stellar evolution (of sun- like stars) how do stars age & die?
TRANSCRIPT
Stellar Evolution (of sun-like stars)
How do stars age & die?
The Main Factor
• The evolution of any star is controlled by its mass.
• Mass controls gravity.• Gravity controls density.• Density controls how fast the star
uses up its available fuel.• Fuel availability controls when a star
goes through changes.
Hydrostatic Equilibrium
• Every star is in a constant balance between 2 forces:– The inward pull of gravity.– The outward push of pressure caused by
the heat given off by nuclear fusion.
• This balance is called hydrostatic equilibrium.
We already know the early stages…
• A rotating nebula collapses & heats.• The knot of hot gas in the center is a
protostar.• Nuclear fusion ignites. The star
becomes a pre-main-sequence star.• Strong bipolar outflows blow away
the surrounding gas & dust, allowing the new star to become visible.
• The star gradually brightens.
The long middle ages…
• The longest part of a star’s life is its middle age, where it normally fuses Hydrogen into Helium (proton-proton chain.)
• For a star like our sun, this stage lasts 8 to 9 billion years.
• During this time, the sun gradually brightens, possibly doubling in brightness. Life on earth ends.
Main Sequence stripMain Sequence strip
Sun starts its middle-agelife here.
9 billion years later,sun ends its middle-
aged life here.
Hydrogen Fuel Runs Out!
• Eventually, the hydrogen fuel in the sun’s core runs out.
• The core no longer produces as much outward pressure, so it contracts.
• Contracting causes the core to heat up.
• Heat from the core causes the outer layers to expand. Sun becomes a red giant.
Red Giant Phase
• Sun expands to 500x its current size (500 million miles in diameter.)
• Mercury, Venus, and Earth are consumed.
• As the outer layers expand, they cool to 3500 Kelvin & become red.
• Sun begins to fuse Hydrogen to Helium in outer layers.
• This stage lasts about 1 billion years.
Helium flash!
• Core continues to shrink and grow hotter until it reaches about 100 million Kelvin.
• Built-up Helium “ash” in the core suddenly ignites & begins fusing into carbon (triple alpha process).
• Sun suddenly brightens (briefly) with heat from this helium to carbon fusion.
Triple-Alpha Process
Yellow Giant Phase
• With fusion now going on in several layers (H He in outer layer, andHe C in core) the sun grows hotter and turns yellow again.
• This phase lasts 100 million years or less.
Fuel runs out again• As the Helium fuel begins to run out
in the core, the core begins to shrink a second time. Helium fusion slows down.
• Hydrogen fusion from the outer layers continues to dump waste Helium into the core.
• Every so often, enough Helium builds up in the core to briefly start the triple alpha process again. The star pulses like a beating heart.
• These are called thermal pulses.
Fuel is all gone…
• When all the sources of fuel are gone, the core contracts one last time and becomes intensely hot.
• The super-hot core causes the outer layers to expand. In the process of expanding, the outer edges of the outer layers cool and turn red.
• The sun is very briefly a red supergiant, larger than Mars’ orbit!
Planetary Nebula
• Within just a few million years, the sun sheds its outer layers into the solar system.
• This expanding cloud of hot, glowing gas is called a planetary nebula.
The sun’s planetarynebula might looklike this from thesurface of Pluto!
White Dwarf• In a fairly short period of time, the
planetary nebula fades.
• The hot exposed core of the dead sun is now exposed. This is a white dwarf star, shining with residual heat.
• The core is about the size of the earth, and hotter than 100,000,000 Kelvin.
What’s it made of?
• The white dwarf is basically a huge, hot liquid crystal of carbon and oxygen – almost like an enormous diamond!
• The white dwarf has a density of about 10 billion kilograms per cubic meter – 200,000 times denser than the earth!
Gravity
• The gravity on a white dwarf’s surface is so enormous (200,000 G’s) that anything on it would be crushed flat.
• A white dwarf can even disrupt other nearby stars! (More on this later.)
Black Dwarf
• Over hundreds of millions of years, the white dwarf cools off and no longer shines from the left-over heat.
• The cinder is now a black dwarf.
A nearly dead black dwarf star.
What about larger stars?
• Larger stars go through this same set of steps - except right near the end.
• That’s for another day!