M100

The hundredth object in Messier's catalogue is a distinctive spiral galaxy in the northern reaches of the Virgo Cluster, within that part of the cluster that falls within the boundaries of Coma Berenices. Lying about 55 million light years from the Milky Way, M100 is one of the largest and most prominent of the Virgo Cluster's galaxies.

This galaxy is oriented almost exactly face-on to observers on Earth, so that its spiral structure is clearly visible. The spiral shape is defined by two long spiral arms that sweep out from the star-forming regions near its centre and loop around much of M100's circumference. This well-defined spiral structure places it within the class of galaxies known as the 'grand design' spirals.

M100 has a diameter of more than 100,000 light years (comparable with the Milky Way Galaxy) and in common with many large galaxies of this type it has a number of satellite galaxies of its own. Among these is the elliptical NGC 4328, and another small galaxy generally known as NGC 4323 (though due to some confusion over its original identification, some sources catalogue it as NGC 4322). However it is catalogued, this small satellite galaxy is joined to M100 by a thick lane of material extending out from one of the main galaxy's immense spiral arms.