stellar luminosity
/ loo-mih-NOS-ih-tee /
A candle held at arm's length can look brighter than a lighthouse miles away, even though the lighthouse pours out far more light. The difference between how bright something looks and how much light it actually gives off is the whole problem of measuring stars. Luminosity is the honest, distance-independent number: the total amount of energy a star radiates into space every second, no matter where you stand to watch it.
Luminosity is a power, measured in watts, just like a light bulb — but a star's wattage is staggering. The Sun's luminosity is about 3.8 x 10^26 watts, a number astronomers use as a yardstick called one solar luminosity (L_sun). Stars range from less than a thousandth of a solar luminosity for dim red dwarfs to a million times the Sun for the most brilliant supergiants. Because light spreads out and dims with the square of distance (the inverse-square law), you can recover luminosity by combining how bright a star looks with how far away it is: luminosity is proportional to apparent brightness times distance squared.
Luminosity is the vertical axis of the Hertzsprung-Russell diagram and one of the two numbers (with temperature) that fix a star's place in the stellar census. It connects to deep physics: by the Stefan-Boltzmann law, a star's luminosity depends on both its surface area and the fourth power of its surface temperature, so luminosity, radius, and temperature are locked together. Knowing a star's true luminosity is also what turns certain stars into standard candles for measuring far greater cosmic distances.
Rigel, a blue supergiant in Orion, looks only modestly bright because it is about 260 parsecs away — yet its luminosity is roughly 100,000 times the Sun's. Account for the distance and its true output is enormous.
Looks dim, shines bright: luminosity strips away the illusion of distance.
Brightness and luminosity are not the same. How bright a star looks (apparent magnitude) mixes its true power with its distance; luminosity is the true power alone. You cannot read luminosity off the sky without also knowing the distance.