Stars

The Sun shines on the Earth every day, but there are trillion and trillions of other stars in the sky as well. Some scientists say there are more stars in the sky than grains of sand on beaches. Stars are made up of extremely hot gases mostly of hydrogen and helium. They are usually sphere-shaped, and in a process called nuclear fusion, the heat and bright light is caused by burning hydrogen into helium.

Nearly all stars start out as a giant cloud of dust called nebulae. The dust is compressed together by gravity and continues to become larger and larger as more dust is added as the gravity gets stronger. It has now become a protostar. As the center becomes hotter nuclear fusion begins and the star is born.

In the next stage of a star's life cycle, it will glow for billions of years burning energy. This is called the main sequence. During this time the star will continue shrinking and expanding but remain relatively the same size. This will continue until it runs out of hydrogen.

When there is no more hydrogen and the outside of the star expands to its largest size becoming a red giant. Finally, in the center of the star iron will be made, which will cause the star to collapse. Depending on the star's size, it can become a white dwarf star like most average stars. If it is larger there can be a nuclear explosion created and it becomes a supernova. Following its life as a supernova, the star may become a black hole or neutron star.

Normal stars in the main sequence stage are classified by their color. The smallest ones are red and do not give off much light, medium like the Sun are yellow, and the biggest and brightest are blue. The larger the main sequence star then the hotter and brighter they are.

Dwarfs are red and yellow, but a brown dwarf is one that did not get large enough for nuclear fusion to take place. When it is a white dwarf, it is mostly the remainder from the collapse of a red giant star. Other giant stars may also be called blue giants, which are stars that expand just like the red giant. Some supergiant stars can get as big as the entire solar system. The neutron star is the tiniest but it can be extremely dense.

A black hole cannot be seen unless a space telescope with special tools is used. It is a star where gravity pulls so much, light cannot escape. The gravity is strong because all of the matter has been squeezed into a tiny space.

The stars in the universe are mostly red dwarfs. The smaller stars are the longer they will live, and even though the red giants are the brightest, they burn out the fastest. Many stars can also be found in pairs called binary stars, and sometimes appear in groups of up to four.

Stars can be seen twinkling in the sky because of the movement of the Earth's atmosphere. The closest star, besides the Sun, to the Earth is Proxima Centauri, which is about 4.2 light years away. This means it would take 4.2 years to travel there, but a person would have to travel the speed of light. It takes 8 minutes for the light from the Sun to reach the Earth. Finally, the Sun is about 4.5 billion years old, about half way through its life cycle.

In summary, a star's life cycle includes its birth; main sequence, which lasts until it runs out of hydrogen; and then red giant. Most stars during the main sequence, become white dwarfs. There can also be a nuclear explosion and the star becomes a supernova. Later, a star may become a black hole or neutron star.