Stellar evolution of sun like stars
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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.

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Stellar Evolution (of sun-like stars)

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Stellar evolution of sun like stars

Stellar Evolution (of sun-like stars)

How do stars age & die?


The main factor

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

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

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 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.


Stellar evolution of sun like stars

Main Sequence strip

9 billion years later,sun ends its middle- aged life here.

Sun starts its middle-age

life here.


Hydrogen fuel runs out

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

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

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

Triple-Alpha Process


Yellow giant phase

Yellow Giant Phase

  • With fusion now going on in several layers (H He in outer layer, and

    He C in core) the sun grows hotter and turns yellow again.

  • This phase lasts 100 million years or less.


Fuel runs out again

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.


Stellar evolution of sun like stars

  • 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

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

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.


Stellar evolution of sun like stars

The sun’s planetary

nebula might look

like this from the

surface of Pluto!


White dwarf

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

What’s it made of?

  • The white dwarf is basically a huge, hot crystal of carbon and oxygen – essentially 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

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

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.


Stellar evolution of sun like stars

A nearly dead black dwarf star.


What about larger stars

What about larger stars?

  • Larger stars go through this same set of steps - except right near the end.

  • That’s for another day!


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