Chapter 13 Stellar Evolution

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Guiding Questions

• Why do astronomers think that stars evolve?
• What kind of matter exists in the spaces between
  the stars?
• Where do new stars form?
• What steps are involved in forming a star like
  the Sun?
• When a star forms, why does it end up with only a
  fraction of the available matter?
• What do star clusters tell us about the formation
  of stars?
• Where in the Galaxy does star formation take
  place?
• How can the death of one star trigger the birth
  of many other stars?

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Low vs. high mass stars

• Mass is important!
• Evolution of a star depends on its original mass.
• Sun-like stars - mass close to Suns
• Massive stars - mass much larger than Suns (M gt
  5 M? )
• Mass determines the location of a star on main
  sequence.

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Review Basic concepts

• Hydrostatic equilibrium - a balance of gravity
  pulling in and pressure from heat pushing out.
• Luminosity - total energy radiated by a star
  (total brightness)
• depends on size and temperature.
• Nuclear fusion - joining of two nuclei together
  to form different one.
• Requires high temp. and density to overcome
  electrical repulsion of protons.

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Birthplace of Stars

• The matter between the stars are collectively
  termed as interstellar medium. It is made out of
  two components
• Gas Dust
• Any interstellar cloud of gas dust is called a
  Nebula (plural Nebulae)
• Evidence of Nebulae
• Spectral lines.
• Reddening of stars.

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Birthplace of Stars
Star-Forming Regions(Nebulae)

• Emission Nebula A nebula with the characteristic
  emission line spectrum of a hot, thin gas.
• Found near hot, luminous (Type O B) stars
• emission nebulae have masses 100 M? to10,000 M?
  
• An example of such Nebula is the Orion Nebula
• The middle star in Orions sword.

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Birthplace of Stars

• About 450 pc from Earth.
• about 300 M?.

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Birthplace of Stars
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Star-Forming Regions(Nebulae)

• The vast amounts of UV radiation emitted by the
  close by Hot, type O or type B stars are absorbed
  by the Hydrogen atoms in the Nebulae
• these high energy photons strips the H atoms of
  its electron leaving H ions - H II.
• Emission nebulae are referred to as H II regions
• H II regions emit visible light (red) when some
  of the free electrons recombine with protons, and
  the re-captured electrons cascade down to lower
  orbits.

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Star-Forming Regions(Nebulae)

• Most important among these transitions is the n3
  to n2 transition.
• emits 656nm - red photons (H? photons).
• This gives the distinctive red color to the H II
  regions.
• Dark Nebula A nebula so opaque that it blocks
  visible light that are emitted from stars behind
  the nebula.
• Higher concentration of Dust grains
• These look like dark patches.
• Example Horsehead Nebula.

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Emission, Dark and Reflection Nebulae near the
Star Alnitak in the Orion constellation.
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Dark Nebula
Bernard 86 nebula in the constellation of
Sagittarius.
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Star-Forming Regions (Nebulae)

• Reflection Nebula Do not produce its own light
  like emission nebulae, but scatters star light.
• The scattering is due to the dust grains.
• Lower concentration of dust grains than dark
  nebulae.
• Scattering gives rise to Blue color (similar to
  the blue sky on Earth).

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Reflection Nebula
NGC 6726-27-29 in the constellation of Corona
Australis.
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Star-Forming Regions(Nebulae)

• Interstellar Extinction the intensity of star
  light is reduced as light passes through the
  interstellar medium.
• Interstellar Reddening When light from a star
  pass through interstellar medium, dust particles
  absorb or scatter blue light allowing red light
  to pass through (like a Sun set on Earth)
• the star appears red.

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Reflection Nebula
Interstellar Reddening
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Interstellar Reddening
NGC 3576, 2400 pc
NGC 3603, 7200 pc
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Spiral Galaxy M83
The reddish regions in the spiral arms are H II
regions.
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Spiral Galaxy NGC891
The dark band is caused by dust.
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Star-Forming Regions

• Giant Molecular Clouds In certain cold regions
  of interstellar space atoms combine to form
  molecules.
• Molecular H is hard to detect, since they do not
  emit light (radiation)
• However, carbon monoxide present in these clouds
  emit millimeter wavelength light, and thus can be
  detected by radio telescopes.
• these giant clouds have masses ranging from 105
  - 106M?.

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Giant Molecular cloud in Orion Constellation

• About 1000 giant molecular clouds are known in
  our galaxy.
• These clouds lie in the spiral arms of the
  galaxy.

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The Formation of Stars

• Stage 1 An interstellar cloud
• Star formation begins when part of the
  interstellar cloud contracts under its own mutual
  gravitational attraction - denser regions in the
  clouds are favorable for star formation
• The gravitational collapse overwhelms the
  pressure - colder regions are more favorable
  since they are low pressure regions.
• These cold dense regions of clouds collapse
  under its own weight to form clumps known as -
  protostars.

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The Formation of Stars

• Stage 1 Contracting Cloud - star formation is
  triggered when a sufficiently massive pocket of
  gas is squeezed by some external event.
• Material flowing out of protostars cause shock
  waves that trigger regions nearby to collapse.
• A supernova explosion of a dying star can
  compress the surrounding gas triggering a
  collapse.

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The Formation of Stars

• Stage 2 Fragmentation - Contracting interstellar
  cloud fragments into smaller pieces due to
  gravitational instabilities.
• The pieces continue to collapse and fragment,
  eventually to form many tens of hundreds of
  separate stars.

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The Formation of Stars

• Stage 3 Several tens of thousands of years after
  its first began contracting, a typical stage 2
  fragment has shrunk by the start of stage 3 to
  roughly the size of our solar system (still 10,
  000 times the size of our Sun).
• The dense, opaque region at the center is called
  a protostar an embryonic object at the dawn of
  star birth.

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The Formation of Stars

• Stage 4 Protostellar Evolution-
• As a protostar evolves, it shrinks, its density
  increases and it temperature rises. .
• Some 100,000 years after start of the cloud
  collapse, its center gets heated to 1 million K -
  purely due to compression of the gas.
• This hot protostar produce substantial luminosity
  and can be plotted on a H-R diagram.
• As the protostar evolves it collapses, and thus
  its luminosity and temp. changes.

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• Pre-main sequence evolutionary tracks in the H-R
  diagram

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The Formation of Stars

• protostar evolution of a Sun like star .
• Outer layer is cooler and opaque - temperature
  does not increase much.
• However, due to the collapsing of the protostar
  the radius decreases and thus the Luminosity
  decreases.
• The star moves to down(less luminous) and
  slightly to the left (hotter) in the H-R diagram.
• Caution this does NOT represent an actual
  movement of the star in space .

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The Formation of Stars

• protostar evolution of a Sun like star
• After about 10 million yrs. the internal
  temperature gets high enough for Hydrogen burning
  to take place and the pressure due to this
  process will stop the collapse of the protostar -
  the evolutionary track now reaches the main
  sequence.

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The Formation of Stars

• protostar evolution of a massive star
• a more massive star collapses faster and it
  heats more rapidly and therefore, the H-burning
  takes place sooner. - evolutionary tracks are
  horizontal.
• protostar evolution of a low mass star
• Does not contain the required mass to develop the
  necessary pressure and temperature to start
  H-burning - end up as Brown Dwarfs - Low
  luminosity low temp. They occupy the bottom
  right corner of the main sequence.

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The Evolution of a Star
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The Formation of Stars

• Stage 5 A Newborn Star
• About 10 million years after its first
  appearance, a protostar (comparable in mass to
  our Sun) will become a true star.
• It would have shrunk to about the size of our Sun
  (starting from about 10,000 times the size of our
  Sun) the contraction would raise the temperature
  to 10 million Kelvin - enough to start
  thermonuclear reactions.
• When thermonuclear reactions start at the center
  of a protostar, we say a new star is born.

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Evidence of these processes

• We have observed
• bipolar flow from young stars.
• star forming regions (e.g. Orion Nebula).
• Young Star clusters.

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Mass Loss from a Young Stars

• When the star forms by collapsing due to its
  gravity, the protostar also emits much of the
  cold dark matter into space.
• This is seen in T Tauri stars.
• Bipolar outflow - many young stars loose mass by
  ejecting gas along two narrow jets that are
  oppositely directed.
• These objects are referred to as Herbig- Haro
  objects.

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Bipolar outflow
Hubble telescope image of a bipolar out flow.

• These are clouds of glowing ionized gas created
  when fast moving gas jets ejected from the
  protostar slams into the surrounding interstellar
  medium.

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Star Clusters

• Star clusters
• A cluster of stars forms when a large gas cloud
  collapses into many stars of many different
  masses. Each cluster is a snapshot of stellar
  evolution.
• Stars in a cluster start forming almost
  simultaneously.
• However, they do not become main-sequence stars
  at the same time.
• That depends on the mass of the star.

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M16 Star Clusters in Eagle Nebula
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Young Star Clusters
A young star cluster in a H II region and its H-R
diagram. Stars are still forming - all are not
main sequence stars yet.
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The Pleiades cluster An older (50 mill. Yrs.)
cluster. All the cool, low mass stars have become
main sequence stars
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