Triangulum Galaxy
(M33, NGC 598)
Due south at 21.56 (GMT) on 15 November.
Position: 01 hr 33.8 min +30 deg 40 min
Image: Martin Gill (HAS)
The Triangulum Galaxy – so-called because it is in the Triangulum constellation, it is not remotely triangular itself – or Messier 33 is relatively nearby, it is large and it is bright. Great, surely it must be easy to see, almost as easy as the Andromeda Galaxy (Messier 31). Alas no, it is in fact very difficult to see with the naked eye. On one level this is easy to explain, it is bright (it has an apparent magnitude of 5.7 which in fact means little when one is dealing with galaxies) but it is also very large, about twice the size of the full Moon (it is roughly two full Moons alongside each other). Our ability to see an extended astronomical object is governed by its magnitude divided by its size. The Andromeda Galaxy has the same problem, but being so much brighter it can be seen with the naked eye in darker areas. The Triangulum Galaxy has another problem: it is face-on to us so its features are not very obvious and it does not stand out against the background sky in the same way that the tilted Andromeda Galaxy does. So you need a good dark sky area to see it and using a telescope can actually make matters worse. I have just barely seen it in a dark sky area in Portugal and it was little more than a dull smudge. It was difficult to see even in binoculars (the best way to see it if you can). But as the above image shows, it is a wonderful sight when photographed. It is a member of our Local Group along with the Andromeda Galaxy and it is slightly further away from us (2.73 million light years against 2.54 Mly for the Andromeda Galaxy). So it is the most distant object that can be seen with the naked eye although experienced observers have claimed to see the far more distant Messier 81 in very dark mountainous areas. It is also smaller than the Andromeda Galaxy being only 60,000 light years across against the Andromeda’s 220,000 ly. It has formed bridges of neutral hydrogen with its big sister and it is slowly falling into the Andromeda Galaxy, eventually merging with it and the Milky Way. The Triangulum Galaxy was probably seen by the Italian astronomer Giovanni Battista Hodierna in the mid-seventeenth century and rediscovered by Charles Messier in August 1764.