Castor (star)
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Triple star castor (α Gem) |
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| CastorA and B | |||
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| Position of Castor in the constellation Gemini | |||
| Observation data epoch : J2000.0 |
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| AladinLite | |||
| Constellation | Twins | ||
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Vis. Brightness (total) |
1.59 likes | ||
| Astrometry | |||
| Radial velocity | +5.2 / −1.2 km / s | ||
| distance | 51.5 ± 1 ly (15.8 ± 0.3 pc ) |
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Absolute visual brightness (total) M vis |
0.54 mag | ||
| Age | approx. 200 million years | ||
| Proper movement | |||
| Rec. Share: | −206.33 mas / a | ||
| Dec. portion: | −148.18 mas / a | ||
| Individual data | |||
| Names | Aa / Ab , Ba / Bb , Ca / Cb | ||
| Observation data: | |||
| Right ascension | Aa / Ab | 7 h 34 m 35.885 s | |
| Ba / Bb | 7 h 34 m 36.105 s | ||
| Ca / Cb | 7 h 34 m 37.406 s | ||
| declination | Aa / Ab | + 31 ° 53 ′ 18.20 ″ | |
| Ba / Bb | + 31 ° 53 ′ 18.59 ″ | ||
| Ca / Cb | + 31 ° 52 ′ 09.79 ″ | ||
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Apparent brightness |
Aa / Ab | 1.88 m • 11.43 m | |
| Ba / Bb | 2.96 m • 9.41 m | ||
| Ca / Cb | 8.35 m • 8.67 m | ||
| Typing: | |||
| Spectral class | Aa / Ab | A1 Vm • M5 V | |
| Ba / Bb | A2 Vm • M2 / V | ||
| Ca / Cb | M0.5 Ve • M0.5 Ve | ||
| BV color index | Aa / Ab | 0.034 | |
| Ba / Bb | 0.002 | ||
| Ca / Cb | |||
| UB color index | Aa / Ab | 0.02 | |
| Ba / Bb | 0.02 | ||
| Ca / Cb | |||
| Astrometry: | |||
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Absolute visual brightness M vis |
Aa / Ab | 0.89 mag • 10.44 mag | |
| Ba / Bb | 1.97 mag • 8.41 mag | ||
| Ca / Cb | 7.36 mag • 7.67 mag | ||
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Absolute bolometric brightness M bol |
Aa / Ab | 0.75 mag • 9.09 mag | |
| Ba / Bb | 1.86 mag • 7.46 mag | ||
| Ca / Cb | 6.65 mag • 6.96 mag | ||
| Physical Properties: | |||
| Dimensions | Aa / Ab | 2.15 M ☉ • 0.5 M ☉ | |
| Ba / Bb | 1.7 M ☉ • 0.5 M ☉ | ||
| Ca / Cb | 0.57 M ☉ • 0.62 M ☉ | ||
| radius | Aa / Ab | 2.3 R ☉ • 0.79 R ☉ | |
| Ba / Bb | 1.6 R ☉ • 0.72 R ☉ | ||
| Ca / Cb | 0.76 R ☉ • 0.68 R ☉ | ||
| Luminosity | Aa / Ab | 37.8 L ☉ • 0.08839 L ☉ | |
| Ba / Bb | 14 L ☉ • 0.08839 L ☉ | ||
| Ca / Cb | 0.18766 L ☉ • 0.13982 L ☉ | ||
| Effective temperature | Aa / Ab | 9,230 K • 3,240 K | |
| Ba / Bb | 8,970 K • 3,580 K | ||
| Ca / Cb | 3,850 K • 3,850 K | ||
| Metallicity [Fe / H] | Aa / Ab | 950% | |
| Ba / Bb | 280% | ||
| Ca / Cb | |||
| Designations and catalog entries | |||
| Bayer-Bez. | α Geminorum | ||
| Flamsteed-Bez. | 66 Geminorum | ||
| Hipparcos catalog | HIP 36850 | ||
| WDS catalog | WDS J07346 + 3153 | ||
| ADS catalog | ADS 6175 | ||
| Other names |
YY Gem ( Ca / Cb ), FK5 287, GJ 278 | ||
| Bonn diam. | Aa / Ab | BD + 32 ° 1581 A | |
| Ba / Bb | BD + 32 ° 1581 B | ||
| Ca / Cb | BD + 32 ° 1582 | ||
| Bright Star Cat. | Aa / Ab | HR 2891 | |
| Ba / Bb | HR 2890 | ||
| Ca / Cb | MR | ||
| HD catalog | Aa / Ab | HD 60179 | |
| Ba / Bb | HD 60178 | ||
| Ca / Cb | HD 60179 C | ||
| SAO catalog | Aa / Ab | SAO 60198 | |
| Ba / Bb | SAO 60198 | ||
| Ca / Cb | SAO 60199 | ||
| Tycho catalog | Aa / Ab | TYC 2457-2407-1 | |
| Ba / Bb | TYC 2457-2407-2 | ||
| Ca / Cb | TYC 2453-1918-1 | ||
Castor (Latin spelling, literally translated " beaver ") or (Greek) Kastor is the second brightest star in the constellation Gemini . Although a bit fainter than its neighbor Pollux in the same constellation, it bears the Bavarian name α Geminorum in astronomy .
It has long been known as a telescopic double star , but it is actually a rarely occurring system of a total of 6 stars. It can be divided into 2 or 3 stars even in the small telescope; free-eyed , it appears like a white single star of the 1st magnitude (1.5 mag), while the closer Pollux (β Geminorum) shines reddish and with 1.2 mag appears about a quarter brighter.
The names "Castor and Pollux" stand for the pair of twins of the Dioscuri , who were moved to the starry sky by Zeus , with Castor being the mortal of the half-brothers in Greek mythology .
In ancient records and in astrology , the stars Castor and Pollux were always seen as belonging together. They mark the heads of the two inseparable twins in the constellation. Both stars together form a corner of the so-called winter hexagon .
A 6-fold star system
It has only been known for a few decades that Castor, who is about 51 light years away from us, represents such a complex multiple star system. The 6 stars are grouped in 3 pairs of stars, which circle closely around each other, while they circle their common center of gravity, the barycentre , on strongly elliptical orbits.
The three pairs of stars themselves are spectroscopic double stars , ie they cannot be separated even in the largest telescope, but only differentiated on the basis of periodic changes in their spectrum . Two of the double stars (i.e. 4 of the 6 stars) account for around 99.8 percent of the total radiation.
These two main components, referred to as α Gem A and α Gem B for short, are 1.9 mag and 2.9 mag bright and belong to the spectral classes A1 and A2m. Their orbital period is 420 years. With their angular distance of currently 4–5 ", they can be separated even in small telescopes. Both are, however, spectroscopic , i.e. not optically resolvable, double stars (called Aa, Ab, Ba, Bb), which belong to luminosity class V. Accordingly, all are 4 stars (just like our sun) so-called main sequence stars .
The third component, abbreviated as α Gem C, with the alternative designation YY Geminorum, is an eclipsing star consisting of two red dwarfs . YY Geminorum is only 8.07 mag bright and circles the Castor system at a distance of about 70 "from components A and B. The position angle to A is 164 °.
The three main stars can be observed with the telescope, whereby the separation of the closer together A and B becomes easier from year to year because they are currently diverging from the earth. At present (2011) a telescope with an aperture of about 5 centimeters is required for resolution .
Change of position angle and angular distance of the two main components Castor A and B:
| year | distance | Position angle |
|---|---|---|
| 1990 | 3 " | 76 ° |
| 1995 | 3.5 " | 69 ° |
| 2000 | 3.9 " | 65 ° |
| 2005 | 4.3 " | 61 ° |
| 2010 | 4.7 " | 57 ° |
| parameter | component | |||||
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| Aa | From | Ba | Port | Approx | Cb | |
| Spectral class | A1 V | ? (possibly M5 V) | A2 Vm | M2 V | M0.5 Ve | M0.5 Ve |
| Mass ( M ☉ ) | 2.15 | 0.4-0.6 | 1.7 | 0.4-0.6 | 0.62 | 0.57 |
| Radius ( R ☉ ) | 2.3 | ? | 1.6 | ? | 0.76 | 0.68 |