Red dwarf

A red dwarf is the least massive, smallest, least luminous, and coolest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are not easily observed. Not one star that fits the stricter definitions of a red dwarf is visible to the naked eye. Proxima Centauri, the star nearest to the Sun, is a red dwarf, as are fifty of the sixty nearest stars. According to some estimates, red dwarfs make up three-quarters of the fusing stars in the Milky Way. The coolest red dwarfs near the Sun have a surface temperature of about 2,000 K and the smallest have radii about 9% that of the Sun, with masses about 7.5% that of the Sun. These red dwarfs have spectral types of L0 to L2. There is some overlap with the properties of brown dwarfs, since the most massive brown dwarfs at lower metallicity can be as hot as 3,600 K and have late M spectral types. Definitions and usage of the term "red dwarf" vary by how inclusive they are on the hotter and more massive end. One definition is synonymous with stellar M dwarfs, yielding a maximum temperature of 3,900 K and 0.6 M☉. Another includes all stellar M-type main-sequence and all K-type main-sequence stars (K dwarf), yielding a maximum temperature of 5,200 K and 0.8 M☉. Some definitions include any stellar M dwarf and part of the K dwarf classification. Other definitions are also in use. Many of the coolest, lowest-mass M dwarfs are expected to be brown dwarfs, not true stars, and so those would be excluded from any definition of red dwarf. Stellar models indicate that red dwarfs less than 0.35 M☉ are fully convective. Hence, the helium produced by the thermonuclear fusion of hydrogen is constantly remixed throughout the star, avoiding helium buildup at the core, thereby prolonging the period of fusion. A low-mass red dwarf therefore develops very slowly, maintaining a constant luminosity and spectral type for trillions of years, until its fuel is depleted and it turns into a blue dwarf. Because of the comparatively short age of the universe, no red dwarfs yet exist at advanced stages of evolution.

Definition The term "red dwarf" when used to refer to a star does not have a strict definition. One of the earliest uses of the term was in 1915, used simply to contrast "red" dwarf stars with hotter "blue" dwarf stars. It became established use, although the definition remained vague. In terms of which spectral types qualify as red dwarfs, different researchers picked different limits, for example K8–M5 or "later than K5". Dwarf M star, abbreviated dM, was also used, but sometimes it also included stars of spectral type K. In modern usage, the definition of a red dwarf still varies. When explicitly defined, it typically includes late K- and early to mid-M-class stars, but in many cases it is restricted to M-class stars. In some cases all K stars are included as red dwarfs, and occasionally even earlier stars. The most recent surveys place the coolest true main-sequence stars into spectral types L2 or L3. At the same time, many objects cooler than about M6 or M7 are brown dwarfs, insufficiently massive to sustain hydrogen-1 fusion. This gives a significant overlap in spectral types for red and brown dwarfs. Objects in that spectral range can be difficult to categorize.

Description and characteristics

Red dwarfs are very-low-mass stars. As a result, they have relatively low pressures, a low fusion rate, and hence, a low temperature. The energy generated is the product of nuclear fusion of hydrogen into helium by way of the proton–proton (PP) chain. Hence, these stars emit relatively little light, sometimes as little as 1⁄10,000 that of the Sun, although this would still imply a power output on the order of 10×1022 watts (10 trillion gigawatts or 10 ZW). Even the largest red dwarfs (for example HD 179930, HIP 12961 and Lacaille 8760) have only about 10% of the Sun's luminosity. In general, red dwarfs less than 0.35 M☉ transport energy from the core to the surface by convection. Convection occurs because of the opacity of the interior, which has a high density compared with the temperature. As a result, energy transfer by radiation is decreased, and instead convection is the main form of energy transport to the surface of the star. Above this mass, a red dwarf will have a region around its core where convection does not occur.

Because low-mass red dwarfs are fully convective, helium does not accumulate at the core, and compared with larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence. As a result, red dwarfs have estimated lifespans far longer than the present age of the universe, and stars less than 0.8 M☉ have not had time to leave the main sequence. The lower the mass of a red dwarf, the longer the lifespan. It is believed that the lifespans of these stars exceed the expected 10-billion-year lifespan of the Sun by the third or fourth power of the ratio of the solar mass to their masses; thus, a 0.1 M☉ red dwarf may continue burning for 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.

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