Kapteyn's Star

© Steve Quirk, Views from Frog Rock
(used with permission).





Apparent motion of Kapteyn's Star over
three years.

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).

JPL, CalTech, NASA



Larger illustration of NASA's
Space Interferometry Mission (SIM).



Astronomers have identified Kapteyn's Star
as a "Tier 1" target for NASA's optical
SIM mission, now indefinitely delayed.

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	Orbital Distance (a=AUs)	Orbital Period (P=days)	Orbital Eccentricity (e)	Orbital Inclination (i=degrees)	Mass (Earths)	Diameter (Earths)	Density (Earths)	Surface Gravity (Earths)	Metallicity (Solar)
Kapteyn's Star	0.0	...	...	...	96,600-129,900	31.7-34.9	...	...	0.074
Inner H.Z. Edge?	0.126	29.8	0.0	?	...	...	...	...	...
Planet "b"	0.168	48.6	0.21	?	=>4.8	...	...	...	...
Outer H.Z. Edge?	0.236	76.5	0.0	?	...	...	...	...	...
Planet "c"	0.311	121.5	0.23	?	=>7.0	...	...	...	...

The Star

Kapteyn's Star is a dim red subdwarf or main sequence (sdM0-1.5 or V), halo star (John E. Gizis, 1997, page 809; and NASA Star and Exoplanet Database), which is thought to be originally a member of the Milky Way galaxy's luminous halo. The star has around 7.4 percent of Sol's abundance of iron and a weighted mean metallicity abundance of elements heavier than hydrogen and helium 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 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.

VLTI, ESO




Larger and jumbo images.




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

Habitable Zone

According to a 2014 analysis, the inner edge of Kapteyn's habitable zone is located relatively close from the star at around 0.126 AUs from the star, while the outer edge lies farther out at around 0.236 AUs (Anglada-Escudé et al, 2014, Table 2 on page 4). Accounting for the relatively greater infrared output of M-stars like Kapteyn's Star, the distance from Kapteyn's where an Earth-type planet would have liquid water on its surface is centered around only 0.158 AU -- well within the orbital distance of Mercury in the Solar System. Assuming that Kapteyn's has 39 percent of Sol's mass, such a planet would complete its orbit around the star in less than 37 days at 0.158 AU -- less than six weeks.

Planet "b"

A super-Earth, Kapteyn b might support liquid water on is surface. It has at least 4.8 Earth-masses and completes its orbit within only 48.6 days at an average orbital distance of 0.17, with an orbital eccentricity of 0.21. (CIS news release; and Anglada-Escudé et al, 2014).

Planet "c"

An even larger super-Earth, Kapteyn c is more massive at a minimum of 7.0 Earth-masses. Its year lasts 121.5 days at an average orbital distance of 0.31, with an orbital eccentricity of 0.23. The planet, however, is located far enough from its dim host start that it should be too cold to support liquid water. (CIS news release; and Anglada-Escudé et al, 2014).

A subdwarf and halo star? - 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" or stellar "moving 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.

Ata Sarajedini and
Giampaolo Piotto,
ESA, NASA






Larger and jumbo
images.






Kapteyn's Star may
have been born as
a member of the
Omega Centauri
Globular Cluster
before being perturbed
into orbit around
the Milky Way (more).

In an October 20, 2009 pre-print, a group of astronomers looking at 16 stars in the Kapetyn stellar moving group hypothesized that Kapteyn's Star is one of 14 halo stars in the group with the same elemental abundance as many members of the Omega Centauri Globular Cluster, which lies some 17,000 light-years from Sol. The most luminous globular cluster observed from Earth, 12-billion-year-old Omega Centauri has stars of different age and trace elemental abundance groups, which suggest that the cluster may be the nucleus of a dwarf galaxy that merged with the Milky Way (Bekki and Freeman, 2005; and 2003). During the ancient merger, most of the outlying stars of the dwarf galaxy were perturbed into new retrograde orbits around the Milky Way's core, which appears to include Kapteyn's Star based on kinematic and chemical analysis (Wylie-de Boer et al, 2009; and Ken Croswell, New Scientist, November 12, 2009).

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