TWINS
Twins? Both Castor and Pollux were sons of Leda, but of different
fathers, Pollux divine from Zeus himself, Castor of mortal
paternity. But twins they are: there is no arguing with myth.
When Castor died, Zeus granted him divinity so that the Twins could
live together forever within the constellation Gemini, represented by stars that bear their names. At
least until their different spatial motions (the "twins"
astronomically quite unrelated) tear them apart. But even within
reality, Pollux has ascendancy, as at first magnitude (1.14,
ranking 17th) it's notably brighter than Castor, which at 1.58 (no.
23 in brightness) lies atop the pile of second magnitude stars.
Nevertheless, Castor sneaks in as Bayer's Alpha star, Pollux taking
second place as Beta Gem for reasons known only to the master
mapmaker. And he is not telling.
The stars themselves are more unrelated even than implied by the
initial divinity issue. While Pollux seems to be an ordinary class
K0 giant of the sort that dots
the skies (and with a radius just 10 times solar, one not all that
big), it does have its merits. At a distance of just 34 light
years, it's the closest helium-fusing giant, the only reason it
makes first magnitude to start with. Far more important, Pollux
has an orbiting planet (one of the few giants
to make the claim), though with more than three Jupiter-masses, not
one you could live on, even though at an average orbital size of
1.7 Astronomical Units it was at least in a reasonably comfortable
zone prior to the star's evolution to gianthood and its current
luminosity of 45 Suns.
In terms of interest, dimmer Castor (51 light years away) may top
it. Not only is Castor double (as befits a twin) as
seen through the telescope, made of two white class A hydrogen-
fusing dwarfs (A1 and A5) currently just 4.8 seconds of arc apart,
it's actually sextuple! The
spectrograph shows that each of the white pair (which average 114
AU from each other and take 467 years to orbit) is accompanied by
a vastly dimmer, low mass red (orange?) dwarf that orbits its
master in just days. Then way off in the distance just over a
minute of arc away is tenth magnitude Castor C, which is made of eclipsing red dwarfs (giving
it the alternative name YY
Gem) that take under a day to circuit each other (but, over 1000 AU
from the inner quartet, take at least 12,000 years to go around
them). Any planet in such a complex system would be most unlikely,
but we have been surprised before.
Twins aside, other Geminid stars cry out for attention. Try
watching Mekbuda, (the Arab's "lion's
paw"), more commonly called Zeta Gem. Varying between magnitudes
3.7 and 4.2 over a period of 10.2 days, Zeta Gem is one of a
handful of naked-eye Cepheid
variables, the most prominent of which are the prototype Delta Cephei, Eta
Aquilae, and of all things, Polaris
(though you won't see it vary as the pulsations have nearly
stopped). Cepheids are powerful tools in the galaxy/cosmology business, as
their periods give their absolute magnitudes, which
when combined with apparent magnitudes yield distances. A try for
Zeta Gem using a standard period-luminosity relation gives 1415
light years (assuming no interstellar absorption), while the
parallax gives a satisfying 1375 l-y, the difference well within
the rather large uncertainties. Then there is Eta Gem (Propus, Castor's "forward foot"),
a semi-regular class M3 red
giant 385 light years away that varies between magnitudes 3.3
and 3.9 over an interval of 234 days. And if you want a well-
hidden secret, turn your eye or binoculars toward 6 Gem. Less than a degree northwest of
Propus, it's an M1 red supergiant that glows softly at
magnitude 6.4. Some 5500 light years away, it's similar to much
closer Betelgeuse, radiating with the
power of between 100,000 and 150,000 Suns from a bloated surface
that could be as large as 85 percent of the orbit of Jupiter. It's
a prime candidate for a supernova, one that if
exploded now would easily surpass the brightness of Venus and shatter the Twin's
outline.
Fittingly (for the Twins) we might highlight a pair of other
notable stellar objects. The most obvious is the sprawling, half-
degree-wide, open cluster
Messier 35. Tucked into a curve of stars that comes off Gemini's
northwest corner, M 35 lies 2700 light
years away, and from the brightest and hottest dwarf stars that
remain after the top end has died off (the cool end of class B) is
some 200 million years old. Were it not for a full magnitude of
dimming by the Milky Way's interstellar dust,
the cluster would be an easy naked eye object.
At the other end of the end of the evolutionary scale is one of the
brightest planetary nebulae in the sky, the
Eskimo Nebula, NGC 2392, which lies in
obvious splendor 2 1/2 degrees east-southeast of Delta Geminorum (Wasat). Planetary nebulae are the leavings of
dying giant stars as they slough off their outer envelopes to
reveal the nascent white
dwarfs within. A classic "double shell" object, the intricate
structure (which has a near-record expansion velocity of more than
50 kilometers per second) was formed by a hot wind from the central
star that has shovelled aeons of mass loss in front of it. And you
have a double treat awaiting, as the hot (65,000 Kelvin), blue,
near-tenth-magnitude central star is one of the few easily visible
in a small telescope. With a guessed distance of 3000 light years
(nebular distances notoriously unreliable), the stellar nucleus
shines with the luster of thousands of Suns, nearly all in the
invisible ultraviolet. And
going along with the "two" theme, look up near Castor and Pollux
for NGC 2371-2, one of four double-lobed
planetaries that carry not one, but two NGC numbers. (The others
are NGC 650-1 in Perseus, also known as
Messier 76, NGC 2474-5 in Lynx, and NGC
6164-5 in Norma. The name NGC 2474-5 was
misapplied to the nebula and actually belongs to a pair of
galaxies, while NGC 6164-5 is no longer listed as a planetary but
as a high mass evolving star with a surrounding ejected nebula,
getting us back to two of them.) NGC 2371's 15th magnitude central
star blisters at 118,000 Kelvin and will shortly begin to cool as
a white dwarf as its ejected gases vanish into the interstellar
gloom.
A final pair of items are as much of the Earth as they are of the
sky. Gemini traditionally carries the Summer
Solstice, the heavenly point
at which the Sun reaches its maximum angle north of the celestial equator to mark the beginning
of astronomical summer. However, it was not always so. Because of
precession, the 26,000 wobble of
the Earth's rotational axis (which brings with it a cycle of pole
stars), in ancient times the solstice fell in Cancer, hence the "Tropic of Cancer" ("Tropic of
Gemini" not quite sounding quite right). Moving steadily westward,
precession caused the solstice to cross the official line into Taurus in 1990. Of course such
boundaries, established by the International Astronomical Union
(the folks who de-planeted Pluto) around 1930, are somewhat
artificial. Since the solstice is still closer to the traditional
sky-figure of Gemini than it is to that of Taurus, at least for a
time we'll let the Twins keep it.
At the end, bring the sky to Earth with the fine Geminid meteor
shower. Usually peaking the night of December 13-14, sending us up
to perhaps a couple morning meteors a minute, it is at least as
good as the better-known Perseid shower of August, but it's hard to
convince people to lie on their backs on winter's ground. Meteor
showers are caused by the debris of comets, the flaked rocky junk
hitting the atmosphere to burn up in streaks of heat and light.
Though travelling on parallel tracks, a shower's meteoroids, which
range in size from grains of sand to ball-sized pieces of fluff,
seem to emanate from a specific radiant point in the sky as a
result of perspective, the location of which depends on the motion
of the Earth relative to that of the meteoroid swarm. The Geminids
are related to what was once thought to be a short-period asteroid,
3200 Phaethon, but which now seems to be a defunct comet that has
lost its volatiles thanks to an orbit that takes it close to the
Sun. None of the streaking matter ever hits the ground, allowing
us to admire Gemini and its notable sets of duos in perfect safety.
Copyright © James B. Kaler, all rights reserved.
These contents are the property of the author and may not be
reproduced in whole or in part without the author's consent
except in fair use for educational purposes. First published in
the January/July 2013 Newsletter of the Lowestoft and Great
Yarmouth Regional Astronomers, who are gratefully acknowledged.