Saturday, 3 August 2013

Extragalactic objects in Triangulum galaxy (M33)

The Triangulum galaxy / Messier 33

The Triangulum galaxy is the best galaxy in the northern hemisphere when it comes to extragalactic objects. The easiest targets in M33 are the numerous H II regions and stellar associations. Globular clusters are numerous, but a lot fainter than the ones in Andromeda Galaxy, with ten brightest ranging between magnitudes 15.9 and 17.6.

There is a little problem when it comes to the H II regions. Finding an appropriate designation for an object can be quite a mess. This is mostly thanks to Guillaume Bigourdan whose original coordinates are inaccurate. Feel free to blame all the modern authors as well. They can't seem to be able to make up their minds which object is which. If you look at two modern sources and try to match the designations together, you are quite often in for a treat! The identities of NGC 588, 592, 595 and 604 are pretty certain. With the IC objects it isn't quite so. Several sources seem to agree on IC 135, IC 136, IC 132 and IC 133 as well. NGC/IC-project has a pretty good base but since you can't ask directly from Bigourdan himself, their fixed are guesses and estimates at best no matter how good, convincing and true they might be. There are numerous differences and errors out there in different books, software and internet sites. Here are some just to name two:

IC 143 is labeled twice on NSOG Vol1 (page 392) obviously the region to the nucleus is IC 142.

IC 137 problem. "Atlas of the Messier Objects" show IC 137 at 01 33 11 +30 29 53. Megastar V5 shows the same object as IC 136 and a map with in article by Steve Gottlieb[2] plots the same region just as A 127 and show IC 137 far away at 01 34 14 +30 34 31. Where is IC 137?

So what to do? In my opinion the best bet is to name the few NGC and IC objects correctly, then use the association designations as listed in the 1991 paper "The stellar populations of M 33" by Sidney Van Den Bergh[1] since they're correctly listed in nearly all publications. At least for me this saves a lot of time and frustration but enough history, time to go forward with the actual objects.

H II region NGC 604

No introduction is needed when it comes down to NGC 604. The H II region is the bomb: it is nearly 1500 light years in size, estimated to come only second to the Tarantula nebula in the entire Local Group of galaxies. The Orion nebula truly pales in front of this nebula. With these characteristics, it is no wonder NGC 604 can be observed with pretty much any telescope and larger binoculars. With a modest size telescope it looks like a slightly elliptical glow with a brighter center. In my opinion, this is the easiest extragalactic object in the northern hemisphere. NGC 206 comes close but its appearance is more diffuse and surface brightness lower. The brightest individual stars in the central region of NGC 604 are roughly 16th magnitude.

Globular cluster C 39 (Mayall "C")

It might not look like much but it is something alright. Also known by its GSC number (2293:1339), C 39 has a visual magnitude of 15.9 so it is right at the edge of a good 8" telescope under pristine conditions. This will be a great challenge.

Globular cluster C 27

This second brightest globular cluster in M33 is considerably fainter than the previous one, being close to 17th magnitude visually although the commonly given v magnitude is 16.5. This will require larger aperture. Coordinates for this one are 01 34 43.7 +30 47 38.

Globular cluster U 49

These two globulars appear very faint in photographs but are usually listed as brighter than C 27. U 49 is located at 01 33 44.3 +30 47 32.9 and listed at 16.2 magnitude. The B magnitude for the object is 16.3.

Globular cluster U 62

U 62 (listed as U 43 in the book Atlas of the Messier Objects, page 155) is slightly fainter and located at 01 34 10.5 +30 45 48.7. This stellar point seems to be a double object, so the combined magnitude of these two are 14.8 (v). The globular is the fainter object of this two, listed as 16.5 magnitude.

Star B 342

This is it: the brightest individual star in M33 - if you exclude the LBV stars. Star known as B 324 is an A-type supergiant and lies just 6' from the center of M33 in the star association 67 (A 67) or IC 142. The v magnitude of this star is 15.2 but is this really in M33 or just another Milky Way star? Lundmark (1921) listed the brightest star in the galaxy to be 15.7 (B) magnitude. Humphreys, Massey & Freedman proved in 1990 that B 342 is indeed part of M33 and also the brightest in single star in the entire galaxy. Considering the magnitude, this should be a fairly easy catch with telescopes 8" and larger. Coordinates: 01 33 55.9 +30 45 30.4.

Star GR 290 (Romano's star)

This object is a LBV (Luminous Blue Variable) star. "It shows eruptions with amplitude of more than 1 mag and timescale of about 20 years and smaller oscillations with amplitude 0.5 mag and a period of about 320 days"[3]. The star varies between magnitude 16.5 and 17.8 so it isn't exactly for medium apertures. This star lies close to A 89. Coordinates: 01 35 09.7 +30 41 57.4.

Don't forget to check out the LBV stars Var B, Var C and Var 83 (absolute magnitude of -11.1!) in the galaxy. As with Romano's star, these are truly massive stars varying between magnitudes 15 down to 16.5 and from time to time display eruptions making them even brighter. For example Var C in M33 is listed as varying between magnitudes 15.2 - 16.5 making it in range to moderate size telescopes[4]. Good luck!


Atlas of Messier Objects. Ronald Stoyan et al. 2008.
[1] The stellar populations of M 33. Sidney Van Den Bergh. 1991.
[3] ROMANO’S STAR IN M33 - LBV CANDIDATE OR LBV? R. Kurtev et al. 2000.
[4] 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.