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M 96
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M 96

February 19, 2005
Spiral Galaxy M96 (NGC 3368), type Sa, in Leo

Right Ascension: 10 : 46.8 (h:m)
Declination: +11 : 49 (deg:m)
Distance: 38000 (kly)
Visual Brightness: 9.2 (mag)
Apparent Dimension: 6x4 (arc min)

Discovered by Pierre Méchain in 1781.

M96 is the brightest member of the Leo I group of galaxies, which is therefore also called the M96 group, and also includes M95, M105 as well as a number of fainter galaxies.

Pierre Méchain discovered M96, together with M95, on March 20, 1781. Consequently, Charles Messier included it in his catalog on March 24, 1781. It was among the first spirals that have been discovered, and listed by Lord Rosse as one of 14 "spiral nebulae" discovered to 1850.

Its distance was determined to be about 41 million light years (after corrections for the distance scale which are implied by the results of ESA's Hipparcos satellite) by Nial R. Tanvir with the Hubble Space Telescope by observing Cepheid variables. Interpolated with the HST result of 35.5 million light years for its neighbor M95, we adopt a value of 38 million light years here for the whole group.

At this distance, the apparent diameter of its brighter central region, 6 arc minutes, corresponds to a linear dimension of 66,000 light years. However, as can be seen e.g. in the Digital Sky Survey image, or the Hubble Atlas of Galaxies, this galaxy has faint extensions, an outer ring of filaments (spiral arm fragments), which are connected to the bright visible part near the northwest end of the major axis. This ring has a diameter of at least about 9 arc minutes in the DSS image, corresponding to about 100,000 light years.

The apparent visual brightness of 9.2 magnitudes corresponds to an absolute magnitude of -21.1.

According to J.D. Wray's Color Atlas of Galaxies, the bright inner disk is composed of a smooth yellow stellar population of old stars, which ends slightly beyond a ring of blue knots. These knots are probably clusters of young, hot stars. As visible in our image, this galaxy contains a significant amount of dust, which is apparently more concentrated on the left side in our image. 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 near side of M96 is on the left in our image.

G. de Vaucouleurs has determined that M96 is inclined by 35 degrees to our line of sight, and that it rotates with the spiral arms trailing.

A bright supernova, SN 1998bu, was discovered by Mirko Villi on May 9, 1998 at 13th magnitude and was quickly brightening to 11.8 mag.