Field guide · Stars
StarMagnitude
How bright something looks. The scale runs backwards — smaller is brighter — and each step is about 2.5 times fainter than the one before.
- Faintest by eye
- About 6 under dark skies, 3–4 in a city
- One magnitude
- 2.512 times brighter
- Five magnitudes
- Exactly 100 times
- Brightest star
- Sirius, −1.4
Around 130 BC the Greek astronomer Hipparchus sorted the stars into six classes: the brightest were of the first magnitude, the faintest he could see were of the sixth. That is why the scale runs backwards, and why it still works the way eyes do.
In 1856 Norman Pogson made it exact: five magnitudes is a factor of 100 in brightness, so each step is about 2.512 times. The brightest objects went off the top of the old scale into negative numbers. Sirius is −1.4, Venus reaches −4.6, the full Moon −12.7.
This is apparent magnitude: how bright a thing looks from here, which mixes how luminous it is with how far away. Deneb and Vega look similar, but Deneb is thousands of times more powerful and about a hundred times farther off.
See it in the sky
Each one opens the sky map pointed at it, for tonight, from where you are.