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Castor

CASTOR (ALPHA (α) GEMINORUM)
BLUE STAR

Right Ascension:
07h 34m 36s

Best Seen:
1/1 - 5/15

Declination:
31º 53' 19"

Magnitude:
1.58

Computer File:
castor

Constellation:
Gemini

Actual

Compared to Sun

Distance

~ 52 l.y.

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Actual Brightness

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34 / 14

Surface Temperature

~ 16,000 ºF

~ 1.4

Diameter

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Mass

--

Surface Gravity

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Surface Composition (by mass)

74% hydrogen 24% helium 2% everything else

same

Spectral Type

A1 V / A2 Vm

G2 V

Density (gram/cubic cm)

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WHAT TO LOOK FOR THROUGH THE TELESCOPE:

Recommended eyepiece: 24mm or 40 mm.
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:

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).
Through a telescope, it can be seen that Castor is more than one star.

a. The two bright stars (mag. 2.0 and 2.8) form a multilple star system.

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

ii. 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.)

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

Each of the three stars are themselves spectroscopic binaries!

a. Star A is really two almost identical stars which take about 9 days to orbit each other.

b. Star B is really two stars which only take 3 days to orbit each other.

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

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

Item
Updated Notes

Coordinates 12-05-2003
SIMBAD 07 34 36 +31 53 18 Hipparcos 07 34 36 +31 53 19

Magnitude
12-05-2003
SIMBAD: 1.59 Hipparcos: 1.58

Distance
12-05-2003
SIMBAD, Hipparcos parallax: 63.27mas = 51.6 ly

Actual Brightness
12-05-2003
from solstation site (see below)

Surface Temperature 12-05-2003
for A stars, 7500 - 10,000K

Diameter

Mass

Surface Gravity

Surface Composition 12-05-2003
OK for all stars

Spectral Type
12-05-2003
SIMBAD, Hipparcos: A2 Vm

Other Information
12-05-2003
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

Composition 7/30/13 Changed the to 74% / 24% / 2%