EXTREME STARS

From the biggest and brightest to the smallest and dimmest, we take a look at some stellar extremes

- Andrew May

The Sun produces heat and light through nuclear fusion, converting hydrogen into helium. In this respect, it’s a pretty typical star. Around 90 per cent of all stars are undergoing the same process, referred to as the main sequence of the stellar life cycle. Even so, there are some striking exceptions, such as stars of very low or very high mass or ones that have exhausted all their nuclear fuel. Here we take a look at some of these extreme stars – from brown dwarfs to supergiants and from neutron stars to weird hybrid stars. But first it’s worth reviewing the basics of stellar evolution.

Although the stars in the night sky look similar to the naked eye, there’s actually a wide variety of stellar types. This is partly because we see different stars at different points in their evolutionary cycles. This proceeds much too slowly for us to observe directly, so each star is like a single snapshot along the evolutionary path. It begins in a cloud of interstellar gas, where knots can form with sufficient mass that they start to collapse under their own gravity. As the collapse proceeds, the material gets hotter and denser, eventually forming protostars. Not all protostars are equal, even those formed in the same interstellar cloud at the same time. They come in a wide range of masses, from much smaller than our own Sun to many times larger. What happens next depends on the mass of the protostar. All but the very lowest mass stars soon become hot enough for nuclear fusion to take place, putting them on the main sequence. Somewhat paradoxically, however, it’s the high-mass stars that burn through their nuclear fuel most rapidly before moving on to the later – and often much more dramatic – phases of stellar evolution.

BROWN DWARFS