luminosity
/ loo-mi-NOS-i-tee /
Two light bulbs can look equally bright — one a faint nightlight right next to you, the other a stadium floodlight far down the street. Their apparent brightness is the same, but their true power is wildly different. Luminosity is the true, intrinsic power of an object: the total amount of energy it radiates every second, regardless of how far away it is or how bright it happens to look from where we stand.
Luminosity is a measure of energy per unit time, so it is just a power, measured in watts, exactly like a light bulb's rating. Astronomers often quote it relative to the Sun, whose luminosity is about 3.8 times ten to the twenty-sixth watts (written L-sub-sun). On this scale, faint red dwarf stars might be a thousandth of a solar luminosity, while the brightest supergiants are hundreds of thousands of solar luminosities, and a quasar can shine with a trillion. The Stefan-Boltzmann law ties luminosity to a star's temperature and size, so knowing any two lets you find the third.
Luminosity is the quantity astronomers really want, because it is a property of the object itself, not an accident of distance. But we cannot measure it directly — all our telescopes see is how much light arrives, the flux. To convert what we receive into the true power, we must know the distance and apply the inverse-square law. This is why luminosity and distance are forever entangled: get one and you can get the other. A 'standard candle' is any object whose luminosity we know in advance, letting us solve for distance.
The bright star Deneb looks only moderately bright in our sky, yet it is one of the most luminous stars known — tens of thousands of times the Sun's power — because it is extremely far away. Its faintness fools the eye; its luminosity reveals its true nature.
Luminosity is true power; how bright a thing looks also depends on its distance.
Luminosity (intrinsic power) and brightness (how it looks to us) are different things. A star can be luminous yet faint if it is far away, or modest yet bright if it is nearby — the inverse-square law links them through distance.