age of m13: 14 billion years. mass of stars leaving the main-sequence ~0.8 solar masses
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Age of M13: 14 billion years. Mass of stars leaving the main-sequence ~0.8 solar masses. Helium core-burning stars. Giants. Sub-giants. Main Sequence. Here is a way to think about it. Outside of star. Plenty of hydrogen. Shrinking core. - PowerPoint PPT PresentationTRANSCRIPT
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Age of M13: 14 billion years. Mass of stars leaving the main-sequence ~0.8 solar masses
Main Sequence
Sub-giants
Giants
Helium core-burning stars
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Here is a way to think about it.
Outside of star
Plenty of hydrogenShrinking core
Where core use to be. And where conditions were right for fusion.
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Result…
• Because the core is shrinking there is hydrogen that is introduced into the area around the core where temperatures and pressures are high enough for hydrogen fusion to take place.
• Hydrogen begins to fuse into helium, in a shell around the shrinking helium core.
• Now there are two energy sources in the star.
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Two energy sources.
• Gravitational potential energy is being used to make radiant energy in the core.
• The shell around the core is producing energy from the fusion of hydrogen.
• The result of all this energy is that the outer envelope of the star expands enormously. The star becomes a red giant. (luminosity class III)
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Age of M13: 14 billion years. Mass of stars leaving the main-sequence ~0.8 solar masses
Main Sequence
Sub-giants
Giants
Helium core-burning stars
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Helium core burning
• The core contraction and hydrogen shell burning until at last the temperature is high enough in the core to begin helium fusion. This is around 100 million degrees.
• When this happens the star is fusing Helium into Carbon in its core, and still is fusing Hydrogen into Helium is a shell around the core.
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The triple alpha process
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Core and shell burning produces more energy than the star produced on the main sequence so the He-
core burning stars are more luminous than when they were main-sequence stars.
Main Sequence
Sub-giants
Giants
Helium core-burning stars
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Helium gone in the core
• Helium fusion rate is much faster than the Hydrogen fusion rate was. Within a few hundred million years the supply is gone in the core.
• The core once again shrinks, releasing gravitational potential energy.
• The material in a shell closest to the core begins to fuse helium into carbon, in bursts, as the temperature increases.
• Above this shell, hydrogen is being converted into helium.
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Here is a way to think about it.
Outside of star
Plenty of hydrogenShrinking core
Hydrogen shell burning
Helium shell burning
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Three energy sources.
• At this point there are three sources of energy in the star, the shrinking carbon core, and two shells.
• The star rapidly expands and heads back up to the giant stage. This is called the asymptotic giant branch, because it asymptotically approaches the red giant branch.
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Asymptotic Giant branch phase
• During this phase the helium core burning is not stable. It rapidly turns on and off in bursts. Small explosions.
• The results of these explosions is to eject shocks into the outer envelope of the star. Material in the envelope is lifted off the star, over and over again.
• When the carbon core can no longer contract, everything stops.
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• The lost envelope becomes an expanding planetary nebula.
• The exposed carbon core is a white dwarf star.
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Planetary nebula & White Dwarf
White Dwarf star
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Summary of evolution of lower mass stars• Star is on main-sequence – Core converting hydrogen
into helium.• Star is a Sub-giant -- Core is contracting releasing
gravitational potential energy• Star is a Giant (III) – Core is contracting releasing
gravitational potential energy and hydrogen into helium in a shell around the core.
• Helium core burning phase – Star is converting helium into carbon in the core and hydrogen into helium in a shell.
• Asymptotic Giant branch phase – Core is contracting releasing potential energy, Helium into Carbon in a shell, and hydrogen into helium is a shell around Helium shell.
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Notice a pattern
• Whenever a star has an inert core that is shrinking, the star is moving up the giant branch. The star grows in radius
• Whenever there is nuclear fusion in the core the star shrinks back down. Smaller radius.
• This will be important in high mass stars.
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Inert core
Core fusing elements
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Low mass stars cannot fuse Carbon
• Core temperature is too low to fuse Carbon into other elements.
• The core shrinks until all the free electrons are trapped in spaces between the Carbon nuclei. They set up energy levels and the core acts like a giant atom. Core cannot shrink any more.
• The core is similar in size to the radius of the Earth, but has a mass of as high as 1.4 times the Sun’s mass.
• From here on the core will just slowly cool off. Like a hot piece of metal, slowly cools down.
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Planetary Nebula
• During the Helium shell burning phase, there are helium flashes occurring. The helium in the shell doesn’t “burn” at a constant rate. It burns in spurts. Each time helium shell burning turns on, there is an eruption.
• The result is the outer envelope of the star gets shocked, over and over. The outer shell is lifted off in layers.
• The result is a planetary nebula. The exposed Carbon core is a white dwarf.
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The Ring nebula – M57
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Cat’s Eye Nebula
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M57 through a small telescope
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Boomerang Nebula
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Butterfly Nebula – Central White Dwarf has T = 250,000 K.
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Cat’s Eye in optical and X-ray light
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Eskimo Nebula
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NGC 2440 – Central White Dwarf has T = 200,000 K
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Ring Nebula – Multiple mass ejections.
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The planetary nebula phase is short lived.
• The radius of a typical planetary nebula is about 1 light year.
• The gas is glowing, so we see an emission nebula.
• Typical elements in at planetary nebula are hydrogen, helium, carbon, oxygen and nitrogen. Also some neon present.
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Spectrum of Ring nebula
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Sirius – The Dog Star
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Sirius is a binary star
Sirius A
Sirius B
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Which star is older in this binary system
1 2 3
33% 33%33%1. Sirius B because it is
already a white dwarf2. Sirius A because it is
more luminous3. They are in a binary
system, they must be the same age
30
0
30
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Which star was originally the most massive?
1 2 3
33% 33%33%1. Sirius B because it is
a white dwarf 2. Sirius A because it is
more luminous3. They formed at the
same time so they must have the same mass
30
0
30
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
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Globular cluster M 4
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• The stars with masses higher than about 0.8 solar masses have died.
• There should be a lot of white dwarfs in the a globular star cluster.
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White Dwarfs in M 4
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• White dwarfs are just the leftover core of the star. It is not producing energy. It is simply cooling off.
• As a WD cools it becomes less luminous because the temperature is decreasing.
• The cooling follows a very simple cooling relation that depends primarily on time. The older the white dwarf, the cooler it is.
• There is a cutoff in the WD temperature. No WD are found that are cooler than the cut off.
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Cooling
Cut-off
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Why is there a cut-off in the white dwarf population?
1 2 3 4
25% 25%25%25%1. Cooler WD are impossible to detect
2. At a certain temperature, WD explode
3. The universe isn’t old enough to have cooler white dwarfs
4. WD come into temperature equilibrium with the universe and remain that temperature
60
0
30
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Age of the universe using WD cooling
• To date hundreds of thousands of White Dwarfs have been observed.
• There is a temperature cut-off beyond which no white dwarfs are found.
• This is because there hasn’t been enough time since the start of the universe for WD to cool any further.
• The age of the universe computed from WD cutoff is about 12 billion years.
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The Death of High Mass Stars
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• When a high mass star runs out of hydrogen in its core, the core begins to shrink. The outside of the star expands and the star moves right on the H-R diagram.
• The temperature is cooling and the radius is growing, but the luminosity is virtually constant.
• Since L = σT4(4πR2); T4 must be changing at the same rate as R2
• The star becomes a supergiant (luminosity class I star)
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The Death of High Mass Stars
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• As the star tracks to the right for the first time the inert helium core is contracting and hydrogen shell burning is occurring.
• At the farthest right, helium core burning begins, converting helium into carbon. And still hydrogen shell burning.
• The star begins to move to the left on the H-R diagram.
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The Death of High Mass Stars
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• When the helium runs out in the core, the core begins to contract again, there is helium shell burning into carbon, and hydrogen shell burning into helium.
• The star moves right again, toward cooler temperatures and larger radii.
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The Death of High Mass Stars
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• Finally the carbon core is hot enough to fuse carbon into oxygen and nitrogen.
• The star moves back to the left on the H-R diagram. There is a core changing carbon into oxygen and nitrogen, a shell changing helium into carbon, and a shell changing hydrogen into helium.
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A rule of thumb.
• Every time a high mass star moves to the right (cooler temp) on the H-R diagram, the core is inert, but contracting.
• Every time a high mass star moves to the left, the core is fusing one element into another.
• Throughout all of this there is shell burning going on.
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Final stage.
• The core of the high mass star fuses:• hydrogen into helium• helium into carbon• carbon into oxygen and nitrogen• oxygen and nitrogen into sulfur and silicon• And finally silicon into IRON.
• At last the core is iron. This is where everything stops with a bang!
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The final core and shells of a high mass star
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Fusing Iron does not release energy.