Have you ever wondered how to find M33 the Triangulum Galaxy? In today's post of our Deep Sky with Dave Messier Marathon series, I will walk you through my four-step method for finding this amazing autumn celestial wonder.
M33, the Triangulum Galaxy, is the third-largest member of the local group of galaxies behind the Milkyway and Andromeda Galaxies. At 2.73 million light-years away, the M33 spiral galaxy is one of the most distant objects visible to the naked eye, and is potentially a satellite galaxy of the larger M31 Andromeda Galaxy and may have interacted with Andromeda Galaxy gravitationally in the past. M33’s name Triangulum is taken from the constellation in which it lies, the Triangulum constellation, and was likely first discovered by Giovanna Battista Hodierna before 1654, but was re-discovered and cataloged by Charles Messier on August 25th-26th 1764 and published in his “Catalog of Nebulae and Star Clusters” in 1771.
Later, William Hershel also cataloged this object on September 11th, 1784 as H V-17. The Triangulum galaxy is actually home to many of Hershel’s “New General Catalogue” or “NGC” objects, including NGC 588, 592, 595, and 604. There is so much to talk about here on these objects, so these will have to wait until a future video.
The brightness and angular size of M33 make it one of the largest and brightest galaxies in the entire sky. At an apparent angular size of 70.8’ by 41.7 minutes of arc (1.18 degrees by 0.695 degrees) in the sky. It’s over twice the width of the full moon!
At magnitude 5.72, M33 is just bright enough to be observed with the naked eye from extremely dark skies. The object’s brightness, however, is concentrated towards the central bulge, so viewing this object without the aid of a telescope will not result in much more than a pinprick size spot in the sky.
Triangulum is one of the best targets for binoculars and telescopes and should be viewed with relatively low magnification. As always, larger diameter optics will provide the best results by increasing the light-gathering and resolving power. With this object, however, it’s important to make sure you keep the focal length of your optics low enough to see the entire object within your field of view.
Due to the high sensitivity of cameras compared to the human eye, astrophotography can be conducted on this object even from light-polluted areas, especially when combined with a light pollution filter. HII narrowband filters can also be used in order to expose bright HII emission nebulae within the galaxy’s central nucleus.
Step 1) Find a Starting Asterism or Constellation
At my location in the Northeast US, we will start our observation by locating the “Great Square of Pegasus” asterism as it rises in the eastern sky just after sunset starting in late July through early August. The “Great Square of Pegasus” asterism is part of the constellations of Pegasus as well as Andromeda and is made of many of these constellations’ brightest stars.
Throughout the autumn, the “Great Square” will move westward as time progresses towards winter. However, due to the shortening of days and nighttime coming earlier each month, the great square remains in the evening sky throughout the fall and into winter and sets just after sunset in the western sky by early to mid-January.
Step 2) Find the Object Using Star Hopping
We are going to use the stars of the “Great Square of Pegasus” to help us find M33, the Triangulum Galaxy.
Starting at the “Great Square”, we first need to identify some of the major component stars of this important asterism.
The Great Square itself is made of four stars oriented in a nearly square shape
Three of these stars found in the constellation of Pegasus are Scheat, Markab, and Algenib
The fourth star to complete the square, Alpheratz is actually part of the constellation of Andromeda, the Princess.
In order to find M33, the Triangulum Galaxy, we will start at the top star of the great square, Scheat, and draw an imaginary line through Alpheratz.
As we pass through Alpheratz, the next bright star we will encounter is Delta Andromedae, in the lower of the two legs of the Andromeda constellation.
We will continue along the lower leg of Andromeda to the very bright middle-star Mirach.
From Mirach, we will draw another imaginary line through Mu Andromedae, the middle-star in the upper leg of Andromeda
Now that we’ve located Mu Andromedae, re-trace the line back through and past Mirach, nearly twice as far as the angular distance between the two stars.
You can use your hand as a basic measurement tool. The distance between these two stars should be slightly more than the width of three fingers held at arm’s length.
If you do not own a Telrad finder (as seen in the image below) move on to step three.
If you own a Telrad finder, you will notice that the distance between these two stars is just under 4 degrees, approximately equal to the width of the Telrad’s Reticle.
Move the reticle, causing Mirach to move from the eastern side of the reticle to the western side of your reticle.
Move the reticle again, so that you are approximately 2x the width of the reticle along the imaginary line from Mirach
Either of these methods should place M33 nicely in the field of view of your finder scope.
Step 3) Move your eye to your Magnified Finder
At this point, you should have M33, the Triangulum Galaxy in your magnified finder scope.
Above is an image as would be seen in my Stellarvue 9 x 50 Deluxe Finder
In dark skies, M33 should be easily visible in a 50mm or larger finderscope or binoculars. In extremely dark skies, it will even be visible with the naked eye. It will appear as a wispy cotton-ball-like object in your finderscope.
Center M33 in your finderscope.
Step 4) Move your eye to your Widest-Field Eyepiece
Always start your observations at your widest-field eyepiece
For this simulation, I’ve chosen my 100-degree apparent field of view Stellarvue Optimus 20mm eyepiece on my Stellarvue SVX130T Premier Apochromatic Refractor.
Center your object in the field of view, and slowly work your way down to smaller and smaller focal length eyepieces, centering each one, until you get the desired field of view for your setup.
Short focal length telescopes and long focal length eyepieces work best on this object due to its very large angular size.
Thank you so much for watching today’s episode of Deep Sky with Dave. This is part of my Messier Marathon series of videos, in which I plan to go through all 110 Messier objects. If you find this blog post helpful, please help me out by liking this blog by clicking the heart at the bottom of the blog post and consider subscribing to the blog by signing up to the email list at the bottom of this page.
If you have a different method for finding M33 the Triangulum Galaxy, want to provide me feedback on this tutorial, have suggestions or requests for future posts or if you have any questions regarding my star-hopping techniques, observational astronomy, telescopes, or astrophotography, please leave them in the comments below.