Triangulum, the Triangle, is a small northern constellation whose three brightest stars form a simple, recognizable triangle. Although it contains no particularly bright stars, it is home to one of the most famous galaxies in the night sky: the Triangulum Galaxy (M33). Covering approximately 132 square degrees, Triangulum is the 78th-largest of the 88 recognized constellations. It was known to ancient astronomers and was included among the 48 constellations cataloged by Ptolemy in the second century AD.
Where and When to Find Triangulum
Triangulum is best observed from the Northern Hemisphere during autumn and early winter, particularly from October through December. The constellation lies between Andromeda to the north and Aries to the south, with Pisces nearby. One way to locate it is to first identify the Great Square of Pegasus, then follow the chain of stars extending through Andromeda. Look south of Andromeda for three moderately faint stars forming a narrow triangle.
The constellation’s brightest stars are around third and fourth magnitude, so they are reasonably easy to identify from suburban skies, although darker conditions make the pattern clearer. Triangulum is especially rewarding for observers with binoculars because of the nearby Triangulum Galaxy.
History and Mythology
Triangulum is one of the oldest recognized constellations, with a triangular star pattern appearing in ancient Babylonian astronomy. The Greeks called it Deltoton, referring to the uppercase Greek letter delta (Δ), which resembles a triangle. The constellation was also associated with the triangular Nile Delta in Egypt. Another ancient interpretation connected its shape with the island of Sicily, which has a roughly triangular outline.
Unlike constellations representing mythological heroes or animals, Triangulum was primarily recognized for its simple geometric shape.
Notable Stars
- Beta Trianguli is the brightest star in the constellation, shining at approximately magnitude +3.0. Located around 125 light-years away, it is a spectroscopic binary system whose primary component is a white giant. The two stars orbit each other approximately every 31 days.
- Mothallah (Alpha Trianguli) is the second-brightest star, with an apparent magnitude of around +3.4. Located approximately 63 light-years away, it is another spectroscopic binary, with an orbital period of less than two days. Its traditional name is associated with the head of the triangle.
- Gamma Trianguli is a blue-white star shining at approximately magnitude +4.0 from around 110 light-years away. It rotates rapidly and is associated with an infrared excess that suggests surrounding circumstellar material.
- Delta Trianguli is a nearby binary system approximately 35 light-years away. Its two stars orbit closely around one another, making them impossible to separate through an ordinary amateur telescope.
Together, Alpha, Beta, and Gamma Trianguli form the constellation’s distinctive triangular outline.
Notable Deep-Sky Objects
Triangulum Galaxy (M33)
The Triangulum Galaxy (Messier 33, NGC 598) is the constellation’s most famous object and one of the nearest major spiral galaxies to the Milky Way. Located approximately 2.7 million light-years away, M33 is the third-largest major galaxy in the Local Group, after the Andromeda Galaxy and our own Milky Way. It measures roughly 60,000 light-years across and contains tens of billions of stars. Its spiral arms are filled with young stars, glowing nebulae, and active regions of star formation.
M33 is sometimes called the Triangulum Pinwheel Galaxy, although it should not be confused with the better-known Pinwheel Galaxy (M101) in Ursa Major.
Despite having an integrated apparent magnitude of around +5.7, M33 is surprisingly difficult to see because its light is spread over a large area of sky. This gives it a low surface brightness. Under exceptionally dark, transparent skies, experienced observers may glimpse it without optical aid. Binoculars often provide a better view than a small telescope at high magnification, revealing a broad, faint oval glow.
How to find M33: Locate Alpha Trianguli (Mothallah), then look approximately 4 degrees northwest toward Andromeda. A low-power binocular view or a wide-field telescope eyepiece is useful for sweeping the area. M33 is also home to an interesting stellar-mass black hole system called M33 X-7, where a black hole orbits a massive companion star. It provides astronomers with an opportunity to study the evolution of massive binary stars.
NGC 604 — A Giant Star-Forming Region
NGC 604 is an enormous star-forming region located within one of M33’s spiral arms. Spanning roughly 1,500 light-years, it is one of the largest known regions of ionized hydrogen gas in the Local Group. For comparison, the familiar Orion Nebula in our own galaxy is much smaller. NGC 604 contains hundreds of young, massive stars whose intense ultraviolet radiation causes the surrounding hydrogen gas to glow. Powerful stellar winds have also carved cavities and intricate structures into the nebula.
Although NGC 604 lies millions of light-years away, it can be detected as a small bright patch within M33 using sufficiently large amateur telescopes under favorable conditions.
Other Galaxies in Triangulum
Triangulum contains several other galaxies, although most are considerably fainter than M33.
These include NGC 925, a barred spiral galaxy; NGC 672, a spiral galaxy interacting with nearby IC 1727; and NGC 784, another relatively faint spiral galaxy. These objects are mainly targets for experienced observers using larger telescopes or astrophotography equipment.
Observing Triangulum
Triangulum is a useful constellation for learning how to locate faint galaxies. Begin by identifying its three principal stars between Aries and Andromeda. Once you have found the triangle, use Alpha Trianguli as a starting point for locating M33. Choose a moonless night with good transparency and use low magnification. The Triangulum Galaxy is large but faint, so a wide field of view and dark skies are more important than high magnification.
For observers interested in galaxies beyond the Milky Way, Triangulum offers one of the best deep-sky targets of the autumn sky.