Saturday, 3 August 2013

Extragalactic objects in Andromeda galaxy (M31)

The Andromeda galaxy / Messier 31

Even though the H II regions in the Andromeda galaxy are not as prominent as the ones in the Triangulum galaxy, this bad boy beats M33 in globulars that's for sure. When compared to the Triangulum galaxy, ten of the brightest globular clusters are between 13.7 and 14.9 magnitude (15.9 - 17.6 in M33). In total over 300 globular clusters have been identified from the galaxy. That's not all: there's a whooping 60 brighter clusters than 16th magnitude. Should you choose to pursue some of these objects, a good photograph is required. The image to the left shows only a handful of globular clusters, is very simplified and presents just a small part of the entire Andromeda Galaxy. A good place to start is the 1981 Atlas of the Andromeda Galaxy by Paul W. Hodge. Observing Handbook and Catalogue of Deep-Sky Objects by Skiff and Luginbuhl has a basic map on page 17. Night Sky Observer's Guide Volume 1 has a fairly good photography map of M31 on page 16. There is an obvious error though: G78 is marked at the wrong position. If you're looking for a very complete list of objects in M31, the best one is Star Clusters by Hynes & Archinal containing literally hundreds of objects.

Even though only the brightest globular cluster is introduced, I encourage everyone to try to observe as many of them as possible. Several of the extragalactic globulars can be seen even with small apertures between 4 to 6 inches. So far I've been able to detect 7 of the brightest globular clusters using my 4.7" Sky-Watcher under typical truly dark skies of Finland. If you live south enough be sure to log NGC 1049 (Hodge 3) located in the Fornax dwarf galaxy. This globular cluster beats Andromeda's G1 in brightness (by over a magnitude) as do the three other globulars in the same galaxy.

OB association NGC 206

RA: 00 40 31 Dec: +40 44 16
NGC 206 is the most prominent object in the entire Andromeda Galaxy. What I can remember about this star cloud is that it should be "kidney-shaped" and it slightly is. Still, have we really gone so low that we're spotting kidneys at the eyepiece these days? Apparently so! In any case, NGC 206 is so bright and obvious under good conditions that I wouldn't be surprised if someone came forth and claimed it to be visible with a pair of binoculars. It is just so conspicuous and cuddly even using a simple 3 inch telescope. With little research it seems like the brightest star in the NGC 206 association and in the entire Andromeda galaxy is #12 by a paper by Odewahn (1987) at magnitude 16.0 (v). The star is marked in the image on the left.

OB association A 54

RA: 00 44 33 Dec: +41 52 30
This association is nearly as large as NGC 206 but a lot fainter. Generally listed as a magnitude 16 object it still is visible with medium apertures such as 8 to 10 inches of aperture. It appears as a NE-SW elongated, slightly mottled patch of light in the near the NE tip of the galaxy. It is even mentioned in the 1998 (2. edition) Observing Handbook and Catalogue of Deep-Sky Objects as being visible in a 10" telescope.

H II region C 410

RA: 00 44 25.1 Dec: +41 20 42
C 410 is the brightest H II region (+ and open cluster) in the Andromeda galaxy according to the book Atlas of Messier Objects. The object is located between the globular clusters G272 and G280. Not much else can be said about it as I have not seen this object myself.

Globular cluster G1 / Mayall II / NGC-224-G1 / SKHB 1

RA: 00 32 46 Dec: +39 34 40
As even the image above shows this is a big boy - especially when you keep in mind the distance of at least 2.5 million light years. As an example the remote globular cluster NGC 2419 is roughly 300 000 light years away. It is still uncertain if this object is actually a dwarf galaxy or simply a giant globular cluster. Compared to the biggest globular cluster in our Milky Way, Omega Centauri, G1 is twice as massive.[1]

G 1 forms a tight triangle with a pair of nearly 15th magnitude stars. With small apertures and small magnification this group forms a single, stellar point of light. Many observers say that in order to claim G1 as seen, one should resolve it from the two nearby stars. This can be done with small apertures such as 4 to 6 inches but requires high magnification and so good seeing as well. However, claiming G1 as seen does not require you to separate these two faint stars from the cluster. The non-stellar appearance is quite easily achieved even under suburban skies using 6 to 8 inches of aperture.

Finding G1 might require a bit of patience if you're unfamiliar with the field or star hopping in general (for some reason). Tom Trusock wrote in his 2006 Small Wonders column that: "While it's not a toughie to see - if you have sufficient aperture - it can be a real pistol to find."[2] I assume this means that G1 difficult to find but it makes me wonder why. Maybe it is aperture related? To me the globular is basically just two star hops away. I will not go into specifics but I will explain you which one I use and the one I think is the easiest. First locate the 5th magnitude star 32 Andromedae which should be visible at least with your finder. From this star simply move to the north-west until you locate the 7th magnitude star HD 2993. This star has two fainter stars to the south the last one in line is 9th magnitude PPM 65429. From here, move less than 15' to the west and here is G 1. The key is just to get the finder and/or stars orientated the right way. Once you get it, rest is easy.

Star AF Andromedae

RA: 00 43 33.1 Dec: 41 12 10

The hypergiant AF And is one of the most luminous stars known with an absolute magnitude of -10.6 (our Sun is +4.5). This irregularly variable LBV is generally fainter than 17th magnitude but has had two brief eruptions in 1970-1974 and 1987-1992 when it was somewhere in the 16th magnitude. When measured between September 1992 and January 1993 (by Szeifert, T. et al.) it showed some variability from 16.91 to 17.14 magnitude.[3]

Star AE Andromedae

RA: 00 43 02.5  Dec: +41 49 12
This too is a bit on the heavy side although not as massive as AF And. With an absolute magnitude of -9.4 it still ranks high (top 20) in the most luminous list. As with AF Andromeda, this is a LBV star which has irregular eruptions. The last eruption was back in 1928 when the star flared up (and was also discovered for the first time) to an impressive object brighter than 15th magnitude. Between September 1992 and January 1993 CCD photometry was taken from the the star and it showed only a little variability: 17.56 - 17.59.[3]

Star A1 Andromedae

RA: 00 44 50.57 Dec: +41 30 38

This star is located in a H II region A41. Out of the three most well known LBV stars in Andromeda galaxy, A1 And is the brightest. Once again photometry obtained between September 1992 and January 1993 showed this giant star at 16.18 - 16.59 magnitude.[3]


Atlas of Messier Objects. Ronald Stoyan et al. 2008.
ELLIPTICAL GALAXY? G. Meylan et al. 2001.
[2] Tom Trusock. Small Wonders: Deep Andromeda Satellite Galaxies, Star Clouds and Globular Clusters of M31
[3] HST and groundbased observations of the `Hubble-Sandage' variables in M 31 and M 33. Szeifert, T. et al.
Images copyrighted by The Digitized Sky Survey.