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Stellar Evolution

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					                   Stellar Evolution

Star birth in the Eagle Nebula Courtesy of the Space Telescope Science Institute
• Human lifetimes vs. ages of stars
• How do we know . . .?
  – Humans via pictures
     • In one day, take pictures of people, then
       piece together human behavior &
       history; similar to finding the life history
       of stars
• Theories tested, modified, some
  completely rewritten
• Many questions remain
         The Birth of a Star
Nebulae = more than
  one nebula
• Vast clouds of gas
  in space
• Mainly hydrogen
• Disturbance
  – Colliding with other
  – Blast from nearby
    supernova explosion
   The Birth
   of a Star
The Birth of a Star
 Rotating cloud collapses in
   on itself
 As center of the cloud                       Particles far
                                              apart don’t exert
   becomes more dense,                        much gravity on
   collapse accelerates due                   each other
   to increased gravitational
   attraction between gas
 Collapsing clouds mark the
   formation of a protostar         The same particles, now
                                    closer together, exert more
   (not yet a true star; no nuclear
                                    gravitational force on each
     reactions occurring yet)
                            The Birth of a Star
    As the clouds continue to
    collapse it begins to warm
                                      Out here, the gas particle
• When the gas particle              has both kinetic & potential
  collides with the center of                            energy
  the cloud,
    – it loses kinetic energy
      because it slows down
    – It loses potential energy
      because it isn’t so far away
      from the middle of the           Center of
      cloud.                           gas cloud
• This energy turns into
The Birth of a Star          As the temperature
                             increases, these
                             hydrogen particles
• Warming occurs slowly      move faster.
                             Eventually, they move
  at first                   so fast that when they
• Center begins to glow,     collide they’ll stick
  dim to bright
                             A helium nucleus has
• When central               been formed!
  temperature is high        When this
  enough (~15 000 K, ~15     “sticking” (fusion)
  273 C) nuclear reactions   occurs, a bit of
                             mass is converted
  can begin                  to energy as in
• Protostar has now          E = mc2
  become a true star
*OK, really . . .
The Birth of a Star
                  • Stars can form from
                    extremely large
                    interstellar clouds
                    that have
                    fragmented into
                    smaller clouds.
                  • These clusters of
                    stars are called . . .
                    Star clusters (!)
                  • Ex: The Pleiades
                    (Seven Sisters)
           The Birth of a Star
• The haze (“nebulosity”) is
  part of the original gas
  cloud that’s left over.
• How long does formation
  – Small low mass stars can
    take billions of years to
  – More massive stars can
    completely form in a few
    hundred thousand years
               Main Sequence
•   Star has settled into the
    most stable part of its life
•   Converts hydrogen to
    helium (H => He)
•   Next step depends on the
    mass of the star
•   Three different examples of
    1. Stars similar to our Sun
    2. Stars several times more
       massive than the Sun
    3. HUGE HUMONGOUS stars,
       VERY massive
     The Life of a Sun-like Star
• Will remain on the main sequence (H to He) for
  about 10 billion years
• As more He is produced, temperature increases and
  core contracts
  – We see this as an increase in brightness
  – Temperature not high enough to sustain He to C fusion
• Central core then expands
as more He is produced
  • Star expands, becoming a
  Red Giant
  • Our sun, as a red giant, will
  be as large as Earth’s
  present orbit
The Life of a Sun-like Star
        • Over thousands of years, the
          star’s central region shrinks &
          heats up.
        • Outer regions are pushed
        • We see:
          – a small, dense central star
          – surrounded by expanding shell
            of gas
        • The star is now a planetary
     The Life of a Sun-like Star
• The object seen at the center of the gas cloud
  is the core of the original star
• Still very hot (~100 000 C)
• Gradually cools & contracts to become a white
• Cools even more to become a black dwarf; not
  much bigger than Earth, but much more dense
The Life of a Sun-like Star
 The Life of a Star Several Times
   More Massive Than the Sun
• Enters main
  sequence (H to He
  process) at a higher
  temperature than
  smaller stars
• Core is hotter than
  smaller stars,
  causing faster
• After all H is
  converted to He, He
  is fused into carbon
  (requires 100 million
The Life of a Star Several Times
  More Massive Than the Sun
• After all the He is used, C fuses into neon (requires 500
  million degrees)
• As each element is used up, star becomes a red giant.
• . . . And so forth, as long as temperatures are high
  enough to fuse that particular element
• As particles that are colliding get larger, much more
  heat (energy) is needed to get them to stick together
  The Life of a Star Several Times
    More Massive Than the Sun
• When an iron core is
  – Reactions STOP
  – Iron fusion requires HUGE
    amounts of energy
  – Eventually, cools to white
    dwarf, then black dwarf
  – Different than smaller
    star’s fate because
    different elements will
    compose the core
     The Life of HUGE Stars
• As with all other stars, follows main
• If the star is still large (>1.4 Suns) when
  the core becomes iron, a supernova
        The Life of HUGE Stars
•Within seconds of running out of nuclear fuel, the
HUGE gravitational force (remember, large mass =
large gravity) attracts all of the atmosphere into the

          The Life of HUGE Stars
• As particles fall to the core they lose kinetic & potential
  energy and more HEAT results
• This heat triggers nuclear fusion in the outer layers,
  and the resulting explosion is the supernova.
• The energy released can fuse iron and other heavier
  elements, up to uranium.
The Life of HUGE Stars
      “This next image is one of the most
        spectacular views of 1987A yet
    acquired by the HST. The single large
         bright light is a star beyond the
        supernova environs. Around the
     central supernova is a single ring but
•      associated with the expansion of
    expelled gases are also a pair of rings
       further away that stand out when
    imaged at a wavelength that screens
          out much of this bright light.”
    The Life of HUGE Stars
• The death of the largest stars results in
  a core more dense than anything we
  know on earth
• This core has such a large gravitational
  force that light cannot escape it.
• . . . Hence the name, black hole
• Picture here
                                         Caption: In this image, X-ray contours
                                           are overlaid on an optical image. The
                                           X-ray contours and the colors in the
                                           optical image represent brightness
                                           levels of the X-ray and optical
                                           emission, respectively. When viewed
                     QuickTime™ and a
                                           with an optical telescope this galaxy,
           TIFF (Un compressed) decompressor
                                           located 2.5 billion light years from
              are neede d to see this picture.

                                           Earth, appears normal. But the
                                           Chandra observation discovered an
                                           unusually strong source of X rays
                                           concentrated in the central regions of
                                           the galaxy. The X-ray source could be
                                           another example of a veiled black
                                           hole associated with a Type 2
CXO 0312 Fiore P3 (CXOUJ031238.9-          Quasar. This discovery adds to a
765134): A possible Type 2 quasar veiled   growing body of evidence that our
black hole.(Credit: X-ray: NASA/CXC/SAO; census of energetic black hole
Optical: ESO/La Silla)                     sources in galaxies is far from

Some artists’ conceptions of a
         black hole
                The Life of HUGE Stars
• How do we know a
  black hole exists?
• Evidence
  – Strong x-ray emissions
    from charged particles
    accelerating REALLY
  – Gravitational lensing
     • Light from stars is bent
       when a black hole is
       between us & the stars
• Usually form in binary
  star systems
We are all made of stars . . .

 For next time: read through Chapter 20, sections 2 &
 3 and answer the questions at the end of each
 section. Quiz next time!

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