52 AUSTRALIAN SKY & TELESCOPE APRIL 2016Isles of light
Let the colourful stars of Gemini brighten your journey along the Milky Way.
... 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 flee away,
And mix with their eternal ray?
— Lord Byron, The Siege of Corinth, 1 816
G
emini is one of the most vividly starred
constellations of the zodiac. 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 its 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
neighbours as galactic distances go.
Let’s begin our journey through starry Gemini
with Zeta (ζ) 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.2266CastorPollux23042331O'Neal 9 2420_`δ¡aκhioζqfpt
GEMINI23927 h 00 m+30°7 h 30 m 6 h 30 m+25°+20°Star magnitudes324
5
6
7The lights of Gemini
Object Type Mag(v) Size/Sep RA Dec.
ζGem Doublestar 4.1,7.7 1.7′ 07 04.1 +20° 34 ′
NGC 2304 Open cluster 10.0 5.0′ 06 55.2 +17° 59 ′
δGem Doublestar 3.6,8.2 5.5′′ 07 20.1 +21° 59 ′
κGem Double star 3.7, 8.2 7.5′′ 07 44.4 +24° 24 ′
NGC 2420 Open cluster 8.3 6.0′ 07 38.4 +21° 34 ′
O’Neal 9 Asterism 8.0 3.2′ 07 41.0 +21° 49 ′
NGC 2331 Open cluster 8.5 19 ′ 07 07.0 +27° 16 ′
NGC 2266 Open cluster 9.5 5.0′ 06 43.3 +26° 58 ′
Angular sizes and separations are from recent catalogues. Visually, an object’s size is often
smaller than the catalogued value and varies according to the aperture and magnification of
the viewing instrument. Right ascension and declination are for equinox 2000.0.Sue French
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