The Triangulum Galaxy, a nearby spiral galaxy

The Triangulum Galaxy, a nearby spiral galaxy

Image Credit & Copyright: Roth Ritter, Dark Atmospheres Astrophotography (

The Triangulum Galaxy (catalogued as Messier 33 or NGC 598) is a spiral galaxy, located approximately 3 million light-years away in the constellation Triangulum. With a diameter of about 50,000 light years (half the diameter of the Milky Way), the Triangulum galaxy is the third largest member of the Local Group (a group of galaxies which also contains the Milky Way, Andromeda and about 30 other smaller galaxies). However, the faintest outlayers seem to reach more far out, so that the true diameter may be some 82 million light-years. It is approaching our Galaxy at 24 kilometers per second.

Triangulum may be home to 40 billion stars, compared to 400 billion for the Milky Way, and 1 trillion (1000 billion) stars for Andromeda. The mass of the Triangulum Galaxy has been estimated between 10 and 40 billion solar masses.

The disk-shaped galaxy, surrounded by a faint halo, has prominent loosely wound arms of gas and dust that spiral out from the nucleus, and dark dust lanes. There is no bulge at the nucleus. The inner part of the galaxy has two luminous spiral arms, along with multiple spurs that connect the inner to the outer spiral features. The main arms are designated IN (north) and IS (south).

Although Triangulum is classified as unbarred, there may be a weak bar-like structure about the galactic nucleus. The nucleus of this galaxy is an H II (star forming) region, and it contains an ultraluminous X-ray source, which is the most luminous source of X-rays in the Local Group. However, the nucleus does not appear to contain a supermassive black hole.

Triangulum appears to be divided into two distinct components with a different star formation history between the inner disk (within a radius of 30 million light-years) and the outer disk and halo, and may be explained by a scenario of “inside-out” galaxy formation. This occurs when gas is accumulated at large radii later in a galaxy’s life space, while the gas at the core becomes exhausted. The result is a decrease in the average age of stars with increasing radius from the galaxy core.

The northern main spiral arm contains four large HII regions, while the southern arm has greater concentrations of young, hot stars. The estimated rate of supernova explosions in the Triangulum Galaxy is one supernova explosion every 147 years, on average. As of 2008, a total of 100 supernova remnants have been identified in the Triangulum Galaxy, and a majority of the remnants lie in the southern half of the spiral galaxy. Similar asymmetries exist for highly luminous concentrations of massive stars. The center of the distribution of these features is offset about two arc minutes to the southwest.

About 54 globular clusters have been identified in this galaxy, but the actual number may be 122 or more. The confirmed clusters may be several billion years younger than globular clusters in the Milky Way, and cluster formation appears to have increased during the past 100 million years. This increase is correlated with an inflow of gas into the center of the galaxy.

At least 112 variables have been discovered in Triangulum, including 4 novae and about 25 Cepheids. In 2007, a black hole about 15.7 times the mass of the Sun was detected in the galaxy. The black hole, named M33 X-7, orbits a companion star which it eclipses every 3.5 days. It is the largest stellar mass black hole known.

Triangulum’s brightest and largest H II region is NGC 604, a diffuse emission nebula containing ionized hydrogen. As seen from Earth, NGC 604 is located northeast of the galaxy’s central core. It is one of the largest H II regions known, with a diameter of nearly 1500 light-years and a spectrum similar to that of the Orion Nebula.

NGC 604, may have undergone a discrete outburst of star formation about three million years ago. Astronomers counted 200 young hot massive stars (of 15 to 60 solar masses) which have recently formed here. This nebula is the second most luminous HII region within the Local Group of galaxies.

Triangulum is also home to the smaller H II regions NGC 588, 592 and 595. Other prominent HII regions in the galaxy include IC 132, IC 133 and IK 53.

One of the small Local Group member galaxies, the Pisces Dwarf (LGS 3), located a little more than 2 million light-years away, lies at a distance of 913 million light-years from both Triangulum and Andromeda. This means it could be a satellite galaxy of either galaxies. The Pisces Dwarf has a core radius of 483 light-years and 26 million solar masses.

Triangulum itself is a remote but gravitationally bound companion of the Andromeda galaxy. The distance of Triangulum from Andromeda is about 750,000 light-years. Triangulum is moving towards the Andromeda Galaxy and a clumpy stream of hydrogen gas and stars is linking Triangulum with Andromeda, what suggests that a past interaction between these two galaxies took place between 2-8 billion years ago, and a more violent encounter 2.5 billion years in the future.

The fate of the Triangulum Galaxy is unclear, but seems to be linked to its larger neighbor Andromeda. Suggested future scenarios for Triangulum range from being torn apart and absorbed by Andromeda fueling it with hydrogen to form new stars or losing all of its gas—thus, the ability to form new stars—to participating in the collision between the Milky Way and Andromeda, most likely ending orbiting the merger remnant of the latter two galaxies and fusing with it much later. Two other possibilities are a collision with the Milky Way before Andromeda arrives or an ejection out of the Local Group.

The Triangulum Galaxy is one of the most distant objects that can be viewed with the naked eye – under exceptionally good viewing conditions with no light pollution. Through a small telescope, the galaxy looks like a diffuse patch about the same size as the full Moon.

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