10 EASY GLOBS!

by Marilyn Head

One of the best things about winter viewing is that it gets dark so early so
that you can put in a few hours observing before it gets too late or too
cold.  Here’s a nice little tour of globular clusters that you can do
between 7 - 9pm - if you remember to stop at 9pm that is!  Globular clusters
are groups of  a few hundred thousand to several million Population II stars
(i.e. old stars) that  are distributed in the spherical halo of the galaxy.
Well, that’s what I thought until I began to find out more and there seem to
be  exceptions to just about every one of those parameters!  However,  the
stars in each globular cluster share a common history and differ from each
other only in mass.  Though globular clusters have been recognised and
studied for centuries, the past eighty years has seen them take centre stage
in the role they have played firstly in establishing  the spiral shape of
the Milky Way galaxy and our less than central position in it; secondly, and
still contentiously, in serving as a reference point in dating the galaxy
and, even more contentiously, the universe; thirdly, in providing both proof
and challenge of stellar evolution theories.

 "The meanest flower that doth blow" - I always think of Wordsworth’s line
when I look at Omega Centauri, the most beautiful celestial "dandelion" but
before you get to this "most superb object" (JH) and the "Great Cluster" 47
Tucanae, enhance your appreciation of these spectacular objects by sampling
a few that are less spectacular but more demanding. 

NGC 2808  in Carina and NGC 3201 in Vela are two very different globs easily
found by using the false cross as a reference. Crosses are always difficult
to ‘read’ because you never know which way up they are, but  the two
adjacent stars most widely separated belong to Carina, and the other pair
belong to Vela.  At present the bottom star in Carina, is Epsilon, a yellow
giant, and the top star is iota, a white supergiant.  To the left and up a
bit is Beta Carinae.  NGC 2808 is halfway between Iota and Beta Carinae and
is like a miniature 47 Tuc., very bright and compact.   Harlow Shapley (who
incidentally used globulars to establish the shape of the Milky Way, albeit
making it twice as large as it really is) classified Globulars by their
concentration of stars correlated with the central surface brightness from I
to XII , class I , to which 2808 belongs, being the most highly condensed.
NGC 3201 provides the contrast, being a class X globular which can be found
by following the line between the two stars, Delta and Kappa Velorum,
upwards.  There is a fainter star about the same distance away, then
slightly further and to the right you will notice three stars in a line; NGC
3201 lies to the right and below these and is quite easy to find because it
covers a wider field of view. At 16,300 l.y. it is almost twice as close as
NGC 2808. It is not as bright but the stars are resolveable and it is very
attractive with some stars in "short curved rays like jets of water from a
fountain" (Malin & Frew).

Having seen the two which will set first, you can now take your time in
looking at the globs in the  constellations that dominate our sky in winter
- Scorpius, Sagittarius and Ophiuchus.  You may think you’ve already seen
variety in the intense glow of the distant 2808, the formless, scattered
grace of 3201 and the perfection of Omega Cen., but M4, just to the left and
up a bit of Antares shows another surprise for here there is a clearly
discerned straight band of stars lying diagonally across  the central haze
of a somewhat oval and loose cluster, the closest globular at 6,800 l.y.
Further to the left , M80 lies directly in line with Antares and the
gorgeous double star Beta Scorpii (actually a quintuple!).  It is tiny and
concentrated with a fuzzy shell which made its discoverer, Mechain, compare
it with a comet. Move to the right of Scorpius and Corona Australis and find
the constellation of Ara which always reminds me of a chair lying on its
side rather than the altar it is supposed to be.   NGC 6397 lies  about
halfway between and down from Alpha and Beta Arae.  Because it is close
(7200 l.y.),  the stars are mostly resolveable and cover a wide field in a
regular, circular pattern , which makes it exceptionally pretty. To the
right and just below Ara lies NGC 6752 in Pavo, my very favourite glob.
Nicknamed the "starfish",  it is 13,700 l.y. away, has a magnitude of 5.3, a
Shapley class of VI and is a stunning view with the  bright stellar ‘arms’,
for which it was named, curving away from the centre.  As Hartung describes:
"a most lovely object".

Tear yourself away from Pavo to take a look at two bright globs in
Sagittarius, M22 and M28. Both are near the "lid" of the teapot, lambda
Sagittarii, a yellow giant with two fainter stars nearby - I mention this
because there is so much going on in this part of the sky, it is sometimes
easy to think you’ve got the wrong star!  M28 is a tiny glob, mag. 6.9,
just to the left of Lambda Sag., whereas M22 is about  twice the same
distance to the right and, at  mag.5.1, is much more luminous.  They are
actually quite similar in appearance with  smallish centres defining
tightly packed clusters, but M22 being 10,000 l.y. away is twice as close as
M28 so it appears bigger and the stars are  more easily resolved.  Indeed on
one memorable night, the red and yellow stars in M22 reminded me far more of
a jewel box than John Herschel’s choice for that epithet - it was absolutely
beautiful!

Swing left now to Ophiuchus on the other side of Scorpius, which bears the
lugubrious nickname of the "coffin", perhaps appropriately as  Ophiuchus was
a mythical healer who could raise the dead!  As usual, we see it ‘upside
down’ and the coffin is pointing downwards.  The middle star of the base of
the coffin is Zeta Ophiuchus, the left side is marked at the base by two
stars close together (Delta & Epsilon) and halfway down by Gamma Ophiuchi.
Draw a line across from Gamma Oph. to meet a line down from Zeta - along
that line are two globs M10 & M12.  I always get them mixed up! Both are
large, well resolved clusters with hazy centres.  The right side of the base
of the coffin is marked by  the bright star Eta Oph. You will find the very
interesting M9 between it and the next star up and to the right, Xi Oph.  M9
is like a faint oval smudge and makes a nice contrast to the more rounded
globulars in the same constellation. Above Xi Oph. are three stars on an
angle, M19 lies to left of the top one.  Although M9 and M19 have the same
Shapley class of VIII, to an observer they differ in overall shape and
‘texture’ in that M9 has lots of what Hartung calls "outliers" i.e. stars
scatttered away from the core, whereas M19 has almost none, the cluster
appearing more homogeneous.
  
Now, finish the night with a good look at Omega Cen and 47 Tuc. and be glad
you live in the Southern Hemisphere where you can echo John Herschel’s
description of the latter’s "most superb…Rose coloured central Mass…most
striking…the definition admirably perfect…Most glorious!"

PS Yes, I know I can’t count!