Barnard's Star is a low mass red dwarf star located 6 light years away from Earth in the constellation of Ophiuchus. It is the fourth closest star to the Sun, after the triple star system of Proxima Centauri and Alpha Centauri A and B. It is the closest star in the Northern Celestial Hemisphere. The tsar has an apparent magnitude of +9.5 and is thus invisible to the naked eye.
The star is named after the American astronomer, E. E. Barnard. While he was not the first to observe the star, in 1916 he measured its proper motion as 10.3 arcseconds per year relative to the Sun, the highest known for any star.
Thanks to its close proximity to Earth and its favorable location for observation near the celestial equator, Barnard's Star is one of the most well studied red dwarfs. 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 ancient it still experiences star flare events. One such event was observed in 1998.
From the early 1960s to the early 1970s, Peter van de Kamp argued that there were one or more gas giants in orbit around the star. However, his specific claims of large gas giants were refuted in the mid-1970s after lengthy debates. A candidate super-Earth extrasolar planet was announced to be orbiting the star in November 2018, however, the planet's existence was refuted in July of 2021 and was confirmed to not exist in 2022.
Name[]
The International Astronomical Union organized a Working Group on Star Names (WGSN) in 2016 to catalogue and standardize proper names for stars. The WGSN approved the name "Barnard's Star" for this star on February 1, 2017.
Description[]
Barnard's Star is a dim, very low mass red dwarf star. It has a spectral type of M4. Although it is close to Earth, it is too dim to be seen with the naked eye and requires a telescope to see. Its apparent magnitude is 9.5.
Barnard's Star is considerably older than the Sun, at 7-12 billion years old, compared to the Sun's approximate 4.5 billion years of age. It may be one of 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 indicates that it rotates about every 130 days. 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, which revealed 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 blueshift to be about -110 km/s. Combined with its proper motion, this gives a space velocity (actual velocity relative to the Sun) of -142.6 km/s. Barnard's Star will make its closest approach to the Sun in the year 11800, where it will be located within 3.75 light years of the Sun.
Proxima Centauri is the closest star to the Sun at 4.24 light years away from it. However, despite Barnard's Star's closer pass to the Sun in 11800 AD, it will still not be the nearest star since by that time Proxima Centauri will have moved to a yet-near proximity to the Sun. Even when the star is closest to Earth, 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, which is 2.5 magnitudes short of being visible to the naked eye.
Barnard's Star has a mass of about 0.14 solar masses, and a radius of 0.196 Solar radius, or 15% to 20% to that of the Sun. Thus, Barnard's Star has roughly about 150 times the mass of Jupiter, while its radius is only about 1.5 to 2.0 times larger, because of its much higher density. It has a visual luminosity of 0.0004 solar luminosities and an effective temperature of approximately 3,100 kelvin. Barnard's Star is so faint that if it were at the same distance from Earth as the Sun is, it would only appear about 100 times brighter than a full moon, comparable to the brightness of the Sun at 80 astronomical units.
Barnard's Star 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.
Search for exoplanets[]
Astrometric planetary claims[]
For a decade from 1963 to 1973, a substantial number of astronomers accepted a claim by Peter van de Kemp that he had detected, by using astrometry, a perturbation in the proper motion of Barnard's Star, consistent with 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 Sproul Observatory at Swarthmore College, to find minuscule variations of one micrometer in its position on photographic plates consistent with orbital perturbations that would indicate a planet. 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 Jupiter mass 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 Kemp suggested that there were two planets of 1.1 and 0.8 Jupiter masses respectively.
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 errors and publihsed a further claim of the two planets' existence as late as 1982. He later 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 because of this.
Barnard's Star b[]
Artist's concept of Barnard's Star b
- See main article: Barnard's Star b
In November 2018, an international team of astronomers announced the detection of a candidate super-Earth orbiting in relatively close proximity to Barnard's Star, using the radial velocity method. The large team was led by Ignasi Ribas of Spain, and their work included two decades of observation, with their observations giving strong evidence that the planet existed. However, the existence of the planet was disputed in 2021, because the radial velocity signal was found to originate from a stellar activity cycle, and a study in 2022 confirmed this.
Dubbed "Barnard's Star b", the planet was thought to be found near the stellar system's snow line, which is an ideal spot for the icy accretion of proto-planetary material. It was thought to orbit about 0.4 AU every 233 days and has a proposed mass of 3.2 Earth mass. The planet was expected to be freezing cold, with an equilibrium temperature of about 105 K (-168 °C; -270 °F), and it was believed to lie outside of Barnard Star's presumed habitable zone.
Stellar flares[]
1998 flare[]
On July 17, 1998, a stellar flare was detected, based on changes in the spectral emissions by William Cochran at the University of Austin in Texas during an unrelated search for variations in the star's proper motion. Four years passed before the flare was fully analyzed, at which point it was suggested that the temperature of the flare was as high as 8,000 K, more than twice the normal temperature of the star.
The flare came as a surprise to astronomers, as such intense activity was not expected from a star of this age. Although flares are not fully understood, astronomers believe they are caused by sudden burst of pent-up plasma in the troposphere caused by a strong magnetic field: strong magnetic fields occur in rapidly rotating stars, while old stars tend to rotate slowly. Therefore, astronomers believe that it is very rare for Barnard's star to undergo an event of such magnitude. Research on the star's periodicity, or changes in stellar activity over a given timescale, also suggest it should be a "quiet star"; research in 1998 showed weak evidence for periodic variation in the star's brightness, noting only one possible starspot over 130 days.
This stellar activity has made Barnard's star a prime object for observation for astronomers to understand similar stars. It is hoped that photometric studies of its X-ray and UV emissions will shed light on the galaxy's large population of old M dwarfs. Such research has astrobiological implications: since the habitable zone of an M-type dwarfs are close to the star, any planet located therein would be strongly affected by solar flares, stellar winds, and plasma ejection events.
2019 flare[]
Two additional ultraviolet stellar flares were detected in 2019, each with far-ultraviolet energy of 3x1022 joules, together with one X-ray stellar flare with energy 1.6x1022 joules. The flare rate observed is enough to cause the loss of 87 Earth atmospheres per billion years.
Environment[]
A map of the closest stars to the Sun, including Barnard's Star
Barnard's Star shared much of 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. It closest neighbor is currently Ross 154, a red dwarf star located 5.41 light years (1.66 parsecs) away from the star. The Sun and Alpha Centauri are, respectively, the next closest systems. From Barnard's Star, the Sun would be visible on the diametrically opposite side of the sky, at the following coordinates: Right Ascension: 5h 57m 48.5s, Declination: -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 Earth.
Proposed exploration[]
Project Daedalus[]
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 as 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.