Red dwarf stars are the smallest and coolest kind of stars on the main sequence. Red dwarfs are the most common type of stars in the Milky Way galaxy.
Key Facts & Summary
- Red dwarf stars have a prevalence of around 73% in the universe.
- They are usually of spectral class K and M.
- Red dwarf stars have temperatures of around 4,000 K, much cooler than our Sun.
- Though red dwarfs are common, their luminosities are around 0.0001 to 0.8 that of the Sun, and thus despite their prevalence, individual red dwarf stars are hard to spot due to their dimness.
- Red dwarf stars live up to trillions of years, they might be the last stars to die in the universe, their light, the last to shun in the eternal darkness.
- Red dwarf stars usually have around 0.08 to 0.45 solar masses.
- The nuclear fusion process in red dwarfs is slowed down and even prolonged, converting hydrogen into helium both in their cores and throughout.
- Red dwarf stars live for so long that not one of them has reached an advanced stage of evolution since the universe was created.
- Some examples of red dwarf stars are Proxima Centauri or Trappist-1.
- Proxima Centauri is also the closest star to the Sun.
- Some astronomers believe that the term “red dwarf” does not refer to a single kind of star, but rather, it is applied to cooler objects, including brown dwarfs, which are often referred to as “failed stars” due to their lack of hydrogen fusion sustainability at their cores.
- The low temperatures of red dwarfs contribute to the slow consumption of their hydrogen supplies.
- Red dwarf stars have relatively low pressures, a low fusion rate, and hence, a low temperature.
- Red dwarfs have a region around their cores where convection does not occur.
- Red dwarfs are quite common in binary or larger star systems.
Formation & Characteristics
Red dwarf stars form just like other main-sequence stars. They can form out of a molecular cloud of dust and gas for example. Gravity pulls the swirling gas and dust together, and it begins to rotate.
The material clumps in the center, and when it reaches the critical temperature, fusion begins. Red dwarf stars include the smallest of stars, weighing only 7.5% to 50% of our Sun’s mass.
Since they are small, they burn at a lower temperature, reaching around 6,380 degrees Fahrenheit / 3,500 degrees Celsius. Since they have low temperatures, red dwarf stars are much dimmer than our Sun.
Their low-temperature also contributes to the slow consumption of their hydrogen supplies. Massive stars are the opposite, they devour their hydrogen supplies very fast.
Another aspect is that massive stars burn through their hydrogen at their core before coming to the end of their lifetimes, while red dwarf stars consume all of their hydrogens, inside and outside their core. This helps them in prolonging their lifetime to trillions of years. This is far more than the usually 10-billion-year lifetime like stars such as our Sun.
Red dwarf stars are very-low-mass stars. Because of this, they have low pressures, low fusion rates, and low temperatures. The energy generated is the product of a nuclear fusion of hydrogen into helium by way of photon-proton chain mechanisms.
Red dwarf stars are very dim, even the largest of them have only around 10% of the Sun’s luminosity. Generally, red dwarf stars less than 0.35 solar masses, transport energy from the core to the surface by convection.
Convection occurs because of the opacity of their interiors, which is typically very dense compared to the temperatures. Because of this, energy transfer by radiation is decreased, and instead, convection is the main form of energy transport to a red dwarf’s surface.
Above this mass, a red dwarf will have a region around its core where convection does not occur. Since low-mass red dwarfs are fully convective, helium doesn’t accumulate at the core, and compared to larger stars such as our Sun, they can burn a larger portion of their hydrogen before leaving the main-sequence.
Hence, red dwarfs have estimated lifespans far longer than the present age of the universe, and stars less than 0.8 solar masses have not had time to leave the main-sequence since the universe's inception.
As a result, it is theorized that the lower the mass of a red dwarf star, the longer the lifespan it experiences. A 0.1 solar mass red dwarf, for example, is believed to live up to 10 trillion years.
As the proportion of hydrogen in a red dwarf is consumed, the rate of fusion declines, and the core starts to contract. The gravitational energy released by this size reduction is converted into heat, which is carried throughout the star by convection.
Classifying Red Dwarfs
It is quite difficult for scientists to distinguish a red dwarf star from a brown dwarf star. Brown dwarf stars are cool and dim, and probably form in the same way that a red dwarf does, however, brown dwarfs never reach the point of fusion due to their small sizes, and thus they’re not even considered stars.
One postulated method of distinguishing a red dwarf from a brown dwarf is by calculating the celestial object’s temperature. Fusion-free brown dwarfs are cooler than 2,000 K, while hydrogen-fusing stars are warmer than 2,700 K. Everything in between could be classified as either a brown dwarf or red dwarf.
Occasionally, chemicals in a celestial object’s atmosphere can reveal clues about what’s happening at heart. According to astronomer Adam Burgasser of the University of California, the presence of molecules like methane or ammonia, that surive only in cold temperatures, suggests that an object is a brown dwarf.
The presence of lithium in a celestial object’s atmosphere also suggests that a red dwarf is a brown dwarf rather than a true star.
Many red dwarfs have exoplanets orbiting around them, however, the planetary habitability of a red dwarf star system is subjected to many debates.
Though red dwarfs have advantages such as their sheer numbers and longevity, some factors may make life difficult on planets orbiting a red dwarf.
Generally, planets that are situated in the habitable zone of a red dwarf star would need to very close to it, and thus the planet would most likely be tidally locked.
This means that half of the planet would experience perpetual day, while the other, perpetual darkness. This would inevitably lead to enormous temperature variations.
Such conditions would make it difficult for forms of life to evolve, especially if they are similar to the ones we know about. Another problem would be the atmosphere since most of it would be frozen, and leave the other side bare and dry.
Variability in stellar energy output may also have negative impacts on the development of life. Red dwarf stars are often flare stars, which can emit gigantic flares, doubling their brightness in minutes.
Such variability would certainly make it difficult for life to develop and persist near a red dwarf star. Some new studies suggest that all red dwarf stars may have this trait, if this is proven to be true, chances for life to develop on planets near them, are low to zero.
Location – Examples of Red Dwarf Stars
Red dwarf stars have a prevalence of around 73% in the universe, thus, they are the most prevalent stars out there. Here are some examples of red dwarf stars:
Lacaile 8760 is a red dwarf star located in the constellation of Microscopium. This red dwarf is very faint, and it is one of the nearest stars to us, at around 12.9 light-years.
It has an apparent magnitude of +6.7. It can be seen with the naked eye under favorable conditions.
Another red dwarf star close to Earth is Gliese 876, situated at only 15 light-years away, in the constellation of Aquarius. It is among the closest stars to us that have a planetary system. Currently, four confirmed exoplanets are orbiting Gliese 876.
Small red dwarf stars have an unfathomably extended lifetime. They will most likely be the last beams of light in the universe to fade away. Eventually, all-stars meet their end.
When a red dwarf star burns through all of its fuel, it will become a white dwarf, and thus they will no longer undergo fusion at their core. Eventually, the white dwarfs will radiate away all of their heat and become black dwarfs.
Did you know?
- In our Milky Way galaxy, about three-fourths of the stars are red dwarfs.
- Red dwarfs will not pass through a red giant phase in their evolution.
- If red dwarf stars are proven to be favorable towards the development of life near them, then they will be the last stars in the universe where life could develop, or survive.