Kapteyn's Star

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Kapteyn's Star
Observational Data
Designation HD 33793
VZ Pictoris
Right ascension 05h 11m 40.58112s[1][2]
Declination -45° 01' 06.2899"[1][2]
Constellation Pictor
Type of object Red dwarf
Magnitude Apparent Mag: +8.853[1][3]
Absolute Mag: +10.88
Distance from Earth 12.78 ly
Radial velocity 245.19±0.1 km/s[1][4]
Proper motion RA: 6505.08 mas/yr[1][2]
Dec.: -5730.84 mas/yr[2]
Parallax 255.66±0.91 mas[1][2]

Kapteyn's Star is one of the closest stars to our Solar System. The star is noted for its unusual features compared to other local stars, such as its retrograde galactic orbital path, extreme age, and origins outside the galactic plane. The star was first discovered by Jacobus Cornelius Kapteyn (1851-1922) and Robert Thorburn Ayton Innes in 1897,[5] and was found to have the greatest proper motion of any star until the discovery of Barnard's Star.

Kapteyn's Star itself is classified as a M0-1.5V(dwarf) or VI(subdwarf) halo star.[6] Because the star is a dim M class dwarf with only 4/1000th of the sun's brightness, it has an apparent magnitude of 8.85, rendering it invisible to the unaided eye. This is despite being located only 12.77 years away, making it the 25th closest star. Kapteyn's Star has only 10.5 percent of the metallicity of Sol, 29 to 39.3 percent of Sol's mass,[7] and less than third (29 to 32 percent) of its diameter.[5] It is also a variable star, and thus is also known by its variable star designation of VZ Pictoris.

Because Kapteyn's Star is so metal-poor (lacking in elements heavier then hydrogen and helium), it appears more bluish and dimmer then the typical younger, main sequence M-class dwarf.[8]

Very few such halo stars are located near our solar system, with such stars accounting for only 0.1 to 0.2 percent of all stars that are near the sun. Kapteyn's Star is believed to be the closest of these. The star is a member of a local cluster of halo stars that is very thinly spread out over a distance of some 6,500 to 9,800 light-years stretching towards the galactic halo.[9] Kapteyn's Star is unusual because most such stars lie far above or below the galactic plane. Like other halo stars, Kapteyn's Star does have an extremely elliptical galactic orbit and is believed to be among the oldest stars in the galaxy and the region around it, estimated to be some 10 billion years old or older.

In October 2009, astronomers observing the group of stars that Kapteyn's star is a member of hypothesized that Kapteyn's Star itself is one of 14 halo stars that share the same elemental abundance as many stars in the globular cluster Omega Centauri, some 17,000 light years away.[10] Because Omega Centauri has stars of different age groups, it is theorized that it is the core remainder of a dwarf galaxy that since merged with the Milky Way. Kapteyn's Star may be one of the ancient stars of Omega Centauri perturbed into a retrograde orbit from this galactic merger.[11]

There are no known sub-stellar companions around Kapteyn's Star at this time. However the star is classified as a "Tier 1" target star for NASA's optical Space Interferometry Mission that will search for rocky worlds, due to its proximity to our system. For an Earth-like world to have liquid water on its surface, the world would have to be around 0.26 AUs from the star, or about half of Mercury's distance from the sun. Such a world would be tidally locked to the star.[5]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 Kapteyn's Star from the SIMBAD Astronomical Database
  2. 2.0 2.1 2.2 2.3 2.4 van Leeuwen, F. (2007). Validation of the new Hipparcos reduction. Astronomy & Astrophysics, 474(2), pp.653-664. arXiv:0708.1752
  3. Koen, C., Kilkenny, D., van Wyk, F. and Marang, F. (2010). UBV(RI)C JHKobservations of Hipparcos-selected nearby stars. Monthly Notices of the Royal Astronomical Society, 403(4), pp.1949-1968.
  4. Nidever, D., Marcy, G., Butler, R., Fischer, D. and Vogt, S. (2002). Radial Velocities for 889 Late‐Type Stars. The Astrophysical Journal Supplement Series, 141(2), pp.503-522. arXiv:astro-ph/0112477
  5. 5.0 5.1 5.2 Kapteyn's Star from solstation.com
  6. Gizis, J. (1997). M-Subdwarfs: Spectroscopic Classification and the Metallicity Scale. The Astronomical Journal, 113, p.806. Online
  7. Woolf, V. and Wallerstein, G. (2004). Chemical abundance analysis of Kapteyn's Star. Monthly Notices of the Royal Astronomical Society, 350(2), pp.575-579. Online
  8. Wing, R., Dean, C. and MacConnell, D. (1976). The temperature, luminosity, and spectrum of Kapteyn's star. Astrophysical Journal, 205, pp.186-193. Online
  9. Eggen, O. (1996). The Ross 451 Group of Halo Stars. The Astronomical Journal, 112, p.2661. Online
  10. Bekki, K. and Freeman, K. (2005). Formation of ω Centauri from an Ancient Nucleated Dwarf Galaxy. Highlights of Astronomy, 13, p.196. Online
  11. Boer, E., Freeman, K. and Williams, M. (2009). Evidence of Omega Cen tidal debris in the Kapteyn moving group. Proceedings of the International Astronomical Union, 5(S265), pp.263-266. arXiv:0910.3735