Sky and Telescope - February 2017 - 60

Starry Siblings

7h 30m

ο
α

Castor

Pollux

υ

6h 00m
+35°

θ

Seeing Double in the Twins

ρ

AURIGA

τ

+30°

ι
GEMINI

κ

ε
ω

2392

μ

δ

63

ζ

M35 +25°
2158
5

η

1

+20°

15

ν

ORI ON

20

γ

λ

ξ
ξ

2

+15°
3

ν

4
5
6

Mekbuda. The bottom-left corner is held by 2nd-magnitude
Gamma (γ), also called Alhena. Along the rectangle's lowerright side are 3rd-magnitude Epsilon (ε), Mu (μ), and Eta
(η) - respectively: Mebsuta, Tejat, and Propus. These eight
Gemini markers all merit closer inspection.
Propus, for example, is a long-period variable star that
fluctuates between 3rd and 4th magnitude in a semi-regular
fashion over about eight months. You won't notice any
change in Propus from one night to the next, but give it a
glance once a month. When Propus reaches maximum, it's
almost as prominent as Mu to the east. At minimum, it's
nearly as faint as 4.2-magnitude 1 Geminorum to the west.
And keep your eye on Mekbuda. A classic Cepheid variable
star, Mekbuda cycles between magnitudes 3.6 and 4.2 every
ten days. At its height, Mekbuda rivals Wasat; at bottom, it's
no brighter than 1 Gem.
Do you have binoculars? My 7×50s reveal a wonderful
range in color among the Gemini Eight. Castor, 52 light-years
away, gleams white tinged with blue. Pollux, at 34 light-years
the closest star of the set, exudes a golden radiance. Wasat
(60 light-years) is cream-colored, while Mekbuda (1,200
light-years) is straw-yellow. Alhena (110 light-years) is as
white as snow. Mebsuta (800 light-years), Tejat (230), and
Propus (400) are all orange.
But there are more stellar splendors here. Northeast of
Castor, 5th-magnitude Pi (π) is dimly reddish. South and
west of Pollux are 3.6-magnitude Kappa (κ), sunny yellow;
4.0-magnitude Upsilon (υ), ruddy-orange; and 3.8-magnitude Iota (ι), deep yellow. Finally, two-thirds of the way from
60

Alhena to Tejat, 4.1-magnitude Nu (ν) seems vaguely bluish.
My 4¼-inch f/6 Newtonian reflecting telescope confirms the
blue tint; indeed, it intensifies all these colors.

M37

Star magnitudes

φ

σ

6h 30m

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

A telescope will also confirm that Gemini has double stars
aplenty. Most show in my scope at just 27×. Nu is one - it
comes with a wide 8.0-magnitude companion star 112″
(arcseconds) approximately northward. In the same lowpower field, northwest of Nu, 6.7-magnitude 15 Geminorum
holds an 8.2-magnitude secondary 25″ to its south. (Don't be
misled by the 6th-magnitude star lying halfway between 15
Gem and Nu.) And near Alhena (Gamma), 20 Geminorum
is a pretty pair boasting 6.3- and 6.9-magnitude components
20″ apart.
Still at 27×, the scope shows the wide companions of
three more bright stars. Mekbuda oversees a 7.6-magnitude
companion 101″ north. Mebsuta has a 9.6-magnitude outlier
111″ eastward, and Castor sports a 9th-magnitude red dwarf,
called Castor C, 71″ southward. This wee star is an easy
catch, but I must increase the power to perceive its fiery hue.
In truth, Castor C is a pair of red dwarfs that look single no
matter how high the magnification. Fortunately, the extra
power produces a much more impressive prize - Castor itself.
Castor is a marquee binary star. Its 1.9- and 3.0-magnitude components, 4.8″ apart, divide beautifully in my trusty
star-splitter at 93×. (Like Castor C, both Castor A and Castor
B are indivisible pairs. In all, Castor is a six-sun system!)
After ogling Castor, I drop southward to 3.5-magnitude
Wasat, which guards an 8.2-magnitude companion only 5.5″
to the southwest. Wasat is one tough twosome, but I can
resolve it at 186×. Splitting this tight, strongly unequal binary
is always satisfying.

S FUZZ AND FLUFF The visual appearance of the planetary nebula
NGC 2392 depends somewhat on aperture and filters. In a 4-inch scope
at moderate power, you'll see a bright blob that's slightly too thick to be
a star. Upping the magnification and adding an O III filter to the eyepiece
will give you a better chance at distinguishing the shell of nebulosity from
the planetary's central star.

JIM R A DA A ND ST UDEN TS / A DA M BLOCK / NOAO / AUR A / NSF

π

β

7h 00m



Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

Contents
Sky and Telescope - February 2017 - Cover1
Sky and Telescope - February 2017 - Cover2
Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
Sky and Telescope - February 2017 - 4
Sky and Telescope - February 2017 - 5
Sky and Telescope - February 2017 - 6
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Sky and Telescope - February 2017 - Cover3
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