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Transcript
Salisbury Plain Observing Group—Deep Sky Objects : September Globular Clusters
The spectacle of the September sky heralds the
that that galaxy lying 1200 times further away
return of the observing season and the delights of
than M13 no doubt also has its own globular
the coming dark nights. To start the new season’s
clusters!
articles, I thought it would be interesting to have a
tour
of
some
of
September’s
finest
globular
clusters from across the constellations.
Whilst in Hercules, visit M92, which, were it not
for M13, would have had the Great Globular title!
M92 lies only a short star hop away from M13 to
The obvious jewel of the northern skies is
the north east and is easily found in binoculars if
Messier 13, the Great Globular or Keystone Cluster
you are careful– look for the non stellar fuzzy and
in Hercules. Lying some 26,000 light years away,
you have M92! This cluster lies another 1100 light
M13
and
years further than M13 and is 110 light years
it is an
across so is not much smaller than M13, but only
is
around
150
light
comprises some 600,000
years
across
solar masses;
easy binocular object (or in the darkest of skies,
has around 400,000 solar masses.
naked eye) and rewards magnification even in
claim to fame of being a very old cluster estimated
smaller apertures.
to be 14 thousand million years old.
Using my 6” reflector, I can
begin to resolve stars at a magnification of around
60x and have gone up to 100x.
Slightly larger
apertures (8”+) can resolve the “propeller”; this is
a shape reminiscent of the Mercedes Benz logo
which can be seen towards the south east of the
cluster. The propeller can be tricky to image but is
a very worthwhile feature to sketch.
Close by is
NGC 6207, a galaxy lying 30 million light years
away, which is visible in an 8”.
Sobering to think
It has the
Slightly more challenging is NGC 6229, a
smaller 9th magnitude cluster colloquially known as
the overlooked globular cluster in Hercules. It is a
small (1’)
globular, but as it is condensed it is
comparatively bright;
I have spotted it from
Kelling fairly easily.
Our next stop is Sagitta which contains M71
lying 18,000 light years away.
M71 was, for a
long time, a matter of some debate over whether
it was a loose globular cluster or older, compact
open cluster. It can be spotted in binoculars, but
does take magnification quite well; Luginbuhl &
Skiff in their catalogue of Deep Sky Objects record
seeing dark patches in the cluster giving the
impression of “eyes” in 8” of aperture.
Pressing onto Lyra now and M56; at only a third
of the mass of M13, it is has been described as a
good “non comet”. It is fairly easy to find, lying
south west of an open chevron shaped asterism.
It is around 55 light years across and is 27,000
light years away. Moderate powers used with a 6”
start to resolve the cluster into its individual stars.
Leaving Lyra, Delphinus is the next destination,
an overlooked constellation lying south east of
Cygnus. Despite it being small, it is conspicuous
when you ignore Cygnus!
Map of the Hercules Globular Clusters
happy
to
hunt
with
For those who are
telescope
rather
than
binoculars, NGC 7006 is to the east of the Dolphins
41,000 light years and a diameter of 190 light
head and is one of the furthest globulars lying
years, comprising some 1,500,000 stars and is the
some 185,000 light years away. It does need large
most concentrated of all the Messier globulars. It
aperture to obtain a sensible observation, but it can
can be found on a straight line drawn down from
be found in a 6” as a non star. To the South of the
M15 to just inside the boundary of Aquarius and is
tail is NGC 6934, again non resolvable visually, but
easily spottable in binoculars - if you happen to be
spottable in a 6”.
in an area with clear mountain air you may just
spot it naked eye!. M72 is even more distant than
M2 and is 58,000 light years away. Like M71, this
globular is classed as a very open globular and is
spread across 98 light years of space. Locating it is
not too tricky but it is farther down than M2. From
M2 come down in a straight line to the 8 th
magnitude
star Sadalsuud then follow a straight
line to beta Capricorni along which M72 is half way
across. M72 is worth seeking as close by lies the
Saturn Nebula, and M73 an open cluster.
Of course, many other globular clusters lie in
and around Saggitarius, which is an article in itself.
If you’d like to learn more about clusters both
Summer Trangle Globulars
globular and open, then “Star Clusters” by Brent
Pegasus holds M15, described by Messier as
Archinell and Steven Hynes seems to be the
standard
“Nebula without star; it is round, the centre of it
work,
but
I
hope
this
article
and
accompanying maps will stimulate you to go and
is brilliant, 3’ diameter”
seek some of these celestial long term denizens
this autumn, and get out under the night sky.
And by John Herschel as
“a magnificent globular cluster … comes up to a
perfect blaze in the centre … it has straggling
streams of stars drawing to a centre”
M15 lies around 30,000 light years away, a
diameter of 200 light years, and like M13 contains
many red giant stars, but with its greater distance
appears fainter and more compact. It’s also noted
as one of the few globulars to contain a planetary
nebula, Pease 1, which at magnitude 14, can be
located
with
a
appropriate filters.
large
aperture
telescope
and
M15 itself is easily seen with
binoculars with stellar resolution being achieved
with apertures of around 4”.
Comet Garrad
(C/2009 P1) was in the vicinity of M15 during the
2nd and 3rd of August and made for a brilliant sight.
Beneath the front legs of Pegasus lies Aquarius,
containing 2 Messier globulars (2 and 72) and a
faint NGC object.
M2 sits at a distance of nearly
Jonathan Gale