Kapteyn's Star
| Observation data Epoch J2000 Equinox J2000 |
|
|---|---|
| Constellation | Pictor |
| Right ascension | 05h 11m 40.58112s |
| Declination | −45° 01′ 06.2899″ |
| Apparent magnitude (V) | 8.853 |
| Characteristics | |
| Spectral type | sdM1 |
| U−B color index | +1.21 |
| B−V color index | +1.57 |
| Variable type | BY Dra |
| Astrometry | |
| Radial velocity (Rv) | +245.2 km/s |
| Proper motion (μ) |
RA: +6,505.08 mas/yr Dec.: -5,730.84 mas/yr |
| Parallax (π) | 255.66 ± 0.91mas |
| Distance | 12.76 ± 0.05 ly (3.91 ± 0.01 pc) |
| Absolute magnitude (MV) | 10.89 |
| Details | |
| Mass | 0.274 M☉ |
| Radius | 0.291±0.025 R☉ |
| Luminosity (bolometric) | 0.012 L☉ |
| Luminosity (visual, LV) | 0.004 L☉ |
| Surface gravity (log g) | 4.96 cgs |
| Temperature | 3550±50 K |
| Metallicity [Fe/H] | −0.99±0.04 dex |
| Rotational velocity (v sin i) | 9.15 km/s |
| Age | ~11 Gyr |
| Other designations | |
| Database references | |
| SIMBAD | The star |
| planet b | |
| planet c | |
Kapteyn's Star is a class M1 red subdwarf about 12.76 light years from Earth in the southern constellation Pictor; it is the closest halo star to the Solar System. With a magnitude of nearly 9 it is visible through binoculars or a telescope.
Its diameter is 30% of the Sun's, but its luminosity just 1.2% 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 — was announced in 2014.
Attention was first drawn to what is now known as Kapteyn's Star by the Dutch astronomer Jacobus Kapteyn in 1898. Under the name CPD-44 612 it was included in the Cape photographic Durchmusterung for the equinox 1875 (-38 to -52) by David Gill and Jacobus Cornelius Kapteyn in 1897. 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, Kapteyn noted that a star, previously catalogued in 1873 by B.A. Gould as C.Z. V 243, seemed to be missing. However, R.T.A. Innes found an uncatalogued star about 15 arc seconds away from the absent star's position. It became clear that the star had, in fact, a very high proper motion of more than 8 arc seconds per year and had moved significantly in the meantime. Later, CPD-44 612 came to be referred to as Kapteyn's Star although it is clear that equal credit should be accorded to Robert Innes. At the time of its discovery, it had the highest proper motion of any star known, dethroning Groombridge 1830. With the discovery of Barnard's Star in 1916, Kapteyn's Star dropped to second place, where it remains. In 2014, two super-Earth planet candidates in orbit around the star were announced.
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