Triangulum is a small constellation in the northern sky. Its name is Latin for “triangle,” derived from its three brightest stars, which form a long and narrow triangle. Known to the ancient Babylonians and Greeks, Triangulum was one of the 48 constellations listed by the 2nd-century astronomer Ptolemy. The celestial cartographers Johann Bayer and John Flamsteed catalogued the constellation’s stars, giving six of them Bayer designations. Triangulum is bordered by Andromeda to the north and west, Pisces to the west and south, Aries to the south, and Perseus to the east. The centre of the constellation lies halfway between Gamma Andromedae and Alpha Arietis. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined as a polygon of 14 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 01h 31.3m and 02h 50.4m, while the declination coordinates are between 25.60° and 37.35°. Covering 132 square degrees and 0.320% of the night sky, Triangulum ranks 78th of the 88 constellations in size.
Three stars make up the long narrow triangle that gives the constellation its name. The brightest member is the white giant star Beta Trianguli of apparent magnitude 3.00, lying 127 light-years distant from Earth. It is actually a spectroscopic binary system; the primary is a white star of spectral type A5IV with 3.5 times the mass of our sun that is beginning to expand and evolve off the main sequence. The secondary is poorly known, but calculated to be a yellow-white F-type main-sequence star around 1.4 solar masses. The two orbits around a common centre of gravity every 31 days, and are surrounded by a ring of dust that extends from 50 to 400 AU away from the stars. The second-brightest star, the yellow-white subgiant star Alpha Trianguli (3.41m) with a close dimmer companion, is also known as Caput Trianguli or Ras al Muthallath and is at the apex of the triangle. It lies around 7 degrees north-northwest of Alpha Arietis. Making up the triangle is Gamma Trianguli, a white main sequence star of spectral type A1Vnn of apparent magnitude 4.00 about 112 light-years from Earth. It is around double the size of and around 33 times as luminous as the sun and rotates rapidly. Like Beta, it is surrounded by a dusty debris disk, which has a radius 80 times the distance of the Earth from the Sun.
The Triangulum Constellation also known as Messier 33, was discovered by Giovanni Battista Hodierna in the 17th century. A distant member of the Local Group, it is about 2.3 million light-years away, and at magnitude 5.8. It is a spiral galaxy with a diameter of 46,000 light-years and is thus smaller than both the Andromeda Galaxy and the Milky Way. A distance of fewer than 300 kiloparsecs between it and Andromeda supports the hypothesis that it is a satellite of the larger galaxy. Within the galaxy, NGC 604 is an H II region where star formation takes place. In addition to M33, there are several NGC galaxies of visual magnitudes 12 to 14. The largest of these include the 10 arcminutes long magnitude 12 NGC 925 spiral galaxy and the 5 arcminutes long magnitude 11.6 NGC 672 barred spiral galaxy. The latter is close by and appears to be interacting with IC 1727. The two are 88,000 light-years apart and lie around 18 million light-years away. These two plus another four nearby dwarf irregular galaxies constitute the NGC 672 group, and all six appear to have had a burst of star formation in the last ten million years. The group is thought connected to another group of six galaxies known as the NGC 784 group, named for its principal galaxy, the barred spiral NGC 784. Together with two isolated dwarf galaxies, these fourteen appear to be moving in a common direction and constitute a group possibly located on a dark matter filament. 3C 48 was the first quasar ever to be observed, although its true identity was not uncovered until after that of 3C 273 in 1963. It has an apparent magnitude of 16.2 and is located about 5 degrees northwest of Alpha Trianguli. NGC 634 is a spiral galaxy in Triangulum. It has an apparent magnitude of 14 and is approximately 250 million light years distant from Earth. The galaxy was discovered by the French astronomer Édouard Stephan in the 19th century. In 2008, a Type Ia supernova, SN 2008a, was observed in the galaxy. Credits: Constellation Guide, Wikipedia.