Sky and Telescope - March 2016 - 54

OBSERVING

Deep-Sky Wonders

Isles of Light
Let the colorful stars of Gemini brighten your journey along the Milky Way.
7h 30m

7h 00m

6h 30m

_
Castor

Pollux

`

Star magnitudes

o

+30°

f
2331

p

q
+25°

2266

2
3
4
5
6
7

¡
κ

t
GEMINI

O'Neal 9

δ

2420

ζ

2392

i

+20°

2304

a

h

The Lights of Gemini
Object

Type

Mag(v)

Size/Sep

RA

Dec.

ζ Gem

Double star

4.1, 7.7

1.7′

07h 04.1m

+20° 34′

55.2m

+17° 59′

NGC 2304

Open cluster

10.0

5.0′

06h

δ Gem

Double star

3.6, 8.2

5.5″

07h 20.1m

+21° 59′

κ Gem

Double star

3.7, 8.2

7.5″

07h 44.4m

+24° 24′

NGC 2420

Open cluster

8.3

6.0′

07h 38.4m

+21° 34′

41.0m

+21° 49′

O'Neal 9

Asterism

8.0

3.2′

07h

NGC 2331

Open cluster

8.5

19′

07h 07.0m

+27° 16′

NGC 2266

Open cluster

9.5

5.0′

06h 43.3m

+26° 58′

Angular sizes and separations are from recent catalogs. Visually, an object's size is often
smaller than the cataloged value and varies according to the aperture and magnification of the
viewing instrument. Right ascension and declination are for equinox 2000.0.

54

March 2016 sky & telescope

. . . the sky
Spreads like an ocean hung on high,
Bespangled with those isles of light,
So wildly, spiritually bright;
Who ever gazed upon them shining,
And turned to earth without repining,
Nor wished for wings to fl ee away,
And mix with their eternal ray?
- Lord Byron, The Siege of Corinth, 1816
Gemini, the Twins, is one of the most vividly starred
constellations of the zodiac and well placed for Northern
Hemisphere observers. Our Sun dwells in the Milky
Way Galaxy near the inner edge of a spiral-arm segment
known as the Orion Spur or Local Arm. When we gaze
at Gemini, we're looking outward through this arm
and are rewarded with the sight of many nearby stars
brightly adorning our sky. All of Gemini's stars bearing Greek letter designations lie within the Orion Spur,
the nearest being Pollux at 34 light-years, and the most
distant being Omega (ω) Geminorum at roughly 1,500
light-years - near neighbors as galactic distances go.
Let's begin our journey through starry Gemini with
Zeta (ζζ) Geminorum, also known as Mekbuda. Viewed
through my 130-mm refractor at 23×, Mekbuda is a very
wide double star whose components gleam yellow. The
4.1-magnitude primary is attended by a 7.7-magnitude
companion to the north-northwest. This is merely an
optical double, with the companion considerably closer
to us than the primary. These stars also have significantly different motions through space.
Zeta is a Cepheid variable star and, according to
Daniel Majaess and colleagues (Astrophysical Journal Letters, 2012), a star-cluster member. Combined with recent
measurements, the determination of this membership
allowed the researchers to refine the distance to Zeta,
yielding a weighted mean of 1,200 light-years. On the sky,
the nearest likely cluster member to Zeta is the 11.5-magnitude star 1.5′ to its east.
On his website Stars (stars.astro.illinois.edu/sow/
sowlist.html), astronomer Jim Kaler writes that Mekbuda
should be 60 times our Sun's diameter. If Mekbuda
replaced our Sun, it would span 30° in our sky (about the
size of Gemini), but it would be far too toasty here on
Earth to enjoy the view.


http://stars.astro.illinois.edu/sow/sowlist.html http://stars.astro.illinois.edu/sow/sowlist.html

Sky and Telescope - March 2016

Table of Contents for the Digital Edition of Sky and Telescope - March 2016

Contents
Sky and Telescope - March 2016 - Cover1
Sky and Telescope - March 2016 - Cover2
Sky and Telescope - March 2016 - 1
Sky and Telescope - March 2016 - Contents
Sky and Telescope - March 2016 - 3
Sky and Telescope - March 2016 - A
Sky and Telescope - March 2016 - B
Sky and Telescope - March 2016 - 4
Sky and Telescope - March 2016 - 5
Sky and Telescope - March 2016 - 6
Sky and Telescope - March 2016 - 7
Sky and Telescope - March 2016 - 8
Sky and Telescope - March 2016 - 9
Sky and Telescope - March 2016 - 10
Sky and Telescope - March 2016 - 11
Sky and Telescope - March 2016 - 12
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Sky and Telescope - March 2016 - Cover3
Sky and Telescope - March 2016 - Cover4
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