Wolf 359

Wolf 359 is a red dwarf star located in the constellation Leo, near the ecliptic. At a distance of 7.86 light-years (2.41 parsecs) from Earth, it has an apparent magnitude of 13.54 and can only be seen with a large telescope. Wolf 359 is one of the nearest stars to the Sun with only the Alpha Centauri system (including Proxima Centauri), Barnard's Star, and the brown dwarfs Luhman 16 (WISE 1049-5319) and WISE 0855−0714 known to be closer. Its proximity to Earth has led to its mention in several works of fiction. Wolf 359 is one of the faintest and least-massive nearby stars known. At the light-emitting layer called the photosphere, it has a temperature of ~2,800 K, low enough for chemical compounds to form and survive. The absorption lines of compounds such as water and titanium(II) oxide have been observed in its spectrum. The star's surface has a magnetic field hundreds of times as strong as that of the Sun, generated by its thorough internal convection. As a result of this significant magnetic activity, Wolf 359 is a flare star that can undergo sudden and great increases in luminosity, which can persist for several minutes. These flares emit strong bursts of X-ray and gamma ray radiation that have been observed by space telescopes. It is a relatively young star with an estimated age of less than a billion years. No planetary companions for Wolf 359 have been confirmed so far, though there is one unverified candidate; as of yet, no debris disks have been found.

Observation history and name

Wolf 359 first came to the attention of astronomers because of its relatively high rate of transverse motion against the background, also known as the proper motion. A high rate of proper motion can indicate that the star is located nearby, as closer stars can achieve the same rate of angular change with a lower relative speed. The proper motion of Wolf 359 was first measured in 1917 by German astronomer Max Wolf, aided by astrophotography. In 1919 he published a catalogue of over one thousand stars with high proper motions, including this one, that are still identified by his name. He listed this star as entry number 359, and the star has since been referred to as Wolf 359, in reference to Max Wolf's work. The first parallax measurement of Wolf 359 was reported in 1928 from the Mount Wilson Observatory, yielding an annual shift in the star's position of 0.407 ± 0.009 arcseconds. From this position change, and the known size of the Earth's orbit, the distance to the star could be estimated. It was the faintest and least-massive star known until the discovery of VB 10 in 1944. The infrared magnitude of the star was measured in 1957. In 1969, a brief flare in the luminosity of Wolf 359 was observed, linking it to a class of variable stars known as flare stars.

Properties

Wolf 359 has a stellar classification of M6, although various sources list a spectral class of M5.5, M6.5 or M8. Most M-type stars are red dwarfs: they are visually red because the energy emission of such stars reaches a peak in the red and infrared parts of the spectrum. Wolf 359 has a very low luminosity, emitting about 0.1% of the Sun's power. If it were moved to the location of the Sun, it would appear ten times as bright as the full Moon. At an estimated 11% of the Sun's mass, Wolf 359 is just above the lower limit at which a star's core can undergo hydrogen fusion through the proton–proton chain reaction: ~8% of the solar mass. (Substellar objects below this limit are known as brown dwarfs.) The radius of Wolf 359 is an estimated 14.4% that of the Sun, or about 100,200 km. For comparison, the equatorial radius of the planet Jupiter is 71,490 km, making the star a mere 40% wider than the planet. The entire star undergoes convection, whereby the energy generated at the core is transported toward the surface by the convective motion of stellar plasma, rather than through electromagnetic radiation. This constant circulation redistributes throughout the star any excess accumulation of helium in the core generated by stellar nucleosynthesis. This process allows Wolf 359 to remain on the main sequence as a hydrogen fusing star for proportionately longer than one such as the Sun, for which helium steadily accumulates in the core and is not diluted. In conjunction with a much lower rate of hydrogen consumption due to its low mass and core temperature, Wolf 359 is expected to remain a main sequence star for about eight trillion years before finally exhausting its hydrogen supply and ending up as a helium white dwarf. A search of this star by the Hubble Space Telescope revealed no stellar companions. No excess infrared emission has been detected, which may indicate the lack of a debris disk around it.

Outer atmosphere The outer, light-emitting layer of a star is known as the photosphere. Estimates of the photospheric temperature of Wolf 359 range from 2,500 K to 2,900 K, which is sufficiently cool for equilibrium chemistry to occur. The resulting chemical compounds survive long enough to be observed through their spectral lines. Numerous molecular bands appear in the spectrum of Wolf 359, including those of carbon monoxide (CO), iron hydride (FeH), chromium hydride (CrH), water (H2O), magnesium hydride (MgH), vanadium(II) oxide (VO), titanium(II) oxide (TiO), and possibly the molecule CaOH. Since there are no lines of lithium in the spectrum, this element must have already been consumed by fusion in the core. This indicates that the star must be at least 100 million years old. Beyond the photosphere lies a nebulous, high temperature region known as the stellar corona. In 2001, Wolf 359 became the first star other than the Sun to have the spectrum of its corona observed by a ground-based telescope. The spectrum showed emission lines of Fe XIII, which is heavily ionized iron that has been stripped of twelve of its twenty-six electrons. The strength of this line can vary over a time period of several hours, which may be evidence of microflare heating.

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