See W. V. Byars, ed., B. and M. Gratz (1916).
Barnard's Star is a very low-mass red dwarf star approximately 6 light-years away in the constellation of Ophiuchus (the Snake-holder). The star is also known under the name Velox Barnardi, (Latin for "the Swift One of Barnard") or Barnard's Runaway as well. In 1916, American astronomer E. E. Barnard measured its proper motion as 10.3 arcseconds per year, which remains the largest known proper motion of any star relative to the Sun. At a distance of about 1.8 parsecs, or just under six light-years, Barnard's Star is the nearest known star in Ophiuchus, the second-closest known star system to the Sun, and the fourth-closest known individual star to the Sun, after the three components of the Alpha Centauri system. Despite its proximity, Barnard's Star is not visible with the unaided eye.
Barnard's Star has been the subject of much study and has probably received more attention from astronomers than any other M dwarf star due to its proximity and favourable location for observation near the celestial equator. Research has focused on stellar characteristics, astrometry, and refining the limits of possible extrasolar planets. Although an ancient star, observations suggest that Barnard's Star still experiences flare events.
The star has also been the subject of some controversy. For a decade, from the early 1960s, Peter van de Kamp's erroneous claim of a gas giant planet (or planets) in orbit around Barnard's Star was accepted by other astronomers; while small terrestrial planets remain possible, massive planets have been largely discounted and Van de Kamp's specific claims overturned. The star is notable as the target for a study on the possibility of rapid, unmanned travel to nearby star systems.
Barnard's Star is a red dwarf of the dim M4 spectral type and is too faint to see without a telescope. Its apparent magnitude is 9.57. This compares to −1.5 for Sirius (the brightest star in the night sky) and 6 for the faintest visible objects (the scale is logarithmic, and so magnitude 9.57 is only about 1/27th of the brightness of the faintest star that can be seen by unaided human eyes under good viewing conditions).
At 7 to 12 billion years of age, Barnard's Star is considerably older than the Sun and may be among the oldest stars in the universe; it has lost a great deal of rotational energy, and periodic changes in its light indicate it rotates just once every 130 days (compared to just over 25 days for the Sun). Given its age, Barnard's Star was long assumed to be quiescent in terms of stellar activity. However, in 1998 astronomers observed an intense stellar flare, making it a surprising flare star. It has the variable star designation V2500 Ophiuchi.
The proper motion of the body corresponds to a relative lateral speed ("sideways" relative to the Sun) of 90 kilometres per second (km/s). The 10.3 seconds of arc covered annually amounts to a quarter of a degree in a human lifetime, roughly half the angular diameter of the full Moon.
Barnard's Star's radial velocity towards the Sun can be measured by its blue shift. Two measurements are given in catalogues: 106.8 km/s in SIMBAD, and 110.8 km/s in ARICNS and elsewhere. These measurements, combined with proper motion, suggest a true velocity relative to the Sun of 139.7 and 142.7 km/s, respectively. Barnard's Star is approaching the Sun so rapidly that it will become the nearest star around AD 11,700, at a distance of some 3.8 light-years. Perhaps disappointingly, the star will still be too dim to be seen with the naked eye at this time as its apparent magnitude will be about 8.5. After this it will steadily recede.
Barnard's Star is approximately 17% of a solar mass and has a radius 15–20% that of the Sun (the higher estimate is most recent). Thus although it is roughly 180 times the mass of Jupiter, its radius is only 1.5 to 2.0 times larger, reflecting the tendency of objects in the brown dwarf range to be about the same size. Its effective temperature is 3134(±102) K, and it has a visual luminosity just 4/10000 of solar luminosity, corresponding to a bolometric luminosity of 34.6/10000. It is so faint that, were it to replace the Sun, it would appear only 100 times brighter than a full moon.
For a decade from 1963 onwards, a substantial number of astronomers accepted a claim by Peter van de Kamp that he had detected 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 1 micrometre in its position on photographic plates consistent with orbital perturbations (wobbles) in the star 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 1.6 Jupiter mass planet at 4.4 AU in a slightly eccentric orbit, and these measurements were apparently refined in a 1969 paper. Later that year he suggested two planets of 1.1 and 0.8 Jupiter masses.
Other astronomers subsequently repeated Van de Kamp's measurements, and two important papers in 1973 undermined the claim of a planet or planets. Gatewood and Eichhorn, at a different observatory and using newer plate measuring techniques, failed to verify the planetary companion. Another paper published by 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 telescopic lens; the planetary "discovery" was an artifact of maintenance and upgrade work.
Van de Kamp never acknowledged any error, and published a further confirmation of two planets' existence as late as 1982. 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 are reported to have become estranged because of this.
While not completely ruling out the possibility of planets, null results for planetary companions continued throughout the 1980s and 90s, the latest based on interferometric work with Hubble space telescope in 1999.
While the controversy may have had a negative effect on the study of extrasolar planets, it did heighten the profile of Barnard's Star. During the period that the planetary claim was accorded credibility, the star's fame among the science fiction community grew (see Barnard's Star in fiction), and it was adopted as a target for Project Daedalus.
Excepting the planet controversy, the best known study of Barnard's Star was part of Project Daedalus. Undertaken between 1973 and 1978, it suggested that rapid, unmanned travel to another star system is 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 Jovian 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 1000 years.
While the research initiated by Van de Kamp and focused on the planetary search has perhaps had the highest profile, Barnard's star is a well-documented object in other respects.
Several papers on mass-luminosity relations appeared prior to Dawson's definitive work in 2003. Along with refining the temperature and luminosity (see above), this paper suggested that previous estimates of Barnard's Star radius consistently underestimated the value; it suggests 0.20 solar radius (±0.008 solar radius), at the high end of the range typically provided.
In a broad survey of the metallicity of M dwarf stars, Barnard's Star's was placed between −0.5 and −1.0 on the metallicity scale, which is roughly 10 to 32% of the value for the Sun. Metallicity, the proportion of stellar mass made up of elements heavier than helium, 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 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.
The Hubble telescope work by Benedict and colleagues has been wide-ranging. In 1999 absolute parallax values and absolute magnitude values were refined. This aided the refinement of planetary boundaries (see below). Another important paper, by Kurster et al. in 2003, reported the first detection of a change in the radial velocity of a star caused by its motion; further variability in its radial velocity was attributed to stellar activity.
Astrometry and the study of other stellar characteristics may also yield further information on the possibility of planets. 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 star. M dwarfs such as Barnard's Star are more easily studied than larger stars in this regard because their lower mass renders perturbations more obvious. Gatewood was thus able to show in 1995 that planets of 10 Jupiter masses (the lower limit for brown dwarfs) 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 Jupiter mass with an orbital period of less than 1000 days, while Kurtzer 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 Earth masses or with a mass greater than 3.1 Neptune masses (much lower than van de Kamp's smallest suggested value).
While 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 and the ESA's Darwin, both scheduled to begin looking for extrasolar Earth-like planets around 2015, have chosen Barnard's Star as a search target.
The observation of a stellar flare on Barnard's Star has added another element of interest to its study. Noted by William Cochran, University of Texas at Austin, based on changes in the spectral emissions on July 17, 1998 (during an unrelated search for planetary "wobbles"), it was four more years before the flare was fully analyzed. At that point Diane Paulson et al., now of Goddard Space Flight Center, suggested that the flare's temperature was 8000 K, more than twice the normal temperature of the star, although simply analyzing the spectra cannot precisely determine the flare's total output. Given the essentially random nature of flares, she noted "the star would be fantastic for amateurs to observe".
The flare was surprising because intense stellar activity is not expected around 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. An event of such magnitude around Barnard's Star 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 Barnard's 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. Photometric studies of its X-ray and UV emissions are hoped to 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 can be viewed directly overhead just north of the equator, at 4° N; theoretically viewable ± 90° from this point, it can thus be observed at most Earth latitudes, although atmospheric extinction will reduce visibility when it is near the horizon in the extreme north and south.
The star shares much the same neighbourhood as the Sun. The neighbours of Barnard's Star are generally of red dwarf size, the smallest and most common star type. Its closest neighbour is currently the red dwarf Ross 154, at 1.66 pc or 5.41 ly 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=, Dec=, 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 an impressively bright first-magnitude star, like Pollux to us.