Absolute Beginners No.1: The colours of stars

2015 May 29

Paul G. Abel

Step outside on any clear night and after a few minutes outdoors, when your eyes have adjusted, you will notice that the stars are not all the same. Some appear much brighter, some look larger and importantly, they are not all the same colour. Take a look at Betelgeuse in Orion the Hunter: it’s orange in colour; Capella in Auriga is yellow. The summer stars Vega and Altair are bluish, while Antares in Scorpius is a deep red colour. Whilst being pretty to look at, there is a much deeper importance in star colour, one which provides an insight into the private lives of stars.

The lives and deaths of stars

Stars are not constant – like people, they are born, they live and they die. How long stars live for and the way that they expire is largely dependent on one thing: size. This particular branch of our science is called stellar evolution and covers everything from star formation to supernova explosions.
All around the night sky we can find examples of stellar nurseries: the places stars are made. The sword handle of Orion (also called the Orion Nebula, M42) is a good example. Inside these vast clouds of hydrogen, material clumps together under gravity. Eventually this protostar becomes heavy enough to allow nuclear fusion to start within its centre: hydrogen is converted into helium and a new star is born. Stars tend to be born in clusters, and slowly over time drift away. A nice example of a young group of stars is the Pleiades star cluster in Taurus the Bull.
Once a star is born, it converts hydrogen at a regular rate, and astronomers chart the lives of stars using a figure called the Hertzsprung-Russell diagram (Figure 1).
The diagram has two axes: running vertically we have luminosity (this is the total amount of energy emitted by the star). On the horizontal axis we have temperature. The main important feature is the diagonal band of stars running from top left to the bottom right; this is called the ‘main sequence’. Stars on the main sequence slowly burn their hydrogen fuel in a regular fashion.
Very young stars are hot and luminous and blue in colour (top left of the diagram). Spica and Bellatrix are good examples. As stars evolve, they continue consuming their hydrogen fuel and start to cool down, causing them to pass from bluish to white in colour. Sirius and Altair are examples of these stars. As stars pass into middle age, they continue to cool down and become yellow in colour: our own Sun is at this point.
Eventually a star will run out of its hydrogen fuel, and burn helium instead. As a result, its atmosphere bloats out, becoming cooler and redder as the star passes into the red giant phase. What happens after this depends on the mass of the star.
Stars like the Sun spend a long time on the main sequence, then pass into the red giant phase. Eventually they shed their outer atmosphere into space and the result is a beautiful object known as a planetary nebula. The night sky is full of such tombstones – M57 in Lyra (the Ring Nebula) is a particularly good example. All that remains of the original star is the hot core which we call a white dwarf. White dwarf stars are very dense and heavy, and this is the ultimate end for stars like the Sun.
Heavier stars (10 times or more heavier than the Sun) end their lives in a far more spectacular fashion. Giants like Antares or Betelgeuse no longer follow the main sequence and after their hydrogen and helium burning phase, they are massive enough to begin converting heavier elements: carbon, neon, oxygen, silicon and eventually iron. As the star burns these heavier elements, so its core gets heavier and heavier. Eventually the core collapses under its own weight and the result is a supernova explosion. When a supernova occurs, so much energy is released that a single star, normally hidden amongst the glow of the combined light of billions of stars in the galaxy, will outshine its parent galaxy.
Eventually the supernova starts to fade, and all the heavy elements which were contained in the core (carbon, iron etc.) are returned to the Universe and will end up recycled in the formation of new stars and planets. All of the heavy metals we have here on Earth originated in the cores of massive stars which became supernovae billions of years ago.
Really massive stars are so heavy that they collapse completely under their own weight; it is thought that these stars become black holes. At the heart of a black hole is a singularity where all of the matter has been crushed into a tiny point of incredible density. This singularity is hidden from the outside Universe by the event horizon. Once anything passes the horizon, it is doomed to fall into the singularity – even light cannot escape.

Colour and temperature

From the Hertzsprung-Russell diagram, it is clear that there is a connection between colour and temperature. Hot young stars like Spica in Virgo are blue in colour, cooler stars like the Sun are yellow, while very cool stars like Arcturus and Antares are orange-red in colour. Astronomers have created groups which classify stars according to their colours: these are the letters O B A F G K M [remembered by O Be A Fine Guy Kiss Me] on the horizontal axis of the Hertzsprung-Russell diagram. The table shows the stellar classification and the corresponding temperature and colour. Here temperature is measured in degrees Kelvin (to convert a temperature in Kelvin to Celsius we simply subtract 272.15).
So the next time the sky is clear, go out and take a look at the stars and their colours. Just by knowing their colour you can now infer how hot these stars are, and which stage of evolution they are in.

Paul G. Abel