Astronomy/Stellar Evolution

For 2012, the Astronomy event will focus on stellar evolution and type Ia supernovae.

Birth
The life cycle differs between stars depending on their mass. Normal-mass stars begin in stellar nurseries, and some matter condenses to create a protostar. This gains more mass until fusion (H -> He) begins, when it becomes a main-sequence star.

Maturity
A helium core builds up inside the star, but there isn't enough heat/pressure to fuse the helium into heavier elements. However, the hydrogen "shell" around the helium core starts to fuse at a higher rate, causing the star to expand into a red giant and become more luminous. Then, as the star uses up its store of hydrogen, the outer layers of the star contract, finally achieving enough heat and pressure for the He in the core to fuse to carbon and oxygen.

Death
Normal-mass stars don't have enough mass to fuse carbon and oxygen into any heavier elements, so once it uses its entire store of hydrogen and helium, the outer layers of the star are ejected at high speed, potentially forming a planetary nebula. The carbon/oxygen core cools to become a white dwarf. If a white dwarf accumulates enough mass (perhaps gas from its partner in a binary system), it will explode in a Type Ia supernova. The "mass limit", so to speak, of a white dwarf is called the Chandrasekhar Limit and is equal to about 1.4 solar masses.

High-mass stars
Larger stars are similar, except they begin with more mass and grow to supergiants. However, high-mass stars DO have enough mass to fuse carbon and oxygen into heavier elements, each step of which temporarily creates enough outwards pressure to keep the star from collapsing under its own mass. Fusion continues all the way to iron - any element heavier than iron releases energy through fission instead of fusion. At the end of their lifetime, they can explode in a massive explosion known as a supernova and/or collapse into a neutron star or a black hole.

Low-mass stars
Smaller mass stars (red dwarfs) don't become giants. Due to a main sequence lifespan longer than the age of the universe, no evolved red dwarf has been observed. Some current models predict the red dwarf increasing in surface temperature while maintaining a constant radius, transforming them into blue dwarfs. Upon the termination of nuclear fusion, the blue dwarf will cool into a white dwarf and eventually cool into a black dwarf.

In general, the mass of a star is inversely proportional to its lifespan - smaller stars (red dwarfs) live much longer than our own Sun (an "average" star), which in turn has a much longer lifespan than massive stars like Vega.