The short answer
Magnitude is the number astronomers use for how bright an object looks in the sky. The scale runs backwards: lower numbers are brighter, and the very brightest objects have negative magnitudes. A difference of 5 magnitudes is exactly a factor of 100 in brightness, so each single step is about 2.5 times. Under dark skies the faintest stars the eye can see are around magnitude 6; from a city the limit is often 3 or 4.
Why the numbers run backwards
The system goes back more than two thousand years, to Greek astronomers who sorted stars into six classes. The brightest were “of the first magnitude”, the next brightest second, and the faintest they could see were sixth. In the 19th century astronomers made the scale precise by defining a 5-magnitude step as a brightness ratio of exactly 100, which kept the old classes roughly where they were.
Once measured carefully, some stars turned out to be brighter than “first magnitude”, so the scale extended past zero into negative numbers. That is why Sirius, the brightest star in the night sky, has a magnitude of about −1.5, and Venus can reach about −4.5.
| Difference | Brighter by about |
|---|---|
| 1 magnitude | 2.5× |
| 2 magnitudes | 6.3× |
| 3 magnitudes | 16× |
| 5 magnitudes | 100× |
| 10 magnitudes | 10,000× |
Familiar objects on the scale
| Object | Magnitude (approx.) |
|---|---|
| The Sun | −26.7 |
| Full Moon | about −12.7 |
| Venus | up to about −4.6 |
| Jupiter | about −1.6 to −2.9 |
| Sirius | −1.5 |
| Vega | about 0 |
| Polaris | about 2 |
| Faintest stars, dark sky, naked eye | about 6 to 7 |
Planets change brightness as their distance from Earth changes, so their magnitudes are ranges. Most stars hold steady, though some, called variable stars, change noticeably over days, months or years. Object pages on this site, such as those in stars, list the magnitude of each.
Your limiting magnitude
Your limiting magnitude is the faintest star you can see from where you are. It depends far more on the sky than on your eyes. Light pollution brightens the background, and faint stars disappear into it. Moonlight, haze, a low altitude and incomplete dark adaptation all lower it further.
- Inner city: often magnitude 3 to 4, enough for the brightest few dozen stars.
- Suburbs: roughly 4.5 to 5.5, depending on the direction and the night.
- Dark rural site: around 6 or a little fainter; experienced observers at the darkest sites can do better.
Because each magnitude adds many more stars than the last, a small change in limiting magnitude makes a big difference to how full the sky looks. The Bortle scale ties limiting magnitude to what you see at each level of darkness. Binoculars and telescopes push the limit much fainter; see why can’t I see it? for typical values.
The catch with fuzzy objects
For a star, the magnitude describes a single point of light. For a galaxy or nebula, the catalog magnitude is the total light of the object as if it were squeezed into a point. Spread over a large patch of sky, the same light is far harder to see.
That’s why the Andromeda Galaxy, listed around magnitude 3.4, is still difficult or impossible to see from light-polluted places, while a magnitude-3.4 star would be easy. Astronomers describe this with surface brightness: how much light falls on each small patch of the object. For galaxies and nebulae, a dark sky matters more than the headline magnitude suggests.
Apparent and absolute magnitude
Everything above is apparent magnitude: how bright something looks from Earth. A star can look bright because it is powerful or simply because it’s close. Absolute magnitude removes distance from the picture: it is the magnitude a star would have if it were placed at a standard distance of 10 parsecs, about 32.6 light-years.
By that measure, the Sun would be an unremarkable star of about magnitude 4.8, faint from a city. Rigel, which looks a bit fainter than Sirius, is in reality tens of thousands of times more luminous than the Sun; it only looks modest because it is hundreds of light-years away. When an observing guide quotes a magnitude, it means apparent magnitude unless it says otherwise.