Messier 74 (also catalogued as NGC 628) is a face-on spiral galaxy in the constellation Pisces. It has the lowest surface brightness of any of the Messier objects, and is a difficult object to observe visually. But its large angular size and face-on orientation make it an archetypal example of a "Grand Design" Spiral Galaxy - and an ideal object for professional astronomers to study spiral structure and density waves.
M 74 was discovered by Pierre Mechain in September of 1780 and confirmed by Charles Messier a month later. M 74 was among the first 14 "Spiral Nebulae" listed by Lord Rosse in 1850.
Amateur Observation
Although faint, M 74 is the most prominent galaxy in Pisces, located 1.5° east-northeast of the double star Eta Piscium. It is a nearly face-on spiral of visual magnitude 9.4, with relatively large dimensions of 10.2' x 9.5'. This makes difficult to observe visually; the galaxy is best seen with a 12-inch or larger telescope, and requires clear, dark skies unaffected by light pollution. Because the disk with the spiral arms is of low surface brightness, the core is the galaxy's most conspicuous portion.
Very good conditions are needed to see more than this nucleus. If present, suggestions of the magnificent spiral arms become apparent in telescopes starting at 4" aperture. In telescopes of this size, the diffuse, hazy, mottled disk can be traced to a diameter of about 6' to 8'. Numerous faint foreground stars are visible in the field around this galaxy. Larger telescopes show the faint spiral arms more clearly, and in large amateur instruments (16" and up), knots of star clouds and nebulae within M 74's disk become recognizable within and between the spiral arms.
Two supernovae have been identified in M74: SN 2002ap and SN 2003gd. M 74 is the brightest member of the M 74 Group, a group of five to seven galaxies that also includes NGC 660, UGC 891, UGC 1176, UGC 1195, and UGCA 20.
Properties and Structure
Messier 74 is similar in structure to M 33, but over fifteen times farther away, at a distance of 35 million light years. Its spiral arms are about 1000 light years wide, and reach out to roughly 95,000 light years from the core. M 74 is nearly the same size as our own Milky Way Galaxy, and is moving away from us at 793 km/sec.
M 74 contains two clearly-defined spiral arms, and is classified as a Sc spiral. Its remarkably symmetric appearance is caused by density waves sweeping around M 74's disk, probably induced by gravitational interaction with neighboring galaxies. When gas clouds orbiting within the disk encounter such density waves, they are accelerated into the spiral-shaped wave crest, and converge into the spiral arm, enhancing the density wave.
Collisions and mergers of neighboring gas clouds occur along the density wave, which induce the observed starbirth activity along the spiral arms. They are traced with clusters of blue, young stars and pinkish colored diffuse gaseous nebulae (H II regions) in color photos. The large number of H II regions - more than 193 - indicate that star formation currently occurs vividly along the pronounced spiral pattern in M 74's disk. These regions also show up as bright knots in the UV part of the spectrum.
In 2005, the Chandra X-ray observatory observed an ultraluminous X-ray source at the center of M 74. This is an indicator of an intermediate-mass black hole, with an estimated mass of around 10,000 Suns. This would be a rather uncommon class of object.