Article Updated: 24 Dec , 2015


Triangulum, located just north of the ecliptic plane, was one of the 48 original constellations listed by Ptolemy, and remains one of the 88 modern constellations. It spans 132 square degrees of sky and ranks 78th in size. Triangulum has 3 mains stars in its asterism and 15 Bayer Flamsteed designated stars within its confines. It is bordered by the constellations of Andromeda, Pisces, Aries and Perseus. Triangulum can be seen by all observers located at latitudes between +90° and ?60° and is best seen at culmination during the month of December.

As one of the very few constellations to be named after an object instead of a mythical figure or animal, one of the first names of this constellation was Sicilia – which represented the island of Sicily. This tale came about because it was believed that Ceres, the patron goddess, had begged Jupiter to immortalize her home in the stars. For a time, this region of sky was also known as Triangulum Minus, as recorded by Johannes Hevelius. It was formed from the southern parts of his Triangula, and the name quickly fell into disuse. It eventually simply took on the Latin term for its three primary stars the “triangle” and has been referred to as Triangulum ever since.

Let’s begin our binocular tour of Triangulum with its brightest star – Beta – the “B” symbol on our chart. Beta often goes by the name Deltotum, which is a Greek letter – Delta – which also resembles a triangle. Beta is a white A-type giant star located about 124 light years from Earth. Now switch off to the second brightest star – Alpha – the “a”. Its name is Mothallah – the head of the triangle. Guess what? It’s a binary star! While you won’t be splitting this spectroscopic yellow-white F-type subgiant binary star with any optics, it’s still fun to know that its diameter is about 3 times as large as the Sun and that its companion orbits it in less than 2 days from a distance of under 4 million miles. That’s almost touching in astronomical terms! By the way… They’re both about 65 light years away from our solar system. For a binary star you can separate in a telescope, have a look at 6 Trianguli. Its 5.3 and 6.9 components are easy to pick apart even with a small telescope because they are separated by almost 40 arc seconds.

Now, you might need to get out your telescope for the next object… A long term variable star named R Trianguli (RA 02: 34 DEC +34: 03). Depending on when you start, you may have a long time to wait to see changes, because R takes 266 days to go from stellar magnitude 5.7 to an almost invisible 12.4! R Trianguli is an “M-class” Red giant star who owes its changes to pulsations. As it expands, it becomes brighter… As it contracts, it becomes faint. What an incredible star to watch!

For binoculars and rich field telescopes, it’s time to head towards the ghostly galaxy, Messier 33 (RA 1 : 33.9 Dec +30 : 39). While this incredible spiral galaxy has an apparent magnitude of 5.7, you’re not going to find it quite as easy to find as you might think. Why? Because a lot of times you’re going to be missing the forest because you’re looking at the trees. M33 is huge! Located some approximately 3 million light-years away, the “Pinwheel Galaxy” contains a host of its own NGC objects and can often be spotted without optical aid from a dark sky location. One of the most positive ways to locate it is to use the very lowest magnification eyepiece you have available and work your way up to study each portion. It is the third largest galaxy in the Local Group, a group of galaxies that also contains the Milky Way Galaxy and the Andromeda Galaxy, and it may be a gravitationally bound companion of the Andromeda Galaxy.

The Triangulum Galaxy was probably discovered by Giovanni Batista Hodierna before 1654, who may have grouped it together with open cluster NGC 752. It was independently discovered by Charles Messier in 1764, who catalogued it as M33 on August 25. M33 was also catalogued independently by William Herschel on September 11, 1784 number H V.17. It was among the first “spiral nebulae” identified as such by Lord Rosse. Herschel also cataloged The Triangulum Galaxy’s brightest and largest H II region (diffuse emission nebula containing ionized hydrogen) as H III.150 separately from the galaxy itself, which eventually obtained NGC number 604. As seen from Earth NGC 604 is located northeast of the galaxy’s central core, and is one of the largest H II regions known with a diameter of nearly 1500 light-years and a spectrum similar to the Orion Nebula. Herschel also noted 3 other smaller H II regions (NGC 588, 592 and 595).

In 2005, using observations of two water masers on opposite sides of Triangulum via the VLBA, researchers were, for the first time, able to estimate the angular rotation and proper motion of Triangulum. A velocity of 190 to 60 km/s relative to the Milky Way is computed which means Triangulum is moving towards Andromeda. In 2007, a black hole about 15.7 times the mass of the Sun was detected in the galaxy using data from the Chandra X-ray Observatory. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days. Although we can never see it, we can certainly enjoy this faint galaxy for all the mysteries it holds!

Keep your telescope handy as you head off for our next galactic designation, NGC 925 (RA 2 : 27.3 Dec +33 : 35). At magnitude 10 and nearly 10 arc minutes in size, it is also fairly easy for a small telescope and large binoculars. This face-on presentation spiral galaxy is also part of the Hubble Space Telescope project for extra-galactic distances which use Cepheid variable stars to help judge that vast expanse of space between us. Look for a bright core region with elongated wispy spiral galaxy structure!

Now try your hand, and your telescope, and NGC 672 (RA 1 : 47.9 Dec +27 : 26). At close to magnitude 11 and 7 arc minutes in size, it is a bit more of a challenge, but large telescopes will find it and interacting galaxy IC 1727 in the same field of view. The pair is believed to be separated by about 88,000 light years – or about their own diameters. While you won’t catch an outstanding amount of detail in either one, you may begin to resolve out some lumpy areas of star birth!

Last, but not least, is NGC 784 (RA 2 : 01.3 Dec +28 : 50). At magnitude 12 and about 6 arc minutes in size, it is the smallest and faintest challenge yet. It is a barred-spiral galaxy presented nearly edge-on, and it is very diffuse. In spite of its expected small distance, NGC 784 has not yet been resolved into stars and is still being studied for velocity and kinematics. Good luck!

Chart courtesy of Your Sky.

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