Stars, such as our sun, are giant balls of burning gas. Hydrogen serves as the bulk of the gas and fuses into helium at the core to produce the thermonuclear reaction that generates tremendous heat and light. Stars have very definite and well documented life cycles that can be identified by the star's appearance.
Creation and Birth: Protostars
Nebulae are the birthplace of stars. Clouds of dust made of carbon and silicon as well as hydrogen and helium gather. Their collective gravity begins to pull in more nebulae material in a process known as accretion . As more mass is gained, the gravitational pull and temperature continue to rise. When the gravity pulling in and the gas pressure pushing out have balanced, equilibrium has been achieved and accretion stops. If the critical core temperature needed for fusion ignition is not achieved, the star will become a brown dwarf. If the critical temperature is achieved, the hydrogen begins fusing into helium and the star is born.
Main Sequence
Once fusion reaction has started, the star is in its main sequence--the childhood through adult stage of its life. Gas pressure pushing out is equal to gravity pulling in and the star slowly shrinks due to fuel consumption. The core slowly increases in temperature as the stars mass is reduced. Our sun is in its main sequence.
Old Age
Stars are 97 percent hydrogen and 3 percent helium, so hydrogen is the primary fuel. When all the hydrogen is depleted, the star begins burning helium and enters a red giant stage. Helium burning at the core and remaining hydrogen burning in the outer shell expands the shell and gives it a red color.
Death
The core contracts as the helium is consumed and gravity increases the density at the core. Carbon burning takes place when there is no more helium. The carbon burning process happens so rapidly that it causes the star to supernova or explode. The pressure from the explosion compacts the core further and can create a neutron star or, in the case of a very large star, a black hole.
Tags: gravity pulling, into helium, pressure pushing