Kapteyn's Star

According to Ken Croswell, the extremely high proper motion of this star was discovered in 1897 by Jacobus Cornelius Kapteyn (1851-1922) of the University of Groningen and by Robert Thorburn Ayton Innes (1861-1933) of Edinburgh, Scotland. Innes was observing from the Cape of Good Hope in South Africa, probably with the 7-inch refractor at the Royal Observatory. Kapteyn, who lacked a telescope, volunteered to measure photographic plates taken by David Gill (1843-1914), also at the Cape Observatory. As a result, Kapteyn developed the Cape Photographic Durchmusterung or CP(D), a catalog of 454,875 southern stars and values for the density of stars in space as a function of distance, brightness, and spectral class. In addition, his discovery of "star streaming" led to the concept of galactic rotation. The star appears to have been renamed for Kapteyn in the 1920s, after his death, possibly because he was the first to notice that the faint star listed in an old catalogue as "Cordoba Zone 5 hours 243" and missing from one of Gill's photographic plates until Innes found a star matching its description to the east of its original position. (More on Kapteyn).

VLTI, ESO




Larger and jumbo images.




Kapteyns Star is larger than
Alpha Centauri C (Proxima)
but significantly smaller
than Sol (more from ESO).

Due to Kapteyn's proximity to Sol and its deduced ancient origin from outside the galactic disk, the star has been an object of high interest among astronomers. It has been selected as a "Tier 1" target star for NASA's optical Space Interferometry Mission (SIM) to detect a planet as small as three Earth-masses within two AUs of its host star (and so some summary system information and images of Kapteyn's Star are available from the SIM Teams).

The Star

Kapteyn's Star is a dim red subdwarf (sdM1.5 /VI) and a halo star (John E. Gizis, 1997, page 809), a member of the Milky Way galaxy's bright halo. The star has around 7.4 percent of Sol's abundance iron and a mean weight metallicity abundance of elements heavier than hydrogen of almost 10.5 percent of Sol's (Woolf and Wallerstein, 2004). Due to the scarcity of heavier elements, the star looks a bit more bluish than a main sequence red dwarf of class M. It may have around 29 to 39.3 percent of Sol's mass (Ken Croswell, 2005; and RECONS), around a third (29 to 32 percent) of its diameter, and less than 4/1,000th of its luminosity. It is a variable star designated VZ Pictoris. Some useful star catalogue numbers and designations for this star are: VZ Pic, Gl 191, Hip 24186, HD 33793, CD-45 1841, CP(D)-44 612, SAO 217223, LHS 29, LTT 2200, LFT 395, GC 6369, U 628, and Cordova Zone 5 hours 243.

Accounting for the great infrared output of M-stars like Kapteyn's Star, the equivalent orbital distance for an Earth-type planet may move out to around 0.26 AUs (summary system informationSIM summary data). At a little over half of Mercury's distance in the Solar System, however, the rotation of the planet could become tidally locked with the star so that one side would have perpetual daylight with the other in darkness. Assuming that Kapteyn's Star has at least 39.3 percent of Sol's mass, a small Earth-type rocky planet would complete its orbit the star in around 77 days.

Subdwarfs and Halo Stars

Subdwarfs, such as nearby Kapteyn's Star, are dimmer and more bluish than younger main-sequence dwarf stars (Wing et al, 1976). They have a lower "metals" content of elements heavier than helium. This low metallicity is thought to be due to their birth in an earlier age (or region) of the galaxy when relatively few supernovae had as yet spewed their metals into surrounding dust clouds (John E. Gizis, 1997). Astronomers had found 15 subdwarfs in the Solar neighborhood by 1998 (Fuchs and Jahreiß, 1998

Most of the stars in the central bulge and in the globular clusters of the galactic halo are old, low metals stars, and halo stars account for only 0.1 to 0.2 percent of the stars near Sol (Ken Croswell, 1995, pp. 62-63). Kapteyn's Star is the closest known halo star, a member of a local cluster of halo stars called the Kapteyn's star group that may be part of a local supercluster of halo stars distributed across 2,000 to 3,000 parsecs (6,500 to 9,800 light-years) into the Milky Way's galactic halo (Olin Jeuck Eggen, 1996).

Halo stars are distributed somewhat spherically around the galactic core but most members of the halo lie far above or far below the galactic plane. With extremely elliptical galactic orbits, they can move as far away as 100,000 light-years from the galactic center and as close a few thousand ly. Including the stars of the distant globular clusters, halo stars are among the galaxy's oldest, thought to be mostly 10 billion years and older. While halo stars are only very weakly concentrated towards the galactic plane, they exhibit a strong concentration towards and including the galactic nucleus but with highly eccentric orbits. As a group, these stars as a group show little if any net rotation around the galaxy, and so a halo member has a very negative V velocity ("retrograde direction"), since the Sun's motion around the galactic center is in the positive V direction.

These stars contain a very low metals abundance of one to 10 percent of Sol's (with a mean around two percent), but Kapteyn's metals deficiency as an M-type subdwarf is surprisingly mild compared with type K and G subdwarfs (J. R. Mould, 1976). While halo stars may total as few as 0.1 percent of the stars in the solar neighborhood, they include local subdwarfs, Kapteyn's Star and Groombridge 1830 (a G8VIp with "superflares" that is now believe to be a single star -- without an M-type flare star companion). Also called Population II stars because of their later discovery, this group also includes RR Lyrae variables with periods greater than 12 hours, subdwarfs and other extremely metal-poor stars, and some red giants.