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Barnard's Star

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Barnard's Star[1]

Barnard's Star
Observation data
Epoch J2000      Equinox J2000
Constellation Ophiuchus
Right ascension 17h 57m 48.5s
Declination +04° 41' 36"
Apparent magnitude (V) 9.57
Characteristics
Spectral type M4 V
U−B color index 1.28
B−V color index 1.74
Variable type BY Draconis
Astrometry
Radial velocity (Rv)-110.8 km/s
Proper motion (μ) RA: -797.84 mas/yr
Dec.: 10326.93 mas/yr
Parallax (π)546.98 ± 1.00 mas
Distance5.96 ± 0.01 ly
(1.828 ± 0.003 pc)
Absolute magnitude (MV)13.26
Details
Mass0.17 M☉
Radius0.15-0.20 R☉
Luminosity0.0004 L☉
Temperature3,134 K
Metallicity10-32% Sun
Rotation130.4 days
Age~1.0 × 1010 years
Other designations
Velox Barnardi, V2500 Oph, BD+04°3561a, GCTP 4098.00, GJ 699, LHS 57, Munich 15040, Gl 140-024, LTT 15309, LFT 1385, Vyssotsky 799, and HIP 87937.
Database references
SIMBADdata
ARICNSdata

Barnard's Star is a very low-mass star in the constellation Ophiuchus which was discovered by the astronomer E. E. Barnard in 1916. Barnard measured its proper motion to 10.3 arcseconds per year, which remains the largest known proper motion of any star relative to the Sun.[2] Lying at a distance of about 1.8 parsecs or 5.96 light-years, Barnard's Star is the nearest star in the constellation Ophiuchus and is also the second closest known star system to the Sun and the fourth closest known individual star after the three components of the Alpha Centauri system.

Barnard's Star is a relatively well-studied astronomical object, and has likely received more attention than any other M dwarf star given its proximity and favourable location for observation near the celestial equator.[3] Research has focused on stellar characteristics, astrometry, and refining the limits of possible planets. It has also been the subject of some controversy. For a decade from the early 1960s onward, an erroneous discovery of a planet or planets in orbit around Barnard's Star was accepted by astronomers. The star is notable as the target for a study on the possibility of rapid, unmanned travel to nearby star systems.

System summary

Barnard's Star is a red dwarf of the faint M4 spectral type and so, despite its proximity, it 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 9.57 is nowhere near the visible range.

A very old star at 11 to 12 billion years, Barnard's Star 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).[4] Given its age, Barnard's Star was long assumed to be quiescent in terms of stellar activity. However, in 1998 astronomers observed an intense solar flare, making it a surprising flare star.[5] 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.[6]

Its radial velocity towards the Sun can be measured by its redshift. 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.[7] In fact, Barnard's Star is approaching the Sun so rapidly that it will be the nearest star around AD 11,800, at a distance of 3.8 light-years.[8]

Barnard's Star, all positions since 1985.

Barnard's Star is 17% of a solar mass and has a radius 15-20% that of the Sun.[9] Its effective temperature is 3134(+/-102)K and it has a visual luminosity just 4/10000 of solar luminosity, corresponding to a bolometric or absolute luminosity of 34.6/10000.[3] It is so faint that, were it to replace the Sun, it would only appear 100 times brighter than a full moon.[9]

Barnard's Star's closest neighbour is currently Ross 154, at 1.66 pc or 5.41 ly away. With the exception of the Sun and Alpha Centauri A and B, all of Barnard's star's neighbours within 10 ly are other faint K or M spectral class red dwarfs.[9]


Supposed planets

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.[8] Van de Kamp had been observing the star from 1938, attempting, with colleagues at the Swarthmore College observatory, to find extremely minute variations of 1 micrometre in its position on photographic plates consistent with "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.[10] Van de Kamps's initial suggestion was a 1.6 Jupiter mass planet at 4.4 AUs in a slightly eccentric orbit, these measurements apparently refined in a 1969 paper. Later that same year he would suggest two planets of 1.1 and 0.8 Jupiter masses.[11]

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

Other astronomers attempted to duplicate Van de Kamp's finding 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.[12] Another paper published by Hershey four months earlier, also using the Swarthmore observatory, suggested a cause for the discrepancy. He found that changes in the astrometric field of various stars correlated to the timing of adjustments and modifications that had been done on the telescopic lens;[13] the planetary "discovery" was an artifact of maintenance and upgrade work.

Van de Kamp refused to acknowledge any error for the rest of his life, publishing a supposed confirmation of two planets as late as 1982.[14] In general a gregarious and well-admired man, he may have felt betrayed by colleagues who disputed his findings. 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.[15]

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.[16]

While the controversy may have dampened work on extrasolar planets, it did have a salutatory effect on the profile of Barnard's Star. During the period that the claim was accorded credibility, it contributed to the star's fame among the science fiction community (see Barnard's Star in fiction) and the star's adoption as a target for Project Daedalus.

Project Daedalus

Main article: Project Daedalus

Excepting the planet controversy, the best known work related to Barnard Star's is 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.[17] Barnard's Star was chosen as a target in part because of the assumption of planetary companions.[18]

The theoretical model suggested that a nuclear pulse rocket employing nuclear fusion (specifically, electron bombardment of deuterium and helium-3) could achieve a velocity of 12% light speed over a four-year acceleration phase. The star could then be reached in 50 years, i.e. within a human lifetime.[18] Along with detailed investigation of the star and any companions, the interstellar medium would be examined and baseline astrometric readings performed.[17]

The initial Project Daedalus model would spark further theoretical research. In 1980, Freitas suggested a more ambitious plan: a self-reproducing interstellar probe 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 much the same parameters as the original Project Daedalus. Once at the star, however, it would begin self-automated replication activities. A factory would be constructed, initially to manufacture exploratory probes and eventually to create a copy of the original spacecraft after 1000 years.[19]

Research

File:Barnard'sStarSize.jpg
Barnard's Star has a diameter only 15 to 20% that of the Sun.[20]

While the research following from 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.

Stellar characteristics and astrometry

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.[3]

In a broad survey of the metallicity of M dwarf stars, Barnard's Star's was placed between -0.05 and -0.1 on the metallicity scale, or roughly 10 to 32% as metal-enriched as the Sun.[20] 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 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 actually 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.[20][21]

The Hubble telescope work by Benedict and colleagues has been wide-ranging. In 1999 absolute parallax values and absolute magnitude values were refined.[16] This aided in refining planetary boundaries (see below). Another important paper, by Kurster et al., appeared in 2003. It showed the first detection of a change in the radial velocity of a star caused by its space motion; further variability in radial velocity was attributed to stellar activity.[21]

Refining planetary boundaries

File:BarnardsStar.JPG
Barnard's Star in NASA's digital star survey.

Work on astrometry and other 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 reduced. More simply, 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.[22] In this way, Gatewood was able to show in 1995 that planets of 10 Jupiter mass (the lower limit for brown dwarfs) were impossible around Barnard's star,[8] in a paper which helped refine the task of providing negative certainty regarding planetary objects in general.[23] The 1999 work with Hubble would further exclude planetary companions 0.8 Jupiter masses with an orbital period of less than 1000 days,[16] while Kurtzer would determine in 2003 that within the habitable zone around Barnard's Star, planets are not possible with an "Msin i" value[24] of 7.5 Earth masses and 3.1 Neptune masses in general (well below van de Kamp's smallest suggestions).[21]

While this research has greatly restricted the parameters of possible planets around Barnard's Star, it has not ruled them out completely; terrestrial planets are still a possibility but 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.[9]

1998 flare

The observation of a solar flare on Barnard's Star has added another element of interest to its study. Noted by Cochran based on changes in the spectral emissions on July 17, 1998 (during an unrelated search for planetary "wobbles"), it took four more years before the flare would be properly analyzed. At that point Paulson, now of Goddard Space Flight Center, suggested that the flare's temperature was 8000 K, more than twice normal for the star, although simply analyzing spectra cannot precisely determine the total output of the flare.[25] Given the essentially random nature of flares, she noted "the star would be fantastic for amateurs to observe."[5]

The flare was surprising because intense stellar activity is not expected around stars of such age. Flares, though not completely understood, are believed to be caused by strong magnetic fields, which suppress plasma convection leading to sudden outbursts; strong magnetic fields require a rapidly rotating star, while old stars tend to rotate slowly. An event of such magnitude around Barnard's Star is thus presumed to be rare.[25] Research on its 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.[26]

See also

Notes and references

  1. ^ SIMBAD is used for observation data, while ARICNS is used for astrometry. More specific numbers from research papers may be employed, but will also be mentioned in the body.
  2. ^ E. E. Barnard (1916). "A small star with large proper motion". Astronomical Journal. 29 (695): 181. Retrieved 2006-08-10.
  3. ^ a b c Dawson, P. C.; De Robertis, M. M. (2004). "Barnard's Star and the M Dwarf Temperature Scale". Astronomical Journal. 127 (5): 2909. doi:10.1086/383289. Retrieved 2006-08-16.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Darling, David. "Barnard's Star". The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved 2006-08-15.
  5. ^ a b Croswell, Ken (November 2005). "A Flare for Barnard's Star". Astronomy Magazine. Kalmbach Publishing Co. Retrieved 2006-08-10.{{cite web}}: CS1 maint: year (link)
  6. ^ Kaler, James B. (November 2005). "Barnard's Star (V2500 Ophiuchi)". Stars. James B. Kaler. Retrieved 2006-09-07.{{cite web}}: CS1 maint: year (link)
  7. ^ or Stars with a large proper motion naturally have large true velocities relative to the Sun, but proper motion is also a function of proximity. While Barnard's star has the largest proper motion, the largest known true velocity of another star in the Milky Way belongs to Wolf 424 at 555 km/s.
  8. ^ a b c Bell, George H. (April 2001). "The Search for the Extrasolar Planets: A Brief History of the Search, the Findings and the Future Implications, Section 2". Arizona State University. Retrieved 2006-08-10.{{cite web}}: CS1 maint: year (link) Full description of the Van de Kamp planet controversy.
  9. ^ a b c d "Barnard's Star". Sol Station. Retrieved 2006-08-10.
  10. ^ "The Barnard's Star Blunder". Astrobiology Magazine. 2005. Retrieved 2006-08-09. {{cite web}}: Unknown parameter |month= ignored (help)
  11. ^ Van de Kamp, Peter. (1969). "Alternate dynamical analysis of Barnard's star". Astronomical Journal. 74 (8): 757. Retrieved 2006-08-10.
  12. ^ Gatewood, George, and Eichhorn, H. (1973). "An unsuccessful search for a planetary companion of Barnard's star (BD +4 3561)". Astronomical Journal. 78 (10): 769. Retrieved 2006-08-09.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ John L. Hershey (1973). "Astrometric analysis of the field of AC +65 6955 from plates taken with the Sproul 24-inch refractor". Astronomical Journal. 78 (6): 421. doi:10.1086/111436. Retrieved 2006-08-09.
  14. ^ Van de Kamp, Peter. (1982). "The planetary system of Barnard's star". Vistas in Astronomy. 26 (2): 141. Retrieved 2006-08-10.
  15. ^ Kent, Bill (2001). "Barnard's Wobble". Bulletin. Swarthmore College. Retrieved 2006-08-09.
  16. ^ a b c G.Fritz Benedict, Barbara McArthur, D. W. Chappell, E. Nelan, W. H. Jefferys, W. van Altena, J.Lee, D. Cornell, P. J. Shelus, P.D. Hemenway, Otto G. Franz, L. H. Wasserman, R. L. Duncombe, D. Story, A. L. Whipple, L.W.Fredrick (1999). "Interferometric Astrometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: Detection Limits for sub-Stellar Companions". Astrophysics. Retrieved 2006-08-10.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  17. ^ a b Bond, A., and Martin, A.R. (1976). "Project Daedalus - The mission profile". Journal of the British Interplanetary Society. 29 (2): 101. Retrieved 2006-08-15.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  18. ^ a b Darling, David (2005). "Daedalus, Project". The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Retrieved 2006-08-10. {{cite web}}: Unknown parameter |month= ignored (help)
  19. ^ Freitas, R.A., JR. (1980). "A self-reproducing interstellar probe". Journal of the British Interplanetary Society. 33: 251. Retrieved 2006-08-15. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  20. ^ a b c Gizis, John E. (1997). "M-Subdwarfs: Spectroscopic Classification and the Metallicity Scale". The Astronomical Journal. 113 (2): 820. doi:10.1086/118302. Retrieved 2006-08-24. {{cite journal}}: Unknown parameter |month= ignored (help)
  21. ^ a b c Kürster, M.; Endl, M.; Rouesnel, F.; Els, S.; Kaufer, A.; Brillant, S.; Hatzes, A. P.; Saar, S. H.; Cochran, W. D. (2003). "The low-level radial velocity variability in Barnard's star". Astronomy and Astrophysics. 403 (6): 1077. Retrieved 2006-08-16.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  22. ^ Michael Endl, William D. Cochran, Robert G. Tull, and Phillip J. MacQueen. (2003). "A Dedicated M Dwarf Planet Search Using the Hobby-Eberly Telescope". The Astronomical Journal. 126 (12): 3099. Retrieved 2006-08-18.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  23. ^ George D. Gatewood (1995). "A study of the astrometric motion of Barnard's star". Journal Astrophysics and Space Science. 223 (1): 91–98. doi:10.1007/BF00989158. {{cite journal}}: Text "http://www.springerlink.com/content/q722p32q2h28kp50/" ignored (help)
  24. ^ "Msin i" represents 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.
  25. ^ a b Diane B. Paulson, Joel C. Allred, Ryan B. Anderson, Suzanne L. Hawley, William D. Cochran, and Sylvana Yelda (2006). "Optical Spectroscopy of a Flare on Barnard's Star". Publications of the Astronomical Society of the Pacific. 118 (1): 227. doi:10.1086/499497. Retrieved 2006-08-21.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  26. ^ Benedict, G. Fritz; McArthur, Barbara; Nelan, E.; Story, D.; Whipple, A. L.; Shelus, P. J.; Jefferys, W. H.; Hemenway, P. D.; Franz, Otto G.; Wasserman, L. H.; Duncombe, R. L.; van Altena, W.; Fredrick, L. W. (1998). "Photometry of Proxima Centauri and Barnard's star using Hubble Space Telescope fine guidance senso 3". The Astronomical Journal. 116 (1): 429. doi:10.1086/300420. Retrieved 2006-08-18.{{cite journal}}: CS1 maint: multiple names: authors list (link)

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