​                                                            M33 - The Triangulum Galaxy

    

    The Triangulum Galaxy (also known as M33) that is about 3 million light-years away from Earth. While its mass is not well understood - one estimate puts it between 10 billion and 40 billion times the sun's mass - what is known is it's the third largest member of the Local Group, or the galaxies that are near the Milky Way. Triangulum also has a small satellite galaxy of its own, called the Pisces Dwarf Galaxy. Under dark sky conditions, M33 is just barely visible with the naked eye in the constellation Triangulum, just west of Andromeda and Pisces.

   

    Among the galaxy''s most distinctive features are ionized hydrogen clouds, also called H-II regions, which are massive regions of starbirth. Sprawling along loose spiral arms that wind toward the core, M33's giant H-II regions are some of the largest known stellar nurseries, sites of the formation of short-lived but very massive stars. Intense ultraviolet radiation from the luminous, massive stars ionizes the surrounding hydrogen gas and ultimately produces the characteristic red glow.

   

    Triangulum's official name of M33 would suggest at first that Charles Messier, who created the Messier catalog, was the one who discovered the galaxy. Messier was cataloging a list of deep-sky objects that were not comets in order to make searching for comets easier. M33 was noted on Aug. 25, 1764.However, the European Southern Observatory adds that the discovery could have been made by Italian astronomer Giovanni Battista Hodierna sometime before 1654, when his book "De systemate orbis cometici" ("About the systematics of the cometary orbit") was published. Some accounts say that Hodierna made an oblique reference in the book that could be interpreted as an observation of the galaxy.

   

    William Herschel (who is best known for discovering Uranus) also observed Triangulum in the late 1700s, and also noted a large region of gas within the galaxy that is today noted as NGC 604. M33 itself was considered a "spiral nebula," and it was unclear if M33 was part of the Milky Way Galaxy.

   

    By the 1920s, observations by Edwin Hubble noted that M33 is a separate "stellar system" based upon observations of Cepheid variable stars. These stars are often used to measure cosmic distances in space because their luminosity is the same no matter where they are located. Later observations of the galaxy revealed that it is approaching the Milky Way Galaxy at about 62,000 mph (100,000 kph). Some astronomers believe that Triangulum is "gravitationally trapped" by the massive Andromeda Galaxy that is also hurtling toward our galaxy, the European Southern Observatory stated.

 

                                                                                                                                                                                                      Elizabeth Howell,Space.com