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Small Wonders:
Deep Andromeda
Satellite Galaxies, Star Clouds and Globular
Clusters of M31
Tom Trusock
11/06
Image Courtesy Jeff
Thrush
| Object |
Type |
Size |
Mag |
RA |
Dec |
| M 31 |
Gx |
189.1'x61.7' |
3.5 |
00h 43m 07.7s |
+41° 18' 40" |
| M 32 |
Gx |
8.5'x6.5' |
8.1 |
00h 43m 05.1s |
+40° 54' 29" |
| M 110 |
Gx |
19.5'x11.5' |
7.9 |
00h 40m 45.4s |
+41° 43' 39" |
| NGC 147 |
Gx |
13.2'x7.8' |
9.4 |
00h 33m 35.0s |
+48° 33' 00" |
| NGC 185 |
Gx |
8.0'x7.0' |
9.3 |
00h 39m 21.2s |
+48° 22' 47" |
| NGC 206 |
Star Cloud |
4.2' |
|
00h 40m 55.5s |
+40° 46' 50" |
| G1 |
GC |
10" |
13.7 |
00h 33m 09.7s |
+39° 37' 14" |
| G119 |
GC |
2" |
15 |
00h 42m 16.3s |
+40° 49' 43" |
| G213 |
GC |
2" |
14.7 |
00h 43m 37.8s |
+41° 09' 52" |
| G272 |
GC |
3" |
14.8 |
00h 44m 37.9s |
+41° 21' 52" |
| G280 |
GC |
2" |
14.2 |
00h 44m 53.2s |
+41° 24' 08" |
| G72 |
GC |
2" |
15 |
00h 41m 16.1s |
+41° 21' 16" |
| G73 |
GC |
|
15 |
00h 41m 18.9s |
+41° 44' 00" |
| G76 |
GC |
3" |
14.2 |
00h 41m 22.3s |
+40° 38' 20" |
| G78 |
GC |
3" |
14.3 |
00h 41m 24.5s |
+41° 16' 17" |
Target List
Introduction: M31 and associated
objects
If you're a long time reader of
this series you're probably asking yourself - "Andromeda? Didn't we do
that already?" The answer is, of course, yes. But not
like this. There are certain areas in the sky that deserve
special treatment, and M31 is one. From time to time, we'll take an observers
"diversion" to cover a few topics that can't be covered in a typical
Small Wonders. This
article covers Messier 31 and some of it's attendant galaxies and
DSOs. Of necessity, the article is therefore somewhat
slanted towards
larger scopes, but a surprising number of these targets can be caught
in fairly small apertures.
Late fall / early winter is a wonderful time to observe in the
northern hemisphere. The skies are clear and crisp with the haze
of summer long gone. It's a time that brings it's own
unique challenges for many of us, but there is something to be said for
being able to observe
by 7pm. Many is the night I've stepped out and looked up
into a velvet black sky. My eye is naturally drawn toward zenith, and I
can hardly overlook M31. The Andromeda galaxy is one
of the best known deep sky objects in the night sky. Easily visible to
the naked eye from a clear, semi-dark site, it’s
fullest
extent is open for debate. Recent, conservative reports (Skiff
and Luginbuhl, French) place
it close to 3 degrees in width, Walter Scott Houston wrote that he'd
once measured it close to 5 (10 full moon diameters!), as did
Robert Jonckheere
in 1953. Take some time to measure it for yourself and
see what you come up with. Could the difference in measured
length be due to a decline in sky conditions over the
years?
Images Courtesy John
Graham - M32 and M110
Many know that Messier 31 is surrounded by its own attendant
host. Indeed,
M32 and M110 greet us in our low power views. I've always found
it ironic that these
two galaxies would be spectacular objects in their own right, but
are relegated to
side-show status when accompanying the king of the local group. Some
astronomers don't realize these aren't the only attendants easily
visible for the amateur. NGC 147 and NGC
185 lie some seven degrees away are often overlooked. From very
dark skies, I've barely spotted NGC 185 as a subtle background
brightening in a 70mm scope. I find it a fair amount easier in my
4"
refractor, although not bright by any means. From my rural skies,
I find NGC 147 is barely visible in 4" of aperture.
M31, M32, M110,
NGC185 and NGC147
I’ve viewed M31 in everything from 12x36mm binos to 20+ inch
Dobsonians, and I've found that each size has something to offer. I
generally prefer smaller apertures so as to take in more than just the
central bulge. My favorite views of M31 have typically come
through one of my 4” refractors, although I do remember one absolutely
spectacular view of the dust lanes though an 18” telescope.
But as beautiful as those objects are though, they aren't the main
focus
for this article.
It’s time to go deep and observe some deep space objects inside
Messier 31 itself.
NGC 206 - The Great Star Cloud
in Andromeda
You may remember the article I previously wrote on
Andromeda. In it I gave G1 (Mayall II) as a challenge object to
cap off your observing run. It may surprise some to know that G1
isn't the most visible DSO in M31 – not by a long shot. That
honor goes to NGC 206, which is visible in small apertures if you know
what
you’re looking for. NGC 206 is a giant star cloud located in
the southwestern arm, large enough and bright enough for William
Herschel to give it a separate catalog number following his observation
of it in October of 1786. Some observers have found this stellar
association is visible
through a 4” scope. To look for it, imagine an isosceles triangle
with two corners at M32 and M110. The third corner, lying away
from the nucleus of M31,
would be NGC 206. If the star cloud isn't immediately obvious
try using moderate
powers and look for a somewhat sharply defined eastern
edge. Reference the image below for assistance.
M31 Original Image
Courtesy Jurij Stare - NGC 206 is the exploded area
According to a 1997 paper in the publication Astronomy and
Astrophysics, authored by Magnier, Prins, Augusteijn, van Paradijs, and
Lewin, 206 is probably the largest and most massive region of star
formation in the local group. It's actually located at the
intersection of two spiral
arms, suggesting that the interaction between the arms is what’s
fueling the star formation. Further, they place the age somewhere
between
20 and 30 Myr. For those interested, the full article
can be accessed here.
There are several other open clusters and stellar associations visible
to the amateur in M31, for an excellent reference, see pg. 17 of
Luginbuhl and Skiff’s: Observing Handbook and Catalogue of Deep-Sky
Objects. Some of the best defined objects however, are the
globular
clusters. A paper published in late May 2006 (http://arxiv.org/abs/astro-ph/0605718)
indicated that there were on the order of 97 currently known
(confirmed) clusters
in M31, with many more suspected. Of these, probably some 23+
globulars are visually detectable by amateurs (with large enough
scopes).
Bright Andromedean Globular Suspects
|
| ID |
Mag
|
G1
|
13.7
|
G76
|
14.2
|
G280
|
14.2
|
G78
|
14.3
|
G213
|
14.7
|
G272
|
14.8
|
G72
|
15.0
|
G119
|
15.0
|
G64
|
15.1
|
G219
|
15.1
|
G257
|
15.1
|
G172
|
15.2
|
G302
|
15.2
|
G244
|
15.4
|
G256
|
15.4
|
G279
|
15.4
|
G96
|
15.5
|
G226
|
15.5
|
G87
|
15.6
|
G305
|
15.6
|
G54
|
15.7
|
G2
|
15.8
|
G287
|
15.8
|
We'll take a look for 9 or so of the brighter and easier to find
Andromedean globulars. For most of the targets in the rest
of the article, I'd recommend a 10 inch or larger telescope. (G1 is the
notable exception here
– I’ve heard reports of it being caught in apertures as small as
5”.) Tracking
ability helps as well, because we'll be working at moderate to high
magnification most of the time.
M31 Globular Resources
Image Courtesy - Pete
Kennett
Excellent charts are a requirement. Although the charts for this
series are usually embedded in the article itself, this isn't really
optimum
for this approach, therefore I’ve provided a link to three full scale
finder charts for the globulars mentioned in this article. I’d
suggest printing them off for reference at the scope:
I still have charts here so the reader can follow along, and my star
hop
comments are really addressed to the ones in this article - I expect
with the full charts, people can plan their own star hops. Please
note that the globular finder charts display stars to around magnitude
15 except as noted. In addition, again, I’d recommend Luginbuhl
and Skiff’s: Observing Handbook and Catalogue of Deep-Sky Objects as a
further reference for those who wish to explore further.
When hunting for the globs in M31, I find I typically use medium powers
(150-180x) to star hop, then confirm the field with high power
(250-300).
Finding Andromedean Globular
Clusters
Central Andromedean
Globulars and NGC 206
G119
Lets start with G119. First center the core of M32
in your scope, then move SW slightly - about 1/3 of the way to NGC 206,
and look for an offset trapezoid
(see the attached charts for a better view) that lies to the south of
G119 lies in. Switch to higher power and
averted vision should bring out a faint point of light in exactly the
right
place. To all visual appearances this is a simple foreground star, but
check your charts to confirm it is one of the remote globulars in the
Andromeda galaxy.
G76
Now lets head a bit further southwest in search of
G76, one of the brightest globulars in M31. Notes
from a session with my 15”, indicate I reached a triangle of stars that
included GSC 2801-2059, in the general area. In the position
indicated by my charts I found not one, but two points of light very
similar in magnitude. If you’re using a large scope try using
high power to determine which one is a foreground star, and which is
the globular - I've seen some reports that indicate in larger apertures
G76 shows some extent. Personally, I believe the western most one
is most likely the
globular – what do you think?
G213
Travel back to M32 again in order to begin another star
hop along the outskirts of the galaxy. From M32 head NE to
G213. In my 15”, I found this faintly visible with averted
vision. Interestingly, it appeared extended, but I this may have
been due to poor seeing (or averted imagination) rather than my
detecting any actual extension in the globular itself.
G280 and G272
On to G280 and G272. In my 15”, at 168X the field containing G280
and G272 is marked by a curious asterism of 5-6 stars that resembles a
mushroom. G280 lies slightly NE of G272, and seems ever so
slightly harder to hold in view. Both appear as faint star like
points of light. My notes indicate that G272 was somewhat hard to
hold at 168x because of the glare off a nearby bright star. Use
of a narrow AFOV eyepiece and increasing the power to 315x tightened
the field and brought globular out better.
G73
From here, head up across M31 to M110. There on the eastern side
of M110 lies our next target: G73. Again, in my 15” scope, I
found G73 quite easy to hold and recognize. Look for a chain of
stars parallel to M110 on the eastern side.
G78 and G72
Drop south to G78 and G72. I'll confess I know I’ve hit the
general area, but
I’ve yet to confirm G78. There were multiple points of light in
the correct general area - all similar magnitude, and none of them
exactly where my charts show G78. Can you pick it out? G72
on the other hand was easily
visible w/ direct vision 90% of the time.
From here, it’s time to begin the long (yet surprisingly easy) hop down
to G1 (Mayall II) the giant globular in the SW suburbs of M31.
G1
According to Wikipedia,
G1 (also known as Mayall II, MII, NGC-224-g1,
SKHB 1, GSC-2788:2139, HBK 0-1 and Andromeda’s Cluster) is the
brightest cluster in the local group, and lies some 170,000 light years
from the galactic core. Fans of the spectacular Omega Centarui
might be surprised to learn that G1 is suspected to have twice that
jewels mass. There is apparently some debate, due to the apparent
indications
of multiple star generations inherent in the cluster, it may not be a
true galactic cluster at all, but
the remains of a dwarf galaxy consumed by Messier 31. Other
astronomers
postulate that G1 could have been formed during tidal interactions with
a dwarf galaxy.
What's truly amazing is that you can easily make out some extent to
G1 through a moderately sized back yard telescope. It's not
just a point source like most of the other globulars we visited this
evening. While one is a long long way from resolving
individual stars, you can clearly see that there is something there -
especially when you compare it to the two foreground stars (mag 14.3
and 13.6 respectively) that flank it. At mag 13.7, it's a
somewhat dim target though, so the more aperture you can throw at this,
the better your chances of finding it. It's certainly doable in a
10" scope from a decent site, and more than likely is cacheable in an
8" scope from a very dark site. I've even heard rumors of folks
grabbing it in a 5" scope.
While it's not a toughie to see - if you have sufficient aperture - it
can be a real pistol to find. Conceptually, this is a spectacular
target.
G1 Star Hop Chart
Start your star hop at the nucleus of M32 and work down to the asterism
indicated in the chart - keep in mind the area indicated is 1/2 to 1/3
of a degree wide. Once you've found it, then star hop over
towards G1.
G1 Eyepiece Finder Chart
I've flipped the image in this chart to aid star hopping at the
eyepiece.
Note the circled grouping of stars in the chart above - in a moderately
sized scope, this grouping bears a strong resemblance to
Cassiopeia. Once you’re in the right area crank up the power and
begin
inspecting multiple stars in the area. G1 lies almost in the
middle of two similar magnitude foreground stars, and that's a big help
when it comes to fishing it out. At moderate powers I’ve found it
can look like a triple star. Under high
magnification, it will resemble Mickey Mouse - the two foreground stars
are the ears, and Micky's head - somewhat extended is G1.
The DSS image should resemble what you see. Be sure to pump up
the magnification, and you can see that it's not quite a stellar point.
While I've caught it in my 10" and noted extent in my 15" and 18"
scopes, the best view
I've ever had was when I was observing with Gary Gibbs
in his 20" with a Collins I3 image intensified eyepiece. The
barest glance showed that it wasn't a star - it presented a star like
core with a dimmer coma around the outside and reminded me of tiny dim
Milky Way globulars I've caught in one of my small scopes.
This view through the Hubble Space
Telescope should help give you a better idea of what you're actually
seeing.
As always, I'm gratified that folks find this column useful.
Till next time -
Tom T.
Addendum 11/26/06
Bob Abraham, an astronomer with the University of Toronto
contributed the following:
"...in the section on G1, it is noted that
this object is more massive than Omega Cen, and that G1 might be the
core of a stripped dwarf galaxy, and not a "true" globular cluster as a
result. Interestingly, Omega Cen itself also shows multiple stellar
populations, and many people have recently argued that it also is a
good bet that Omega Cen is the core of a dwarf galaxy that has suffered
multiple collisions with the Milky Way. At this point, it's probably
premature to suggest that either object is not a "true" globular
cluster, because we don't yet know much about the rate of dwarf galaxy
destruction, and some popular models for galaxy formation actually
require the rate of dwarf galaxy destruction to be very very high, so
quite a few similar objects might be out there. (That said, most
globular clusters have very uniform old stellar populations, so its
unlikely that most formed via dwarf galaxy accretion unless this
happened a very long time ago)."
".... there is quite a bit of
controversy about the subset of supposedly "young" globulars in M31
(where "young" in this context means "less than 5 billion years old").
As noted above, most M31 globulars are rather older than this, but a
few years ago some astronomers claimed to have discovered a population
of young globulars in M31. Last year some observations with Keck's new
adaptive optics system were able to resolve these objects and showed
quite convincingly that most of these objects were open clusters or
asterisms, and not globulars at all. Here's a link to a PDF version of
the article:
http://arxiv.org/pdf/astro-ph/0510631
Comparing Figure 1 of this
paper to the HST image of G1 shows that adaptive optics is really
beginning to deliver the goods."
Note: Figure 1 can be found at the end of the paper.
Additional Resources / References
Island Universe: Sue
French, Sky and Telescope,
December 2005
Observing Handbook and Catalog of Deep Sky Objects: Luginbuhl
and Skiff
Online
Papers / Abstracts:
G1 in
M31 - Giant Globular Cluster or Core of a Dwarf Elliptical Galaxy?
Cepheids
as tracers of star formation in M 31. II. NGC 206: evidence for spiral
arm interactions.
Formation
of giant globular cluster G1 and the origin of the M 31 stellar halo
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