What’s the closest galaxy? If you answered the Milky Way, you’re right. It belongs to the Local Group, a pack of more than 50 galaxies bound together by gravity. Andromeda is the largest galaxy in the group with a diameter of 200,000 light years with the Milky Way in second place at 100,000 light years. Both are probably familiar to you. But I’d be willing to bet a fair share of people have never heard of the galaxy in that third place slot — the Triangulum Galaxy.
It gets its name from the constellation Triangulum the triangle in which it resides. Most of us use the more informal “Pinwheel Galaxy” which nicely describes its appearance. The Pinwheel isn’t much further than Andromeda, just 3 million light years from Earth, and if you have very dark skies, it’s faintly visible with the naked eye. I’ll often point out that the Andromeda Galaxy is most distant thing you can see at 2.5 million light years, and it is for most of humanity. Those blessed with velvety black nights can go a little further and glimpse Triangulum! Some 40 billion stars blend together to fashion this wisp of light.
The Hubble Space Telescope recently zeroed in on the galaxy to create the most detailed view ever of its central region and inner spiral arms. 54 separate photos were stitched together to create the panoramic image showcasing the galaxy’s central region and spiral arms, comprising 10 to 15 million stars. Like Andromeda and our own galaxy, Triangulum is a spiral with a more compact center framed by a whorl of starry arms like some spinning octopus. At 60,000 light years across, it’s the smallest spiral galaxy in the Local Group.
Unlike the Milky Way and Andromeda, the Pinwheel’s central bulge is small, but it contains huge amounts of gas and dust that are collapsing under gravity’s attractive force to create lots and lots of brand new stars. Astronomers estimate that one solar mass worth of stars gets cranked out every two years. Since most stars in the universe are red dwarfs with masses of one-tenth to one-half that of the sun, we’re talking around several new stars a year. I encourage you to use the zoom tool to dig into the Hubble image for a taste of this stellar richness. No exaggeration — it’s incredible.
I can look through my telescope and see nearly a half-dozen gas clouds lit up by newborn suns in the Pinwheel. There are many, many more, the reason astronomers were interested in getting a better look with the Hubble. When stars are born they use up the material in these clouds, leaving less fuel for new stars to form. Scientists take these “deep,” multiple-exposure images to better understand star formation and stellar evolution and how it compares to the same processes in the Milky Way and Andromeda. At the bottom of all good science is baseline data. Get the information, and then we’ll see where it takes us. Like me, I hope you’re enjoying the ride!
To help get you there, try finding the Triangulum Galaxy on clear, dark night with binoculars. As I wrote, it’s not easy with the naked eye, but 8×40, 10×50 and the like binoculars will show it well under a reasonably dark sky. If you sought and saw Comet 46P/Wirtanen last month, the two look very similar. Make sure you observe with the moon either a crescent or out of the sky altogether. Give it a try and see as far as you can see!