Gravity is the Key to Why Stars Change Throughout Their Lives?
Gravity pulls the outer layers of the dust and gas cloud toward the center of the cloud. The center of the cloud is not very hot yet. There is nothing at the center strong enough to hold up the weight of the outer layers So the cloud continues to collapse. This is similar to the egg in the left-hand panel. The weight of the foot is like the weight of the outer layers of the star. The pressure in the egg is like the pressure in the core of the star. What do you think will happen? Click on the panel to see if you are right.
When the temperature at the center gets very hot, nuclear reactions begin, converting hydrogen to helium with the release of energy. The temperature increases even further and the gas at the center presses outward holding up the outer layers of the star. Like the bowling ball on the left, high internal pressure means that the weight of the outer layers can no longer crush the star. The star enters the main sequence and is stable for a large part of its life. Click on the bowling ball to see what happens.
The end, however, is inevitable. When the star's fuel supply is used up and nuclear reactions stop, the star will collapse. Gravity always wins. What's left at the end depends on how massive the star was to begin with.
So let's look in more detail at the lives of low- and high-mass stars, separately.