Star magnitude is the scale astronomers use to describe how bright a star or other object appears in the sky. The unusual thing about the scale is that it runs backward: the lower the magnitude, the brighter the object. Sirius, for example, has an apparent magnitude of −1.46, while stars near magnitude +6 are about the faintest most people can see with the naked eye under a dark sky.
There’s another twist: the magnitude scale is logarithmic rather than linear, so a difference of five magnitudes represents a 100-fold difference in brightness. Once you understand these two ideas, the seemingly strange magnitude numbers used throughout astronomy become much easier to read.
Why Do Brighter Stars Have Lower Magnitudes?
The backwards scale is a legacy of ancient astronomy. Around the second century BC, the Greek astronomer Hipparchus grouped visible stars according to their brightness. The brightest were described as first magnitude, while the faintest stars visible to the naked eye were sixth magnitude.
Modern astronomy retained that basic convention even after brightness could be measured precisely. The scale was also extended beyond its original limits, which is why especially bright objects can have zero or negative magnitudes. Sirius is magnitude −1.46, while Venus can become brighter than magnitude −4.
How Much Difference Does One Magnitude Make?
The magnitude scale is logarithmic, meaning that each step does not represent an equal change in brightness.
A difference of one magnitude corresponds to a brightness ratio of about 2.512 to 1, while a difference of five magnitudes corresponds to exactly 100 to 1.
- A magnitude 1 star is about 2.5 times brighter than a magnitude 2 star.
- A magnitude 1 star is about 6.3 times brighter than a magnitude 3 star.
- A magnitude 1 star is 100 times brighter than a magnitude 6 star.
Examples of the Star Magnitude Scale
Some familiar objects make the magnitude scale easier to visualize:
| Object | Approximate apparent magnitude |
|---|---|
| Sun | −26.7 |
| Full Moon | −12.7 |
| Venus at its brightest | about −4.7 |
| Sirius | −1.46 |
| Vega | about 0 |
| Typical faintest naked-eye stars under dark skies | about +6 |
The scale isn’t limited to stars. Astronomers use apparent magnitude for planets, the Moon, the Sun and other objects in the sky, allowing their apparent brightness to be compared using the same system.
Apparent vs Absolute Magnitude
The magnitudes above are apparent magnitudes — measurements of how bright objects appear from Earth.
But apparent brightness doesn’t tell us how much light a star actually produces. Distance matters enormously. A nearby star can look bright simply because it is close to us, while an extremely luminous star can appear much fainter because it is hundreds or thousands of light-years away.
Astronomers therefore also use absolute magnitude. This describes how bright a star would appear if it were placed at a standard distance of 10 parsecs, or 32.6 light-years, from Earth.
Apparent magnitude tells us how bright a star looks from Earth. Absolute magnitude lets us compare the intrinsic brightness of stars without their different distances getting in the way.
Sirius is a good example. It dominates our night sky partly because it lies only 8.6 light-years away. Some stars that look much fainter than Sirius actually produce far more light but are vastly farther away.
How Faint a Star Can You See?
Under an exceptionally dark sky, the traditional naked-eye limit is around magnitude +6, although some observers can see fainter stars under excellent conditions.
Light pollution makes faint stars harder to see. From a brightly lit city, many fifth- and sixth-magnitude stars disappear from view entirely.
The faintest star you can see is therefore a useful indication of the quality of your night sky. Astronomers call this the naked-eye limiting magnitude.
And there’s a simple way to estimate yours using a familiar part of the autumn sky.
Test Your Sky With the Great Square of Pegasus
The Great Square of Pegasus provides a convenient naked-eye test of how faint a star you can see.
First, find the Great Square of Pegasus and allow your eyes time to adapt to the darkness. Then count the stars you can see inside the Square, not including the four bright stars marking its corners. The stars inside the Square have different brightnesses, so the number you can detect provides an estimate of the faintest magnitude visible under your observing conditions.
The more stars you can see, the higher your limiting-magnitude number — meaning you can see fainter stars.
The test isn’t a precision measurement. Your eyesight, atmospheric transparency, nearby lights and the height of Pegasus above the horizon can all affect the result. But it provides a simple way to turn the magnitude scale from an abstract set of numbers into something you can actually test with your own eyes.