Red Dwarfs
A red dwarf is the smallest, coolest type of star that still shines by fusing hydrogen into helium. They are also by far the most common stars in the universe: around three out of every four stars in the Milky Way is a red dwarf. None of them can be seen with the naked eye, not even the closest one to us, but every modern telescope is pointed at them, partly because they live so long and partly because more and more planets are being found orbiting them.
- % of all starsapprox. 73%The most common type by far
- Surface temperatureapprox. 2,000 to 3,500 °CMuch cooler than the Sun (5,500 °C)
- Mass0.08 to 0.5 SunsA small star is still huge
- LifespanUp to 10 trillion yearsFar longer than the universe has existed
- Closest to EarthProxima Centauri4.24 light years away
- Visible to naked eye?NoEven the closest needs binoculars
What is a red dwarf?
A red dwarf is a fully working star, but a very small and faint one. Astronomers classify it as a type M main-sequence star: cool (less than 3,700 °C at the surface), reddish in colour, and with less than half the mass of our Sun. Above that mass, the star burns hotter and shines yellow or white. Below about 0.08 solar masses, the object is not heavy enough for hydrogen fusion to start at all, and you get a brown dwarf instead, an in-between body that is neither star nor planet.
Why are they so common?
The simple answer is that small things are easier to make. When a cloud of gas collapses to form stars, it tends to break up into many small clumps and a few big ones. The same rule shows up in lots of places in nature: there are far more small rocks in the solar system than big ones, far more grains of sand on a beach than pebbles. Stars work the same way. About 73% of all stars are red dwarfs. By comparison, only around 7% are stars similar to our Sun and less than 0.1% are giants like Betelgeuse.
Why red dwarfs live so long
Red dwarfs are tiny stars, so they have less fuel. But they also burn their fuel much more slowly than bigger stars do, because their cores are cooler and less crushed by gravity. They also have a useful trick: a red dwarf's whole interior is fully convective, meaning hot gas churns from the centre to the surface and back like water boiling in a pot. That mixing brings fresh hydrogen into the core throughout the star's life, so the star can keep fusing for far longer.
The total lifespan of the smallest red dwarfs is estimated at up to 10 trillion years, more than 700 times the current age of the universe. No red dwarf has ever died of old age yet. Every single red dwarf ever born is still shining.
Proxima Centauri: our nearest neighbour
The nearest star to our Sun is a red dwarf called Proxima Centauri, just 4.24 light years away. It is part of the triple system Alpha Centauri (with two larger Sun-like stars), but Proxima itself is so faint that it cannot be seen with the naked eye, despite being so close.
In 2016, astronomers found a planet about the same size as Earth orbiting Proxima Centauri, called Proxima Centauri b. It sits in the right zone of distance from its star for liquid water to exist on its surface, the so-called habitable zone. A second smaller planet, Proxima c, was found a few years later. They are the closest known exoplanets to us, although a trip there with current rockets would take tens of thousands of years.
Stormy little stars
Despite their cool, steady appearance from Earth, red dwarfs can be surprisingly violent. Many of them throw off huge stellar flares: sudden bursts of X-rays and ultraviolet light hundreds of times more powerful (relative to the star's normal output) than anything our Sun ever does. This is good news for understanding red dwarfs, but possibly bad news for any planet trying to keep an atmosphere around them. Some scientists think red dwarf flares could strip the atmosphere off a nearby planet over millions of years, making true habitability harder than we first hoped.
Deeper dive: why red dwarfs may host most of the habitable planets in the universe
Until quite recently, astronomers thought the most likely places to find life were planets around Sun-like stars. The thinking was simple: the Sun has life, so other stars like the Sun would be the best bet. But there are good reasons to think red dwarfs might be even better.
First, red dwarfs are far more common: about 10 times more numerous than Sun-like stars. Second, they live far longer: a red dwarf gives any planet around it trillions of years for life to evolve. By contrast, the Sun will only be habitable for another billion years before its growing brightness boils the oceans. Third, planet-hunting telescopes find it much easier to detect planets around red dwarfs, because the planet is bigger compared to its star and so blocks more light when it passes in front. As a result, most of the small Earth-like planets discovered so far are around red dwarfs.
The big questions are whether red dwarf planets can keep their atmospheres against the constant flaring of their star, and whether being tidally locked is good or bad for life. Future telescopes like the James Webb Space Telescope are studying the atmospheres of planets around nearby red dwarfs right now to try to answer exactly those questions.
For other star types, see white dwarfs, supergiants and neutron stars. Or follow a star from birth to death in life cycle of a star.