apart, but have similar properties Must have formed close together, then sudden drastic expansion (inflation) occurred briefly, THEN normal expansion resumed During the inflation era space ballooned up by a factor of 1050 in a fraction of a second This is easily faster than light but that is okay because nothing was moving through space that fast Future of the BB Universe Depends on which version we live in: o Closed: universe stops expanding after a finite time, then a contracting phase occurs (the “Big Crunch” or oscillation) o Open: universe still expanding after infinite time, eventually everything will get dark and cold everywhere o Flat: universe coasts to a halt after infinite time, this is a unique version of the BB There are three possible geometries for a BB universe o Spherical space: two lasers come together (closed) o Flat space: parallel beams remain parallel forever (flat) o Hyperbolical space: two beams spread apart (open) Which version is it? Measure mass of all things seen in universe, divide by the volume If the average density is: > Critical density >> closed universe < Critical density >> open universe = Critical density >> flat universe Originally we calculated <, which implies an open universe Discovery of dark matter increases the average density to nearly the critical density We think we are in a flat universe Relativistic Evolving BB Models of the Universe (Assumes H = 25 km/s per MLY, critical density = 2x10-29 gm/cm3) Version Age (BY) Curvature Average Density Extent Closed t<9 + > critical finite Open 9<t<14 - < critical infinite Flat t=14 0 =critical infinite Measures of luminous mass indicate an average density less than critical (open universe), BUT including dark matter pushes the average density near the critical density (flat universe) In order to visualize the 3 possible curvatures of the universe, consider these two- dimensional analogs Resolutions of Olbers’ Paradox If any of the underlying assumptions can be shown false, then there is no paradox I. The paradox implicitly assumes a static universe, but we now have evidence that the universe is expanding Thus, light from distant stars can be redshifted to invisibility in the infrared and radio part of the spectrum II. The paradox assumed an infinitely old universe in which light from infinitely far away has had time to get here BB says universe began 13.7 BY ago, so light from states say, 400 BLY away hasn’t had time to get here yet This resolves the paradox The distance of 13.7 BLY is called the radius of the “cosmic horizon” It is equal to the distance in LY given by the age of the universe in years The “observable universe”, on the other hand, is how far we can see with current technology. This has increased dramatically since about 1600! Chapter 29 Life in the Universe Life = carbon based, water based, i.e. “life as we know it” This would seem to require: planets like the Earth, orbiting stars like the Sun The Doppler effect has discovered more than 300 Jovian-like planets orbiting nearby stars None of these are terrestrial type planets yet! Probability of Extra-Terrestrial Life Drake equation (an estimate of the current number, Ncc, of intelligent, communicative civilizations in the galaxy) Ncc is the product of several factors: The first two are astronomical in nature and are reasonably well known The next several are biological in scope and not so well known The last factor is the most uncertain: the longevity of civilizations It is the most important factor and the one for which we have only one known case (ours!) Optimistic Ncc = (1)(0.5)(0.1)(1)(1)(0.5)(40x106) = 1 million! Pessimistic Ncc = (1)(0.5)(0.1)(0.1)(0.1)(0.1)(4x103) = 1!