Red Dwarf Stars
Red dwarf stars are small, cool, and low-luminosity stars, primarily classified in spectral class M, although some late K and L-type stars are also included. They have surface temperatures ranging from approximately 2,100 to 3,800 kelvins, causing them to emit more energy in the infrared spectrum than in visible light. This makes them challenging to detect with the naked eye; despite being the most common stars in the universe—comprising up to 80% of all stars—they are largely invisible to us. Red dwarfs exhibit distinctive absorption spectra and are characterized by low luminosity and mass, making them fundamentally different from larger red giants.
Interestingly, red dwarfs can have lifespans ranging from billions to trillions of years, far exceeding that of the Sun, and they exhibit stellar activity similar to the Sun, including flares and spots. Recent studies have sparked renewed interest in their potential to host habitable planets, as findings indicate that the conditions around these stars may support life, despite initial assumptions that their close proximity would hinder habitability. The exploration of planets around red dwarfs, including the detection of potentially Earth-sized planets in habitable zones, is an active area of research, enhancing our understanding of the diversity of planetary systems in the universe.
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Red Dwarf Stars
Red dwarf stars have the lowest mass and luminosity but largest population of stars in our galaxy. Developing theoretical models of the evolution of these stars and their planetary systems has expanded astronomers’ understanding of stellar evolution and the circumstances under which planets might support life.
Overview
Red dwarf stars are low-mass, low-temperature, low-luminosity stars that occupy the lower, “tail” end of the main sequence on the Hertzsprung-Russell (HR) diagram. The majority are in spectral class M, although some authors also include late K stars (K5 onward) as well as some L-type stars (so-called ultracool dwarfs). Their red color is due to a low surface temperature, between approximately 2,100–3,800 kelvins, and they actually emit more energy in the infrared portion of the Electromagnetic spectrum than in visible light. Therefore they are most easily found via infrared (IR) surveys, such as the Deep Near Infrared Survey (DENIS) and the 2-Micron All Sky Survey (2MASS) of the late 1990s.
Because of their low surface temperature, the spectra of red dwarfs have distinctive absorption lines, especially titanium oxide (TiO), water, carbon monoxide, and vanadium oxide (VO). Their low temperature is caused by their low mass (0.08-0.6 times that of the Sun, a main sequence star), which limits their ability to generate energy through nuclear fusion. As a result, although red dwarfs are thought to be the most common type of star—comprising up to 80 percent of the total population of stars (and as much as half of total stellar mass of the Milky Way)—none is visible to the unaided eye. The brightest known red dwarf, AX Microscopium, has a disappointing 6.7 visual magnitude.
Not only do these stars have low luminosity (between 0.001 and 0.01 times that of the Sun), but their radii are also small, between 0.1 and 0.6 that of our sun. The designation “red dwarf,” therefore, is commonly used to prevent confusion between these “underachieving” stars and the impressively large “red giants” and supergiants that share a similar surface temperature (and hence color) with these stars. Given their low luminosity, it is not surprising that many red dwarfs remain undiscovered in our own celestial neighborhood. It is estimated that up to eight thousand red dwarfs are within 25 parsecs of the Sun, the majority of them undiscovered.
Among the nearby red dwarfs, two are of historical importance. Proxima Centauri is the third star in the Alpha Centauri multiple system and technically lies 10,000 astronomical units (AU) closer than its much brighter G and K class companions, making it the closest known star outside our solar system (4.22 light-years). Barnard’s star, discovered by E. E. Barnard in 1916, has the largest proper motion of any known star, 10 arc seconds per year.
From the 1930s to the 1950s, astronomers struggled with models of energy generation and transportation in these stars. Because of their low mass, their central pressure and temperature are much less than in solar-type stars, leading to a significantly lower rate of hydrogen-to-helium fusion and hence less energy production. Another important difference is the mechanism used to transport energy from the core to the surface. In solar-type stars, radiative transfer is used outside the core until convection becomes more efficient in the outer quarter of the Sun. In red dwarf stars, convection occurs much deeper within the star, and in the coolest M dwarfs (M4 and later) the entire star is convective. This large-scale movement of material means that nearly 100 percent of the star’s initial hydrogen is available as fuel over the course of the star’s life (in contrast to an estimated 10 percent for the sun). Greg Laughlin and his colleagues have computed the lifespans of these miserly stars to be hundreds to thousands of times greater than the current age of the universe: between 1011 and 1013 years. In their study of the future of the universe, Laughlin and Fred Adams have defined the end of the Stelliferous (star-filled) Age in which we now live by the death of the last generation of red dwarfs, predicted to be 1014 years after the universe’s birth.
Many red dwarfs exhibit stellar activity reminiscent of the Sun, including spots, flares, and coronal mass ejections. UV Ceti or flare stars were discovered by Ejnar Hertzsprung, Adriaan van Maanen, and others in the 1920s, 1930s, and 1940s. Later shown to be red dwarfs, these variable stars suddenly brighten by several magnitudes and then decline over minutes to hours. For example, Proxima Centuri is a flare star. During these outbursts, a star can increase its ultraviolet and X-ray output by a hundred times or more, and some stars also exhibit radio outbursts. The general mechanism is similar to that of solar flares, although more energetic by a factor of a thousand or greater. BY Draconis red dwarf variables (named for the multistar system BY Draconis) vary in visual brightness by only tenths to hundredths of magnitudes over days or months. This behavior is caused by the rotation of the star, carrying changing numbers of “starspots” and chromospheric areas of activity into view. For some stars, as much as 10-40 percent of the surface is covered in spots at any given time.
Knowledge Gained
The study of red dwarf stars has added to our understanding of stellar structure and evolution, but even more important, it has caused astronomers to take a second look at long-held assumptions about planet formation and the definition of a habitable planet. Observations of young red dwarfs have found that the formation of protoplanetary disks (proplyds) is as likely around these stars as it is around higher-mass stars. For example, AU Microscopium, which lies 10 parsecs away and is about 12 million years old, has a disk of cold dust that shows signs of being in the late stages of planetary formation. However, because of the narrow definition of “habitable planet” (with its myopic view of solar-type systems), planets around red dwarfs were not considered objects of interest until the mid-1990s.
According to the most general definition, a planet is considered to be in a star’s habitable zone if its surface temperature allows for the existence of liquid water. For a star of given temperature and luminosity, this designates a possible range of orbital distances. Given red dwarfs’ low luminosity and surface temperature, any habitable zone would be narrow (about 0.02 to 0.2 AU) and lie very close to the star. This was thought to present insurmountable challenges for life. For example, it was assumed that planets close to a red dwarf would be tidally locked in such a way that they would keep one (exceedingly hot) face permanently turned toward the sun and the other (brutally cold) face forever locked in darkness. Initial calculations suggested that on the cold side any atmosphere would be permanently frozen. In addition, astrobiologists were concerned about the effects of the stellar flares on life.
In 1994, the First International Conference on Circumstellar Habitable Zones revisited suppositions about habitable planets and found red dwarfs worth a second look. This led to the First Workshop on Habitability of M Star Planets, sponsored by the Astrobiology Institute of the National Aeronautics and Space Administration (NASA), in 2005. These conferences led to a dramatic rethinking of red dwarfs as hosts for life-sustaining planets. For example, more detailed calculations demonstrated that planets in close orbits might not be tidally locked, and even if they were, a planetary atmosphere with a pressure one-tenth that of Earth would have sufficient heat circulation to keep the atmosphere from freezing out, and if the pressure were similar to that of Earth (about 1,000 millibars), liquid water could be present. Development of an ozone layer could theoretically protect life from excessive ultraviolet exposure during flares. Flare activity, however, might not present any difficulty for the evolution of life around red dwarfs, since it has been shown that such activity decreases with the age of the star and lasts for a few million years for higher mass stars up to approximately a billion years for low-mass stars. Since these time frames are an insignificant fraction of the star’s total lifespan, flare activity poses a serious threat to life only during the star’s youth.
Even the presumed problem of the low luminosity (and hence small habitable zone) of red dwarfs was found to be resolvable. Because of red dwarfs’ low mass, the way they die differs from that of other stars. Early-type M stars with a quarter solar mass or greater leave the main sequence to become red giants (as they exhibit a brief period of hydrogen-shell burning), but no red dwarfs will ever reach the necessary central core temperature to convert helium into carbon via the triple-alpha cycle. The surface temperature as well as the radius of a lower-mass red dwarf star actually increases near the end of its life, turning the red dwarf into a “yellow giant” and thereby increasing the size of the habitable zone. With surface temperature and luminosity comparable to those of the sun, this yellow giant can survive for billions of years, yielding sun-like conditions that could facilitate the evolution of life and the development of technologically advanced civilizations. Therefore, these unassuming low-mass stars may in fact represent ideal targets for Search for Extraterrestrial Intelligence (SETI) searches.
Prior to the mid-1990s, both theoretical investigations and observational searches for planets around red dwarfs were largely neglected. Beginning in 1998, with the discovery of red dwarf planets ranging in mass from several times that of Earth to masses similar to Neptune and Jupiter, the question of planet formation around low-mass stars became relevant. A major problem was the prediction that the core accretion method of forming large gas giant planets would not be feasible in red dwarf systems because material would accumulate too slowly to grow large planets before the gas in the protoplanetary disk would dissipate. A new model, proposed by Alan Boss, called the “disk instability mechanism,” allows for the rapid creation of massive gas giants but predicts that red dwarf systems should tend to support fewer gas giant planets than more massive stellar systems. This prediction is borne out by current observations. Most prevalent appear to be “super Earths” and “failed Jupiters” (exposed high-mass cores that either never accumulated a gaseous envelop or had it stripped off by ultraviolet radiation).
Context
The discovery of red dwarf planets and theoretical considerations of their formation are ongoing areas of research, both of which add to our understanding of the wide variety of possible planetary systems and the methods by which different types of planets can be created and discovered. For example, because of the small masses of red dwarf stars and the small size of their habitable zones, habitable planets will cause greater (and more easily measurable) variations in radial-velocity surveys than in comparable solar-type systems. Transits of planets in front of their star are also more probable in red dwarf systems and will create greater variations in the star’s apparent brightness. Microlensing experiments have also detected several planets around red dwarfs, including a possible 7 Jupiter-mass planet, the largest yet found orbiting a red dwarf. The orbital transit mission Corot (which launched on December 27, 2006 and ended on March 31, 2013) orbited Earth and took images of exoplanets. The Kepler mission, which launched in 2009, confirmed more than 2,600 exoplanets before its “retirement” in 2018. A year earlier, Kepler found evidence for seven Earth-sized planets orbiting in the habitable zone of a red dwarf called TRAPPIST-1. Astronomers hope that observing the star and the planetary system with the state-of-the-art James Webb Space Telescope, which was launched in 2021, will solve the mystery of how Earth-like the planets are.
Bibliography
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