M33 M33 (NGC598) is a spiral galaxy located in the constellation Triangulum, and, as the most prominent galaxy in that constellation, is often called the Triangulum Galaxy.  It is located about 2.73 million light years from Earth and with a diameter of 61,100 light years is the third largest member of the Local Group of galaxies, with the Milky Way and M31, the Andromeda Galaxy, being the two largest.  M33 contains an estimated 40 billion stars, compared to 100-400 billion for the Milky Way and about a trillion for the Andromeda Galaxy. 

Under exceptionally clear and dark skies, keen-eyed observers can spot M33 with the unaided eye as a diffuse, extended object.  It is the farthest galaxy visible without optical aid, but it is a serious test of visual acuity and sky conditions.  (I have personally not seen it with the naked eye.) 

Although Charles Messier discovered the galaxy in 1764 and was published as object 33 in his catalogue, there is evidence that M33 may have been discovered over a century earlier by the Italian astronomer Giovanni Battista Hodiema, as he described an object in Triangulum fitting the description of M33 in his 1654 work De systemate orbis cometici, deque admirandis coeli characteribus.

M33 and M31 likely interacted with one another some 2 to 8 billion years ago and there is the possibility that M33 will have a more violent encounter with M31 in about 2.5 billion years.  Meanwhile M31 is heading towards our Milky Way galaxy and will eventually collide.  How M33 will participate in this gravitational dance of behemoths is uncertain. 

M33 has an unusually large number of HII regions of active star formation.  Many of these can be seen visually through moderate to large aperture amateur telescopes.  The largest of these is NGC604, a vast region of star formation that spans 1500 light years across, which is about 40 times the size of the Orion Nebula in our galaxy.  Other important HII regions include NGC588, NGC592, NGC595, IC131, IC132, IC133, IC136, IC137, IC139, IC140, IC142, and IC143. 

M33 also contains 54 known globular cluster, of which C39 is the brightest.  Some of these can be identified visually through large aperture amateur telescopes.  

This image was produced from data I acquired in 2016, shortly after I installed the Paramount MX+ in my observatory.  For a few months, while waiting for a larger instrument, I had mounted my 4” Borg refractor on the mount and a friend loaned me his color SBIG camera for some test imaging.  I acquired over nine hours of data for M33, but I was not happy with the result I achieved in processing the data.  I set it aside, intending to process it again when I found the time.  Over eight years later, I finally revisited the data and was able to produce a more satisfactory result.