Triangulum Galaxy

The Triangulum Galaxy, or Messier 33, is a spiral galaxy about three million light-years away in the constellation Triangulum. It is the third-largest member of the Local Group.

Local Group: The collection of galaxies gravitationally bound to the Milky Way, Andromeda Galaxy, and their smaller companions. M33 is one of the Local Group’s major spiral galaxies.
H II region: A cloud of ionized hydrogen surrounding young, hot stars. M33’s disk contains prominent regions where young stars ionize surrounding gas.
Variable star: A star whose apparent brightness changes over time. Variable stars in M33 help establish its distance.
Andromeda Galaxy: The nearest large spiral galaxy to the Milky Way and the most massive member of the Local Group. Andromeda is far larger and more massive than M33 despite their shared group.
Spiral galaxy: A galaxy with a flattened disk and a spiral pattern of stars, gas, and dust. M33’s defining shape places it in this galaxy class.
NGC 604: A giant star-forming H II region in the Triangulum Galaxy. This exceptionally large nebula shows star formation on a dramatic scale within M33.
Cepheid variable: A pulsating star whose period of brightness variation is related to its intrinsic luminosity. Cepheids in M33 provide a standard-candle distance measurement.
Large Magellanic Cloud: A satellite galaxy of the Milky Way and the largest of its satellite galaxies. Its irregular form contrasts with M33’s prominent spiral disk.
Messier 33: The Messier catalog designation for the Triangulum Galaxy. This alternate name appears in observations and catalogs of M33.
Stellar population: A group of stars studied together by properties such as age, composition, or location. M33’s stellar populations reveal how its disk formed and changed.

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