Stellar Evolution:The Life Cycle of a Star

Essential Question: How does the life cycle of a star vary based upon its

mass?

Star Sequence (Life Cycle)

Stars develop in a stellar nebula - a giant cloud of dust and gas.

Sometimes part of the cloud compresses and shrinks because of gravity.

As it compresses it becomes hotter and when it is hot enough, nuclear fusion can start in the center, creating a protostar.

The protostar then forms a star.

Once nuclear fusion is producing heat in the center of the new star, this heats stops the rest of the star from collapsing.

The balance between gravity trying to make the star shrink and heat holding it up is called Thermodynamic Equilibrium.

The star then stays almost exactly the same for a long time (about 10 billion years for a star like the Sun).

Depending on the star’s mass it becomes one of three general types of stars:

Brown Dwarf – never fully develops into a star – not enough mass (not enough hydrogen for fusion to continue)

Average Star (Our Sun) MASSIVE Star

QUESTIONS

• What factor affects which type a star will develop into?

• Describe the 3 different options that the stars can develop into.

Average Stars

Also known as Main Sequence stars – make up 90% of stars in space

Become Red Giants as hydrogen resources are depleted

Then can break up into a ring nebula (also called Planetary Nebula, even though it has nothing to do with planets)

Remaining part of the star becomes a White Dwarf as it cools

Once all heat is gone it then becomes a Black Dwarf and dies.

Massive Stars

Become Red Supergiants as hydrogen resources are depleted

Then explodes into a Supernova Remaining part of the star

becomes either a Neutron Star, or a Black Hole and dies.

The life of a star

During its “life” a star will not change very much.

However, different stars are different colors, size and brightness.

The bigger a star, the hotter and brighter it is. Hot stars are Blue. Smaller stars are less bright, cooler and Red.

Because they are so hot, the bigger stars actually have shorter lives than the small, cool ones.

Death of a star

Eventually, the hydrogen (the “fuel” for the nuclear fusion) in the center of the star will run out

• No new heat is made and gravity will take over and the center of the star will shrink.

• This makes the very outside of the star “float up” and cool down, making the star look much bigger and redder - a Red Giant star.

The second Red Giant stage As the center collapses, it becomes very

hot again, eventually getting hot enough to start a new kind of nuclear fusion with Helium as the fuel.

Then the Red Giant shrinks and the star looks “normal” again.

This does not last very long, though, as the Helium runs out very quickly and again the star forms a Red Giant.

The end of a Sun-like star

For a star like the Sun, no more nuclear fusion can take place, so the center of the star will then keep collapsing.

• Eventually it can become almost as small as the Earth, but with the same mass as a whole star! This very dense object is called a White Dwarf.

• A piece of White Dwarf the size of a cell phone would weigh as much as an elephant on the Earth!

Hertzsprung- Russell (H-R) Diagram Illustrates important things about

stars: Brightness Absolute Magnitude

usually shown on right-hand Y-axis Luminosity

usually shown on left-hand Y-axis Temperature/Color Spectral Class

shown on X-axis

Supernovae occur when a massive star ends its life in an amazing blaze of glory. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant. A typical supernova remnant is at most few light-years across. (M 1 shown)

Supernovae

M 1 Supernova Remnant

M 1 Supernova was discovered in 1054 and was visible for 23 days during the daytime, and easily seen for 2 years at night. It apparently was depicted in “cave paintings”