Red Dwarfs: The Most Common and Longest-Lived Stars
NASA's Hubble Space Telescope captures the image of the red dwarf star CHXR 73 and its companion, thought to be a brown dwarf. CHXR 73 is one-third less massive than the sun.
Credit: NASA, ESA and K. Luhman (Penn State University).

The largest population of stars in the galaxy hide in the shadows, too dim to be seen with the naked eye from Earth. Their limited radiance helps to extend their lifetimes, which are far greater than that of the sun.

Scientists think that 20 out of the 30 stars nearest to Earth are red dwarfs; however, none of them can be seen with the naked eye. The closest star to the sun, Proxima Centauri, is a red dwarf.

Despite its frequent appearance, the term "red dwarf" does not refer to a single kind of star. It is frequently applied to the coolest objects, including K- and M-dwarfs, which are true stars, and brown dwarfs, often referred to as "failed stars" because they do not sustain hydrogen fusion in their cores.

"There is no true definition of red dwarfs," astronomer Michaël Gillion of the University of Liège in Belgium told Space.com by email. Gillion, who studies stellar objects at the cooler end of the spectrum, was part of the team that identified the ultracool star TRAPPIST-1. "This term generally refers to dwarf stars with a spectral type ranging from K5V to M5V."

Red dwarfs form like other main-sequence stars. A cloud of dust and gas is drawn together by gravity and begins rotating. The material clumps at the center, and when it reaches the critical temperature, fusion begins.

But red dwarfs include the smallest of the stars, weighing in at 7.5 to 50 percent the mass of the sun. Their reduced size means that they burn at a lower effective temperature, reaching only 6,380 degrees Fahrenheit (3,500 degrees Celsius). The sun, by comparison, has an effective temperature of 9,900 F (5,500 C). The low temperatures of red dwarfs means they are far, far dimmer than stars like the sun.

Their low temperature also means that they burn through their supply of hydrogen less rapidly. While other, more massive stars only burn through the hydrogen at their core before coming to the end of their lifetimes, red dwarfs consume all of their hydrogen, in and out of their core. This stretches the lifetime of red dwarfs out to trillions of years, far beyond the 10-billion-year lifetime of sun-like stars.

Some scientists also apply the term to brown dwarfs, which aren't actually stars. Brown dwarfs are thought to form the same way as their stellar cousins, but they never reach the point of fusion because they are too small. Other scientists only use the term to refer to stars; Gillon said it never refers to brown dwarfs, while Adam Burgasser, an astronomer studying cool stars at the University of California, San Diego, said that it often does.

"Red dwarfs being a vague notion, it lacks a clear definition," Gillon said.

Because cool stars and brown dwarfs are so dim, they can be troublesome to classify when they are first discovered. Both objects of subclasses of red dwarfs.

"When we observe a red dwarf and measure its atmosphere, we don't necessarily know whether it's a brown dwarf or a star — young brown dwarfs look almost exactly like ultracool stars," Burgasser told Space.com. 

To figure out the difference, scientists take the temperature of the atmosphere. Fusion-free brown dwarfs are cooler than 2,000 Kelvin (3,140 F or 1,727 C), while hydrogen-fusing stars are warmer than 2,700 K (4,400 F or 2,427 C). In between lies a gray area that can be made up of both.

By studying the atmosphere of the object, scientists can measure its temperature. Sometimes, the light shining from the star, or its spectrum, can reveal clues about what's happening at its heart. According to Burgasser, the presence of molecules like methane or ammonia, which can only survive at cold temperatures, suggest an object is a brown dwarf. Lithium in the atmosphere also suggest that a red dwarf is a brown dwarf rather than a true star. 

"So 'red dwarf' describes how some looks, 'brown dwarfs' and 'stars' describe whether it is capable of fusing hydrogen," Burgasser said.

Planets form from the material left over in a disk after their star has been created. Many red dwarfs have been found with planets surrounding them, though enormous gas giants are rare.

For a long time, scientists considered red dwarfs to be unfit for habitability. Their limited light and heat meant that the habitable zone — the region where liquid water could form, and thus life would be considered most likely to evolve — of planets around them would be very close to the star, putting them in range of radiation from the tiny stars. Other planets may find themselves tidally locked to the star, with one side constantly facing the sun. [VIDEO: Earth-Like Planets Orbit Nearby Red Dwarf Stars Say Scientists]

But new models have shown that some planets could develop in ways that would potentially allow life to evolve. Because red dwarfs make up more than three-quarters of the stars in the galaxy, this significantly increased the possibilities for the evolution of life in the universe. When its discovery was announced around a red dwarf in 2010, Gliese 581g was called the "first potentially habitable" alien planet.

Tiny red dwarfs may have an extended lifetime, but eventually they, like all other stars, will burn through their supply of fuel. When they do, they will become white dwarfs, dead stars that no longer undergo fusion at their core. Eventually, the white dwarfs will radiate away all of their heat and become black dwarfs.

But unlike the sun, which will become a white dwarf in a few billion years, red dwarfs will take trillions of years to burn through their fuel. This is significantly longer than the age of the universe, which is less than 14 billion years old. Red dwarfs may be a bit dim, but like the tortoise, they will eventually win the survival race.

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