Stellar Evolution

The Life Cycles of Stars

Stage 1- NEBULA

• This is a large cloud of dust and gas in space.

Stage 2- PROTOSTAR

• Gravity compresses the nebula, and it spins and flattens into a disk shape.

Stage 3- MAIN-SEQUENCE STAGE

• This stage occurs when a star begins the process of fusion (combining lighter nuclei into one heavier nucleus).

• A star spends the majority of its life fusing hydrogen into helium.

• It is this fusion that keeps the star from collapsing under its own gravity and causes the star to shine and send out heat into space.

MAIN-SEQUENCE (cont.)

• Once the star runs out of hydrogen fuel, it will begin fusing helium atoms, then the largest stars will fuse carbon atoms after it runs out of helium atoms, becoming hotter and hotter during this process.

DIFFERENT PATHS- SIZE MATTERS

• At this point, the star life cycle splits into two separate paths based on the size of the star.

Stage 4- RED GIANT- path for small to average stars

• As the star gets older and begins to run out of fuel, the core begins to quickly contract and the star is forced to begin fusing helium into carbon to fight gravity.

• While this is happening, the outer layers of the star begin to expand quickly and the star takes on a much darker red color.

Stage 5- PLANETARY NEBULA

• The star’s core contracts as far as gravity can force it to, and the remaining gases cause a haze to surround the star.

Stage 6- WHITE DWARF

• Gravity continues to collapse the star inward until it cannot be compressed any further.

• The haze of gas dissipates, and the temperature in the core is extremely hot.

• After millions of years, the star will no longer give off light and become a black dwarf.

Stage 4- RED SUPERGIANT- path for larger stars

• This process is the same as in a red giant, just on a much larger scale.

• Also, although larger stars have more fuel, they burn through it quicker and actually progress faster through this stage than smaller stars.

Stage 5- SUPERNOVA

• Fusion continues in these large stars, forming heavier elements including iron.

• When fuel runs out, the material contracts and a huge explosion occurs. These explosions are so large that they outshine their entire galaxy.

Stage 6- NEUTRON STAR

• The core of a supernova left behind will contract into an incredibly dense small ball.

• The protons and electrons have smashed together due to this tremendous force and formed neutrons.

• In some cases, a very massive star will have so much matter left after a supernova that the force of gravity is so strong a black hole is formed.