September 16, 2013
Messier 96, a spiral galaxy in Leo
Messier 96 (also known as NGC 3368) is an intermediate spiral galaxy of some 100,000 light-years across (about the size of our Milky Way), located about 31 million light-years away from Earth in the constellation of Leo (the Lion), while it is receding from us at approximately 897 kilometers per second. (The bright central area has a diameter of about 66,000 light-years).
It is the brightest and largest galaxy in the M96 Group (or the Leo I Group), a group of galaxies that contains between 8 and 24 members in the constellation Leo, including Messier 95 and Messier 105.
The bright inner disk of Messier 96 is composed of a smooth yellow stellar population of old stars, while the spiral arms are patchy rings of blue knots, which are clusters of young, hot stars. Several dark dust lanes around the compact core move in a delicate swirl towards the nucleus.
This significant amount of dust is apparently more concentrated on the right than on the left side of the galaxy. It is common that dust appears with greater contrast on the near side of a galaxy than on the far side, so this asymmetry indicates that the right side of this galaxy is the side of the galaxy nearer to us.
Messier 96 is far from being symmetrical. Its core is displaced from the center, its gas and dust are distributed asymmetrically and its spiral arms are ill-defined and also asymmetric, with a top looping arm way out of proportion to the other side of the galaxy, an indication that this arm is been tugged out due to the gravitational pull of other members in the group, or perhaps due to past galactic encounters.
On May 9, 1998 a Type Ia supernova explosion, designated SN 1998bu, was observed in this galaxy. A Type Ia supernova is a result from the violent explosion of a white dwarf star (a compact star that has ceased fusion in its core). SN 1998bu reached maximum light on May 21 then steadily declined in magnitude thereafter. Observations of the ejecta a year later showed that the explosion created 0.4 times the mass of the Sun worth of iron.
One interesting feature of the region is a huge ring of cold neutral hydrogen gas that spans some 650,000 light-years and surrounds most of the galaxies in the M96 Group. A 2010 study of the ring with the CFHT telescope reveals its (very faint) visible structure, and in combination with computer modeling, indicates that the ring consists of material blown out of Messier 96 and the elliptical galaxy NGC 3384 in a direct collision between the two galaxies, about a billion years ago. During that time the two galaxies have moved nearly 40 million light years apart.
A multitude of background galaxies peers through the dusty spiral. Perhaps the most striking of these objects is an edge-on galaxy that — because of a chance alignment — appears to interrupt the outermost spiral arm to the upper left of Messier 96’s core, and which appears to be about 1/5 the size of Messier 96. If the spiral is similar in actual size to Messier 96, then it lies about 5 times farther away.