Star Colour, Magnitude and the Hertzsprung-Russell Diagram

Explained

Star colour, magnitude, and reading the diagram

Two measurable things about a star are enough to place it on a chart that reveals its whole life story: how hot its surface is, and how much light it puts out. The Hertzsprung Russell diagram plots one against the other.

Colour tells you temperature

A star's colour depends on its surface temperature, and the order is the opposite of the everyday association with hot and cold taps.

Blue stars are the hottest, at around 30 000 degrees Celsius or more. White is next, then yellow, like our Sun at about 5 500 degrees. Orange follows, and red stars are the coolest, at around 3 000 degrees.

Blue, white, yellow, orange, red, running from hottest to coolest. The reason is that hotter objects emit more of their energy at shorter wavelengths, and blue light has a shorter wavelength than red.

Two kinds of magnitude

Apparent magnitude is how bright a star looks from Earth. It depends on how much light the star actually emits and on how far away it is, so a dim nearby star can look brighter than a brilliant distant one.

Absolute magnitude is how bright a star would look if it were placed at a standard distance of 10 parsecs. Putting every star at the same distance removes distance from the comparison, so absolute magnitude describes the star itself.

The scale runs backwards, which is the detail that catches people out. A lower number means a brighter star, and the very brightest have negative values. The system dates from ancient astronomers who called the brightest stars first magnitude and the faintest sixth, and the arithmetic was fitted to the naming afterwards.

The regions of the diagram

Temperature runs along the horizontal axis, decreasing to the right, so blue stars are on the left and red on the right. Absolute magnitude runs up the vertical axis, with brighter stars higher up.

The main sequence is a broad band running from the top left to the bottom right, and about nine stars in ten sit on it, including the Sun. Along it, hotter stars are brighter and cooler stars are dimmer, and a star spends most of its life here fusing hydrogen into helium.

Red giants and supergiants sit in the top right: cool, so red, but very bright, which they can only be by being enormous. A large enough surface radiates a great deal of light even at a low temperature.

White dwarfs sit in the bottom left: hot, so white, but dim, which means they must be very small. They are the exposed cores left behind when a star like the Sun sheds its outer layers.

Those two corners are the whole logic of the diagram. Anything bright and cool must be big, and anything hot and dim must be small, because brightness depends on temperature and surface area together.

Answering with the diagram

When you are asked to identify a region, work from both axes rather than from memory of the picture. Read the temperature from the horizontal position, the brightness from the vertical, then reason to the size.

When you are asked to describe a star's future, find it on the main sequence and follow the path its mass dictates. A star like the Sun moves right and up to become a red giant, then down and left to a white dwarf. A much more massive star becomes a red supergiant, explodes as a supernova, and leaves a neutron star or a black hole, neither of which appears on the diagram at all.