Stellar Evolution

1
Stellar Evolution

• We have lots of information about stars, but we
  still need to consider two more areas before we
  begin to put this all together and see if we can
  see some kind of stellar life cycle (also
  called stellar evolution). Those last two areas
  are
• interstellar material atoms, dust, and nebula
• and variable stars.

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Stellar Evolution

The Crab Nebula, M1, as imaged by Hubble Space
Telescope and the Mount Palomar telescope.
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How do we know what is in interstellar space?

• Gas and dust in space can
• scatter light
• absorb light, heat up, and then re-emit light

4
Scattered Light

• In scattering light, blue light scatters more
  than red light. This gas and dust will then tend
  to redden starlight that passes through it.
  This effect is seen on the earth the sky is
  blue because the blue light is scattered more
  than the red light but the sunrise and sunsets
  appear red because most of the blue has been
  scattered out of the direct sunlight.

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Absorb Light

• Atoms will selectively absorb light of particular
  frequencies called an absorption spectrum.
  They will later re-emit that light, but in
  different directions the emission spectrum.
• Dust particles will absorb light of most any
  frequency and tend to heat up. They will emit
  blackbody radiation based on their temperature.

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Nebula

• This combination of absorption and emission of
  light by gas and dust results in different
  types of nebula (areas of relatively high gas
  and dust) dark nebula and glowing nebula.
• See web sites
• http//nssdc.gsfc.nasa.gov/photo_gallery/photogall
  ery-astro-nebula.html
• http//www.robgendlerastropics.com/Nebulas.html

Image of the youngest known planetary nebula, the
Stingray nebula (Hen-1357).
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Interstellar Space

• Liquid water has about 3 x 1022 water molecules
  per cubic centimeter (in English about 30 billion
  trillion). Most solids and liquids have similar
  numbers.
• At the earths surface our atmosphere has about
  2.4 x 1019 molecules per cubic centimeter (about
  a thousand times less dense than liquid water).
• In most of interstellar space, there is about 1
  hydrogen atom per cubic centimeter.
• There are regions of interstellar space, though,
  that have much higher densities. In nebula, that
  number can reach a million atoms per cubic
  centimeter.

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Variable Stars

• While most stars appear to be quite stable, at
  least on a human time frame, some stars do show
  variations in brightness. A few show huge
  changes that appear to be catastrophic events.
  Others have brightness changes in a very periodic
  manner, some on the order of seconds, others on
  the order of days.

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Cepheid Variables

• One type of star, a Cepheid Variable, has a
  brightness that varies by up to about half a
  magnitude with a period that ranges from 1 to 100
  days.
• By looking at star clusters where all of its
  stars appear to be about the same distance away,
  we find that the period of the star is related to
  the luminosity of the star! The lower the
  period, the lower the average luminosity.
  Cepheid variables with a period of 1 day have an
  absolute magnitude (luminosity) of about 2. On
  the other end, Cepheid variables with a period of
  100 days have an absolute magnitude of about
  8.

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Cepheid Variables

• These are all very luminous stars (giants), and
  can be seen from very far away.
• What makes this important are the following
  relations. We can always measure brightness. It
  is also easy to measure the period of a Cepheid
  Variable. With the period-luminosity
  relationship, we can then get the luminosity.
  Finally, knowing the brightness and luminosity,
  we can calculate the distance!
• This distance determination will be an important
  tool in Part 5 of the course where we look at the
  overall size and structure of the universe.

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Stellar Evolution

• Lets now try to put all of this info together
  into a theory that will explain our observations
  and lead us to make further observations to
  support, refine, or refute our theory.

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1. Beginning Gravitational formation

• Most of space is fairly empty of matter and cold.
  However, there are areas of relatively high gas
  and dust (nebula). Over time, gravity will tend
  to pull the gas and dust together. As it does,
  it will tend to convert the gravitational energy
  into heat energy (the speed of falling is
  converted into heat).

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1. Beginning Gravitational formation

• The cloud of gas and dust will tend to get
  smaller and hotter. A smaller size tends to
  reduce the luminosity, but hotter tends to
  increase luminosity. The position of the newly
  forming star on the H-R diagram will move to the
  left as it heats up but wander up and down
  somewhat as its size shrinks.
• This process takes about 50 million years for a
  star like the sun, but may take a much shorter
  time for a more massive star since there will be
  more gravity. A ten solar mass star will only
  spend about 200,000 years in this initial stage.

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H-R DiagramGravitational formation
-10

Luminosi ty
-5
1
0
Sun G2 at 4.8 Magnitude
5
10
15
O0
B0
A0
F0
G0
K0
M0
Temperature / Color
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Nuclear Fusion of HydrogenStability on the Main
Sequence

• When the temperature and pressure at the core of
  the newly forming star reaches a certain point,
  the hydrogen atoms will collide with one another
  so hard that nuclear fusion will occur (basically
  four hydrogen atoms combine to form one helium
  atom plus LOTS of energy). This hydrogen bomb
  process tends to blow the star apart, but gravity
  continues to try to collapse the star.

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2. Nuclear Fusion of HydrogenStability on the
Main Sequence

• The result of these competing tendencies is a
  stable star, both in size and in temperature (and
  hence in position on the H-R diagram on the Main
  Sequence).
• More massive stars have more fuel, but they also
  have more gravity that causes the core to burn
  the fuel at a faster rate than less massive
  stars. The result is that more massive stars are
  hotter and more luminous and are higher on the
  Main Sequence than less massive stars, and they
  remain stable on the Main Sequence for less time.

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2. Nuclear Fusion of HydrogenStability on the
Main Sequence

• A star like the sun will last about 10 billion
  years on the Main Sequence.
• A star with 15 times the mass of the sun will
  only last about 10 million years on the Main
  Sequence. In the same way, stars with less mass
  then the sun will stay on the main sequence much
  longer than 10 billion years.

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Red GiantNuclear Fusion of Helium

• When the hydrogen starts to run out in the core,
  the explosive energy production of nuclear fusion
  no longer can balance the gravitational tendency
  to collapse, and so the core of the star will
  again start to collapse while hydrogen is still
  burning on the outside of the core. This gravity
  collapse of the core will again heat up the core,
  and this extra heat will cause the stars surface
  to expand. As the surface expands, it will tend
  to cool. The result is a red giant state
  higher luminosity but a little cooler surface.

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3. Red GiantNuclear Fusion of Helium

• For a star like the sun, this expansion of the
  surface will be large enough to reach the orbit
  of Venus or even the Earth.
• When the core gets hot enough, it will start to
  have the helium atoms (ashes of the hydrogen
  fusion) combine in nuclear fusion to form carbon
  and release energy.
• This process takes roughly about 10 of the time
  of the Main Sequence hydrogen burning.

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H-R DiagramRed Giant
-10

Luminosi ty
-5
3
1
0
Sun G2 at 4.8 Magnitude
5
2
10
15
O0
B0
A0
F0
G0
K0
M0
Temperature / Color
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4. Unstable stars

• After the helium fuel in the core runs out, there
  are different scenarios for different masses of
  stars.
• For a star with about the mass of the sun or
  less, the core will again collapse and the
  gravitational energy of the collapse will eject
  some of the outer layers of the star (called
  planetary nebula ejection) and the core (now at
  about 0.6 of the original mass of the star) will
  heat up (move to the left and tend to move up)
  and shrink (tend to move down).

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H-R DiagramUnstable
-10
4

eject planetary nebula
Cepheid Variables
Luminosi ty
-5
3
1
0
Sun G2 at 4.8 Magnitude
5
2
10
15
O0
B0
A0
F0
G0
K0
M0
Temperature / Color
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4. Unstable stars

• For more massive stars, the situation is more
  complicated. With the higher gravity, the core
  can get hot enough to start burning the carbon to
  get even heavier elements. This proceeds until
  the core turns into iron. Since the nucleus of
  iron is tightest bound of all atoms, iron cannot
  undergo nuclear fusion to release energy like the
  less massive atoms can.

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4. Unstable stars

• When the core cannot continue with fusion, there
  is nothing to balance gravity, and the core will
  totally collapse. The implosion of the core will
  release so much energy that it will blow the
  outer parts of the star completely away in a
  supernova explosion.

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Final Stage Death of the Star

• There are three possibilities for the collapsed
  core depending on the mass of the remaining core
• 1) If the final mass after the planetary nebula
  release is less than 1.4 solar masses, the
  remaining mass of the star will collapse down to
  a size about that of the earth. It will be a
  white dwarf star, and then as it cools it will
  become a brown dwarf and then eventually cool
  even further.

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H-R DiagramFinal Stage Death
-10
4

eject planetary nebula
Cepheid Variables
Luminosi ty
-5
3
1
0
Sun G2 at 4.8 Magnitude
5
2
5
10
White dwarf
15
O0
B0
A0
F0
G0
K0
M0
Temperature / Color
27
Final Stage Death of the Star

• If the final mass (after the supernova explosion)
  is more than 1.4 solar masses but less than about
  3 solar masses, the core will stop collapsing
  when the atoms are so compacted that the
  electrons are shoved into the protons and the
  whole mass becomes neutrons that stick together
  by gravity. This is called a neutron star. Its
  diameter is only about 20 kilometers (compared to
  about 12,000 kilometers for a white dwarf!).

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Pulsar

• 2-continued) If the original star had an
  appreciable magnetic field and a rotation, the
  resulting neutron star may still have that
  magnetic field and it will have a much higher
  rotational speed due to the collapse. The
  magnetic field may cause light to be emitted in a
  beam, and with the rotation this beam may rotate
  at a high angular speed. We have seen pulses of
  light with periods of a few seconds from these
  spinning neutron stars and so we call them
  pulsars.

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Final Stage Death of the Star

• 3) If the final mass of the core after the
  supernova explosion is more than about 3 solar
  masses, then gravity is so strong it will
  collapse the matter even beyond the neutron star
  size. We know of nothing that would stop the
  collapse. This is called a black hole.

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Black Holes

• Note that the mass of a black hole is still there
  and its gravity will affect things around it.
• But gravity is so strong near it that even light
  can be trapped so that it does not escape from
  the black hole.
• Further away, though, other stars will feel the
  gravity just like they feel the gravity of other
  massive objects.

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Mass back to nebula and space

• In the ejection of the planetary nebula and in
  supernova explosions, some and sometimes most of
  the mass of the star is ejected back into space.
  There is a difference, though. The initial mass
  of the collapsing nebula consisted of mostly
  hydrogen. The final mass of the expanding nebula
  is enriched in the heavier elements. The energy
  in a supernova is so high that elements heavier
  than iron are made.