Star brightness and magnitude

An ancient scale that runs backwards

More than 2,000 years ago the Greek astronomer Hipparchus sorted stars into six classes. The brightest were 'first magnitude' and the faintest he could see were 'sixth magnitude'. We still use his idea, which is why a bigger number means a fainter star.

In 1856 the English astronomer Norman Pogson put the scale on a precise footing. He defined a difference of five magnitudes as exactly a factor of 100 in brightness. That makes each single step about 2.512 times.

Because the scale had to stretch to fit very bright objects, it now goes below zero. Negative numbers are simply brighter than magnitude 0.

Real examples from bright to faint

The Sun shines at about magnitude −26.7, by far the brightest object in our sky. Never look at it directly or through any optics without a certified solar filter that meets the ISO 12312-2 standard; it can damage your eyes in seconds.

The full Moon is about −12.7, and Venus can reach about −4.6 when it is at its best. Sirius, the brightest star of the night, is −1.46. Polaris, the North Star, is a modest +2, which surprises many people who expect it to be dazzling.

At the faint end, the unaided eye under a very dark sky tops out near +6. Binoculars push that to about +9 or +10, and large telescopes go far deeper.

Apparent versus absolute magnitude

Apparent magnitude is how bright an object looks from Earth. It depends on two things: how much light the object gives off and how far away it is.

Absolute magnitude removes distance from the picture. It is the apparent magnitude a star would have if it sat exactly 10 parsecs away, which is about 32.6 light-years. This lets astronomers compare stars fairly.

The Sun shows the difference well. Its apparent magnitude is −26.7 only because it is so close. Its absolute magnitude is about +4.8, so from 32.6 light-years it would be a faint, easily missed star. Some stars that look dim to us are actually far more powerful than the Sun, just much more distant.

How light pollution changes your limit

Artificial light scattered in the air makes the sky glow, and that glow drowns out faint stars. Astronomers call the faintest star you can see your limiting magnitude.

From a remote dark site you may see stars to about magnitude 6, which means a few thousand stars across the whole sky at any moment. In a suburb the limit is often around 4 to 5, and in a bright city center it can fall to 2 or 3, leaving only a few dozen stars.

The Moon also matters. A bright full Moon can wash out faint stars even in the countryside, so new-Moon weeks are best for deep views.

How to judge brightness yourself

Start by comparing stars with known magnitudes. Vega and Arcturus are close to 0, the stars of the Big Dipper are mostly around +2, and Polaris is about +2.

To estimate your sky's limit, pick a familiar constellation, then find the faintest star in it you can see steadily. Check its magnitude on a star chart app. Doing this on different nights shows how haze, the Moon and nearby lights change your view.

Give your eyes about 20 to 30 minutes in the dark before you judge. Using averted vision, looking slightly to one side of a faint star, helps because the edge of your eye is more sensitive in low light.

Why magnitude matters

Every star chart, comet forecast and meteor report uses magnitudes. If a forecast says a comet may reach magnitude 5, you now know it should be visible to the eye from a dark site but probably not from town.

The same scale works for planets, satellites and even the International Space Station, which can approach about −6 at its brightest. Once you know a few reference stars, you can read any sky event and decide whether it is worth going out for.