What are Red Dwarf Stars? Characteristics, exoplanets and lifespans

Red dwarf stars are small, cool, long-lived stars with temperatures between 2,500 and 4,000 K. They are the most common stars in the Milky Way, making up around three-quarters of all stars. Although faint and difficult to see from Earth, many host exoplanets, including systems like Proxima Centauri and TRAPPIST-1.

In addition to being the name of the spaceship in the television series of the same name, red dwarf is the name for some of the smallest stars out there in the universe. They get their name from their colour and size. Red dwarf stars will have a spectral type starting with an 'M', but beware UY Scuti, the largest star, also has an M or K spectral type; the key is the rest of the spectral type. Red dwarf stars have temperatures of roughly 2,500-4,000K, which might seem hot to you and me, but compared to other stars, that's pretty cool.

brown dwarf stars have also been referred to as red dwarf stars because they are amongst the coolest stars out there. Brown dwarfs are commonly called failed stars because they lack the mass to sustain nuclear fusion. Nuclear fusion, the process of converting hydrogen into helium, has not yet begun.

Red dwarf stars are created in the same way that any other star is made from dust cloud. It's just that the cloud that formed it wasn't as large as other clouds that formed larger stars.

Whilst difficult to see from the earth, they are prevalent in the universe and are said to make up at least three-quarters of the stars in the milky way. The nearest and most well-known is Proxima Centauri is also the nearest star to our own.

Orbiting exoplanets

Red dwarfs are known to have extrasolar planets (exoplanets) orbiting them, such as Gliese 581b and Proxima Centauri. Compared to other types of stars, it is easier to spot planets around red dwarf stars as they exert more gravitational influence on the star than a planet does on a larger star. Proxima Centauri is known to have a planet, meaning any potential home for life there is small, if anything. We have spotted planets around giant stars as well, but not as prevalent.

A problem with living on a red dwarf's orbiting planet is that the planet could be tidally locked, meaning that only one side of the planet would ever have access to the star's light. The planet would have to be nearer to the sun proportionally compared to what our planet is to our star.

The nearest star to us, excluding the Sun, is a red dwarf star which was only discovered in 1915 by Robert T.A. Innes. Although our nearest neighbour, the star, is not bright enough to be seen here on Earth.

The search for life: the TRAPPIST-1 system

One of the most exciting discoveries in modern astronomy is the TRAPPIST-1 system, an ultra-cool red dwarf hosted in the constellation Aquarius. This star has seven Earth-sized rocky planets, three of which reside in the habitable zone. However, life around a red dwarf faces unique challenges:

• Tidal Locking: Because the Goldilocks Zone is so close to the cool star, planets are often tidally locked, with one side in permanent daylight and the other in eternal night.
• Stellar Flares: Red dwarfs can be "active," frequently emitting violent solar flares that could strip the atmosphere off nearby planets.

Lifespan of a Red Dwarf

Red dwarf stars are extremely energy efficient compared to giant stars, which burn through their fuel quickly. A giant star could exist for billions of years, and a dwarf star can expect to live trillions of years. The Sun has an expected five billion years left, whereas Proxima has trillions of years. When the sun goes down on Earth, our species will hopefully have moved off and colonised planets elsewhere, maybe around a red dwarf star. Dwarf stars are cooler stars, so they convert their fuel more slowly. Red dwarf stars are convective, meaning that heat from the core is carried to the surface, cools and then returns to the core. Swinbourne,

When a red dwarf has exhausted its fuel, it will hypothetically turn into a blue dwarf star before cooling again and becoming white dwarf star

Red stars are of two different sizes at two different ends of the spectrum. Red stars can be amongst the smallest and largest stars in the universe. The red refers to the heat and luminosity that the star generates. Red stars are the second-coolest, with brown stars being the coolest.

The mechanics of longevity

The secret to a red dwarfs multi-trillion-year lifespan lies in its convective interior. In stars like the Sun, only the hydrogen in the core is used for fusion. In a red dwarf, the entire star is convective; the helium produced by fusion is carried away from the core, and fresh hydrogen from the outer layers is constantly cycled back in. This ensures that the star uses nearly 100% of its hydrogen fuel before dying.

Comparing red dwarfs with other stellar objects

Common red dwarf examples

• teegarden's star in the constellation of aries
• Proxima centauri in the constellation of centaurus
• ez aquarii in the constellation of aquarius

Summary of red dwarf facts

• Red dwarf stars are the most common type of star in the universe.
• The nearest star to Earth other than the Sun is a red dwarf called Proxima Centauri, a mere four light-years away.
• Red stars, regardless of size, are the coolest stars of any type.
• Red stars can be the colour of the smallest and largest stars in the universe.
• Red dwarf stars are fuel efficient, and whilst blue stars might last for millions of years, red dwarf stars could live for trillions of years.
• The goldilocks zone for a red dwarf is closer than it would be for hotter stars like our own sun.
• Red dwarf stars are main sequence stars are they are fusing hydrogen into helium. When you look at the Hertzsprung-Russell diagram, you will see them on the bottom left of the picture.