The matter stops falling on the star A star becomes hot enough to sustain the pressure of gravity
Contraction stops when the gravity is balanced by thermal pressure Stars are held together by gravity. Gravity tries to compress everything to the center. What holds an ordinary star up and prevents total collapse is thermal and radiation pressure. The thermal and radiation pressure tries to expand the star layers outward to infinity.
Surface temperature 6000 K Temperature at the center 14,000,000 K!
A puzzle: the Sun and other stars radiate away huge amounts of energy. They should lose all their heat in less than a million years! There must be an internal energy source: nuclear fusion reactions However, the Sun lives 4.6 billion years
“Planetary” model of atom Proton mass: 1.7x10 -27 kg Electron mass: 9x10 -30 kg
Nuclear reactions Fission: decay of heavy nuclei into lighter fragments Fusion: synthesis of light nuclei into a heavier nucleus
A star will live until all hydrogen is exhausted in its core star mass (solar masses) Lifetime (years) 60 3 million 30 11 million 10 32 million 3 370 million 1.5 3 billion 1 10 billion 0.1 1000's billions Our Sun will live 5 billion years more
What happens when all hydrogen is converted into helium in the core?? Mass defines the fate of the star
Fate of the collapsed core White dwarf if the remnant is below the Chandrasekhar limit 1.4 solar mass White dwarf if the remnant is below the Chandrasekhar limit 1.4 solar mass Neutron star if the core mass is less than ~ 3 solar masses Neutron star if the core mass is less than ~ 3 solar masses Black hole otherwise Black hole otherwise
Death of Stars “All hope abandon, ye who enter here” Dante
Outer layers expand due to radiation pressure from a hot core The star becomes a Red Giant Surface temperature drops by a factor of ~ 2 The radius increases by a factor of ~ 100 Luminosity increases ~ R 2 T 4 ~ 100-1000 times
In only about 200 million years it will be way too hot for humans on earth. And in 500 million years from now, the sun will have become so bright and big, our atmosphere will evaporate, the oceans will boil off, and surface dirt will melt into glass.
What is left?? A stellar remnant: white dwarf, composed mainly of carbon and oxygen
It is extremely dense All atoms are smashed and the star is supported by pressure of free electrons White dwarf
White Dwarfs Degenerate stellar remnant (C,O core) Extremely dense: 1 teaspoon of WD material: mass ≈ 16 tons!!! White Dwarfs: Mass ~ M sun Temp. ~ 25,000 K Luminosity ~ 0.01 L sun Chunk of WD material the size of a beach ball would outweigh an ocean liner!
As it cools, carbon crystallizes into diamond lattice. Imagine single diamond of mass 10 30 kg! Don’t rush, you would weigh 15,000 tons there!
Formation of Neutron Stars Compact objects more massive than the Chandrasekhar Limit (1.4 M sun ) collapse further. Pressure becomes so high that electrons and protons combine to form stable neutrons throughout the object: p + e - n + e Neutron Star
Properties of Neutron Stars Typical size: R ~ 10 km Mass: M ~ 1.4 – 3 M sun Density: ~ 10 14 g/cm 3 Piece of neutron star matter of the size of a sugar cube has a mass of ~ 100 million tons!!!
Isolated neutron stars are extremely hard to observe Neutron stars have been theoretically predicted in 30s. Landau, Oppenheimer, Zwicky, Baade
However, there are two facts that can help: Neutron stars should rotate extremely fast due to conservation of the angular momentum in the collapse Neutron stars should rotate extremely fast due to conservation of the angular momentum in the collapse They should have huge magnetic field due to conservation of the magnetic flux in the collapse They should have huge magnetic field due to conservation of the magnetic flux in the collapse
Jocelyn Bell Discovery of pulsars: Bell and Hewish, 1967
Schwarzschild radius: event horizon for a nonrotating body To make a black hole from a body of mass M, one needs to squeeze it below its Schwarzschild’s radius RsRs Gravitational collapse: the body squeezes below its event horizon
Black holes are NOT big cosmic bathtub drains! Far from a black hole R >> R s weak field Approaching a black hole R R s (strong field): gravity pull very strong If our Sun collapses into a black hole, we won’t see any difference in the gravitational pull (but it will be VERY cold)
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