Stars are made of very hot gas, mostly hydrogen and helium, which are the two lightest elements. The core of the Sun, for example, is a 15 million degree Celsius plasma, a soup of electrons and protons that are stripped from hydrogen atoms. Stars shine by burning hydrogen into helium in their cores, and later in their lives create heavier elements. Most stars have small amounts of heavier elements like carbon, nitrogen, oxygen, and iron, which were created by stars that existed before them. After a star runs out of fuel, it ejects much of its material back into space, and new stars are formed from this material. So the material in stars is recycled.

The process of star formation takes around a million years from the time the initial gas cloud starts to collapse until the star is created and shines like the Sun. Stars form from an accumulation of gas and dust, which collapses due to gravity and starts to form stars. The leftover material from the stars birth is used to create planets and other objects that orbit the central star. The disk is crucial for the proto-star to grow into a properly sized star. The disk is mainly composed of gas, which rotates with the disk and slowly approaches the surface of the proto-star. When the gas comes close enough to the star, it falls onto the surface of the star because of gravity, and the star grows. This process of growing is called an accretion process and the star is said to accrete (accumulate) matter from the disk.