absolute magnitude
/ AB-so-loot /
Comparing how bright two stars look from Earth is unfair, because they sit at wildly different distances — a faint-looking star might just be far away. Astronomers wanted a fair contest, so they imagined a thought experiment: line every star up at the same standard distance and ask how bright each would then appear. The result of that imaginary lineup is the absolute magnitude, a true measure of a star's intrinsic power on the familiar magnitude scale.
The chosen standard distance is 10 parsecs (about 32.6 light-years). Absolute magnitude is defined as the apparent magnitude a star would have if it were placed exactly 10 parsecs away. Because that distance is the same for all stars, absolute magnitude depends only on a star's true luminosity, with no contamination from distance. The same backward number rule applies: a smaller (more negative) absolute magnitude means a more luminous star. Our Sun has an absolute magnitude of about plus 4.8 — modest, and it would be just barely visible to the naked eye from 10 parsecs.
Absolute magnitude is simply a logarithmic way of expressing luminosity, and it is the y-axis of the Hertzsprung-Russell diagram, the single most important chart in stellar astronomy. Crucially, the gap between a star's apparent and absolute magnitude encodes its distance: that gap is the distance modulus. So if you can find a star's absolute magnitude by other means (for instance, recognizing it as a standard candle), measuring its apparent magnitude immediately gives its distance.
Sirius looks far brighter than Deneb in our sky, but that is only because Sirius is close. By absolute magnitude — placing both at 10 parsecs — Deneb (about minus 7) would utterly outshine Sirius (about plus 1.4): Deneb is intrinsically tens of thousands of times more powerful.
Absolute magnitude lines all stars up at 10 parsecs, so it compares true power, not appearance.
Absolute magnitude is just luminosity expressed on the magnitude scale, fixed at a standard 10-parsec distance. It is not something you observe directly — you must know the distance (or use a standard candle) to obtain it.