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 strip 9 billion years later, sun ends its middle- aged life here. Sun starts its middle-age 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, 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 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 planetary nebula might look like this from the surface 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!