Stars in the universe come in all sorts of sizes, from comparatively small neutron stars to massive supergiants. By far the most abundant type of star, however, is the red dwarf. Smaller than our Sun but with a much longer lifetime, these balls of burning gas are extremely important in our understanding of the cosmos.
Red dwarf stars typically have a mass of between 7.5% and 40% of the Sun. Less massive stars are known as brown dwarfs, owing to their comparatively low luminosity, while more massive stars (including our own star) are yellow dwarfs. Their reduced mass means that red dwarfs have a cooler surface temperature than the Sun, typically around 3,500 Kelvin (3,230 degrees Celsius) compared to over 5,750 Kelvin (5,475 degrees Celsius) for the Sun.
Energy is generated in a red dwarf in the same way that it is in the Sun, namely through the fusion of hydrogen into helium. Because of their lower mass and core temperature, though, the rate of nuclear fusion is much less, and thus they emit a smaller amount of light. Even the largest red dwarfs emit only 10% of the Sun’s light, while the smallest have just one ten thousandth of the Sun’s luminosity.
In all stars energy from the core is radiated out from the surface through a process known as convection, losing a large amount of mass in the process. Red dwarfs, on the other hand, are fully convective. This means helium does not accumulate at the core, and the stars can continue to burn hydrogen for a much longer time than other stars.
So long is the process, in fact, that the lifespan of a red dwarf can be far longer than the expected age of the universe, thought to be about 14 billion years. More massive stars burn through their fuel much faster and thus have shorter lifespans, sometimes just a few million years, so the lower the mass of a red dwarf the longer it will live. A red dwarf with a tenth of the Sun’s mass will continue burning fuel for 10 trillion years. Therefore, there are no red dwarfs that we know of in the universe that are nearing the end of their lives, so we will likely never observe what happens in the last throes of their lives.
You can find out more about red dwarfs in issue 7 of All About Space, on sale now.