The Hertzsprung-Russell diagram

ESA's Gaia satellite placed more than four million stars on one chart of colour against brightness.

A chart that sorts stars

In the early 1900s two astronomers, Ejnar Hertzsprung in Denmark and Henry Norris Russell in the United States, separately plotted stars by their brightness and their colour or spectral class. If stars were random, the dots would scatter everywhere. They did not. Most fell along one diagonal band, with a separate group of bright cool stars above it and, later, a group of faint hot stars below it. The chart is now called the Hertzsprung-Russell diagram, or HR diagram, and it is one of the most used tools in astronomy.

Two habits of the chart catch people out. Temperature runs backwards, hot on the left, because the old spectral sequence O B A F G K M was written left to right. And both axes are logarithmic: every step up the luminosity axis is ten times more light, from a ten-thousandth of the Sun to a million Suns. Without that, the Sun and the faint stars would be squeezed into an invisible line at the bottom.

The groups

• Main sequence: the diagonal band from hot, bright stars at the upper left to cool, faint red dwarfs at the lower right. These stars fuse hydrogen in their cores. NASA says about 90% of stars are main-sequence stars, and the Sun is one.
• Giants: cool but bright stars above the main sequence, such as Arcturus and Aldebaran. ESA describes the red giant branch as the arm that leaves the main sequence and turns to the right as stars age and swell.
• Supergiants: the top of the chart, such as Rigel and Betelgeuse, tens of thousands of times the Sun's output or more.
• White dwarfs: hot but faint stars at the lower left, such as Sirius B, the cores left behind by Sun-like stars.

Why are white dwarfs faint if they are hot? Use the formula from the colour unit: luminosity = radius² × (temperature ÷ 5,772)⁴. Sirius B is about 25,200 K, which on its own would make each square metre 360 times as bright as the Sun's surface. But its radius is only 0.0086 Suns, and 0.0086² is about 0.000074. Multiply and the whole star gives off about 0.027 Suns. Betelgeuse works the other way: it is cooler than the Sun, so each square metre is dimmer, yet it has about 524,000 times the surface area, and it shines like tens of thousands of Suns.

That is what the dashed lines in the model show. Along each line every star has the same radius, from 0.01 Suns at the bottom left to 1,000 Suns at the top right. White dwarfs sit near the 0.01 line, the main sequence crosses from about 0.1 to about 10, and supergiants reach towards 1,000. Reading a star's position therefore tells you three things at once: temperature, luminosity and size.

A snapshot, not a path

It is tempting to read the main sequence as a road that every star travels along. NASA's educators point out that it is instead a portrait of many stars of different masses at one moment. A star's mass fixes where it sits on the main sequence, heavy stars at the upper left and light ones at the lower right, and it stays near that spot for most of its life. It leaves only when its core hydrogen runs out, moving up and to the right to become a giant. Turn on "Sun's future path" in the model to see the rough route the Sun is expected to take, up the giant branch and later down to the white dwarfs; the line is a sketch, not a calculation.