Triangulum the Triangle in northern skies

Triangulum: Star chart: A long narrow triangle with a couple of other dots and the star Mizan labeled.
The constellation of Triangulum is overhead in the Northern Hemisphere on December evenings. Chart by Chelynne Campion/ EarthSky.

There are lots of collections of stars in the night sky that can form the shape of a triangle. The giant Summer Triangle is one, and the Triangulum Australe in the Southern Hemisphere is another. Triangulum the Triangle is a constellation in northern skies that you can best spot in late autumn or early winter. It lies to the northeast of the Great Square of Pegasus and below the “W” shape of Cassiopeia.

Ancient Romans saw the small and somewhat dim shape of a triangle. The Romans viewed Triangulum as Sicily, a nearby island with a triangular shape. For ancient Hebrews, Triangulum was a shalish, a musical instrument with three strings.

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Major stars of Triangulum

The key stars in Triangulum are the three that mark the corners of the triangle. The star Alpha Trianguli shines at magnitude 3.42. Alpha lies 64 light-years from Earth. The Alpha stars are not always the brightest in constellations, and this is true for Triangulum. Alpha is only the second brightest star in this constellation.

The next two stars in the corners of the triangle are close together. Beta and Gamma Trianguli lie two degrees apart. The pair lie a little less than seven degrees from Alpha. Beta Trianguli, or Mizan, is the brightest star in Triangulum. It’s magnitude 3.0, and it lies 124 light-years away. Gamma Trianguli is magnitude 4.02 and lies 118 light-years away.

White star chart with black dots marking stars, blue ecliptic line in corner and galaxies as red ovals.
Star chart for Triangulum. Chart via IAU.

The Triangulum galaxy

While there are a number of dim galaxies in Triangulum, there is really only one here that draws the attention of amateur astronomers. It’s one of the brighter galaxies in the sky: the Triangulum galaxy. It also has the nickname of the Pinwheel galaxy, but that can get confusing because there’s another galaxy with the same name in Ursa Major. The Triangulum galaxy is M33 or NGC 598. M33 is just over four degrees from Alpha Trianguli, in the direction of the Great Square of Pegasus.

At magnitude 5.7, you can spot M33 through a steadily held pair of binoculars under a dark sky. The face-on spiral galaxy is a beautiful sight through a telescope or in a photograph.

If you try to observe M33 through a telescope, it has low surface brightness, so make sure you are in a site without light pollution and your eyes have had a chance to adjust. You should see a bit of a brighter central oval shape and a dimmer glow smudged around it. See if you can make out any of its curving arms.

A nearby galaxy

As far as galaxies go, M33 is rather close to us, at 3 million light-years distant. Thus, we are seeing the galaxy not as it appears today, but as it did 3 million years ago, which is how long it took the light traveling from the galaxy to reach us.

M33 is part of the Local Group of galaxies. The Local Group is a family of galaxies including our Milky Way, the Magellanic Clouds, and the Andromeda galaxy.

Bottom line: The constellation Triangulum lies close to overhead in northern skies on December evenings. It contains the Triangulum galaxy, a gorgeous pinwheeling spiral.

December 6, 2022

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