Triangulum is a small constellation in the northern sky. Its name is Latin for triangle, and it should not be confused with Triangulum Australe in the southern sky. Its name derives from its three brightest stars, of third and fourth magnitude, which form a nearly isosceles long and narrow triangle. Triangulum was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and remains one of the 88 modern constellations.
Triangulum has no stars of the first magnitude. Its brightest star is the white giant star β Trianguli (3.00m) with a close, dimmer companion. Its second-brightest star, the yellow-white subgiant star α Trianguli (3.41m) with a close dimmer companion, is also known as Caput Trianguli, and is at the apex of the triangle. 6 Trianguli, (known in some older sources as ι), is an "attractive double star with a noticeable color contrast" that can be split by medium-sized telescopes into a strong yellow and a pale blue star. Both components are themselves close binaries.
Deep sky objects
Triangulum is the location of the Triangulum Galaxy, also known as Messier 33. A distant member of the Local Group, it is about 2.9 million light years away, and at magnitude 5.8, it is bright enough to be seen by the naked eye under the darkest skies. Under light-polluted skies, it is challenging or invisible even in a small telescope or binoculars. Because of its low surface brightness, low power is required.
In addition to M33, there are several NGC galaxies, all with magnitudes fainter than 11. The largest of these include the 10 arcminute long magnitude 12 NGC 925 spiral galaxy and the 5 arcminute long magnitude 11.6 NGC 672 barred spiral galaxy.
History and mythology
In the Babylonian star catalogues, Triangulum together with γ And formed the constellation known as MULAPIN (𒀯𒀳) "The Plough". It is notable as the first constellation listed in the pair of tablets containing the canonical star lists compiled around 1000 BC, which are thus known as the MUL.APIN by their incipit. The Plough was the first constellation of the "Way of Enlil", i.e. northernmost quarter of the Sun's path, corresponding to the 45 days on either side of summer solstice.
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