The nuclear fusion reactions in stars release tremendous amounts of energy. The size of a star affects how much energy it releases, with small star releasing less energy than large stars. The amount of energy coming off the surface of a star is called its luminosity. Maybe you've heard objects described before as being luminous. The word comes from Latin, meaning "full of light." The most luminous stars are those that radiate the most energy.
Luminosity
What is the most luminous star that we know of?
The star Eta Carinae is about 150 times more massive than our own sun and is thought to be blasting out 1 million times more energy than our sun. It's the most luminous star we know of. In this picture you can see that Eta Carinae is blue--the most energetic, most luminous stars typically shine blue.
A star's luminosity should not be confused with how bright it appears to someone on earth. Remember that how bright a star appears to us will depend not only on how much energy the star is blasting out (luminosity) but also on how far away the star is from us. It's much like the headlights of a car that appear brightest when they are very close to us but dimmer when they are far away. The amount of energy the headlights are emitting doesn't change, but our perception of the brightness does change depending on our distance from them. Luminosity is an intrinsic value for a star, but a star's brightness depends on our perception of it. The perceived brightness of a star is called its apparent magnitude.
