This month, we're taking a sabbatical from northern
skies
and heading to the deep south, where we will visit the finest globular
star
cluster in the entire sky.� Which
cluster has earned that coveted title?� Some
would argue 47 Tucanae, while others would say M13 in Hercules or
possibly M22
in Sagittarius.� While they are all
wonderful sights, there is only one king of the globulars, to my eyes
anyway.�
Omega Centauri.
Above: Spring star map from Star
Watch
by Phil Harrington.
Above:
Finder chart for this month's Binocular Universe.� Chart
adapted from Touring
the Universe
through
Binoculars Atlas (TUBA)
Omega
is bright enough to be visible to the naked eye as a faint star-like
point
within the lopsided hexagonal body of the Centaur.�
In fact, it was thought to be just that -- just another faint
star --
before the invention of the telescope.� Ptolemy
listed it as such in his Almagest catalog of 140 AD, while
Johann Bayer
assigned it the Greek letter Omega (�)
in his monumental Uranometria atlas of 1603.
It
wasn't until nearly seven decades after the invention of the telescope
that its
true nature was revealed.� In 1677,
Edmund Halley, most famous for his work on and predictions concerning
the comet
that bears his name, became the first to see Omega Centauri's true
inner self;
that it's not just one star, but rather a huge swarm of them.�
Halley, apparently a man of few words, described this
magnificent object
as merely a "luminous spot or patch."
Current
estimates tell us that Halley's patch contains several million stars,
more than
any of the other 150 globular clusters in the Milky Way's family.�
Those millions of stars are all crammed into a spherical area no
more
than 150 light years across, which means that the density inside
Omega's core is
about 180 stars per cubic light year.� That's
about 80,000 times higher than the rural surroundings that the Sun
enjoys.�
Astronomers place Omega Centauri at 15,800 light years away,
which also
makes it one of the closest of its kind.
These
two factors -- size and distance -- work together to create a grand
view through
binoculars and telescopes alike.� While
a telescope's higher magnification will resolved Omega to its core,
some stars
around the fringes can be spotted with as little as 11-power.�
In fact, even 7-power binoculars show a "not-quite-resolved"
graininess to the cluster.
All
binoculars clearly show that Omega is not uniformly round, like most
other
globulars.� Instead, it's
surprisingly oval, or oblate.� This
effect is due to centripetal force generated by the cluster's rate of
rotation,
similar to what causes Jupiter to flatten at its poles and bulge at its
equator.
Regardless
of the instrument used to spot it, your first trip to Omega Centauri
will be
remembered for a lifetime.� I fondly
recall my first encounter way back in 1986, when my family and I had
traveled to
the Florida Everglades, ironically to see Halley's Comet (and
Disneyworld
, but I digress).�
Although the comet left a lot to be desired (in fact, I'd use
Halley's
own words and describe his comet as just a "luminous spot or patch"),
Omega was a wondrous sight.� Going
back and forth between it and M13 -- often heralded as the finest
globular north
of the celestial equator -- left little doubt of who is the reigning
champ of
the globulars.
Incidentally,
Omega is theoretically visible from as far north as the latitude of
New York City
(40� north), but
naturally, it will suffer
greatly due to atmospheric interference.� I've
tried spotting it from the south
shore
of
Long Island
without success, although I
have spotted stars
matching Omega's declination, so it is possible.�
If you've seen Omega from a latitude close to 40� north,
I'd
love to hear from you.
 |
�
Left:
The author made this sketch of Omega Centauri through 11x80 binoculars
back in
1986.
North
is up.
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If Omega is a little too far south for you,
head 5� north and try your luck with a
galactic enigma.� On August 4, 1826, Scottish
astronomer
James Dunlop discovered what would prove to be a puzzle for the ages.
While
observing from Parramatta,
New South Wales, Dunlop
found what he later
described as "a very singular double nebula. These two nebulae are
completely distinct from each other, and no connection of the nebulous
matters
between them." A singular double nebula? Sounds more like an
astronomical
oxymoron than an observation. What had Dunlop found?
Dunlop's riddle is now known as NGC
5128, a fascinating galaxy that�s
also known as the radio source Centaurus A for its strong emissions.
When we
look at NGC 5128, we are looking at a case of galactic cannibalism.
Inside the
heart of this huge elliptical galaxy lies a massive black hole that is
consuming a smaller spiral galaxy. The two are believed to have
collided
between 160 and 500 million years ago.
Your binoculars won't detect any of the
internal strife that NGC 5128 is enduring, but they can show you this
intergalactic train wreck that continues to fascinate astronomers. That
is, if
you can see the galaxy at all. It never rises more than 7� above my
own horizon
here on Long Island.� That�s better
than
Omega Centauri, but not by much. If you�re further north, its
maximum altitude
shrinks. Of course, travel southward and NGC 5128 will climb higher in
the sky.
Overlooking the Atlantic
Ocean along Long Island's
south
shore, I can just make out NGC 5128 through my 10x50 binoculars. It's
tough,
but by waiting until it lies on the meridian and is highest in the sky,
it's
unmistakable.
As luck would have it, the bright star
Spica [Alpha (�) Virginis] has
nearly the same right ascension as both NGC 5128 and Omega Centauri, so
we can
use it as a gauge for when both are at culmination.
If NGC 5128 is still too far south, you'll
notice that we have M83 on the top
edge of this month�s finder chart, as well.�
Even with a clear view, however, finding it can take some extra
effort.
But if you follow these steps, you should spot it the first time out.
Again
starting from Spica, head south 12�, or roughly two binocular
fields, to
3rd-magnitude Gamma (�)
Hydrae. Swing to Gamma's southeast and keep an eye out for a right
triangle of
stars resembling a deformed arrowhead. M83 lies just to the south of
the
arrowhead's eastern tip.
M83 is visible through nearly all
binoculars as a bright stellar point surrounded by the soft glow of its
spiral
arms. Many amateurs seem to ignore M83 because of its out-of-the-way
location,
but don't you make the same mistake. In fact, thanks to the attractive
star
field created by that crooked arrowhead, it's become one of my favorite
spring
binocular targets.
Before we close this month, many readers
have asked me to include some viewing tips from time to time in this
column. I
think that's a great idea, and would like to begin with a basic one:
how to
hold binoculars by hand. That sounds easy enough, but there is a right
way and
a wrong way. A few years ago, Robert Andrews from Lonetree, Colorado,
sent me
the following tip. "While in the Navy, I was taught to hold them
cradling
the sides of the barrels in my fingers with the hands open, and rest
the
extended thumbs on my cheeks or the side of my face. This results in
much more
stability." This is a slightly different technique than I'm used to,
but I
tested it for myself and found that it works well. Give it a try for
yourself.
Have a favorite binocular tip or trick?
Send them to me, either by posting them to this column's discussion
forum or
using the e-mail link at the top of this article, and I'll try to
include them
in a future column.
Meanwhile, here are few more southern
targets within this month�s Binocular Universe.
Until we meet
again next
month under the stars, remember that two eyes are better than one.
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About the Author:
Phil
Harrington is the author of nine books on astronomy, including Star
Ware and Star Watch.� Visit his web
site at www.philharrington.net .
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| Phil
Harrington's Binocular
Universe is
copyright 2013 by Philip S. Harrington.� All
rights reserved.� No
reproduction, in whole or in part, beyond single copies for use by an
individual, is permitted without written permission of the copyright
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