Triangulum
What is Triangulum?
Constellations· Autumn evenings (N) · small triangle between Andromeda and Aries. Exactly what the name says: a slim, long triangle. It is a small constellation squeezed between Andromeda and Aries, but it holds M33, the Triangulum Galaxy, the third-largest galaxy in our local group.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Beta Trianguli | 3.0 |
| Mothallah | 3.4 |
| Gamma Trianguli | 4.0 |
The 3 brightest of the 3 stars used in the figure (3 lines). Lower magnitude means brighter.
Constellation basics
| Season | autumn evenings (N) |
|---|---|
| Brightest star | Beta Trianguli (3.0) |
| How to find | between Mirach and Hamal |
| Area rank | 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
In 1926 a distance measured in M33 settled what galaxies are
Until the early 1920s astronomers did not know what the spiral ‘nebulae’ in the sky were. Some thought they were clouds of gas inside the Milky Way; others thought they were huge star systems far beyond it. Edwin Hubble set out to answer the question by studying Andromeda and the Triangulum Galaxy, M33. In 1926 he reported finding 35 Cepheid variable stars in M33.
The distance to a Cepheid can be worked out from how long it takes to vary. The distance Hubble found was hundreds of thousands of light-years, far larger than the Milky Way was then thought to be. So M33 had to be an independent island of stars: a galaxy. His figure was only about a third of today's value, but the conclusion still stands.
Inside M33 is NGC 604, a giant cloud where stars are being born. It is about 1,500 light-years across, dozens of times larger than the Orion Nebula, and more than two hundred hot young stars have formed in it together. Although it belongs to another galaxy, a mid-sized amateur telescope shows it as a small knot. It is a rare chance to see star birth beyond the Milky Way.
The constellation in depth
Triangulum is an ancient constellation from Ptolemy's list of 48. The Greeks called it Deltoton because it looks like the Greek letter delta (Δ). Some said it showed the triangle of land at the mouth of the Nile, others the three-cornered island of Sicily. Because the shape is so simple, its name has hardly changed over the centuries.
The most important object here is M33, the Triangulum Galaxy, a spiral galaxy about 2.7 million light-years away that faces us almost head-on. It is the third-largest member of the Local Group, the family of galaxies that includes the Milky Way and Andromeda. Its light is spread thinly, so to the naked eye it is visible only under very dark skies, and binoculars show it as a soft smudge.
On October to December evenings, look between Mirach in Andromeda and Hamal in Aries for three 3rd-magnitude stars forming a slim triangle. The brightest is Beta; Alpha, whose name means ‘head of the triangle’, is slightly fainter. The constellation passes nearly overhead, so it is easy to see from mid-northern latitudes. Turn on deep-sky objects in the Sky view to see M33 marked.
Frequently asked
- When can I see Triangulum?
- Triangulum is best seen in autumn evenings (N).
- How do I find Triangulum?
- To find Triangulum, look for between Mirach and Hamal.
- What is the brightest star in Triangulum?
- The brightest star in Triangulum is Beta Trianguli, at magnitude 3.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.