The tiny northern constellation Triangulum contains the gorgeous face-on spiral galaxy Messier 33, also known as the Triangulum Galaxy. A photogenic target for expert astrophotographers, the Triangulum Galaxy is a next-door neighbor of our Milky Way Galaxy and the most distant object you can see with your unaided eye. As you can see in the above image by Terry Hancock and Ron Brecher, the spiral arms of M33 are festooned with pink star-forming nebulae. The largest, NGC 604, is some 100x the size of the Orion nebula and hosts more than 200 massive stars at its center.
The spiral arms of M33 are loosely bound, and the galaxy is a type-Sc spiral using Hubble’s galaxy classification system. The galaxy spans a diameter of 50,000 light years, about half the diameter of the Milky Way. Indeed, M33 belongs to the so-called Local Group of Galaxies which includes the Andromeda Galaxy, the Magellanic Clouds, and our own Milky Way. M33 may itself be a large satellite of the much larger Andromeda galaxy, Messier 31.
(Note: The Triangulum Galaxy is one of the dozens of deep-sky sights featured in The Armchair Astronomer, a collection of astoundingly beautiful images of clouds of gas and dust where new stars are born. If you haven’t yet downloaded this e-book, you can find it here…)