last time we talked about how stars are formed from nebulas clouds of gas and dust collapsing due to gravity and then heating up enough to start a nuclear fusion this time we're going to talk about what happens next when a protostar finally begins fusion it becomes the star it then joins a class of stars known as the main sequence which makes up about 90% of all stars in the universe the defining feature of a main-sequence star is the presence of hydrogen fusion in the core a process that takes hydrogen ions and turns them into helium creating light and energy in the process aside from that main sequence stars can vary greatly in size color and temperature they range anywhere from 10% the mass of the Sun to 200 times more massive eventually a main-sequence star runs out of hydrogen in its core when this happens in a smaller main sequence star the force of gravity overpowers the outward radiation pressure causing the star to collapse a layer of hydrogen just outside the core ignites causing the outer layers of the star to expand outwards turning it into a red giant the core will continue to collapse until the temperature reaches about 200 million Kelvin fusion begins again this time turning helium into carbon and oxygen but the helium runs out quickly and the core collapse is one last time radiation pressure becomes so strong that it ejects the rest of the star far into space forming a planetary nebula all that's left is the remnant of the core a white dwarf it will spend the rest of its life cooling down as it slowly radiates Heat the life and death of smaller stars is an exciting process but it pales in comparison to what happens to extremely large stars next time we'll talk about super giants and what happens when they die you