Star Constellation Facts: Triangulum

Star Constellation Facts: Triangulum
Triangulum as depicted by Johannes Hevelius in his Celestial Atlas (1690)

Triangulum was known to the ancient Babylonians as long ago as 1000 BC, and by 150 A.D. the Greco-Egyptian astronomer Ptolemy catalogued it as one of the 48 known constellations at that time. While to the Greeks it was known as “Deltoton”, after they linked it to the Nile Delta, and named it after their upper case letter Delta, Roman stargazers, on the other hand, associated it with the island of Sicily, which is somewhat triangular in shape.


Triangulum is a northern hemisphere constellation that can be seen by observers located between latitudes +90° and -60°. This tiny constellation takes up an area of only 132 square degrees of the night sky, ranking it 78th in terms of size, and can be seen in late autumn and winter in the northern hemisphere, although it is best observed at 9 PM (Local Time) during the month of December. As its name suggests, Triangulum means “Triangle” in Latin, and there is no mistaking its shape, as its three principal stars mark out a shallow triangle, with the sharp end pointing southwest toward the constellation Pisces. Look for Triangulum about halfway between Andromeda to the northeast, and Aries to the southwest.


Although Greek astronomers insisted that the constellation represents the Nile River delta system, there were many dissenting opinions at the time, and as a result these days Triangulum is not recognised as representing anything specific.

Notable Stars

Triangulum may not have first or even second magnitude stars as seen from Earth, but it does have some spectacular stars nevertheless. Below are some details on a few of them:

– Beta Trianguli is an A5III-class white giant star about 127 light years distant with an apparent visual magnitude of 3.00, making it the most luminous star in the constellation. The star is believed to be a spectroscopic binary system in which the component stars are too close to each other to be resolved optically. Spectroscopic measurements have however determined the distance between the component stars to be less than 5 AU, and an orbital period of 31.39 days. The system is also showing an excess of infrared radiation, which suggests that both stars are enveloped in a debris disc.

– Caput Trianguli (Alpha Trianguli) is the second most luminous star in the constellation, with an apparent visual magnitude of 3.42 as seen from a distance of about 63 light years away. It also is a close binary system, so close in fact that the components cannot be split optically, but spectroscopic measurements have determined an orbital period of only 1.736 days. This system is estimated to be 1.6 billion years old, with its primary star believed to be either a sub giant, or a fully evolved giant.

Star Constellation Facts: Triangulum, the Triangle– HD 13189 is a fully evolved class K1II-III orange giant with an apparent visual magnitude of 7.57 as seen from a distance of about 1,800 light years away. In absolute terms, its distance and 2 to 7 solar masses combine to make this star a whopping 3,980 times as bright as the Sun.

Notable Deep Sky Objects

– Triangulum Galaxy (M33, NGC 598), sometimes known as the Pinwheel Galaxy, should not be confused with the “official” Pinwheel Galaxy in Ursa Major, which has the designation M101. Nonetheless, M33 is spiral galaxy, and at a distance of between 2,380 and 3,070 light years, it is one of the most distant deep sky objects that can be observed without optical aid. It is also famous for other reasons, such being the 3rd largest galaxy in the Local Group of galaxies, after Andromeda and the Milky Way.

It spans about 50,000 light years, has about 40 billion stars, and at least 54 large globular star clusters. By comparison, the Milky Way has an estimated 400 billion stars, while Andromeda has more than a trillion stars.
M33 also contains the largest known black hole formed by the gravitational collapse of a massive star. M33 X-7, as this black hole is known, is about 15.7 times as massive as the Sun, and it is one component of a binary system. The other component is a massive star that has about 70 solar masses, making it the most massive star in a binary system in which the other component is a black hole. The star and the black hole orbit each other once every 3.45 days.

– NGC 604 is one of the largest H II (star forming) regions known, and is located just to the northeast of the Triangulum Galaxies’ core. It is also the second most luminous H II region in the entire Local Group of galaxies. This vast cauldron of roiling gas and dust spans a distance of between 1,300 and 1,500 light years, and is at least 6,300 times as bright as the Orion Nebula. The central cavity in the cloud is caused by the extremely energetic solar winds of a group of stars that have blown away the surrounding material, thus leaving a “bubble” of clear space around them.
Some of these massive stars are only about 3 million years old, but some are as much as 120 times as massive as the Sun, and have surface temperatures in excess of 72,0000F (39 9820C).


As of 2016, Triangulum has 4 recognized planets, with one Sun-like star, HD 9446, hosting two; one planet being about 0.7 times as massive as Jupiter, while the other weighs in at 1.8 Jupiter masses. The other two stars have one planet each; one weighing in at about 0.6 Jupiter masses, and the other at between 8 and 20 Jupiter masses.