Kapteyn's star

Kapteyn's star

Kapteyn's Star

The red dot shows the location of Kapteyn's Star in Pictor.
Observation data
Epoch J2000      Equinox J2000
Constellation Pictor
Right ascension 05h 11m 40.58112s[1]
Declination −45° 01′ 06.2899″[1]
Apparent magnitude (V) 8.853[2]
Spectral type sdM1[2]
U−B color index +1.21[3]
B−V color index +1.57[3]
Variable type BY Dra[4]
Radial velocity (Rv) +245.2[5] km/s
Proper motion (μ) RA: +6,505.08[1] mas/yr
Dec.: -5,730.84[1] mas/yr
Parallax (π) 255.66 ± 0.91[1] mas
Distance 12.76 ± 0.05 ly
(3.91 ± 0.01 pc)
Mass 0.274[6] M
Radius 0.291 ± 0.025[7] R
Surface gravity (log g) 4.96[6] cgs
Temperature 3,570[6] K
Metallicity [Fe/H] –0.99 ± 0.04[8] dex
Rotational velocity (v sin i) 9.15[9] km/s
Age ~10[6] Gyr
Other designations
VZ Pictoris, GJ 191, HD 33793, CD-45°1841, CP(D)-44°612, SAO 217223, LHS 29, LTT 2200, LFT 395, GCTP 1181, HIP 24186.[3]
Database references

Kapteyn's Star is a class M1 red dwarf about 12.76 light years from Earth in the southern constellation Pictor, and the closest halo star to the Solar System. With a magnitude of nearly 9 it is visible through binoculars or a telescope.[3]

Its diameter is 30% of the Sun's, but its luminosity just 0.1% that of the Sun's. It may have once been part of the globular cluster Omega Centauri, itself a likely dwarf galaxy swallowed up by the Milky Way in the distant past. The discovery of two planets—Kapteyn b and Kapteyn c—has been announced in 2014.

Comparison with Sun, Jupiter and Earth.
German text in illustration: Kapteyns Stern/Kapteyn's Star; Erde/Earth; Sonne/Sun; Jupiter/Jupiter

History of observations

Jacobus Cornelius Kapteyn, the Dutch astronomer who discovered Kapteyn's star

The star now known as Kapteyn's Star was originally cataloged by the Dutch astronomer, Jacobus Kapteyn, in 1898.[10] Also, since it has identifier CPD-44 612, it was included in Cape photographic Durchmusterung for the equinox 1875 (-38 to -52) by David Gill and Jacobus Cornelius Kapteyn in 1897.[11] This catalogue was based on Gill's observations from the Cape Observatory in 1885—1889 and was created in collaboration with Kapteyn. While he was reviewing star charts and photographic plates he noted the star's very high proper motion of more than 8 arc seconds per year. Later, the star became referred to as Kapteyn's Star, in honor of its discoverer.[12] At that time, it had the highest proper motion of any star known, dethroning Groombridge 1830. With the discovery of Barnard's Star in 1916,[13] Kapteyn's Star dropped to second place, where it remains.[6][12] In 2014, two super-Earth planet candidates in orbit around the star were announced.[14]


Kapteyn's Star distance estimates
Source Parallax, mas Distance, pc Distance, ly Ref.
Woolley et al. (1970) 256 ± 7 3.91+0.11
Gliese & Jahreiß (1991) 258.3 ± 7.6 3.87+0.12
12.6 ± 0.4 [16]
van Altena et al. (1995) 256.7 ± 9.2 3.9+0.14
Perryman et al. (1997) (Hipparcos) 255.26 ± 0.86 3.918 ± 0.013 12.78 ± 0.04 [18]
Perryman et al. (1997) (Tycho) (absents) [19]
van Leeuwen (2007) 255.66 ± 0.91 3.911 ± 0.014 12.76 ± 0.05 [20]
RECONS TOP100 (2012) 255.67 ± 0.91[note 1] 3.911 ± 0.014 12.76 ± 0.05 [21]

Non-trigonometric distance estimates are marked in italic. The best estimate is marked in bold.


Based upon parallax measurements with the Hipparcos astrometry satellite,[1] Kapteyn's Star is at a distance of 12.76 light-years (3.91 parsecs) from the Earth.[1] It came within 7.00 light-years (2.15 parsecs) of the Sun about 10,800 years ago and has been moving away since that time.[22] The star is between one quarter and one third the size and mass of the Sun and much cooler at about 3500 °K, with some disagreement in the exact measurements between different observers.[6] The stellar classification is sdM1,[2] which indicates that it is a subdwarf star with a luminosity lower than that of a main sequence star at the same spectral type of M1. The abundance of elements other than hydrogen and helium, what astronomers term the metallicity, is about 14% of the abundance in the Sun.[8][23] It is a variable star of the BY Draconis type with the identifier VZ Pictoris. This means that the luminosity of the star changes because of magnetic activity in the chromosphere coupled with rotation moving the resulting star spots into and out of the line of sight with respect to the Earth.[4]

Kapteyn's Star is distinctive in a number of other regards: it has a high radial velocity,[12] orbits the Milky Way retrograde,[6] and is the nearest known halo star to the Sun.[24] It is a member of a moving group of stars that share a common trajectory through space, named the Kapteyn moving group.[25] Based upon their element abundances, these stars may once have been members of Omega Centauri, a globular cluster that is thought to be the remnant of a dwarf galaxy that merged with the Milky Way. During this process, the stars in the group, including Kapteyn's Star, may have been stripped away as tidal debris.[6][26][27]


The star is at an apparent magnitude of 9 and is visible through binoculars or a telescope in the constellation of Pictor, in the southern sky, on a clear night.[28]

Planetary system

In 2014, Kapteyn's Star was announced to host two low-mass planets, Kapteyn b and Kapteyn c. Kapteyn b is the oldest-known potentially habitable planet, estimated to be possibly 11 billion years old.[14]

The planets are close to a 5:2 period commensurability, but resonances could not be confirmed at the time. Dynamical integration of the orbits suggests[14] that the pair of planets are in a dynamical state called apsidal co-rotation, which usually implies that the system is dynamically stable over very long time-scales.[29] The announcement of the planetary system was accompanied by a science-fiction short-story, "Sad Kapteyn", written by writer Alastair Reynolds.[30]

The Kapteyn's star planetary system[14]
(in order from star)
Mass Semimajor axis
Orbital period
Eccentricity Inclination Radius
b ≥4.8+0.9
c ≥7.0+1.2

See also


  1. ^ a b c d e f g van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics 474 (2): 653–664,  .
  2. ^ a b c Koen, C. et al. (April 2010), "UBV(RI)C JHK observations of Hipparcos-selected nearby stars", Monthly Notices of the Royal Astronomical Society 403 (4): 1949–1968,  
  3. ^ a b c d "V* VZ Pic -- Variable Star", SIMBAD ( .
  4. ^ a b "VZ Pic",  
  5. ^ Nordström, B. et al. (May 2004), "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ˜14 000 F and G dwarfs", Astronomy and Astrophysics 418 (3): 989–1019,  
  6. ^ a b c d e f g h Kotoneva, E. et al. (2005), "A study of Kapteyn's star", Astronomy & Astrophysics 438 (3): 957–962,  .
  7. ^ 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,  
  8. ^ a b Woolf, Vincent M.; Wallerstein, George (January 2005), "Metallicity measurements using atomic lines in M and K dwarf stars", Monthly Notices of the Royal Astronomical Society 356 (3): 963–968,  
  9. ^ Houdebine, E. R. (September 2010), "Observation and modelling of main-sequence star chromospheres - XIV. Rotation of dM1 stars", Monthly Notices of the Royal Astronomical Society 407 (3): 1657–1673,  
  10. ^  .
  11. ^ Dictionary of Nomenclature of Celestial Objects, CPD entry. SIMBAD. Centre de Données astronomiques de Strasbourg.
  12. ^ a b c Kaler, James B. (2002), "Kapteyn's Star", The Hundred Greatest Stars, Copernicus Books, pp. 108–109 .
  13. ^  .
  14. ^ a b c d Anglada-Escudé, Guillem et al. (2014), Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf,  
  15. ^ Woolley R.; Epps E. A.; Penston M. J.; Pocock S. B. (1970). "Woolley 191". Catalogue of stars within 25 parsecs of the Sun. Retrieved 2014-10-21. 
  16. ^ Gliese, W. and Jahreiß, H. (1991). "Gl 191". Preliminary Version of the Third Catalogue of Nearby Stars. Retrieved 2014-10-21. 
  17. ^ Van Altena W. F., Lee J. T., Hoffleit E. D. (1995). "GCTP 1181". The General Catalogue of Trigonometric Stellar Parallaxes, Fourth Edition. Retrieved 2014-10-21. 
  18. ^ Perryman et al. (1997). "HIP 24186". The Hipparcos and Tycho Catalogues. Retrieved 2014-10-21. 
  19. ^ Perryman et al. (1997). "HIP 24186". The Hipparcos and Tycho Catalogues. Retrieved 2014-10-21. 
  20. ^ van Leeuwen F. (2007). "HIP 24186". Validation of the new Hipparcos reduction. Retrieved 2014-10-21. 
  21. ^ "RECONS TOP100". THE ONE HUNDRED NEAREST STAR SYSTEMS brought to you by RECONS (Research Consortium On Nearby Stars). 2012. Retrieved 2014-10-21. 
  22. ^ Bobylev, Vadim V. (March 2010), "Searching for Stars Closely Encountering with the Solar System", Astronomy Letters 36 (3): 220–226,  .
  23. ^ The abundance is given by taking the metallicity to the power of 10. From Woolf and Wallerstein (2005), [M/H] ≈ –0.86 dex. Thus:
    10–0.86 = 0.138
  24. ^ Woolf, V. M.; Wallerstein, G. (2004), "Chemical abundance analysis of Kapteyn's Star",  .
  25. ^ Eggen, O. J. (December 1996), "The Ross 451 Group of Halo Stars", Astronomical Journal 112: 2661,  
  26. ^ Wylie-de Boer, Elizabeth; Freeman, Ken; Williams, Mary (February 2010), "Evidence of Tidal Debris from ω Cen in the Kapteyn Group", The Astronomical Journal 139 (2): 636–645,  
  27. ^ "Backward star ain't from round here",  
  28. ^ "Kapteyn b and c: Two Exoplanets Found Orbiting Kapteyn’s Star". Sci-News. Retrieved 23 July 2014. 
  29. ^ Michtchenko, Tatiana A. et al. (August 2011), "Modeling the secular evolution of migrating planet pairs", Monthly Notices of the Royal Society 415: 2275,  
  30. ^ "Sad Kapteyn", Science fiction story released with the announcement of planetary system, Jun 4, 2014, retrieved 2014-06-04 


  1. ^ Weighted parallax based on parallaxes from van Altena et al. (1995) and van Leeuwen (2007).

Additional reading

External links

  • SolStation.com: Kapteyn's Star
  • Press release on planetary system

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