Triangulum Galaxy

Lore & Background

The Triangulum Galaxy was probably discovered by the Italian astronomer Giovanni Battista Hodierna before 1654, who listed it as a cloud-like nebulosity. It was independently discovered by Charles Messier on the night of August 25–26, 1764, and published in his Catalog of Nebulae and Star Clusters (1771) as object number 33. William Herschel cataloged it on September 11, 1784, as H V-17, and also cataloged its brightest H II region, NGC 604, separately. It was among the first 'spiral nebulae' identified as such by Lord Rosse in 1850. In 1922–23, John Charles Duncan and Max Wolf discovered variable stars in the nebulae, and Edwin Hubble showed in 1926 that 35 of these stars were classical Cepheids, allowing distance estimates consistent with spiral nebulae being independent galactic systems.

The galaxy is believed to be a satellite of the Andromeda Galaxy or on its rebound into the latter due to their interactions, velocities, and proximity. It has an H II nucleus. The Triangulum Galaxy is sometimes informally referred to as the 'Pinwheel Galaxy' by some amateur resources, but the SIMBAD Astronomical Database indicates that Pinwheel Galaxy refers to Messier 101. The galaxy may be home to 40 billion stars, and its disk has an estimated mass of (3–6) billion solar masses, with a gas component of about 3.2 billion solar masses. The dark matter component out to a radius of 55×10^3 ly is equivalent to about 50 billion solar masses.

Reader's Guide

The Triangulum Galaxy holds significance as a key object in the study of the Local Group and spiral galaxy structure. Its visibility to the naked eye under dark skies makes it a reference object on the Bortle Dark-Sky Scale, and its distance—about half again as distant as the Andromeda Galaxy—marks it as the farthest permanent entity visible without magnification. The galaxy's classification as type SA(s)cd in the revised Hubble Sandage system describes it as a disk-shaped spiral galaxy without a bar, with loosely wound arms emerging directly from the nucleus. Its H II region NGC 604 is one of the largest known, with a diameter of nearly 1500 light-years. The discovery of variable stars and Cepheids in the galaxy by Duncan, Wolf, and Hubble was instrumental in establishing that spiral nebulae are independent galactic systems. Evidence of a clumpy stream of hydrogen gas linking the Andromeda Galaxy with Triangulum, announced in 2004 and confirmed in 2011, suggests past tidal interactions. The galaxy's motion toward Andromeda, estimated at 190 ± 60 km/s relative to the Milky Way, supports the hypothesis that it may be a satellite of the larger galaxy.

Did You Know?

• The Triangulum Galaxy is the third-largest member of the Local Group, behind the Andromeda Galaxy and the Milky Way.
• It is sometimes informally called the 'Pinwheel Galaxy,' but the SIMBAD Astronomical Database reserves that name for Messier 101.
• Its brightest H II region, NGC 604, has a diameter of nearly 1500 light-years and a spectrum similar to the Orion Nebula.
• In 2004, evidence was announced of a clumpy stream of hydrogen gas linking the Andromeda Galaxy with Triangulum, suggesting past tidal interactions.