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Stellar evolution is the study of how a star changes over time. Stars can change very much between when they are first created and when they run out of energy. Because stars can produce light and heat for millions or billions of years, scientists study stellar evolution by studying many different stars in different stages of their life.
How a star is born
A star starts its life as a cloud of dust and gas called a nebula. This is pulled together by gravity which causes it to heat up. It also starts to spin and to look like a ball. When it gets hot enough, it starts to release energy through nuclear fusion, changing hydrogen to helium. This makes it shine very brightly and become what astronomers think of as a main-sequence star. It may stay a main-sequence star, looking about the same, for billions of years.
How a star enters old age
Sooner or later, almost all of the hydrogen at the center has changed to helium. This causes the nuclear reaction in the middle of the star to stop and the center will start to get smaller due to the star's gravity. The layer of the star just outside the center will begin to change hydrogen to helium, releasing energy.
The outer layers of the star will get much, much bigger. The star will make much more light, sometimes as much as ten thousand times as much as it did at first. Since the surface of the star will get bigger, this energy will be spread out over a much larger area. Because of this, the temperature of the surface will go down and the color will change to red or orange. It will become a red giant. It can swallow up any planets that orbit around it.
How a star dies
Later, the red giant that was left over from a star like ours stops burning. A cloud of gas is given off and a smaller star called a white dwarf is left behind. After a really long time, the white dwarf cools down into a black dwarf.
But, when a big red giant explodes, the explosion is a lot larger and is called a supernova. Instead of a white dwarf, it leaves behind a much smaller, much denser ball called a neutron star. A neutron star is created because the force of gravity is so strong that the atoms left behind would not have any electrons orbiting the nucleus of the atoms. A teaspoon of that matter might weigh as much as the entire Earth.
A much bigger red giant leaves behind a black hole. A black hole is created because gravity is so strong that even the protons and neutrons collapse in on themselves. Even light can no longer escape a black hole. Since there is nothing we know of stronger than the force that holds atomic nuclei (the plural of 'nucleus') together, some physicists think that a black hole collapses all the way down to a mathematical point called a singularity.