Barnard's Star

Barnard's Star

The location of Barnard's Star, c. 2006 (south is up)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Ophiuchus
Pronunciation /ˈbɑːrnərd/
Right ascension 17h 57m 48.49803s[1]
Declination +04° 41′ 36.2072″[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]
Variable type BY Draconis
Astrometry
Radial velocity (Rv)−110.6 ± 0.2[5] km/s
Proper motion (μ) RA: −802.803[6] mas/yr
Dec.: 10362.542[6] mas/yr
Parallax (π)547.4506 ± 0.2899[6] mas
Distance5.958 ± 0.003 ly
(1.8266 ± 0.0010 pc)
Absolute magnitude (MV)13.21[2]
Details
Mass0.144[7] M☉
Radius0.196 ± 0.008[8] R☉
Luminosity (bolometric)0.0035[9] L☉
Luminosity (visual, LV)0.0004[9] L☉
Temperature3,134 ± 102[9] K
Metallicity10–32% Sun[10]
Rotation130.4 d[11]
Age≈ 10 billion [12] years
Other designations
"Barnard's Runaway Star", "Greyhound of the Skies",[13] BD+04°3561a, GCTP 4098.00, Gl 140-024, Gliese 699, HIP 87937, LFT 1385, LHS 57, LTT 15309, Munich 15040, Proxima Ophiuchi,[14] V2500 Ophiuchi, Velox Barnardi,[15] Vyssotsky 799
Database references
SIMBADdata
ARICNSdata

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.[16] Despite its proximity, because of its 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.[17] 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 proximity to Earth as well as its actual space velocity) as 10.3 arcseconds per year, which remains the largest proper motion of any star relative to the Sun.[18] This is likely to be the fastest star in terms of proper motion, as its 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)[19] 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 included in the List of IAU-approved Star Names.[20]

Barnard's Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator.[9] 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 argued 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.

Overview

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 Milky Way galaxy.[12] 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[11] (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.[21] 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.[22]

Barnard's Star, showing position every 5 years in the period 1985–2005
Distances to the nearest stars from 20,000 years ago until 80,000 years in the future

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.[17]

The radial velocity of Barnard's Star towards the Sun is measured from its blueshift 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 11,800 AD, when it will approach to within about 3.75 light-years.[7]

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.[23] 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☉),[7] and a radius 15% to 20% of that of the Sun.[9][24] 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.[9] 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.[25]

Barnard's Star has 10–32% of the solar metallicity.[10] 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.[10][22]

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 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.[26] 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,[27] and these measurements were apparently refined in a 1969 paper.[28] Later that year, Van de Kamp suggested that there were two planets of 1.1 and 0.8 MJ.[29]

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.[30] 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;[31] 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.[32]

Van de Kamp never acknowledged any error and published a further claim of two planets' existence as late as 1982;[33] 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.[34]

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.[35] 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.

Stars closest to the Sun, including Barnard's Star (25 April 2014)[36]

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.[37] Gatewood was thus able to show in 1995 that planets with 10 MJ were impossible around Barnard's Star,[32] in a paper which helped refine the negative certainty regarding planetary objects in general.[38] In 1999, work with the Hubble Space Telescope further excluded planetary companions of 0.8 MJ with an orbital period of less than 1,000 days (Jupiter's orbital period is 4,332 days),[35] while Kuerster determined in 2003 that within the habitable zone around Barnard's Star, planets are not possible with an "M sin i" value[39] 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).[22]

In 2012, 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 Lick and Keck Observatories and applying 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.[40][41]

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.[25] This mission was shut down in 2010.[42] ESA's similar Darwin interferometry mission had the same goal, but was stripped of funding in 2007.[43]

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.[44] Barnard's Star was chosen as a target partly because it was believed to have planets.[45]

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.[45] Along with detailed investigation of the star and any companions, the interstellar medium would be examined and baseline astrometric readings performed.[44]

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.[46] 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.[46]

1998 flare

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.[47] 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".[21]

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.[47] 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.[11]

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.[12]

Environment

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.[25] From Barnard's Star, the Sun would appear on the diametrically opposite side of the sky at coordinates RA=5h 57m 48.5s, 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.[48]

See also

Notes and references

  1. 1 2 Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  2. 1 2 3 4 5 6 7 8 Koen, C.; Kilkenny, D.; Van Wyk, F.; Marang, F. (2010). "UBV(RI)C JHK observations of Hipparcos-selected nearby stars". Monthly Notices of the Royal Astronomical Society. 403 (4): 1949. Bibcode:2010MNRAS.403.1949K. doi:10.1111/j.1365-2966.2009.16182.x.
  3. ↑ Gizis, John E. (1997). "M-Subdwarfs: Spectroscopic Classification and the Metallicity Scale". The Astronomical Journal. 113: 806. arXiv:astro-ph/9611222. Bibcode:1997AJ....113..806G. doi:10.1086/118302.
  4. 1 2 3 Cutri, R. M.; Skrutskie, M. F.; Van Dyk, S.; Beichman, C. A.; Carpenter, J. M.; Chester, T.; Cambresy, L.; Evans, T.; Fowler, J.; Gizis, J.; Howard, E.; Huchra, J.; Jarrett, T.; Kopan, E. L.; Kirkpatrick, J. D.; Light, R. M.; Marsh, K. A.; McCallon, H.; Schneider, S.; Stiening, R.; Sykes, M.; Weinberg, M.; Wheaton, W. A.; Wheelock, S.; Zacarias, N. (2003). "VizieR Online Data Catalog: 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)". VizieR On-line Data Catalog: II/246. Originally Published In: 2003yCat.2246....0C. 2246: 0. Bibcode:2003yCat.2246....0C.
  5. ↑ Bobylev, Vadim V. (March 2010). "Searching for Stars Closely Encountering with the Solar System". Astronomy Letters. 36 (3): 220–226. arXiv:1003.2160. Bibcode:2010AstL...36..220B. doi:10.1134/S1063773710030060.
  6. 1 2 3 Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051.
  7. 1 2 3 Bobylev, V. V. (March 2010), "Searching for stars closely encountering with the solar system", Astronomy Letters, 36 (3): 220–226, arXiv:1003.2160, Bibcode:2010AstL...36..220B, doi:10.1134/S1063773710030060
  8. ↑ Demory, B.-O.; et al. (October 2009), "Mass-radius relation of low and very low-mass stars revisited with the VLTI", Astronomy and Astrophysics, 505 (1): 205–215, arXiv:0906.0602, Bibcode:2009A&A...505..205D, doi:10.1051/0004-6361/200911976
  9. 1 2 3 4 5 6 Dawson, P. C.; De Robertis, M. M. (2004). "Barnard's Star and the M Dwarf Temperature Scale". The Astronomical Journal. 127 (5): 2909. Bibcode:2004AJ....127.2909D. doi:10.1086/383289.
  10. 1 2 3 Gizis, John E. (February 1997). "M-Subdwarfs: Spectroscopic Classification and the Metallicity Scale". The Astronomical Journal. 113 (2): 820. arXiv:astro-ph/9611222. Bibcode:1997AJ....113..806G. doi:10.1086/118302.
  11. 1 2 3 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. arXiv:astro-ph/9806276. Bibcode:1998AJ....116..429B. doi:10.1086/300420.
  12. 1 2 3 Riedel, A. R.; Guinan, E. F.; DeWarf, L. E.; Engle, S. G.; McCook, G. P. (May 2005). "Barnard's Star as a Proxy for Old Disk dM Stars: Magnetic Activity, Light Variations, XUV Irradiances, and Planetary Habitable Zones". Bulletin of the American Astronomical Society. 37: 442. Bibcode:2005AAS...206.0904R.
  13. ↑ "Barnard's Star and its Perturbations". Spaceflight. 11–12: 170. 1969.
  14. ↑ Perepelkin, E. (April 1927). "Einweißer Stern mit bedeutender absoluter Größe". Astronomische Nachrichten (in German). 230 (4): 77. Bibcode:1927AN....230...77P. doi:10.1002/asna.19272300406.
  15. ↑ Rukl, Antonin (1999). "Constellation Guidebook". Sterling Publishing: 158. ISBN 978-0-8069-3979-7.
  16. ↑ "Archived copy". Archived from the original on 2013-06-26. Retrieved 2013-05-05.
  17. 1 2 Kaler, James B. (November 2005). "Barnard's Star (V2500 Ophiuchi)". Stars. James B. Kaler. Archived from the original on 2006-09-05. Retrieved 2018-07-12.
  18. ↑ Barnard, E. E. (1916). "A small star with large proper motion". The Astronomical Journal. 29 (695): 181. Bibcode:1916AJ.....29..181B. doi:10.1086/104156.
  19. ↑ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  20. ↑ "Naming Stars". IAU.org. Retrieved 16 December 2017.
  21. 1 2 Croswell, Ken (November 2005). "A Flare for Barnard's Star". Astronomy Magazine. Kalmbach Publishing Co. Retrieved 2006-08-10.
  22. 1 2 3 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. arXiv:astro-ph/0303528. Bibcode:2003A&A...403.1077K. doi:10.1051/0004-6361:20030396.
  23. ↑ Matthews, R. A. J.; Weissman, P. R.; Preston, R. A.; Jones, D. L.; Lestrade, J.-F.; Latham, D. W.; Stefanik, R. P.; Paredes, J. M. (1994). "The Close Approach of Stars in the Solar Neighborhood". Quarterly Journal of the Royal Astronomical Society. 35: 1–9. Bibcode:1994QJRAS..35....1M.
  24. ↑ Ochsenbein, F. (March 1982). "A list of stars with large expected angular diameters". Astronomy and Astrophysics Supplement Series. 47: 523–531. Bibcode:1982A&AS...47..523O.
  25. 1 2 3 "Barnard's Star". Sol Station. Archived from the original on 20 August 2006. Retrieved August 10, 2006.
  26. ↑ "The Barnard's Star Blunder". Astrobiology Magazine. July 2005. Retrieved January 26, 2014.
  27. ↑ Van de Kamp, Peter. (1963). "Astrometric study of Barnard's star from plates taken with the 24-inch Sproul refractor". The Astronomical Journal. 68 (7): 515. Bibcode:1963AJ.....68..515V. doi:10.1086/109001. Archived
  28. ↑ Van de Kamp, Peter. (1969). "Parallax, proper motion acceleration, and orbital motion of Barnard's Star". The Astronomical Journal. 74 (2): 238. Bibcode:1969AJ.....74..238V. doi:10.1086/110799.
  29. ↑ Van de Kamp, Peter. (1969). "Alternate dynamical analysis of Barnard's star". The Astronomical Journal. 74 (8): 757. Bibcode:1969AJ.....74..757V. doi:10.1086/110852.
  30. ↑ Gatewood, George & Eichhorn, H. (1973). "An unsuccessful search for a planetary companion of Barnard's star (BD +4 3561)". The Astronomical Journal. 78 (10): 769. Bibcode:1973AJ.....78..769G. doi:10.1086/111480.
  31. ↑ John L. Hershey (1973). "Astrometric analysis of the field of AC +65 6955 from plates taken with the Sproul 24-inch refractor". The Astronomical Journal. 78 (6): 421. Bibcode:1973AJ.....78..421H. doi:10.1086/111436.
  32. 1 2 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. Archived from the original on 13 August 2006. Retrieved August 10, 2006. Full description of the Van de Kamp planet controversy.
  33. ↑ Van de Kamp, Peter. (1982). "The planetary system of Barnard's star". Vistas in Astronomy. 26 (2): 141. Bibcode:1982VA.....26..141V. doi:10.1016/0083-6656(82)90004-6.
  34. ↑ Kent, Bill (2001). "Barnard's Wobble" (PDF). Bulletin. Swarthmore College. Archived from the original (PDF) on June 2, 2010. Retrieved June 2, 2010.
  35. 1 2 Benedict, G. Fritz; McArthur, Barbara; Chappell, D. W.; Nelan, E.; Jefferys, W. H.; Van Altena, W.; Lee, J.; Cornell, D.; Shelus, P. J.; Hemenway, P. D.; Franz, Otto G.; Wasserman, L. H.; Duncombe, R. L.; Story, D.; Whipple, A. L.; Fredrick, L. W. (1999). "Interferometric Astrometry of Proxima Centauri and Barnard's Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions". The Astronomical Journal. 118 (2): 1086–1100. arXiv:astro-ph/9905318. Bibcode:1999AJ....118.1086B. doi:10.1086/300975.
  36. ↑ Clavin, Whitney; Harrington, J.D. (25 April 2014). "NASA's Spitzer and WISE Telescopes Find Close, Cold Neighbor of Sun". NASA. Archived from the original on 2014-04-26. Retrieved 2014-04-25.
  37. ↑ Endl, Michael; Cochran, William D.; Tull, Robert G.; MacQueen, Phillip J. (2003). "A Dedicated M Dwarf Planet Search Using the Hobby-Eberly Telescope". The Astronomical Journal. 126 (12): 3099–107. arXiv:astro-ph/0308477. Bibcode:2003AJ....126.3099E. doi:10.1086/379137.
  38. ↑ George D. Gatewood (1995). "A study of the astrometric motion of Barnard's star". Journal Astrophysics and Space Science. 223 (1): 91–98. Bibcode:1995Ap&SS.223...91G. doi:10.1007/BF00989158.
  39. ↑ "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.
  40. ↑ Gilster, Paul (August 16, 2012). "Barnard's Star: No Sign of Planets". Centauri Dreams. Retrieved April 11, 2018.
  41. ↑ Choi, Jieun; McCarthy, Chris; Marcy, Geoffrey W; Howard, Andrew W; Fischer, Debra A; Johnson, John A; Isaacson, Howard; Wright, Jason T (2012). "Precise Doppler Monitoring of Barnard's Star". The Astrophysical Journal. 764 (2): 131. arXiv:1208.2273. Bibcode:2013ApJ...764..131C. doi:10.1088/0004-637X/764/2/131.
  42. ↑ Marr, James (8 November 2010). "Updates from the Project Manager". NASA. Archived from the original on 2 March 2011. Retrieved January 26, 2014.
  43. ↑ "Darwin factsheet: Finding Earth-like planets". European Space Agency. October 23, 2009. Archived from the original on May 13, 2008. Retrieved September 12, 2011.
  44. 1 2 Bond, A. & Martin, A.R. (1976). "Project Daedalus — The mission profile". Journal of the British Interplanetary Society. 29 (2): 101. Bibcode:1976JBIS...29..101B. Archived from the original on October 20, 2007. Retrieved August 15, 2006.
  45. 1 2 Darling, David (July 2005). "Daedalus, Project". The Encyclopedia of Astrobiology, Astronomy, and Spaceflight. Archived from the original on 31 August 2006. Retrieved August 10, 2006.
  46. 1 2 Freitas, Robert A., Jr. (July 1980). "A Self-Reproducing Interstellar Probe". Journal of the British Interplanetary Society. 33: 251–264. Bibcode:1980JBIS...33..251F. Retrieved October 1, 2008.
  47. 1 2 Paulson, Diane B.; Allred, Joel C.; Anderson, Ryan B.; Hawley, Suzanne L.; Cochran, William D.; Yelda, Sylvana (2006). "Optical Spectroscopy of a Flare on Barnard's Star". Publications of the Astronomical Society of the Pacific. 118 (1): 227. arXiv:astro-ph/0511281. Bibcode:2006PASP..118..227P. doi:10.1086/499497.
  48. ↑ The Sun's apparent magnitude from Barnard's Star, assuming negligible extinction: .

Notes

    • "Barnard's Star". SolStation.
    • Darling, David. "Barnard's Star". The Encyclopedia of Astrobiology, Astronomy, and Spaceflight.
    • Schmidling, Jack. "Barnard's Star". Jack Schmidling Productions, Inc. Amateur work showing Barnard's Star movement over time.
    • Johnson, Rick. "Barnard's Star". Animated image with frames approx. one year apart, beginning in 2007, showing the movement of Barnard's Star.

    Coordinates: 17h 57m 48.5s, +04° 41′ 36″

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