Messier 83, NGC 5236 - Southern Pinwheel Galaxy

Messier 83, also catalogued as NGC 5236, is a spiral galaxy in the constellation Hydra. It is one of the closest and brightest spiral galaxies in the sky, and sometimes called the "Southern Pinwheel".

Messier 83 was discovered by Nicholas Louis de Lacaille at the Cape of Good Hope in 1752. It was the third galaxy discovered, after M 31 and M 32, and the first discovered beyond our Local Group. Charles Messier added M 83 to his catalogue in 1781; its spiral structure was first noted by William Lassell.

Amateur Observation

M 83 is one of the showpieces of the southern sky, but difficult for mid-northern observers to view due to its southern declination. With a visual magnitude of 7.6, M 83 is visible in binoculars from southerly latitudes. It is a large (11' x 10') face-on barred spiral galaxy with an oval core and a bright bar encircled by an interesting spiral arm pattern.

The spiral arm that springs from the northeast end of the bar arcs through only 90 degrees, ending south of the galaxy's center. The arm that springs from the bar's southwest end wraps all the way around the north side of the galaxy to its east, thus giving the galaxy an asymmetrical look. The spiral arms are divided by tiny dark lanes. A dozen foreground stars are superimposed upon the galaxy's disk.

For years, M 83 was the galaxy with most supernovae discovered (SN 1923A, SN 1945B, SN 1950B, SN 1957D, SN 1968L and SN 1983N). But recently, NGC 6946 passed this mark, and as of 2009 it holds the current record, with a total of nine. M 61 has now also caught up, with six supernovae as of late 2008.

Properties and Structure

M 83 lies 15 million light years away, and its true diameter is over 100,000 light years. M 83's absolute magnitude is -21.6, a luminosity of 36 billion suns. It is receding at 337 km/sec.

M 83 is at the center of one of two subgroups within the Centaurus Group of galaxies. These two groups are sometimes identified as one, as they are physically close to each other, and share a common motion. M 83 is part of the northern Centaurus Galaxy Group, along with NGC 4945, NGC 5102, NGC 5253, and NGC 5068 in Virgo. NGC 5128 (Centaurus A) marks the center of the southern group.

M 83 is classified as intermediate barred spiral of type SAB(s)c. It displays a very dynamic appearance, with red and blue knots tracing out its arms. The red knots are diffuse gaseous nebulae where star formation is taking place, energized by very hot, young stars within them. The blue regions represent young stellar populations which have formed just a few million years ago.

Between the pronounced spiral arms are regions with fewer stars. Dark dust lanes follow the spiral structure throughout the disk, and may be traced into the nucleus. The nucleus is composed of an older, yellowish stellar population which dominates the whole central region, and extends along the barlike structure. In 2008, NASA reported large numbers of new stars in the outer reaches of the galaxy, which had previously been thought to lack the materials necessary for star formation.