Barnard's Star /ˈbɑːrnərd/ is a very-low-mass red dwarf about 6 light-years away from Earth in the constellation of Ophiuchus. It is the fourth-nearest known individual star to the Sun (after the three components of the Alpha Centauri system) and the closest star in the Northern Celestial Hemisphere.  Despite its proximity, with a dim apparent magnitude of +9.5, the star is invisible to the unaided eye ; it is much brighter in the infrared than in visible light.
The star is named after the American astronomer E. E. Barnard. He was not the first to observe the star (it appeared on Harvard University plates in 1888 and 1890), but in 1916 he measured its proper motion (which is a function of its close proximity to Earth, and not of its actual space velocity) as 10.3 arcseconds per year, which remains the largest proper motion of any star relative to the Sun.  This is likely to be the fastest star in terms of proper-motion, as its close proximity to the Sun, as well as its high velocity make it unlikely any faster object (in terms of proper-motion) remains undiscovered. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)  to catalogue and standardize proper names for stars. The WGSN approved the name Barnard's Star for this star on 1 February 2017 and it is now so entered in the IAU Catalog of Star Names. 
Barnard's Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator.  Historically, research on Barnard's Star has focused on measuring its stellar characteristics, its astrometry, and also refining the limits of possible extrasolar planets. Although Barnard's Star is an ancient star, it still experiences star flare events, one being observed in 1998.
The star has also been the subject of some controversy. For a decade, from the early 1960s to the early 1970s, Peter van de Kamp claimed that there were one or more gas giants in orbit around it. Although the presence of small terrestrial planets around Barnard's Star remains a possibility, Van de Kamp's specific claims of large gas giants were refuted in the mid-1970s.
At 7–12 billion years of age, Barnard's Star is considerably older than the Sun, which is 4.5 billion years old, and it might be among the oldest stars in the Milky Way galaxy.  Barnard's Star has lost a great deal of rotational energy, and the periodic slight changes in its brightness indicate that it rotates once in 130 days  (the Sun rotates in 25). Given its age, Barnard's Star was long assumed to be quiescent in terms of stellar activity. In 1998, astronomers observed an intense stellar flare, showing that Barnard's Star is a flare star.  Barnard's Star has the variable star designation V2500 Ophiuchi. In 2003, Barnard's Star presented the first detectable change in the radial velocity of a star caused by its motion. Further variability in the radial velocity of Barnard's Star was attributed to its stellar activity. 
The proper motion of Barnard's Star corresponds to a relative lateral speed of 90 km/s. The 10.3 seconds of arc it travels annually amount to a quarter of a degree in a human lifetime, roughly half the angular diameter of the full Moon. 
The radial velocity of Barnard's Star towards the Sun is measured from its blue shift to be 110 km/s. Combined with its proper motion, this gives a space velocity (actual velocity relative to the Sun) of 142.6 ± 0.2 km/s. Barnard's Star will make its closest approach to the Sun around AD 11,800, when it will approach to within about 3.75 light-years.
Proxima Centauri is the closest star to the Sun at a position currently 4.24 light-years distant from it. However, despite Barnard's star's even closer pass to the Sun in 11,800 AD, it will still not then be the nearest star, since by that time Proxima Centauri will have moved to a yet nearer proximity to the Sun.  At the time of the star's closest pass by the Sun, Barnard's Star will still be too dim to be seen with the naked eye, since its apparent magnitude will only have increased by one magnitude to about 8.5 by then, still being 2.5 magnitudes short of visibility to the naked eye.
Barnard's Star has a mass of about 0.14 solar masses ( M☉), and a radius 15% to 20% of that of the Sun.   Thus, although Barnard's Star has roughly 150 times the mass of Jupiter ( MJ), its radius is only 1.5 to 2.0 times larger, due to its much higher density. Its effective temperature is 3,100 kelvins, and it has a visual luminosity of 0.0004 solar luminosities.  Barnard's Star is so faint that if it were at the same distance from Earth as the Sun is, it would appear only 100 times brighter than a full moon, comparable to the brightness of the Sun at 80 astronomical units. 
Barnard's Star's has 10–32% of the solar metallicity. Metallicity is the proportion of stellar mass made up of elements heavier than helium and helps classify stars relative to the galactic population. Barnard's Star seems to be typical of the old, red dwarf population II stars, yet these are also generally metal-poor halo stars. While sub-solar, Barnard's Star's metallicity is higher than that of a halo star and is in keeping with the low end of the metal-rich disk star range; this, plus its high space motion, have led to the designation "intermediate population II star", between a halo and disk star. 
Claims of a planetary system
For a decade from 1963 to about 1973, a substantial number of astronomers accepted a claim by Peter van de Kamp that he had detected, by using astrometry, a perturbation in the proper motion of Barnard's Star consistent with its having one or more planets comparable in mass with Jupiter. Van de Kamp had been observing the star from 1938, attempting, with colleagues at the Swarthmore College observatory, to find minuscule variations of one micrometre in its position on photographic plates consistent with orbital perturbations that would indicate a planetary companion; this involved as many as ten people averaging their results in looking at plates, to avoid systemic individual errors.  Van de Kamp's initial suggestion was a planet having about 1.6 M J at a distance of 4.4 AU in a slightly eccentric orbit,  and these measurements were apparently refined in a 1969 paper.  Later that year, Van de Kamp suggested that there were two planets of 1.1 and 0.8 M J . 
Other astronomers subsequently repeated Van de Kamp's measurements, and two papers in 1973 undermined the claim of a planet or planets. George Gatewood and Heinrich Eichhorn, at a different observatory and using newer plate measuring techniques, failed to verify the planetary companion.  Another paper published by John L. Hershey four months earlier, also using the Swarthmore observatory, found that changes in the astrometric field of various stars correlated to the timing of adjustments and modifications that had been carried out on the refractor telescope's objective lens;  the claimed planet was attributed to an artifact of maintenance and upgrade work. The affair has been discussed as part of a broader scientific review. 
Van de Kamp never acknowledged any error and published a further claim of two planets' existence as late as 1982;  he died in 1995. Wulff Heintz, Van de Kamp's successor at Swarthmore and an expert on double stars, questioned his findings and began publishing criticisms from 1976 onwards. The two men were reported to have become estranged from each other because of this. 
Refining planetary boundaries
While not completely ruling out the possibility of planets, null results for planetary companions continued throughout the 1980s and 1990s, the latest being based on interferometric work with the Hubble Space Telescope in 1999.  By refining the values of a star's motion, the mass and orbital boundaries for possible planets are tightened: in this way astronomers are often able to describe what types of planets cannot orbit a given star.
M dwarfs such as Barnard's Star are more easily studied than larger stars in this regard because their lower masses render perturbations more obvious.  Gatewood was thus able to show in 1995 that planets with 10 M J were impossible around Barnard's Star,  in a paper which helped refine the negative certainty regarding planetary objects in general.  In 1999, work with the Hubble Space Telescope further excluded planetary companions of 0.8 M J with an orbital period of less than 1,000 days (Jupiter's orbital period is 4,332 days),  while Kuerster determined in 2003 that within the habitable zone around Barnard's Star, planets are not possible with an " M sin i " value greater than 7.5 times the mass of the Earth ( M⊕), or with a mass greater than 3.1 times the mass of Neptune (much lower than van de Kamp's smallest suggested value). 
Even though this research has greatly restricted the possible properties of planets around Barnard's Star, it has not ruled them out completely; terrestrial planets would be difficult to detect. NASA's Space Interferometry Mission, which was to begin searching for extrasolar Earth-like planets, was reported to have chosen Barnard's Star as an early search target.  This mission was shut down in 2010.  ESA's similar Darwin interferometry mission had the same goal, but was stripped of funding in 2007. 
Barnard's Star was studied as part of Project Daedalus. Undertaken between 1973 and 1978, the study suggested that rapid, unmanned travel to another star system was possible with existing or near-future technology.  Barnard's Star was chosen as a target partly because it was believed to have planets. 
The theoretical model suggested that a nuclear pulse rocket employing nuclear fusion (specifically, electron bombardment of deuterium and helium-3) and accelerating for four years could achieve a velocity of 12% of the speed of light. The star could then be reached in 50 years, within a human lifetime.  Along with detailed investigation of the star and any companions, the interstellar medium would be examined and baseline astrometric readings performed. 
The initial Project Daedalus model sparked further theoretical research. In 1980, Robert Freitas suggested a more ambitious plan: a self-replicating spacecraft intended to search for and make contact with extraterrestrial life.  Built and launched in Jupiter's orbit, it would reach Barnard's Star in 47 years under parameters similar to those of the original Project Daedalus. Once at the star, it would begin automated self-replication, constructing a factory, initially to manufacture exploratory probes and eventually to create a copy of the original spacecraft after 1,000 years. 
In 1998 a stellar flare on Barnard's Star was detected based on changes in the spectral emissions on July 17 during an unrelated search for variations in the proper motion. Four years passed before the flare was fully analyzed, at which point it was suggested that the flare's temperature was 8000 K, more than twice the normal temperature of the star.  Given the essentially random nature of flares, Diane Paulson, one of the authors of that study, noted that "the star would be fantastic for amateurs to observe". 
The flare was surprising because intense stellar activity is not expected in stars of such age. Flares are not completely understood, but are believed to be caused by strong magnetic fields, which suppress plasma convection and lead to sudden outbursts: strong magnetic fields occur in rapidly rotating stars, while old stars tend to rotate slowly. For Barnard's Star to undergo an event of such magnitude is thus presumed to be a rarity.  Research on the star's periodicity, or changes in stellar activity over a given timescale, also suggest it ought to be quiescent; 1998 research showed weak evidence for periodic variation in the star's brightness, noting only one possible starspot over 130 days. 
Stellar activity of this sort has created interest in using Barnard's Star as a proxy to understand similar stars. It is hoped that photometric studies of its X-ray and UV emissions will shed light on the large population of old M dwarfs in the galaxy. Such research has astrobiological implications: given that the habitable zones of M dwarfs are close to the star, any planets would be strongly influenced by solar flares, winds, and plasma ejection events. 
Barnard's Star shares much the same neighborhood as the Sun. The neighbors of Barnard's Star are generally of red dwarf size, the smallest and most common star type. Its closest neighbor is currently the red dwarf Ross 154, at 1.66 parsecs (5.41 light-years) distance. The Sun and Alpha Centauri are, respectively, the next closest systems.  From Barnard's Star, the Sun would appear on the diametrically opposite side of the sky at coordinates RA=5 h 57 m 48.5 s , Dec=−04° 41′ 36″, in the eastern part of the constellation Monoceros. The absolute magnitude of the Sun is 4.83, and at a distance of 1.834 parsecs, it would be a first-magnitude star, as Pollux is from the Earth.