The+life+of+stars



The picture above shows the life cycle of two types of star: Large mass stars //bottom//** //All// stars are born in a stellar nebula. A stellar nebula is a huge cloud of Hydrogen gas. Gravity pulls the gas inwards to form a young star, or protostar. Protostars are not big enough to fuse Hydrogen. When the star is big enough (and gravity is big enough) Hydrogen fusion begins. When Hydrogen fusion begins, Helium is made, and the star is born! Fusion creates heat and light.
 * Small mass stars (like our Sun) //top//

What happens next depends on the //star's mass//: When small stars run out of hydrogen they start to fuse Helium. Helium releases a lot of heat and the star swells up (expands) to become a red giant. When the helium layer expands it also cools down and this makes it contract (get smaller). The star pulsates (gets bigger and smaller) for a while. But eventually the pulsations become large enough to throw off the out layers of the star into space! This is called a planetary nebular. The hot core which is left behind is called a white dwarf.
 * //Small stars//**

When a large star stars to fuse heavier elements it swells up to become a red supergiant. When it expands this also makes the star cool down and contract (get smaller). At the end of the big star's life, the outer layers contract very quickly and bounce off the star's core! This results in a huge explosion called a supernova. What's left behind is a //neutron star// or a //black hole//.
 * //Large stars//**