The Life Cycles of Stars All Stars start in a stellar nursery, called a nebula Something upsets the nebula and the gas and dust begin to swirl and collapse forming a protostar. As the protostar grows and collects more matter it gets hotter and hotter until it reaches 15 million C˚ At this temperature nuclear fusion begins and the star begins to shine.

Main sequence Stars Stars undergoing nuclear fusion of hydrogen into helium are called main sequence stars. The stars spend most of their lives in the main sequence. The time the star spends in the main sequence is inversely proportional to the size of the star. The larger mass stars have a shorter life cycle. Example: a star 60 times larger than our sun will only stay in the main sequence for a few million yrs.

The size of stars There are two types of giant stars. Red giants are cooler than white stars. Blue giant stars are hot and much more massive than our sun.

27.2 The size of stars Stars that are smaller than the sun come in two main categories, dwarfs and neutron stars. Sirius, the Dog Star, is the largest known white dwarf.

Black Holes