Stars are balls of hot ionized (electrically charged) gas called plasma.
Their energy comes from the complex process of thermonuclear fusion, similar to nuclear bombs. Stars begin as nebulae, or clouds of gas and dust.
A nova is a star system that suddenly becomes incredibly bright and then slowly decreases in brightness again. This happens when one star gathers the material of a local star into itself.
A supernova is a nova on a big scale, much larger than regular novas.
The nearest star is 4.4 light-years, or 25.872 trillion miles (41.654 trillion kilometers), away.