Red dwarfs are stars that have a very low mass and emit very little light, in comparison to the Sun. They make up 70% of all stars and can live for one to ten trillion years.
Our universe contains trillions of stars, but there is an uneven distribution among them, with the majority being Red Dwarfs. Red dwarfs make up approximately 70% of all stars in the universe.
Artist representation of a red dwarf (Photo Credit: Jurik Peter/ Shutterstock)
Despite their abundance, red dwarfs are not visible to the naked eye because they have low luminosity and are significantly cooler. Even the brightest red dwarf is only one-tenth as bright as the Sun, while the dimmest red dwarf is 1000 times less bright.
However, red dwarfs have incredibly long lifespans. They will exist for much longer than our Sun, with an estimated lifespan of one trillion years. They may be the last objects to die as the universe comes to an end.
What Qualifies a Star as a Red Dwarf?
To categorize different types of stars, Ejnar Hertzsprung and Henry Norris Russell developed a system known as the Hertzsprung-Russell diagram. This diagram plots the brightness of a star against its color index.
Hertzsprung-Russell diagram with the main sequence (Photo Credit: Richard Powell/Wikimedia Commons)
The main sequence on the Hertzsprung-Russell diagram consists of stars that are mostly dwarf stars. Red dwarfs belong to the main sequence.
Stars can also be differentiated based on their temperature using the Morgan-Keenan (MK) system. This system classifies stars into categories such as O, B, A, F, G, K, and M, with O being the hottest and M being the coolest. Each category is further subdivided from 0 to 9, with 0 being the hottest and 9 being the coolest.
Spectral class showing a red dwarf (Photo Credit: Rursus/Wikimedia Commons)
The definition of a red dwarf is not precise and can vary. Originally, the term was used to distinguish red dwarf stars from much hotter blue dwarf stars. Researchers use different criteria to classify red dwarfs, such as the sequence from K8 to M5 or anything after K5. Some classifications even include certain K-type stars as well.
Nowadays, there is also a difference in the interpretation. Precise explanations of red dwarfs encompass stars ranging from late K to mid-M type. However, in most instances, stars from the K type are not accounted for, while some definitions incorporate the entire K series. The red dwarf star that is nearest to the Sun is Proxima Centauri, located at a distance of 4.2 light-years.
Red Dwarfs: Description and Characteristics
The energy produced in a red dwarf star is a result of hydrogen fusion into helium. This fusion process occurs through the proton-proton chain mechanism. Red dwarfs have a very low mass, which results in low pressure, a low fusion rate, and low temperature. Consequently, they emit light that can be as faint as 1/10,000th of the light emitted by the sun, resulting in extremely low luminosity, up to 1,000 times less than the Sun.
Due to their low mass, red dwarfs are completely convective, with most of the molecules in motion. This prevents the accumulation of helium in the core and allows them to burn large amounts of hydrogen before leaving the main sequence, unlike the Sun.
As a result, red dwarfs have significantly longer lifespans compared to the current age of the universe (13.7 billion years). Their estimated lifespans exceed that of the Sun (10 billion years). The lifespan of a red dwarf star is directly proportional to its mass, meaning that lower-mass stars have longer lifespans. According to some estimates, a red dwarf with a solar mass of 0.1 M☉ could live for 10 trillion years, which is astonishing considering the relatively young age of the universe. This also explains why no red dwarf stars exist in their advanced evolutionary stages.
Planets Orbiting Red Dwarfs and Their Potential Habitability
Exoplanets do orbit red dwarfs, but their sizes tend to be no larger than Jupiter. Doppler surveys indicate that only about 1 in 40 red dwarfs have an exoplanet as large as Jupiter in orbit. However, microlensing surveys suggest that 1 in 3 red dwarfs have a planet with a mass similar to that of Neptune. Observations made with HARPS (high accuracy radial velocity planet searcher) show that approximately 40% of red dwarfs have a “super-Earth” class planet in the habitable zone, where liquid water could potentially exist on the planet’s surface.
While this may sound promising, there are several challenges for planets orbiting red dwarfs. Due to the star’s luminosity, planets in the habitable zone would need to be very close to the red dwarf, resulting in tidal locking. Tidal locking means that the planet would not rotate on its axis, causing one side to always face the red dwarf, similar to the Moon orbiting the Earth. This would create extreme temperature differences between the two sides of the planet, with one side being extremely hot and the other side very cold.
Representation of a tidally locked planet (Photo Credit: Dotted Yeti/ Shutterstock)
Additionally, red dwarfs often have intense flares and sudden increases in brightness within minutes. This would have catastrophic effects on the atmosphere of any nearby exoplanet, greatly reducing the chances of life surviving on these planets.