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The end stages of stars
• No homework due next week
Review
The Hertzsprung-Russell diagram
Review
A star’s spectral type simply depends on its surface temperature
Review
Stars spend most of their life on the main sequence of the Hertzsprung-Russell diagram, where they are fusing hydrogen into helium in their cores.
Review
The fusion rate depends strongly on the mass of the star; the most massive stars (>100 Msun) burn their fuel very rapidly, so they are very hot and don’t live long.
Review
Low-mass stars (down to ~0.1 Msun) burn their fuel very slowly, so they are cool and live very long.
Review
After the core of the star has been converted into pure helium the fusion stops (although it will still occur in a shell around the core). The star leaves the main sequence, expanding to become a red giant or a supergiant.
Review
What happens next depends on the mass of the star:• If the star is massive enough, then the core will be
hot and dense enough to star fusing the helium into carbon. But it stops there.
• If the star is even more massive (a supergiant), then the carbon core will start to fuse into heavier elements.
• And even more massive stars will fuse even heavier elements in their cores. The most massive stars will eventually build up an iron core, which can no longer fuse.
Q: Which is brighter?A. K-type main sequence starB. K-type giantC. they are about the same
Q: Which is brighter?A. K-type main sequence starB. K-type giantC. they are about the same
Q: Which is larger?A. K-type main sequence starB. K-type giantC. they are about the same
Q: Which is larger?A. K-type main sequence starB. K-type giantC. they are about the same
Q: Which is more luminous?A. A-type supergiantB. K-type supergiantC. they are about the same
Q: Which is more luminous?A. A-type supergiantB. K-type supergiantC. they are about the same
Q: Which is larger?A. A-type supergiantB. K-type supergiantC. they are about the same
Q: Which is larger?A. A-type supergiantB. K-type supergiantC. they are about the same
Review
Red giants, and supergiants:
Homework problem 13.47
The distance from Earth of the red supergiant Betelgeuse is approximately 643 light-years. If it were to explode as a supernova, it would be one of the brightest stars in the sky. Right now, the brightest star other than the Sun is Sirius, with a luminosity of 26LSun and a distance of 8.6 light-years.
• How much brighter in our sky than Sirius would the Betelgeuse supernova be if it reached a maximum luminosity of 8.0×109 LSun?
apparent brightness = luminosity4𝛑×distance2
How stars die — when M<8Msun
So what happens when all fusion finally stops in a red giant?
• The core (which is now made of either helium or carbon) contracts and heats.
• Through strong stellar winds and other processes, the diffuse out layers of the star are ejected creating a planetary nebula.
• The dense stellar core remains, and is now called a white dwarf.
How stars die — when M<8Msun
Hot ionized gasejected from the star
white dwarf
How stars die — when M<8Msun
(Note that planetary nebulaehave nothing to do with planets)
How stars die — when M<8Msun
• The white dwarf starts out very hot, but there is no energy source so it cools off over time
• The gas also starts out hot, and initially is kept hot by the white dwarf, but it will also cool off over time. So the lifetime of a PN is relatively short, just a few 10s of thousands of years.
How stars die — when M<8Msun
• The central core (which, in the most massive stars, is made of iron) undergoes a sudden gravitational collapse, reducing in size until all the electrons in the atoms are smashed down into the nulcei.➡ All of the protons in those nuclei become neutrons.
• The outer layers of the star explode outward (although how exactly this happens is not completely known). This is a supernova. The dense core remains as a neutron star.
How stars die — when M>8Msun
How stars die — when M>8Msun
Hot ionized gasejected from by the supernova
neutron star
The crab nebula: the supernova was observed in 1054
How stars die — when M>8Msun
How stars die — when M>8Msun
This is the remnant of the supernova observedby Tycho Brahe in 1572
How stars die — when M>8Msun
This is the remnant of the supernova recorded byChinese astronomers in 386 A.D.
How stars die — when M>8Msun
Supernovae are incredibly bright, perhaps 1010Lsun.
Stellar remnants
What is left of the dense stellar core after the planetary nebula or the supernova explosion depends on the mass of the core. It can be a
• white dwarf• neutron star• black hole
• A white dwarf is incredibly dense, more than 2000 pounds per cubic centimeter.
• Consists of either helium or hydrogen, and is support by electron degeneracy pressure.
Stellar remnants — white dwarfs
• A white dwarf is incredibly dense, more than 2000 pounds per cubic centimeter.
• Consists of either helium or hydrogen, and is support by electron degeneracy pressure.
Stellar remnants — white dwarfs
Ordinarily, electrons can have a choice of occupying different energy states in their atoms
Stellar remnants — white dwarfs
• A white dwarf is incredibly dense, more than 2000 pounds per cubic centimeter.
• Consists of either helium or hydrogen, and is support by electron degeneracy pressure.
But if the density is high enough, you would expect that the electrons will all get pushed down to the lowest energy state. But this can’t happen!
Stellar remnants — white dwarfs
• A white dwarf is incredibly dense, more than 2000 pounds per cubic centimeter.
• Consists of either helium or hydrogen, and is support by electron degeneracy pressure.
There is a limit to how many electrons can fit in each energy level (for instance, the ground state can only have two electrons in it). This gives rise to electron degeneracy pressure.
• But if the core is more massive than 1.4Msun, then the gravitational force is strong enough to overcome the degeneracy pressure, and the electrons are smashed down into the nuclei.
Stellar remnants — neutron stars
• But if the core is more massive than 1.4Msun, then the gravitational force is strong enough to overcome the degeneracy pressure, and the electrons are smashed down into the nuclei.
Stellar remnants — neutron stars
Subrahmanyan Chandrasekhar(1910-1995)
• But if the core is more massive than 1.4Msun, then the gravitational force is strong enough to overcome the degeneracy pressure, and the electrons are smashed down into the nuclei.
• What is left is a neutron star — basically a ball of pure neutrons — which is supported by neutron degeneracy pressure. A neutron star is about 108 times denser than a white dwarf (think of something with the mass of the Sun compressed into a sphere just a few kilometers across).
Stellar remnants — neutron stars
Stellar remnants — neutron stars• Neutron stars rotate very rapidly due to the
conservation of angular momentum (the current record-holder is 716 times per second). They can also emit pulses of radio waves every time the rotate.
• They were discovered in 1967 by Jocelyn Bell and Antony Hewish
Stellar remnants — black holesIf the original star is more massive than about 25Msun, then the neutron degeneracy pressure isn’t strong enough to support the neutron star from further gravitational collapse. What is left is a black hole.
• This is an object so dense that — if you’re close enough — it’s impossible to escape. Not even light can escape.
• This point of no return is called the event horizon. It has radius of 10km for a 1Msun black hole.
• What goes on inside of the event horizon? Since we can’t see past the event horizon, we have no idea… and we may never know!
Stellar remnants — black holes
event horizon
??
Stellar remnants — black holes
Black holes are perhaps the most mysterious objects in the universe.
• According to Einstein’s theory of gravity, they are literal “holes” in spacetime. Once an object falls in, it is cut off from the rest of the Universe forever.
• If you do have the misfortune to fall into a black hole, you won’t even live to see what goes on inside the event horizon because you’ll be ripped apart by tidal forces before you even get there.
Stellar remnants — black holes
Black holes
What would happen if you fell into a black hole, other than the fact that you’d get ripped apart?
• Time would slow down for you relative to outside observers. By the time you reached the event horizon, your time would actually stop for those outside observers — you would appear frozen.
• Light gets redshifted as it leaves a gravitational field. So outside observers would also see you become infinitely redshifted.
Black holes
Yes, they really do exist
Black holesBut it is now generally believed that Einstein’s theory of gravity doesn’t fully describe what happens in a black hole. If we could somehow combine Einstein’s theory with quantum physics, then we might realize that they’re much more than a big, voracious ball of nothing.
Black holesBut it is now generally believed that Einstein’s theory of gravity doesn’t fully describe what happens in a black hole. If we could somehow combine Einstein’s theory with quantum physics, then we might realize that they’re much more than a big, voracious ball of nothing.
For instance, Stephen Hawking used quantum arguments to show that black holes actually emit light with a perfect thermal spectrum. A 1Msun black hole has a temperature of 6x10-8 Kelvin
Q: If the Sun was suddenly squeezed small enough to become a black hole, (it won't happen, but just suppose…!)
A. Earth would get sucked inB. Earth would continue in orbit pretty much as
beforeC. Earth would get very coldD. B and CE. None of the above
Q: If the Sun was suddenly squeezed small enough to become a black hole, (it won't happen, but just suppose…!)
A. Earth would get sucked inB. Earth would continue in orbit pretty much as
beforeC. Earth would get very coldD. B and CE. None of the above
Chemical evolution of the universe
• Shortly after the big bang, the universe consisted almost entirely of hydrogen and helium, with just a tiny amount of lithium. Heavier elements are created by nuclear fusion within the cores of stars.
• Planetary nebulae and supernovae are responsible for expelling these elements out into the universe, which can then be recycled into subsequent generations of stars and planets.
• By the time the Sun was formed, about 2% of the matter in the Milky Way was converted into heavier elements — the stuff that we are made of.
Chemical evolution of the universe
We are made of starstuff — Carl Sagan
