Brightness and colour: what your star's numbers mean
· 2 min read · Named in Light
Magnitude, spectral type and colour index sound technical, but they tell a simple story about how bright and how hot your star is.
Each star page shows a few numbers from the catalogue. Here's what they mean, and what they say about your star.
Magnitude: how bright it looks
Astronomers measure brightness on the magnitude scale, which dates back over two thousand years to the Greek astronomer Hipparchus. It has two quirks:
- Lower numbers are brighter. A magnitude 1 star is brighter than a magnitude 2 star.
- It's logarithmic. Each step of one magnitude is about 2.5 times in brightness, so five steps make exactly 100 times.
Some examples:
| Star | Magnitude |
|---|---|
| Sirius, the brightest star in the night sky | −1.44 |
| Vega | 0.03 |
| Polaris, the North Star | 1.97 |
| Faintest stars visible from a dark site | about 6 |
The magnitude on your star page is its apparent magnitude: how bright it looks from Earth. A star can look faint because it's small, or because it's very far away.
Distance: how long its light has travelled
Distances are given in light years, the distance light travels in a year (about 9.5 trillion kilometres). When you look at a star 500 light years away, the light reaching your eyes left it 500 years ago.
Colour and spectral type: how hot it is
Stars aren't all white. Their colour reveals their surface temperature:
- Blue-white stars are the hottest, over 10,000 °C.
- White and yellow-white stars are a little cooler.
- Yellow stars are like our Sun, about 5,500 °C at the surface.
- Orange and red stars are the coolest, around 3,000–4,500 °C.
Astronomers sort stars into spectral types by temperature, from hottest to coolest: O, B, A, F, G, K, M. Our Sun is a G star. On your star page you might see something like B2III or M2Ib: the letter and number give the temperature, and the Roman numeral says whether it's a dwarf like the Sun (V), a giant (III) or a supergiant (I).
On our sky map, each star is drawn in its real colour, worked out from a measurement called the colour index. That's why Betelgeuse glows orange and Rigel shines blue-white, even though both are in Orion.