M101 Pinwheel Galaxy

Full Size    Image from 2007

This spiral galaxy in Ursa Major has a large apparent size of 22 arc minutes. It's also quite bright at magnitude 7.9, but due to the large area the surface brightness is quite low.   The distance to this galaxy has determined by observing Cepheid variables with the Hubble telescope back in 1994 / 1995. According to the recent recalibration of the Cepheid distance scale, the distance is about 27 million light years. Such a large apparent size at this distance indicates that this is an enormous spiral galaxy (170,000 light years in diameter – almost twice the diameter of our own galaxy).

In photographs the spiral structure is very prominent. Galaxies with this type of spiral structure are referred to as ‘grand design’ spirals. On short exposure photographs, this galaxy looks very normal - only the brighter spiral arms show and these appear very symmetrical. Longer exposures reveal more of the disk and although the disk is fairly symmetrical in shape, the nucleus is not even close to the centre of this disk.

The spiral arms contain many star formation regions. These HII emission nebula appear as pink regions within the spiral arms in the photograph. Recent star formation has created hot blue super luminous stars. It is these stars that trace out the spiral arms and provide the blue colour.

Why is this galaxy peculiar? It has probably suffered a close approach from another large galaxy recently (on an astronomical timescale). The gravitational tidal interactions both enhance the spiral structure (hence the ‘grand design’) and created the non symmetrical nucleus / disk. There is also evidence of gas falling into the disk at high speed.