Home > Stars > Blue > Castor

Castor

Alpha (α) Geminorum
Blue Star
Non-Contrast Multiple Star System
Right Ascension 07h 34m 36s Best Seen 1/1 - 5/15
Declination 31° 53' 19" Magnitude 1.58
Constellation Gemini

Actual		 Compared
to Sun
Distance ~52 ly --
Orbital Period 467 yrs --
Separation 107 au
about the distance from the Sun to Pluto		 --
Actual Brightness -- 34 / 14
Surface Temperature 16,000 ºF 1.4
Diameter (average) -- --
Mass -- --
Surface Gravity -- --
Surface Composition (by mass) 74% hydrogen
24% helium
2% everything else		 same
Spectral Type A1 V / A2 Vm G2 V
Density (gram/cubic cm) -- --

What To Look For Through The Telescope

  1. Recommended eyepiece: 24mm or 40 mm.

  2. When people look through the telescope, two bright bluish points of light should be seen. A third faint star may also be seen.

Castor Information:

  1. Castor is the 17th brightest star in the Northern Hemisphere’s night sky (24th brightest in the night sky when including Southern Hemisphere’s bright stars).

  2. Through a telescope, it can be seen that Castor is more than one star - there are actually six stars in this system.

    1. The two visible bright stars (mag. 2.0 and 2.8) form a multiple star system. Both stars are very similar in appearance, but a medium sized telescope is necessary to split them.

      a. They are separated by about 107 AU (Pluto’s distance from our sun is 40 AU).

      b. This was the first real multiple star to be recognized. (Accounts differ as to who realized it first. Either Reverend John Michell realized it in 1767, or Sir William Herschel realized it and announced it in 1803. It may have been resolved as a double star as early as 1678 by Giovanni Cassini.)

      c. It was actually the first gravitationally bound objects observed beyond our solar system.

      d. A faint third star may be seen (mag. 9.0). It doesn’t seem to be physically associated with the two bright stars.

  3. Each of the three stars are themselves spectroscopic binaries -- a double-double-double star system!

    1. Castor A is really two almost identical stars which take about 9 days to orbit each other.
      Separation 0.12 AU (about the distance from Mercury to the Sun)
      Orbital Period 9 days

      a. Castor Aa
      Temperature 15,700 °F
      Diameter 1.66 Suns
      Luminosity 34 Suns
      Spectral Type white (main sequence)
      B-V Color Index +0.04, white
      Magnitude +2.88
      Mass 2.15 solar masses

      b. Castor Ab
      Spectral Type red dwarf
      Magnitude +9.83
      Mass 0.5 solar masses

    2. Castor B is really two stars which only take 3 days to orbit each other.
      Separation 0.03 AU (about 1/4th the distance from Mercury to the Sun)
      Orbital Period 2.93 days

      a. Castor Ba
      Temperature 16,500 °F
      Diameter 2.84 Suns
      Luminosity 56 Suns
      Spectral Type white (main sequence)
      B-V Color Index +0.03, white
      Magnitude +1.58
      Mass 1.7 solar masses

      b. Castor Bb
      Spectral Type red dwarf
      Mass 0.5 solar masses

    3. Castor C is really two, much cooler and lower mass stars (M1), which take a mere 20 hours to orbit each other.

      • A pair of red dwarfs that lie about 1,100 AU from the brighter pair.

      • Castor C orbits A & B about every 10,000 years.

      • This is one of only a few known eclipsing binary stars that are both red dwarfs.

      • One or both of them are flare stars, meaning they will sometimes suddenly give off large amounts of surface magnetic energy. (Proxima Centauri is also a flare star.)

      • Combined luminosity of 0.05 Suns
      Separation 0.018 AU
      Orbital Period 19.5 hours

      a. Castor Ca
      Temperature 5,4000 °F
      Magnitude +9.1
      Mass 0.62 solar masses

      b. Castor Cb
      Temperature 5,4000 °F
      Magnitude +9.6
      Mass 0.60 solar masses

  4. In ancient Greece, sailors looked to the twins Castor and Pollux for protection against the dangers of being at sea.

Gemini – the Twins

  1. Castor & Pollux are not related gravitationally. Pollux is 34 light years away, Castor is 52. Pollux is an orange giant star. Castor is a white star.

  2. Castor’s designation is Alpha Gem, but it is actually fainter than Beta Gem (Pollux). It’s likely that either Castor or Pollux has changed in luminosity over the centuries.

  3. In Greek mythology Castor & Pollux are really half-brothers, with their mother being Leda. Pollux’s father is Zeus, so Pollux is immortal. Castor’s father was King Tyndareus, king of Sparta. Their sister was Helen of Troy.

  4. Both Castor A & B are about 370 million years old. Castor C presents a problem however. At 370 million years old, Castor C should be 20% larger than it is. Otherwise, it looks to be only 30-85 million years old ... and we’re not sure how it got connected with this system.  
Home > Stars > Blue > Castor top
References
Item Updated Notes
Coordinates 2003-12-05 SIMBAD 07 34 36 +31 53 18
Hipparcos 07 34 36 +31 53 19
Magnitude 2003-12-05 SIMBAD: 1.59
Hipparcos: 1.58
Distance 2003-12-05 SIMBAD, Hipparcos parallax: 63.27mas = 51.6 ly
Actual Brightness 2003-12-05 from solstation site (see below)
Surface Temperature 2003-12-05 for A stars, 7500 - 10,000K
Diameter --
Mass --
Surface Gravity
Surface Composition 2003-12-05 OK for all stars
Spectral Type 2003-12-05 SIMBAD, Hipparcos: A2 Vm
Other Information 2003-12-05 http://www.astro.uiuc.edu/~kaler/sow/castor.html
http://www.solstation.com/stars2/castor6.htm
SIMBAD (see “Castor A” & “Castor B”)
Burnham’s Celestial Handbook
http://stars.astro.illinois.edu/sow/castor.html
Composition 2013-07-30 Changed the to 74% / 24% / 2%