44 AUSTRALIAN SKY & TELESCOPE APRIL 2016
W
hat can planets do that stars can’t? One thing
is to move into conjunction with another
celestial object. Much of the beauty — and
allofthedrama—ofconjunctionsisthatthey’re
temporary and evolving celestial events.
But some of the beauty is also just in the vision
of two luminaries posing close to each other in the
sky. The stars canoffer us this too. And let’s not
forget that ‘close’ is relative — how close together,
forinstance,dotwo1st-magnitudestarshavetobeto
constitute an astronomical marvel?
LastyearwastheoneIcalled‘theYearofthe
Conjunctions’. Several times, pairings featured
bright planets less than ½°degree apart when they
werehighinafullydarksky.Onemoresuchreally
close and readily observable conjunction — that of
VenusandSaturn—occurredinJanuarythisyear.
An ultra-close conjunction of Venus and Jupiter
will take place this coming August. And Mars and
Saturn, though reaching opposition a mere12 days
apart in late May–early June, won’t get closer than
7 °apartintheautumn.NotuntilAugustwillMars
finallyhaveatrueconjunctionwithSaturn,and
when it does, the two will be, briefly, 4°apart.
Comparethattotheexactly4° 30 ′separation of a
1st-magnitude and bright2nd-magnitude star that
happen to be close in the sky for more than a single
day—infact,they’vebeenthiscloseforthousands
of years. I’m talking about the twin stars named for
history’s most famous (albeit mythical) twins, Pollux
and Castor.
Mars and Saturn will certainly be brighter than
Pollux and Castor this autumn and August. But that’s
not always so. Mars is often dimmer than Castor
(magnitude1.58), let alone Pollux (magnitude 1.16).
And Pollux and Castor’s pairing is so noticeable it has
determined the identity of one of the most prominent
constellations of the zodiac, Gemini.
Pollux and Castor are attention-grabbing, even in a
region of sky which features the brightest constellation
(Orion), the brightest star (Sirius), and the two brightest
naked-eye star clusters (the Hyades and Pleiades).
PolluxandCastoraren’tbrightenoughtosimulate
perfectly a truly dazzling conjunction of planets.
But bright planetary conjunctions are all too liable
tooccurneartheSun(foronething,twoofthe
five classic bright planets, Venus and Mercury,
are incapable of preceding or following the Sun by
morethanafewhoursatmostinoursky).Pollux
and Castor, on the other hand, are easily seen for
long periods by observers at most latitudes.
But if you want much brighter star-pairings,
all you need do is be down here in the Southern
Hemisphere.
By an amazing coincidence, two tight pairings
of very bright stars are almost equal to Pollux and
Castor. At present Alpha (α)andBeta(β)Centauri
(magnitudes – 0 .28 and0.58) are 4° 25 ′apart. And
in the Southern Cross, Alpha and Beta Crucis
(magnitudes 0.77 and 1.25) are 4° 15 ′ apart.
In the next issue’s column we’ll discuss how the
rankings of the tightest bright pairs are changing
— amazingly soon. But we’ll end this column with
the fact that we can simulate a bright planetary
conjunction by looking at a bright double star
through the telescope — and none better than
Alpha Centauri. The brighter two of its three
components are magnitudes –0.01 and 1.35. Their
separation changes rapidly and is usually wide —
but in 2016 is rather close. ✦The power of two
Not to be outdone by planetary conjunctions, stellar pairings offer
a celestial beauty all of their own.
AKIRA FUJIIFred Schaaf
Tonights’s Sky