Gemini, the Twins, are really only
half-brothers. They share the same mother (Leda) but have different
fathers. Castor's father was a king of Sparta, Tyndareus - who would
be chased from his throne but later rescued by Heracles (who nevertheless
wound up killing him).
The father of Pollux was none other than Zeus, or Jupiter. Zeus visited
Leda on her wedding night in the guise of a swan. Thus the twins would
be born. (In fact two twins came from this double union, but let's not
complicate the matter even more...)
It should be said, however, that Pollux had a sister as well by Leda and
Zeus: the beautiful Helen, who would become Queen of Sparta, and whose
abduction by Paris would lead to the Trojan War.
Castor was a great horseman and fighter. One of his pupils was
Heracles. Like Heracles, both Castor and Pollux would become Argonauts,
that is, join Jason in his quest for the golden fleece.
The twins spent their time raiding cattle and abducting young women, as
Greek gods were wont to do. During one such cattle raid a cousin (Idas)
became enraged at Castor and killed him. Zeus threw a thunderbolt at
Idas, killing him instantly.
Since Pollux was the son of Zeus, he was immortal. But Pollux mourned
over his brother's loss to such a point that he wanted to follow Castor
into Hades. Zeus was so stricken by Pollux's love for his brother, he
allowed them both to share Hades and Olympus, (on alternate days). Later
Greek writers had Zeus place the two in the heavens side by side.
The stars of Gemini include two of the most
recognisable in the heavens: the twins Castor and Pollux.
Castor (alpha Geminorum) is the slightly dimmer star. It has a visual magnitude of 1.93 and is 52 light years distant. It isn't a particularly large star, at about twice the Sun's diameter. The star is a noted binary, discussed below.
Pollux is the brighter of the two stars with a visual magnitude
of 1.16 and a distance of 33.7 light years. It is also considerably
larger, with an estimated diameter of about ten Suns.
Castor and Pollux are 4.5 degrees apart, which helps observers estimate
separation distances between other stars.
Epsilon Geminorum is a supergiant at about 30 Sun diameters. This star
may be as far away as 950 light years, but the combination of visual and
absolute magnitudes suggests a much closer star, at only 190 light years.
Zeta Geminorum is the most distant of the bright stars in this
constellation, at over 1200 light years. This is a cepheid variable
Eta Geminorum is a red giant, about 50 times the size of the Sun,
at a distance of 280 light years. It is a visual binary and a variable
Double stars in Gemini
Alpha Geminorum is a well-known binary with the companion currently (2000.0)
at a PA of 65º and separation 3.9". The visual magnitudes are 1.9 and 3.0.
There is some disagreement over the precise period of the companion;
one observer has it at 420 years, another at 511. More recent measurements
put the orbit at 467 years and the orbit we've prepared uses this revised value.
This was the first binary system that was so recognised, in 1802 (or
1803, accounts vary) by William Herschel. However there is considerable
speculation that the star was a known double long before that, perhaps
even a century before Herschel made his announcement.
The companion, Castor B, is also a spectroscopic binary, with its
companion revolving around Castor B every three days.
In fact, the entire system is comprised of six stars, including a red dwarf,
Castor C, which slowly revolves around both Castor A and Castor B. This
star is also a variable (and therefore catalogued as YY Gem).
Delta Geminorum: visual magnitudes 3.5, 8.2, PA 225º, separation
5.8". The period is estimated at 1200 years; the companion is an orange dwarf which may be difficult to resolve in smaller telescopes.
Eta Geminorum is a visual binary that takes some work to resolve; the
companion is only 8.8 (primary is 3.3), the PA is 266º and separation
1.4". This is nearly a fixed binary, with very little movement.
Variable stars in Gemini
Zeta Geminorum is a cepheid variable, from 3.62 to 4.18 every
Eta Geminorum is a semi-regular variable with an average period
of 232.9 days. It ranges from 3.2 to 3.9.
R Geminorum is a Mira-type long-period variable, with large
variation from 6.0 to 14.0 every 370 days. The 2000 maximum should
arrive in mid October.
Deep Sky Objects in Gemini:
The only Messier object in Gemini is M35 (NGC 2168). This is an open
cluster easily enjoyed in small scopes. It lies just 2.5 degrees northwest
of eta Geminorum.
This cluster is extremely attractive, with gently curving rows of glittering
stars. Several hundred stars make up the group, which is perhaps 2500
light years away.
The Eskimo Nebula (NGC 2392) is one of the more distant nebulae at
an estimated distance of 10,000 light years. There is a tenth-magnitude
central star. If you do have a large enough scope, be prepared for anything:
Burnham thought the Eskimo Nebula suggested "the classic and unforgettable
features of W. C. Fields."
While you can locate this blue-green object in small scopes, it takes a
very large telescope to see the "face" of this nebula, the eyes, nose, and
mouth and the "fur collar" that gave it its name.
To find this rather small planetary nebula draw an imaginary line
between kappa Geminorum and lambda Geminorum. Now draw a perpendicular
line from delta Geminorum, and just about where this line meets the
other one is where you'll find the Eskimo Nebula.
For a closer look at Gemini, visit the Binocular Section.
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