1. Death of Stars
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2. Questions of Interest What happens to old stars?
How does death differ for small and large stars?
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3. Death of Low-Mass Stars
Toward the end of life of a star whose final mass just before death is like the Sun’s mass,
the star sheds some of its outer layers to form a planetary nebula
the star’s core continues shrinking until it reaches a density equal to nearly a million times the density of water!
What remains of the star becomes an object called a white dwarf
Since white dwarfs are far more dense than any substance on Earth, the matter inside them behaves in a very strange way
unlike anything we know from experience
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4. White Dwarfs
In the extremely hot, dense gas inside a white dwarf, the electrons resist being squeezed closer together and set up a powerful pressure
Such a gas is said to be degenerate
White dwarfs, then, are stars with degenerate electron cores that cannot contract any further
Theoretical calculations show that stars with final masses (just before death) less than 1.4 MSun end up as white dwarfs
This number is called the Chandrasekhar limit, after the scientist who first calculated it
Low-mass stars, with initial masses up to 10 MSun, can lose enough mass (during their dying process) to become white dwarfs
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5. Masses & Radii of White Dwarfs
Theoretical calculations show that the more massive a white dwarf is, the smaller its radius
This is contrary to the behavior of regular matter, which gets bigger in size when its mass is greater
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6. Sun-Like Star: From Death to White Dwarf
After the star becomes a giant again (A), it will lose more and more mass as its core begins to collapse
The continued mass loss exposes the hot inner core, making the remaining core’s surface temperature increase (B)
At first the luminosity remains nearly the same, but as the star begins to cool off, it becomes less and less bright (C)
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7. Ultimate Fate of White Dwarfs
Since a white dwarf can no longer produce energy by gravitational contraction (or by nuclear fusion), it shines simply from the release of the (substantial) energy left over inside its core
Gradually the white dwarf radiates away all its heat into space
The white dwarf will eventually stop shining and become a black dwarf
This is a cold, dense stellar corpse with the mass of a star and the size of a planet
It is composed mainly of carbon and oxygen
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8. Death of Massive Stars
High-mass stars, with initial masses larger than 10 solar masses, end their lives in other ways
In a massive star, after helium fusion in its core has ended, the weight of the star’s outer layers is sufficient to force the carbon and oxygen core to contract until it becomes hot enough to fuse carbon into oxygen, neon, and magnesium
This cycle of contraction, heating, and fusion repeats several more times
Each cycle produces heavier elements
These cycles cause a massive star, near the end of its life, to develop a structure resembling an onion
Depending on the mass of the star, the cycles continue until it has exhausted all of its sources of energy
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9. Old Massive Star: Stellar Onion
As we get farther from the center, we find shells of decreasing temperature, in which nuclear reactions involve nuclei of progressively lower mass (silicon, sulfur, oxygen, neon, carbon, helium, hydrogen)
The cycles stop after all the silicon has fused into iron because iron is the most stable
Nuclear reactions involving iron would require energy instead of producing energy
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10. Collapse into a Ball of Neutrons
No longer able to generate energy, the star now faces catastrophe
The iron core starts to collapse, overcoming the resistance of the degenerate electron gas
As a result, the electrons are squeezed into the atomic nuclei, where they combine with protons to form neutrons
As is true for electrons, the neutrons in the collapsing core eventually become degenerate and resist being squeeze further
Theoretical calculations show that a star with a final mass (just before death) less than 3 MSun can end up as a crushed ball made mainly of neutrons, which is called a neutron star
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11. Supernova
When the collapse is stopped by the degenerate neutrons, the core is saved from further destruction, but the rest of the star is literally blown apart
The core collapse occurs very rapidly
Its size changes from that similar to Earth’s to that of a midsize town in less than a second!
When the collapse is abruptly halted by the degenerate neutrons, the shock of the jolt initiates a powerful wave that propagates outward and is quickly absorbed by the outer layers of the star
This huge, sudden input of energy reverses the infall of these layers and drives them explosively outward
The resulting explosion is called a supernova
This type of stellar explosion, which signals the death of a massive star, is also called Type II supernova
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12. Supernova Explosion
When a supernova occurs, heavy elements up to iron, produced in stellar nucleosynthesis, are ejected into space
During a supernova, elements heavier than iron, such as gold, silver, uranium, can also be produced
They are built up from the ejected neutrons and the protons produced when the neutrons are absorbed by the ejected iron and other nuclei
Supernovae are believed to play a major role in the creation of chemical elements in the universe
Supernovae are also thought to be the source of cosmic rays (high-energy charged particles)
The ejected gas from a supernova crashes into the surrounding interstellar gas at thousands of km/s
The shock wave heats up the interstellar gas to very high temperatures, making it glow
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13. The Crab Nebula A famous supernova remnant is the Crab Nebula
Chinese astronomers recorded the explosion in 1054
Anasazi Indians painted a picture of it
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14. Remnant of Cygnus Loop Supernova

15. Neutrinos from Supernovae
According to nuclear physics, each time an electron and a proton merge to form a neutron, a neutrino is released
When a neutron star’s dense core forms, an enormous number of neutrinos are released from the core
They carry most of the energy of the explosion
The detection of such neutrinos provides a way to measure the core temperature of the star
In the first second of the explosion, the power carried by the neutrinos is greater than the power put out by all the stars in all the galaxies that we can see!
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16. Supernova 1987a
A supernova occurred in a satellite galaxy of the Milky Way at the star of 1987
It was called SN 1987a
The Kamiokande neutrino detector saw a burst of neutrinos
Studies of SN 1987a confirmed models of supernovae
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1997
AST 2010: Chapter 22
2002

17. Supernova Rate in the Universe
One average, one supernova occurs somewhere in our Milky Way Galaxy every 25 to 100 years
Unfortunately, none has been detected in our Galaxy since the invention of the telescope
Supernovae are very rare because the massive stars that produce them are rare
But there are billions of galaxies in the universe
Simple probability suggests that there should be a few supernovae happening somewhere in the universe during a year
And that is what is seen!
Since supernovae are so luminous and the energy is concentrated in a small area, they stand out and can be seen from hundreds of millions of light years away
Supernova images from the Chandra X-Ray Observatory

18. Pulsars
In the late 1960s, astronomers discovered sources of radio waves that emit rapid pulses of radiation at very regular intervals
Their periods range from to 10 seconds
The great degree of regularity of the signals led some scientists to speculate that they were picking up signals from extra-terrestrial intelligent civilizations
But the discovery of several other similar sources discounted that idea
Now more than a thousand of them have been found
They are now called pulsars, short for “pulsating radio sources”
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19. Discovery of Neutron Stars
By combining theory and observation, astronomers concluded that pulsars must be spinning neutron stars
Neutron stars are the ideal candidates because the core collapse has made them so small that they can turn very rapidly
The rotating neutron star acts like a lighthouse, sweeping its beam in a circle and giving us a pulse of radio radiation when the beam sweeps over the Earth
The magnetic poles are located in different places from the rotation poles
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20. Pulsar in the Crab Nebula
Pulsars lose energy as they age, the rotation slows, and their periods increase
A pulsar has been discovered at the center of the Crab Nebula
Animation

21. Novae
An isolated white dwarf has a boring future: it simply cools off, dimming to invisibility
A white dwarf in a binary-star system, where the companion is still a main sequence or red- giant star, can have a more interesting future
If the white dwarf is close enough to its red-giant or main-sequence companion, gas expelled by the star can fall onto the white dwarf
The hydrogen-rich gas from the companion star's outer layers builds up on the white dwarf's surface and gets compressed
As more and more hydrogen gradually accumulates and heats up on the white dwarf's surface, the new layer eventually reaches a temperature that causes fusion to begin in a sudden explosive way, blasting away much of the new material and creating what is called a nova (“new” in Latin)
To early astronomers, a nova was a new star that suddenly appeared and faded away after a few months or years
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22. Type Ia Supernovae
If the white dwarf accumulates matter from a companion star rapidly, the dwarf’s mass may exceed 1.4 MSun (the Chandrasekhar limit)
If this limit is exceeded, the dwarf can no longer support itself as a white dwarf and begins to collapse
As a result, it heats up and new nuclear reactions begin in its core
In less than a second, an huge amount of fusion occurs that causes the dwarf to explode completely
This kind of explosion is called a Type Ia supernova
to be distinguished the Type II supernova, which signals the death of a massive star
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23. Summary of Ultimate Fates of Stars
Stars with final masses < 1.4 MSun end their lives as white dwarfs
Stars with final masses between and 3 solar masses become neutron stars
Stars with final masses > 3 MSun become black holes
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AST 2010: Chapter 22