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!