Stellar Processes

Figure 1: The Hertzsprung-Russell diagram 
Credit: European Southern Observatory (ESO)

When we study stars, it appears that the length of their lifetimes is almost entirely determined by their mass. In fact, the more mass that they have, the less time they live for. In a way, this feels a little odd; surely more massive stars should live longer as they have more fuel? But our theories and observations suggest the opposite is true - the may have more fuel, but they burn through it much faster.

If we consider our Sun, we believe it will live for around 10 billion years. Stars of half this mass might well live for 40 or more billion years, longer than our Universe has so far existed. If we look at more massive stars than our Sun, we expect lifetimes to be measured in millions of years.

We can understand something of the way stars behave from a famous theoretical diagram known as the Hertzsprung-Russell diagram (Figure 1; observational versions of this plot are often called colour-magnitude diagrams). When we consider the stars that are likely to become supernovae, we look at the supergiants and the very hot, bright stars (of spectral types O and B) towards the top of the Main Sequence. This corresponds to masses of greater than 10 times that of our Sun but astronomers believe stars may be as massive as 200 or so solar masses.

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