- M 33 (NGC 598) Triangulum Galaxy
M 33 is a grand spiral galaxy in the constellation Triangulum, not far from the Andromeda Galaxy (M 31). It is also known as the Pinwheel Galaxy. Along with M 31, it is a member of our Local Group of galaxies, which includes the Milky Way Galaxy. Estimates obtained from observations of Cepheid variable stars in M 33 indicate a distance of around 2.74 million LY, which is slightly more distant than M 31 (around 2.5 million LY). The main body of M 33 has a diameter of around 50,000 LY. The distinctive spiral arms of this galaxy give a real impression of rotation— the galaxy is rotating in a clockwise direction (as seen by us) and takes around 200 million years to make one revolution. M 33 exhibits a beautiful range of colors— older stars in the center and along the main arms give a soft, whitish-yellow glow; huge star-forming regions glow bright red in Hα emissions; clouds of young blue giant stars dust the outer reaches of the spiral arms. Such a separation of colors is lost in images of more distant galaxies.
With a visual magnitude of 5.7, M 33 can be seen with the naked eye under good sky conditions. This makes it the most distant object that can be viewed with the unaided human eye. It may have been observed by Giovanni Batista Hodierna before 1654, although the records are uncertain. It was observed by Charles Messier in 1764 using a 3-inch refractor.
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