Constellations: Triangulum 'the Triangle' Content from the guide to life, the universe and everything

Constellations: Triangulum 'the Triangle'

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The shield of the Science, Mathematics and Engineering faculty of the h2g2 University.Constellations: Overview | Andromeda | Antlia | Apus | Aquarius | Aquila | Ara | Aries | Auriga | Boötes | Caelum
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Si les triangles faisoient un dieu, ils lui donneroient trois côtés1. (If the triangles were to make a god, they would give him three sides.)
– Charles-Louis de Secondat, Baron de La Brède et de Montesquieu

Name:Triangulum (Latin: 'triangle')
Short form:Tri
Area:132 sq deg
Co-ordinates2:Right Ascension 02h, Declination +30°

Triangulum is tiny, the 78th of the modern 88 internationally recognised constellations, so not quite the smallest. It has three main stars which form an isosceles triangle shape when one imagines them joined up. Triangulum shares its borders with Andromeda, Pisces, Aries and Perseus. With such a small constellation, there are but a few objects of interest: one strikingly beautiful one is Messier catalogue M33, commonly known as the Triangulum Galaxy or the Pinwheel Galaxy.

Roman Mythology

Ceres, Roman goddess of agriculture and mother love, lived on the triangle-shaped island of Trinacria, which we now know by the name of Sicily. Her brother-husband Jupiter was father to their daughter Proserpina, a beautiful young maiden, adored by all. When Proserpina was kidnapped by Pluto, god of the Underworld, Ceres walked the Earth searching for her, creating a desert in her footsteps. Jupiter ordered Pluto to surrender Proserpina, whom he had recently married. Rather than lose her, Pluto forced Proserpina to eat the seeds of a pomegranate in order that she would return to him for conjugal rights. Being bound by marital fidelity, Proserpina descends to the Underworld at the end of summer for six months, and no crops grow while her mother awaits her return. At the beginning of every spring Proserpina rejoins her mother on Trinacria (Sicily). So delighted was Ceres to have her daughter back home, that the island's triangular shape was placed in the heavens as a constellation.


The scientific star names are simple to understand (if you know your Greek alphabet). For example, the 'alpha' star means that it is the brightest star in that constellation. The next brightest is designated 'beta', etc. Combined with the genitive name, this is known as the 'Bayer designation'. That's the way it's supposed to work, but sometimes the measurements were a little out of tune or the stars are variable, as in this case, the beta star Deltotum has a brighter magnitude than the designated alpha.

Some stars have proper names as well; for example, alpha Trianguli is Mothallah3. Other stars are known by their catalogue number.

Alpha Trianguli, Mothallah, is actually a binary star system; that is two stars which are gravitationally bound to each other. This is a regular occurrence throughout the universe; solo stars like our own Sun are a lot less common. Around 85% of known stars are part of a binary or multiple star system.

Delta Trianguli is a binary star system comprising two dwarf stars, one yellow and one orange, which orbit each other every ten days or so.

Epsilon Trianguli comprises two main sequence white dwarf stars; separately they are magnitude +4.4 and +11.3, giving a mean average brightness of +5.5.

Yet another binary system, 6 Trianguli is really pretty in telescopes: a yellow 5th-mag giant with a 7th-mag blue dwarf companion.

White dwarf gamma Trianguli spins extremely fast, at least 100 times the speed of our Sun.

Star Table

StarDesignationName or
catalogue number
(light years4)
Spectral classification
and/or comments
β Tribeta TrianguliDeltotum+3120White giant
α Trialpha TrianguliMothallah+3.4 var64Binary star system
γ Trigamma Trianguli9 Trianguli+4 var118White dwarf
δ Tridelta Trianguli8 Trianguli+4.8 var35Binary star system
6 TriHD 134806 Trianguli+4.9 var305Binary star system
ε Triepsilon Trianguli3 Trianguli+5.5 var350Binary star system

New General Catalogue (NGC)

The NGC was compiled by John Louis Emil Dreyer (the director of the Armagh Observatory from 1882 to 1916).

Triangulum Galaxy

As already mentioned, the awesome Triangulum Galaxy (M33/NGC 598) is a staggeringly beautiful, clockwise, grand design spiral, which has pink star-forming regions and blue star clusters; it's doubtful any Earthly artist could produce a more glorious image. M33 is over 50,000 light-years across and three million light years away, but being magnitude +5.7 means it can be seen by naked eye, away from city lights. Within M33 are giant stellar nurseries which have their own NGC designations.

The Triangulum Galaxy has featured in many a science fiction plot, including the Star Trek universe and Andromeda stories of Gene Roddenberry. In seaQuest DSV, the fictional planet Hyperion was located in the Triangulum Galaxy. Vineans, blue-skinned aliens, are befriended by the heroes in a Belgian comic book series called Yoko Tsuno, which was authored by Roger Leloup (a former partner of Hergé).

NGC Table

(light years)
NGC 598
CW spiral galaxy+5.73m5Triangulum Galaxy
NGC 592Diffuse nebula+183mPart of M33
NGC 604Diffuse nebula+153mPart of M33
NGC 672Galaxy+10.818mBarred ACW spiral
NGC 925Galaxy+10.530mCW spiral

Extrasolar Planets

There is a yellow dwarf star HD 9446 exactly the same mass and size as our Sun in Triangulum. Two planets in attendance are a hot gas giant HD 9446 b orbiting in 30 days and a superjovian world HD 9446 c completing its year in 193 days. Another extrasolar planet, WASP-56 b, was discovered by the WASP detecting system in 2011. HD 13189 b is a brown dwarf in orbit around an orange giant star at a distance of 1.85 AU (approx 471 days).

Triangles in Modern Culture

While this article is primarily about the Triangulum constellation, the author thought you might like to see some of the other instances where triangles have appeared.

The ancient Egyptians knew the significance of the triangle shape. They built the pyramids based on connecting four triangular-sided figures forming a point at the apex, usually topped by a Benben stone. These were expensive to build and time-consuming, so generally they were only built for the pharaoh and usually commenced when he was enthroned. When the Great Pyramid (one of the original wonders of the world) was finished it was the tallest building in the world, and it's the only one left — no doubt thanks to the strength of its design.

  • Triangular numbers has its own Edited Entry.

  • A black triangle in the sky is a type of UFO according to some witnesses.

  • The Rushdon Triangular Lodge of Rushton Hall, Northamptonshire, UK, was visited by David Dimbleby during his BBC series How We Built Britain.

  • The Red Triangle, situated between San Francisco and Monterey, is Great White shark territory, so swimming and surfing in these waters is not advised unless you're a thrill-seeker.

  • A 'green triangle' is a chocolate-covered sweet made by Nestle, which used to appear only in their 'Quality Street' assorted selection. It's called a green triangle because the foil wrapping is coloured green - the chocolate is the usual brown colour. Now the green triangle, along with a few other favourites, have been 'supersized' and are available to purchase separately.

  • The Summer Triangle is a group of three stars known as an asterism, as is the Winter Triangle. Asterisms are groups of stars which aren't officially recognised constellations, like the 'Plough' (aka 'Big Dipper') in Ursa Major, the Great Bear.

  • The 'Rhubarb Triangle' is an area of West Yorkshire formed by joining the rhubarb farms bordered by Leeds, Wakefield and Bradford. Rhubarb has been cultivated for 4,000 years and arrived in Great Britain in the 13th Century. Considered a luxury item, it was four times more expensive than opium, which was the recreational drug of choice then.

1Lettres Persanes, 1721.2Current IAU guidelines use a plus sign (+) for northern constellations and a minus sign (−) for southern ones.3Alpha Trianguli is sometimes called Metallah or Rasalmothallah.4A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.5This figure means three million light years.

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