Download Death of Stars • Models of Star behavior can give estimates of how

Death of Stars
• Models of Star behavior can give estimates of how hot a star of mass M
can get.
• From this we can see what happens when a star dies.
• Three classes of stars:
– Low mass stars (red dwarves)
– Medium mass stars (our Sun)
– Large mass stars.
They all die differently:
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• Found that Time =
1
.
(M/M )5/2
But what actually happens?
• Eventually all the hydrogen nuclei in the core are fused into helium. At
this point, the core is all 4 He, but is not hot enough to fuse the helium
(this requires T > 100, 000, 000K).
• So nuclear fusion stops for some time, and without the energy flow and
pressure pushing out, gravity begins to collapse the core.
This compression begins to heat it up. But before the core gets hot enough
to fuse helium, something else happens. Hydrogen shell fusion:
• Hydrogen that had been located near the core but never was hot enough
to fuse now finds that the temperature has risen and can begin to fuse.
But because it fuses in a thin shell, the P-T thermostat does not come
into play here, and it releases a lot of energy quickly which heats the outer
layers of the star and causes them to expand.
• These outer layers then begin to cool off, and what we see is a star that
is very bright, yet cooler, because of the larger surface area.
• The star has entered the red giant or supergiant phase. The process
happens “quickly”, like a million years!
• On the HR diagram, the star leaves the main sequence and follows a path
to the right.
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• But all this time, the core of the star, which is all helium, is still shrinking.
Still not hot enough to start fusing helium. The core gets denser and
denser.
• The behavior gets strange. Get what is called degenerate matter.
• The interior of the star is composed not of atoms, but nuclei of atoms
(helium and some higher mass elements) and free electrons. (Plasma)
• The density is very high. (This structure can be created in laboratories)
• The electrons begin to act differently (Quantum Mechanics)
• The behavior is like that of cars on the highway.
• The cars are free to move around for low density traffic
• But if the amount of traffic increases, the freedom to move around is
diminshed.
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• Can only compress these cars so much now.
• And as the cars speed up, initially the spacing between them doesn’t
change much (increased energy doesn’t change the pressure) If they can
get going very fast or spaced out further, then they go back to ”normal”
behavior.
• When electrons get compressed into a smaller and smaller place, they can
become degenerate, and can only occupy specific energy states.
• Electrons (and the core too) behave differently:
• The core resists compression
• Pressure in the core does not depend on temperature.
• This kills the P-T thermostat and prevents the star from controlling its
fusion reaction which will be a big problem.
• What happens to the helium core depends on how much mass the star
has.
• If M > 3M , the core will heat up to >100,000,000 Kelvin, and helium
fusion in the core starts up, and no degenerate matter starts. (P-T thermostat still functions)
• Big stars are already hot at their centers and don’t need to compress their
cores much to get to this temperature.
• If 0.4M < M < 3M , the degeneracy point is hit first, then the Temperature rises above 100 Million K and Helium fusion starts.
• But the pressure doesn’t change with temperature, so the added energy
just increases the Temperature and fusion goes faster and faster.
• Get Helium Flash: The core releases energy at a rate several thousand
times more than normal. The energy is absorbed by the outer layers of
the star, so we don’t see it directly.
• Outer layers heat up, and get a yellow giant.
• For M < 0.4M , they never get hot enough to fuse Helium.
• For M > 0.4M Helium fusion in core, and Hydrogen shell fusion take
place, with Carbon and Oxygen “ash” being made in the core due to
Helium fusion
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Low Mass Stars
• For M < 0.4M , lifetimes are very long (greater than 100 billion years),
and their luminosity is low (less than 0.04 times the Sun).
• Their internal structure is all convection. So they mix up the gas within
themselves continuously. They “stir up the ash”, so they never get the
100% Helium core that heavier stars do.
• As a consequence, they will live longer and do not get the H shell fusion
that would produce a giant star.
• They will burn all their Hydrogen, and be left with a star mostly Helium, yet unable to compress it sufficiently to get Helium fusion to start.
(Gravity not strong enough)
• They go then to white dwarf stage (They do get hot but just not hot
enough).
Medium Mass Stars
• For 0.4M < M < 3M (roughly) stars, you don’t have as effective
convection, so the star doesn’t mix the Hydrogen that is too cool to fuse
with the core. The core eventually becomes all Helium, and then you get
Hydrogen shell fusion and giant phase.
• Cannot get hot enough to fuse Carbon and Oxygen (Need 600 Million
Kelvin).
• Can get helium shell fusion as core begins to collapse again.
All the energy released can cause the outer layers to be “blown” off the star
Called a planetary nebula (Because of their color)
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• Can still see the hot bright region underneath the layers that were blown
off.
• Star can loose a lot of its mass in this process, in some cases over half.
• The remainder will no longer be able to fuse and begin to collapse, heat
up, and eventually cool off.
• They will become a Carbon based White Dwarf.
Variable Stars
What we find is that some stars vary in their brightness in a way that is not
like eclipsing binaries.
These are classified according to the time it takes to vary.
RR Lyrae : 1/2 day
Cephid variables: 2 weeks
Mira variables: Year
Find these all live in a certain range in the HR diagram, called the “Instability Strip”
• Behavior is tied to a range where the outer layers of the star are on the
edge of being opaque to the radiation.
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• They then trap the heat, and the layers underneath expand. This then
changes the opacity of the outer layers, which then cool off and release
more light from underneath. Eventually they cool off to begin trapping
the light again.
• Like a pot on the stove with a loose fitting lid. Steam puffs out periodically.
Turns out there is a relationship between how bright the star is and how
long it takes to vary.
Will see that this also gets used as a way to see how far away a star is.
White Dwarves
• Again, these are the remainders of stars that could not proceed any further
because they could not raise their internal temperatures high enough to
fuse the remaining elements.
• Dense hot balls of Carbon (or Helium) nuclei and electrons.
• With the advent of Einstein’s General Relativity Theory, people began to
study what would be the behavior of these dense objects.
For white dwarves, the work was done by Subrahmanyan Chandrasekhar.
He found there was an interesting behavior predicted by Einstein’s equations.
If gravity is the main force, combined with degenerate matter that can’t fuse,
then you get the following:
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As you add more mass, the radius shrinks!
For M > 1.4M , the system predicts the radius goes to zero!
density!
Infinite
Will see this comes into play with Supernovas.
But we have a BIG question! How does star with mass greater than 1.4M
become a white dwarf?
Mass loss, as described in the ejection of the outer layers.
Massive stars and their Death
Dealing with stars having M > 4M .
Similar behavior as before, with the core running out of Hydrogen and becoming Helium, contracting, H-shell fusion, expansion to giants and supergiants.
• But for these stars, the core can heat up sufficiently (enough gravity) to
fuse Helium into Carbon.
• Again, the core will shrink when all Helium fused, getting He-shell fusion.
• But it will collapse and raise the temperature enough to fuse Carbon, at
1 Billion Kelvin.
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• In fact the core can collapse enough to raise the temperature to fuse Carbon to Neon to Oxygen to Silicon to Iron. (Remember, Fe is the most
stable nucleus)
• So the idea is you will get a layer cake structure.
• But there is a problem:
• 4 nuclei of Hydrogen fuse to give 1 Helium nucleus
• 3 nuclei of Helium fuse to give 1 Carbon nucleus.
• We are quickly reducing the total number of nuclei to fuse!
• Not only that, but the amount of energy each of these fusion events give
off is going down as they get heavier.
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• To keep the contraction from collapsing the core, you need to keep the
pressure up and that means energy flowing out.
• As the fusion processes goes on, it needs to produce the same energy, but
now each event gives off less. So you have to raise the temperature to fuse
faster.
• Begins to fuse very very rapidly:
– Say the star burns through the Hydrogen in 7 million years (≈ 19M )
– Then it will take 1/2 million years to burn the Helium
– 600 years to burn the Carbon
– 150 days to burn the Oxygen
– 1 day to burn the Silicon!
• It fuses everything it can in the core to Iron (Fe), the most stable nuclei.
Iron will not fuse, so no more energy can be made in the core.
• Gravity is the only game left now, and the core is too massive
• Even the best simulations cannot predict exactly what happens.
• At present, the thought is the Fe core collapses within 1/10 of a second.
The core will become either:
– A neutron star or
– A black hole
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– We will discuss these later.
• But with the collapse of the core, you get a major shock wave.
• The layers above have no layer supporting them.
• They collapse in as well, and then slam into the wave of energy coming
from the core collapse. (The core releases large amounts of energy as it
collapses under gravity)
• The temperature rise is tremendous and the outflowing energy blasts the
outer layers off.
• The star in a few hours releases more energy than all the stars in a galaxy
combined.
• Called a Supernova.
• Historical Supernova
• Crab Nebula in 1057 (Chinese observed it)
• Tycho’s star in 1527
• Kepler’s star in 1604
• SN1987A in the Large Magellanic Cloud. The neutrino flux from this
event was seen before the visible light was observed. The star was known
as Sanduleak 69-202, a blue supergiant 25 times more massive than the
Sun.
Most supernova are observed in other galaxies. Can only be studied by
telescopes.
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Can divide supernova into two broad categories: Type I: Brighter, No Hydrogen absorption lines: These are believed to be due to white dwarves which
have had mass added to them to bring the total mass to greater than 1.4M
Type II: Dimmer, Show Hydrogen lines:
that have collapsed.
These are due to massive stars
Can we test any of this info?
• We do see evidence that Supernovas produce heavier elements in greater
quantities than stars have originally in them (Cobalt and Nickel, which
are heavier than Iron)
• But if stars “evolve” in this way, then we should see that clusters of stars
will exhibit the following behavior:
• Clusters are groups of stars in the same region of space, like the Pleiades.
We assume (big assumption!) that the stars in a cluster all began to form
from the same shock wave hitting the same cloud of gas and dust. In this cluster,
you may have a range of star sizes, so they will not all become full fledged stars
at the same time.
Theory would predict the HR diagram would vary with the age of the cluster:
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What do we observe in clusters?
What happens in a binary star system?
If the two stars are far enough apart, then the death of one will not influence
the other.
But if they get close enough, life can get interesting:
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• At the L1 Lagrangian point, mass can go from one star to another.
• How would star matter get from one star to the L1 point?
• Solar wind, but more importantly, expansion of the star.
The Algol Paradox
• The Algol binary system: Algol (β Perseus) is a B8-V star about 93 ly
away, which shows a variable intensity.
• Eclipsing Binary: From period and time of intensity drop, can estimate
mass and size of the other star. Find the following:
5M!
Main Sequence
1M!
Giant
• What is the problem with this?
• If they started at the same time, and if the bigger you are the faster you
go, why is the heavier star still main sequence?
• Idea is mass transfer from one star to the other as one star expands and
fills its Roche limit.
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• How does matter go about falling into the other star?
draining.
Like a bathtub
• Matter swirls in and forms an accretion disk.
• As it gets closer it gets going faster. Material heats up.
• Can get so hot it emits X-rays, which we observe from the Algol system.
• Can also lead to supernova type Ia if one of the stars is a white dwarf.
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