Triangulum: the faint third spiral

How to find it: start at Mirach, the same star used to find Andromeda. Andromeda lies on one side of it; Triangulum lies about the same distance on the opposite side, towards the small triangle of stars that names the constellation. Use binoculars and look for a large, very faint glow rather than a point.

Triangulum is the easiest Local Group spiral to study face-on — almost nothing blocks the view. Zoom in to find NGC 604, the bright knot up and to the left of the centre.

On paper Triangulum is nearly as bright as the faintest stars you can see by eye. But its light is smeared over an area larger than the full Moon, so its surface brightness is very low. Spotting it without optical help is a classic test of sky darkness and observing skill — on the Bortle scale of light pollution, it generally needs a class 3 sky or better. If you can see it, you are looking at the most distant object visible to the unaided eye that most observers will ever see, about 230,000 light-years farther away than Andromeda.

Most large galaxies have a supermassive black hole at their centre: millions of Suns for the Milky Way, about a hundred million for Andromeda. Triangulum appears to have none. Measurements of how fast stars move in its core set an upper limit of only about 1,500 solar masses for any central black hole — tiny by galactic standards. Astronomers think that's partly because Triangulum is a pure disk galaxy with almost no central bulge, and the size of a central black hole tends to track the bulge.

Triangulum does host a famous small one. M33 X-7 is a black hole of about 15–16 solar masses orbiting a giant companion star, which eclipses it every 3.45 days — one of the most massive stellar black holes known when it was measured in 2007.

Triangulum makes new stars at a far higher rate per unit area than Andromeda. Its showpiece is NGC 604, a glowing cloud of ionised hydrogen about 1,500 light-years across, lit by more than 200 hot young stars — one of the largest star-forming regions in the Local Group. In 2019 Hubble released a mosaic of the galaxy assembled from 54 fields, resolving around 25 million individual stars.

It is the third-largest galaxy in the Local Group and may be a satellite of Andromeda, though recent motion measurements suggest it could be on its first approach rather than a long-term orbit. In the 2025 Milky Way–Andromeda simulations, Triangulum's mass slightly raised the chance of a merger. Its earliest probable record is by the Sicilian astronomer Giovanni Battista Hodierna before 1654; Charles Messier catalogued it as number 33 on 25 August 1764.

If you add up all the stars and gas you can see in Triangulum, the outer disc should rotate more slowly than the inner parts, the way the outer planets orbit the Sun more slowly. It doesn't: radio measurements of hydrogen show its rotation stays fast far beyond the visible stars. Triangulum's "flat rotation curve" is one of the cleanest in the sky, and it's a textbook piece of evidence that galaxies sit inside much larger, invisible halos of dark matter — in Triangulum's case outweighing the visible matter many times over.

In 2005 a team using the Very Long Baseline Array of radio telescopes tracked tiny, intensely bright water-maser spots in two star-forming regions of Triangulum. Over a few years they could see them shift as the galaxy rotated — a movement about as hard to see as a snail crawling on the Moon. Comparing that real sideways motion with the rotation speed measured from Doppler shifts gave a purely geometric distance to the galaxy, independent of Cepheids, and the first measurement of how Triangulum as a whole is moving across our sky. That motion is what tells astronomers it is bound to Andromeda.