Barnard's Star

Coordinates: Sky map 17h 57m 48.5s, +04° 41′ 36″
Source: Wikipedia, the free encyclopedia.

Barnard's Star

The location of Barnard's Star, c. 2006 (south is up)
Observation data
J2000.0
Constellation Ophiuchus
Right ascension 17h 57m 48.49847s[1]
Declination +04° 41′ 36.1139″[1]
Apparent magnitude (V) 9.511[2]
Characteristics
Spectral type M4.0V[3]
Apparent magnitude (U) 12.497[2]
Apparent magnitude (B) 11.240[2]
Apparent magnitude (R) 8.298[2]
Apparent magnitude (I) 6.741[2]
Apparent magnitude (J) 5.24[4]
Apparent magnitude (H) 4.83[4]
Apparent magnitude (K) 4.524[4]
U−B color index 1.257[2]
B−V color index 1.713[2]
V−R color index 1.213[2]
R−I color index 1.557[2]
Variable type BY Draconis[5]
Distance
5.9629 ± 0.0004 ly
(1.8282 ± 0.0001 pc)
Absolute magnitude (MV)13.21[2]
Details
Gyr
Latin: Velox Barnardi[14]
Database references
SIMBADdata
ARICNSdata
Barnard's Star is located in the constellation Ophiuchus.
Barnard's Star is located in the constellation Ophiuchus.
Barnard's Star
Location of Barnard's Star in the constellation Ophiuchus

Barnard's Star is a small

unaided eye; it is much brighter in the infrared than in visible light
.

The star is named after

photographic plates in 1888 and 1890.[17]

Barnard's Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the

extrasolar planets. Although Barnard's Star is ancient, it still experiences stellar flare events, one being observed in 1998.[18]

Barnard's Star has been subject to multiple claims of planets that were later disproven. From the early 1960s to the early 1970s, Peter van de Kamp argued that planets orbited Barnard's Star. His specific claims of large gas giants were refuted in the mid-1970s after much debate. In November 2018, a candidate super-Earth planetary companion known as Barnard's Star b was reported to orbit Barnard's Star. It was believed to have a minimum mass of 3.2 ME and orbit at 0.4 AU.[19] However, work presented in July 2021 refuted the existence of this planet.[20]

Naming

In 2016, the

Working Group on Star Names (WGSN)[21] 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 included in the List of IAU-approved Star Names.[22]

Description

Barnard's Star is a red dwarf of the dim

spectral type M4, and it is too faint to see without a telescope; Its apparent magnitude
is 9.5.

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

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.[25]

Barnard's Star, showing position every 5 years in the period 1985–2005

The proper motion of Barnard's Star corresponds to a relative lateral speed of 90 km/s. The 10.3 arcseconds it travels in a year amount to a quarter of a degree in a human lifetime, roughly half the angular diameter of the full Moon.[16]

The radial velocity of Barnard's Star is −110 km/s, as measured from the

blueshift due to its motion toward the Sun. Combined with its proper motion and distance, this gives a "space velocity" (actual speed relative to the Sun) of 142.6±0.2 km/s. Barnard's Star will make its closest approach to the Sun around 11,800 CE, when it will approach to within about 3.75 light-years.[26]

nearest stars
from 20,000 years ago until 80,000 years in the future

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 CE, 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.[27] 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 0.16 solar masses (M☉),[6] and a radius about 0.2 times that of the Sun.[7][28] Thus, although Barnard's Star has roughly 150 times the mass of Jupiter (MJ), its radius is only roughly 2 times larger, due to its much higher density. Its effective temperature is about 3,220 kelvin, and it has a luminosity of only 0.0034 solar luminosities.[6] 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.[29]

Barnard's Star has 10–32% of the solar

disk star range; this, plus its high space motion, have led to the designation "intermediate population II star", between a halo and disk star.[3][25] Although some recently published scientific papers have given much higher estimates for the metallicity of the star, very close to the Sun's level, between 75 and 125% of the solar metallicity.[30][31]

Search for planets

Astrometric planetary claims

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 Sproul Observatory at Swarthmore College, 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.[32] Van de Kamp's initial suggestion was a planet having about 1.6 MJ at a distance of 4.4 AU in a slightly eccentric orbit,[33] and these measurements were apparently refined in a 1969 paper.[34] Later that year, Van de Kamp suggested that there were two planets of 1.1 and 0.8 MJ.[35]

Artist's conception of a planet in orbit around a red dwarf

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.[36] 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;[37] 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.[38]

Van de Kamp never acknowledged any error and published a further claim of two planets' existence as late as 1982;[39] 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 because of this.[40]

Barnard's Star b

Artist's impression of the surface of a super-Earth orbiting Barnard's Star[41]

In November 2018, an international team of astronomers announced the detection by radial velocity of a candidate super-Earth orbiting in relatively close proximity to Barnard's Star. Led by Ignasi Ribas of Spain their work, conducted over two decades of observation, provided strong evidence of the planet's existence.[19][42] However, the existence of the planet was refuted in 2021, because the radial velocity signal was found to originate from a stellar activity cycle,[20] and a study in 2022 confirmed this result.[43]

Dubbed Barnard's Star b, the planet was thought to be 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 at 0.4 AU every 233 days and had a proposed minimum mass of 3.2 ME. The planet would have most likely been frigid, with an estimated surface temperature of about −170 °C (−274 °F), and lie outside Barnard Star's presumed habitable zone. Direct imaging of the planet and its tell-tale light signature would have been possible in the decade after its discovery. Further faint and unaccounted-for perturbations in the system suggested there may be a second planetary companion even farther out.[44]

Refining planetary boundaries

For the more than four decades between van de Kamp's rejected claim and the eventual announcement of a planet candidate, Barnard's Star was carefully studied and the mass and orbital boundaries for possible planets were slowly tightened.

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.[45]

Null results for planetary companions continued throughout the 1980s and 1990s, including

habitable zone around Barnard's Star, planets are not possible with an "M sin i" value[note 1] greater than 7.5 times the mass of the Earth (ME), or with a mass greater than 3.1 times the mass of Neptune (much lower than van de Kamp's smallest suggested value).[25]

In 2013, a research paper was published that further refined planet mass boundaries for the star. Using radial velocity measurements, taken over a period of 25 years, from the

Monte Carlo analysis for both circular and eccentric orbits, upper masses for planets out to 1,000-day orbits were determined. Planets above two Earth masses in orbits of less than 10 days were excluded, and planets of more than ten Earth masses out to a two-year orbit were also confidently ruled out. It was also discovered that the habitable zone of the star seemed to be devoid of roughly Earth-mass planets or larger, save for face-on orbits.[48][49]

Even though this research greatly restricted the possible properties of planets around Barnard's Star, it did not rule them out completely as

Darwin interferometry mission had the same goal, but was stripped of funding in 2007.[51]

The analysis of radial velocities that eventually led to discovery of the candidate super-Earth orbiting Barnard's Star was also used to set more precise upper mass limits for possible planets, up to and within the habitable zone: a maximum of 0.7 ME up to the inner edge and 1.2 ME on the outer edge of the optimistic habitable zone, corresponding to orbital periods of up to 10 and 40 days respectively. Therefore, it appears that Barnard's Star indeed does not host Earth-mass planets or larger, in hot and temperate orbits, unlike other M-dwarf stars that commonly have these types of planets in close-in orbits.[19]

Stellar flares

1998

In 1998 a

spectral emissions on 17 July 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 8,000 K, more than twice the normal temperature of the star.[52] 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".[24]

Artist's conception of a red dwarf

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.[52] 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.[23]

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

astrobiological implications: given that the habitable zones of M dwarfs are close to the star, any planet located therein would be strongly affected by solar flares, stellar winds, and plasma ejection events.[9]

2019

In 2019, two additional ultraviolet

stellar flare with energy 1.6×1022 joules. The flare rate observed to date is enough to cause loss of 87 Earth atmospheres per billion years through thermal processes and ≈3 Earth atmospheres per billion years through ion loss processes on Barnard's Star b.[53]

Environment

Stars closest to the Sun, including Barnard's Star (25 April 2014)[54]

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

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.[note 2]

Proposed exploration

The position of Barnard's Star on a radar map among all stellar objects or stellar systems within 9 light years (ly) from the map's center, the Sun (Sol). The diamond-shapes are their positions entered according to right ascension in hours angle (indicated at the edge of the map's reference disc), and according to their declination. The second mark shows each's distance from Sol, with the concentric circles indicating the distance in steps of one ly.

Project Daedalus

Main article: Project Daedalus

Barnard's Star was studied as part of Project Daedalus. Undertaken between 1973 and 1978, the study suggested that rapid, uncrewed travel to another star system was possible with existing or near-future technology.[55] Barnard's Star was chosen as a target partly because it was believed to have planets.[56]

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.[56] Along with detailed investigation of the star and any companions, the interstellar medium would be examined and baseline astrometric readings performed.[55]

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.[57] 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.[57]

See also

Notes

  1. ^ "M sin i" means the mass of the planet times the sine of the angle of inclination of its orbit, and hence provides the minimum mass for the planet.
  2. ^ The Sun's apparent magnitude from Barnard's Star, assuming negligible extinction: .

References

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External links

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