Red Dwarf Stars
Red dwarf stars are more properly known as M-type main-sequence stars, with the designation M V. The “M” means that they have a surface temperature less than 4,000 degrees Kelvin. The Roman numeral “V” signifies a main-sequence star. “Main-sequence” refers to stars that fuse hydrogen into helium.
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.
The phrase “red dwarf” is quite famous, of course, due to the celebrated sit-com of the same name.They’re quite dim, due to their low temperature, but observations have determined that they are extremely abundant in the Universe. Indeed, they are thought to comprise three-quarters of the total stars in the Milky Way Galaxy. Our nearest stellar neighbour, Proxima Centauri, is a red dwarf.
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.
As well as being dim, they are quite small. At most, they can be half the size of the Sun, but they can be as small as one-tenth. They can outlive a yellow dwarf such as our Sun by many factors. It is thought that they will remain within the main-sequence for several trillion years. The Universe hasn’t been in existence long enough for any red dwarfs to have reached an advanced evolutionary state.
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.