Mayall II (G1)
The Brightest Globular in M31
Imaging Globular Clusters in M31
Atlas of the Andromeda Galaxy
With a special opportunity to observe faint deep sky objects in August at the San Francisco State field campus at the Sierra Buttes (2200m elevation), I found myself spending several hours exploring one of the most familiar objects in the sky, M31-the great spiral galaxy in Andromeda. Sure, M31 is always worth a look to see its large bright core, immense halo and striking dust lanes, but as we view our galactic neighbor only 12.5° from edge-on we unfortunately miss the stunning spiral arms visible in M51 and the numerous HII regions of M33 seen in a large scope. Jim Shields mentioned to me that he had brought along two issues of Deep Sky (#8 and #32) which discussed in detail the globular and open cluster system of M31 and provided finder charts for tracking down these obscure objects (similar information can be found in Luginbuhl & Skiff's Observing Handbook) and I immediately knew I had an interesting observing project for the week!
Edwin Hubble, whose pioneering work on the M31 Cepheid variables led to resolving the debate over the nature of the "spiral nebulae", also published the first catalogue of 140 globulars in 1932. The globulars he found ranged from 15-18 blue photographic magnitude and 4-10 arcseconds in diameter. At 2.2 million light years, the globulars of M31 are roughly 100 times as distant as M13 in Hercules and hence are 1 / 10,000 as luminous if placed at a similar distance. This translates into a ten magnitude difference and at 14.3 (visual) magnitude, the brightest five globulars in M31 are comparable in absolute magnitude to 4th magnitude Omega Centauri (Mv = -8.5), the brightest within our galaxy.
As many as 509 globulars in M31 have been catalogued based on spectra plates taken at Mauna Kea with the 3.6 meter Canada-France-Hawaii telescope, (Crampton et al, 1985), although recent work indicates that some of these are faint background galaxies. In any case, the M31 globular cluster system does appear to be more numerous than our own galaxy by roughly a 2:1 ratio. This important article provides identifications, positions, magnitudes, colors and radii although no finder charts are given.
As the M31 globulars will appear stellar except for Mayall II, you won't be able to identify the clusters without a good finder chart. Useful photos can be found in the Deep Sky articles as well as Paul Hodge's Atlas of the Andromeda Galaxy which contains 41 large scale photographic charts. These should provide ample ammunition for tracking down the M31 globulars with 10-inch or larger scopes.
I found it really quite easy to star hop around in the vicinity of M31 as the star fields are not densely packed and very easy to recognize small asterisms on the photos. In fact, most globulars were picked up by star hopping at low power (x100 for me) with a 20mm Nagler within a minute or two and then the identification was pinned down at x220 with a 9mm Nagler by comparison with a photo. . After three hours working over the field on two nights with my 44cm telescope, I had picked up a dozen globulars as well as five open clusters , three stellar associations and of course the giant star cloud NGC 206. It was also clear that I will have to return to M31 again, as I really just started working on the fainter globulars.
All the globular and open clusters appeared stellar (since I was viewing the brighter core), except Mayall II, although the catalogued dimensions are generally 3-4 arcseconds. I found the brightest globulars after Mayall II to be #76, #78 and #280 in Hodge's atlas which had photometrically determined magnitudes of 14.2-14.3V and all were easily visible with direct vision. In some cases, two or more globulars are visible in the same field. As you move closer to the core, the globulars are more difficult to pick out as they are reddened by dust and gas and lose contrast being superimposed on the brighter glow from the galaxy. The fainter globulars I viewed ranged from 15.0-15.5V and required averted vision, though I was able to confirm the sightings after carefully checking the charts. Finally, I also easily tracked down #73, which turns out to be a 15th magnitude stellar globular in M110 = NGC 205, the large companion galaxy northwest of M31. If you are looking for a challenging observing project with a 30cm or larger scopes, this is a great one for the fall as M31 passes high overhead and it was an exciting project to be exploring extragalactic globulars at immense distances.
M31-G2 00 33 33.8 +39 31 19 V = 15.8
17.5" (9/28/02): at 324x, this M31 globular appears as a mag 15.5 (V = 15.8)
"star" just 51" SSW of a mag 14 star and 2' SW of UGC 330! M31-G1, the
brightest globular in M31, lies 10' WNW.
M31-G52 00 40 20.3 +40 43 59 V = 15.7
17.5" (01/12/02): this 16th magnitude M31 globular was extremely faint, stellar, just
visible continuously with averted and concentration at 380x. Two mag 13 stars
lie 2.0' S and 3.7' SW. Situated just 2' W of the starcloud N206 at the SW
end of M31!
M31-G64 00 40.5 +41 22 2.3" 15.1
Very faint, visible with direct vision as a 15th magnitude "star", a 13th
magnitude star is 45" northwest, located 20' west of the core of M31 and
20' south of M110.
M31-G72 00 40 52.8 +41 18 54 2.2" 15.0
This M31 globular appears as a mag 15 star located 1.5' WSW of mag 12.5 star. There is a 1' pair of mag 12.5 stars situated 5' S which are nearly collinear with the globular.
M110-G73 00 40.9 +41 41 15.0
Very faint, visible with direct vision as a 15th magnitude star, on line
with a mag 8.5 star 2.5' north and mag 10 star 6.5' north. Located 6' east
of the center of M110. This is the brightest globular cluster within M110.
M31-G78 00 41.0 +41 14 3.2" 14.3
Faint, easily visible as a star of magnitude 14.3, forms west vertex of
slightly obtuse triangle with 2 stars 12.5 magnitude oriented north-south
and situated 2' east. Located roughly 30' west of the core of M31. One of
the 2 or 3 brightest globulars in M31.
M31-G76 00 41.1 +40 36 3.6" 14.2
Faint, easily visible as a stellar object 14.2V located in small "W"
asterism and very close northwest of a similar star 14.2-14.4, three
similar brighter stars within this "W" also form a distinctive obtuse
triangle. Located on south side of southwest tip of M31, C107 is in the
field 6' west. Brightest globular in M31 along with G78.
M31-G87 00 41 14.7 +40 55 52 V = 15.6; Size 2.9"
This extremely faint M31 globular was not seen at 220x but
picked up at 280x. Forms the eastern vertex of a small right triangle with a
mag 13 star due W and a mag 14 star to the SW. Located 17' WNW of the core
of M32.
M31-G96 00 41 28.3 +40 53 50 V = 15.5; Size 2.7"
This mag 15.5 M31 GC appeared extremely faint at 280x but easier than
M31-G87 situated just 3' NW.
M31-G119 00 41.9 +40 47 2.7" 15.0
Faint, appears as a 15th magnitude "star" about 2' north of a trapezoid
formed by four stars 13th magnitude. Mag 7 SAO 36585 is in the field 7'
south-southeast.
M31-G156 00 42 25.3 +40 57 18 V = 15.6; Size 2.5"
This M31 globular appeared as an extremely faint mag 15.6 "star" just
1' W of a mag 9.5 star and 6' NNW of the core of M32!
M31-G172 00 42 33.3 +41 03 29 V = 15.2; Size 2.4"
Very faint, stellar, mag 15-15.5. This M31 globular forms the south
vertex of an equilateral triangle with mag 10.5 and 12.5 stars ~2.5' NW and
NE. Located 12' N of the core of M32 and 13' S of the core of M31.
M31-G213 00 43.2 +41 07 2.5" 14.7
Very faint, visible continuously with direct vision as a mag 15 "star", an
11.5 mag star is 1.5' east-northeast. Located ~10' southeast of the center
of M31 near the edge of the central haze.
M31-G219 = Mayall IV = IV Zw 30 = Mrk 959 00 43 17.8 +39 49 13
V = 15.1
17.5" (01/12/2002): picked up at 220x as a very faint mag 15 "star", possibly
quasi-stellar. At 380x, appears as barely nonstellar glow, ~2" in diameter.
Easily visible at this power.
M31-G226 00 43 30.3 +41 38 56 V = 15.5; Size 3.8"
17.5" (9/28/02): extremely faint, threshold globular in M31 at 324x. Located
43" SE of a mag 123 star. Seemed fainter than listed V magnitude of 15.4,
although transparency was mediocre.
M31-G233 00 43.6 +41 08 2.6" 15.4
Extremely faint, appears as a threshold "star" mag 15.5 visible 70% of time
with averted vision, a mag 12 star is 1.5' northwest and G213 is 4.5' west.
M31-G244 00 43.8 +41 37 2.6" 15.4
Extremely faint, appears as a mag 15.5 star, cannot hold continuously but
easy to locate just 1.5' south of distinctive line of three equally spaced
mag 13 stars of 2' length, a mag 9 star is 5' southeast.
M31-G256 00 43 58.2 +41 24 38 V = 15.4; Size 3.1"
17.5" (9/28/02): extremely faint, appears as a mag 15.5 "star" situated 1.6'
ENE of a mag 10.5-11 star. Viewed at 324x and only visible part of the time
with averted.
M31-G257 00 44.0 +41 30 3.2" 15.1
Very faint, although listed as 15.0V this globular is fairly difficult as
it is superimposed on obvious haze at the northeast edge of the core of
M31, can just hold steadily with averted vision, mag 9 SAO 36609 is just 4'
north.
M31-G272 00 44.2 +41 19 3.4" 14.8
Very faint, appears as a mag 15 "star", just visible continuously with
direct vision, a mag 10.5 star is 1.5' northwest, C410 is just 2.5'
northeast and brighter G280 is 4' northeast.
M31-G280 00 44.5 +41 22 2.7" 14.2
Faint, appears as an easily visible mag 14.3 star, perhaps slightly fainter
than G76 and G78, a similar star is 1' east-southeast, forms a pair with
C410 just 1.2' southwest, also G272 is in field 4' southwest. Located along
east side of northeast extension of M31outside of prominent central region.
M31-G279 00 44.5 +41 29 4.9" 15.4
Extremely faint, appears as mag 15.5 star located midway between two stars
separation 1.5' oriented west-southwest/east-northeast, just visible at
x220, better at x280 although cannot hold continuously.
M31-G287 00 44 42.1 +41 43 55 V = 15.8; Size 2.2"
17.5" (9/28/02): marginal 16th magnitude globular in M31 located 2.5' WSW of
a mg 13/14 double in the NE portion of the outer halo. Only glimpsed
occasionally at 324x.
M31-G302 00 45 25.1 +41 06 22 V = 15.2; Size 2.5"
17.5" (9/28/02): faint, stellar. This is a fairly easy mag 15.2 globular
located 6' SSW of mag 9.1 SAO 36631 and collinear with mag 12 and 13 stars 5'
and 6' NE.
M31-G305 00 45 41.9 +41 45 33 V = 15.6; Size 2.2"
17.5" (9/28/02): identified at 324x as a very faint, stellar mag 15.6
globular at the NE end of M31. Located 47" NW of a mag 13.5 star.
M31-C107 00 40.5 +40 36
Very faint, very small hazy spot with faint stellar core, forms right
triangle with two 13th magnitude stars 1.5' north and 1' north-northwest,
can hold continuously with averted and definitely nonstellar, in field with
G76 5' east.
M31-C202 00 42.1 +40 57
Faint, extremely small, ~6", easily visible as the slightly fainter
southern member of a very close nebulous pair with C203 15" north, just
cleanly resolved at x220. Located southwest of core of M31 and 9' northwest
of M32.
M31-C203 00 42.1 +40 57
Faint, extremely small but nonstellar, similar size but slightly brighter
than C202 just 15" south.
M31-C410 00 44.4 +41 21
Faint, very small but easily visible as a clearly nonstellar haze,
elongated 2:1 north-south, ~20"x10", a mag 10.5 star is 3' west, located
between G280 1.2' northeast and G272 2.5' southwest within the association
33.
M31-C285 00 44.7 +41 27
Threshold object just glimpsed at moments, located between two stars
separation 1.2' oriented southwest-northeast, faintest of 3 with C306 and
C406 in fieldto northeast. On the Hodge's M31 atlas , this group consists
of C284, C285, C286 located within association 40.
M31-C406 00 44.8 +41 29
Very faint, substellar spot, forms a close double with star 14.5 just
south, a close faint double star with similar magnitudes is 2.5'
east-southeast, 2nd brightest of three with C306 4' northeast and C285 3'
southwest. G279 is located just 4' west. This is the dominant cluster
within association 41 on Hodge's atlas.
M31-C306 00 44.9 +41 31
Fairly faint, appears as a mag 13 star with a mag 12 star located 2'
northwest, easily visible at 100x and prominent at 220x, brightest of three
with C406 and C285 on line to southwest, located along eastern flank north
of core. On the atlas, this description may apply to a mag 13.5 star or
C306 located within association 42.
M31-C312 00 45.2 +41 37
Threshold object, very small elongated glow requiring averted, located 2'
north of line connecting two stars 12-13 magnitude oriented WNW/ESE with a
separation 6'. Based on the atlas, I probably glimpsed the unresolved glow
of C311, C312, C313 with a total length of 1.5'.
N0206 = M31-A78 00 40.6 +40 44 4.2x1.5
Fairly faint, fairly large, elongated 5:2 north-south, 4.0'x1.6', low and
uneven surface brightness, a few very faint stars are just visible over the
surface including a brighter star at the south tip, located over 1°
southwest of the core of M31. This object is a huge star cloud in M31.
