Colour and brightness

Look closely on a clear night and you will see that the stars are not all white. Betelgeuse, in the shoulder of Orion, is orange. Rigel, at Orion’s foot, is blue-white.

The colour is a thermometer. Think of a piece of metal in a fire. First it glows dull red, then orange, then yellow-white. If you could make it hotter still it would turn blue.

Stars work the same way. Red stars are the coolest. Blue stars are the hottest. The Sun is in between.

Brightness is harder. A star can look bright because it really is powerful, or just because it is near. To know which, you have to find out how far away it is.

Colour is temperature. Anything hot and dense glows, and the hotter it is the bluer its light. So the colour of a star gives the temperature of its surface, with no need to go there. Temperatures here are in kelvin: the same size of degree as Celsius, counted from absolute zero.

Astronomers sort stars into seven main classes by the pattern of dark lines in their light, which turns out to be an order of temperature.

Class	Colour	Surface temperature	An example
O	Blue	Above 30,000 K	Alnitak, in Orion’s Belt
B	Blue-white	10,000 to 30,000 K	Rigel
A	White	7,500 to 10,000 K	Sirius, Vega
F	Yellow-white	6,000 to 7,500 K	Procyon
G	Yellow	5,200 to 6,000 K	The Sun (5,772 K)
K	Orange	3,700 to 5,200 K	Arcturus
M	Red	Below 3,700 K	Betelgeuse, Proxima Centauri

The odd order of the letters is an accident of history. The classes were first lettered by the strength of one set of lines, and were shuffled into order of temperature afterwards.

Brightness needs distance. How bright a star looks depends on how much light it gives out and how far away it is. Light thins out with the square of distance: move a star twice as far away and it looks four times fainter. So once the distance is known, the star’s true output can be worked out. Distances to the nearer stars come from parallax, the small shift in a star’s position as the Earth goes round the Sun. The Gaia spacecraft has measured it for more than a billion stars.

The chart. Around 1910 two astronomers, Ejnar Hertzsprung and Henry Norris Russell, each had the idea of plotting true brightness against temperature. If the two had nothing to do with each other the dots would scatter everywhere. They do not.

About nine stars in ten fall on one band running from hot and bright to cool and faint. This is the main sequence, and it holds every star that is steadily burning hydrogen in its core, the Sun included. Where a star sits along it depends on its mass.

Two groups sit off the band. Above it to the right are stars that are cool but very bright, which can only mean they are enormous: the giants and supergiants. Betelgeuse is one. Put in place of the Sun it would reach out past the orbit of Mars. Below the band to the left are stars that are hot but very faint, so they must be tiny: the white dwarfs, about the size of the Earth.

What the chart is for. The chart is a picture of how stars live. The main sequence is crowded because stars spend most of their lives there. The giants and white dwarfs are stars that have left it. The next lesson follows a star along that route.