Red dwarf

A red dwarf is the smallest, coolest, and least bright type of star that shines on its own. These stars are very common in the universe, especially in our own Milky Way galaxy. Many of the stars closest to the Sun, like Proxima Centauri, are red dwarfs. But because they don’t shine very brightly, we usually need special tools to see most red dwarfs.

Red dwarfs are smaller and cooler than our Sun. The smallest ones are about one-tenth the width of the Sun and have temperatures around 2,000 K. They are also less massive, weighing only about 7.5% as much as the Sun. Even though they are small and cool, red dwarfs can last for a very, very long time—much longer than the current age of the universe.

Because red dwarfs use their fuel so slowly, they stay the same for trillions of years. They don’t change much until they finally run out of fuel and become a different kind of star called a blue dwarf. Scientists think red dwarfs might be good places to look for planets that could support life.

Definition

The term "red dwarf" for a star doesn't have one single, exact meaning. It was first used in 1915 to describe cooler, red stars compared to hotter, blue ones. Over time, scientists have used the term in different ways.

Today, the idea of a red dwarf still changes depending on who you ask. It usually means certain cooler stars, but sometimes it includes even more types. Some of the coolest stars we know fall into special groups, and it can be hard to tell them apart from other very cool objects.

Description and characteristics

Red dwarfs are very small and not very bright stars. They have less mass than bigger stars, so they do not burn as hot or shine as brightly. Even the brightest red dwarfs give off only about one-tenth as much light as our Sun. Because they are so small, red dwarfs can last much longer than the age of the universe. Some might keep burning for trillions of years!

These stars produce energy by mixing hydrogen into helium. Because they are cool and dense inside, they move this energy to their surfaces in a process called convection, unlike bigger stars. Scientists use red dwarfs to help figure out how old star groups are and even how old the universe might be.

Properties of typical M-type main-sequence stars
Spectral type	Mass (M_☉)	Radius (R_☉)	Luminosity (L_☉)	Effective temperature (K)	Color index (B − V)
M0V	0.59	0.588	0.069	3,850	1.42
M1V	0.51	0.501	0.046	3,660	1.49
M2V	0.45	0.446	0.029	3,560	1.51
M3V	0.36	0.361	0.016	3,430	1.53
M4V	0.24	0.274	7.2×10⁻³	3,210	1.65
M5V	0.155	0.196	3.0×10⁻³	3,060	1.83
M6V	0.105	0.137	1.0×10⁻³	2,810	2.01
M7V	0.090	0.120	6.5×10⁻⁴	2,680	2.12
M8V	0.086	0.114	5.1×10⁻⁴	2,570	2.15
M9V	0.079	0.102	3.0×10⁻⁴	2,380	2.17

Subdwarfs

Main article: Subdwarf

Some small stars called subdwarfs are a bit different from red dwarfs. They burn hydrogen like other stars but have less of some elements. This makes them dimmer and hotter. Because of this, they give off more ultraviolet light. Subdwarfs are not as common as regular stars, while red dwarfs are very common.

Spectral standard stars

Some important red dwarf stars help scientists study others. These include Lalande 21185, GJ 229A, Gliese 581, Gliese 402, Wolf 359, van Biesbroeck 8, VB 10, and LHS 2924. They are used as examples when scientists find new red dwarfs.

Planet formation

Gas-rich disks have been found around small stars and brown dwarfs that are as old as 45 million years. This is surprising because bigger stars usually lose their disks after only 10 million years. These long-lasting disks are called Peter Pan disks, with J0808 being a good example. The gas in these disks might help form special groups of planets, like those seen around TRAPPIST-1.

Studies using special instruments have helped scientists learn about the materials in the inner parts of these disks around very small stars. Some disks are rich in hydrocarbons, while others are rich in water. Younger disks tend to have more oxygen, while older ones have more carbon. This change happens because materials move inward over time. These materials can affect the composition of planets that form, especially their atmospheres. After the gas is gone, what remains is called a debris disk. Examples of debris disks around small stars include AU Microscopii, CE Antliae, and Fomalhaut C.

Planets

Many red dwarf stars have planets, called exoplanets, around them. Big planets like Jupiter are not common. Only about 1 in 40 red dwarfs have these large planets close by.

Smaller planets, like the size of Neptune, are found around one in three red dwarfs. About 40% of red dwarfs have a planet the size of Earth in the "habitable zone." This is where liquid water could exist.

One well-known red dwarf star system is the Gliese 581 planetary system. Several planets were found there between 2005 and 2010. Some of these planets are in the habitable zone and might be able to have liquid water.

In 2017, NASA found seven Earth-sized planets around a red dwarf star named TRAPPIST-1. Three of these planets are in the habitable zone and could possibly have liquid water.

Habitability

Main article: Habitability of red dwarf systems

Scientists think it is hard for planets around red dwarf stars to support life. Even though there are many red dwarfs and they live a long time, there are problems for planets near them. Planets in the right place for life would be very close to the star. One side would always face the star, while the other side would always be dark. This could cause big temperature differences, making it tough for life like ours to exist. The dark side might freeze the air, leaving the lit side dry.

Red dwarf stars can also change how bright they are very quickly. This can make it hard for life to grow and stay safe. These changes in light might blow away a planet's air, making it even harder for life to survive.