Observing Skills · 3 of 11

Understanding Star Brightness

Why does the brightest star in the night sky get a negative number?

Why smaller numbers mean brighter stars — and how to read magnitudes off the sky with nothing but your eyes.

You’ll be able to estimate a star’s magnitude by eye, against stars you know.

13 steps · about 10 min

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Three tags: Polaris, the North Star, and two stars of the Big Dipper. Look before you answer on the card: which of the three is the brightest?

Not Polaris. At 1.97 it is the 48th brightest star in the sky, outshone by three stars of the Dipper alone. It is famous for standing still, and fame and brightness are not the same axis. And on this scale, the smaller number is the brighter star.

The scale runs backwards. Its six classes, usually credited to Hipparchus, ran from "first magnitude", the brightest, to "sixth", the faintest the eye could see. Sirius, the brightest star in the night sky, is so far past first that it needs a negative number.

In 1856 Pogson put exact numbers on the classes: five magnitudes is exactly ×100 in brightness, so one magnitude is ×2.512. The scale needs a zero, too, and Capella is as close as the winter sky gets.

Compare Sirius with Mintaka, the right-hand star of Orion's Belt. Guess on the card: how many Mintakas would it take to send as much light as one Sirius?

The tags give it away: Sirius to Mintaka is 3.69 magnitudes, so by Pogson's rule it takes 30 Mintakas — more than most guess. The eye responds to light logarithmically, and so does the scale.

Read the distances on the tags. Rigel is about 100 times farther than Sirius and only a magnitude and a half fainter, so it must give out thousands of times more light. A magnitude is how bright a star looks, not how bright it is.

The skill to take outside: to judge a star, find two known ones that bracket it and decide where it falls between them. Orion gives you a ladder of rungs, on the tags. Not Betelgeuse: it wanders.

Now use it. Find Algol, "the ghoul", east of Almach: at full light the two are level. It is an eclipsing binary: every 2.9 days a dim companion passes in front, and for about ten hours it is more than a magnitude down, well below Almach.

Betelgeuse is the other kind of variable: a red supergiant that wanders. In 2019–20 it sank to 1.6, fainter than anyone alive had seen it, behind a cloud of its own dust — while Rigel, across Orion from it, held steady. A magnitude is a snapshot, not a property.

Under a rural sky the faintest star you can see is about magnitude 6.5. Here is every catalogue star above your horizon, dropped into a column by magnitude. The columns grow toward the faint end.

A suburban sky: slide the limit from 6.5 to 5.0. It takes almost the whole faint end; what you keep is a thin slice off the top.

The Bortle scale, from Dark Skies, Dark Eyes

Magnitude is how a star looks from here, and some stars vary. Bracketed, a practised eye is good to ±0.2; your retina only needs a reference.

Smaller is brighter; five magnitudes, ×100
Bracket a star between two you know

Understanding Star Brightness

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