M51

M51 is a favorite to show to folks at star parties. It is easily seen in even the smallest of telescopes. In large aperature light buckets, its spiral arms are quite easily detected. There are actually two galaxies in this picture. At first glance, it appears that the two galaxies are interacting (colliding). But as can be seen in the photograph, the galaxy known as NGC5195 is partially obscured by the spiral arm extending from the Whirlpool's core. Astronomers speculate that M51's spiral structure is primarily due to it's gravitational interaction with this smaller galaxy.

Due to this interaction, the gas in the galaxy has been disturbed and compressed in some regions, resulting in the formation of new young stars. As is common in galactic encounters, spiral structure is preferably induced in the more massive galaxy.

M51 in the Constellation Canes Venatici

In 1845, Lord Rosse, with his behemoth reflector telescope set up in the back yard of his castle, first discovered the spiral nature of this galaxy. His drawing of the object is crude but well matches the photograph. For this reason, the galaxy is sometimes referred to as Lord Rosse's Question mark.

Sketch of M51 by Lord Rosse

For star parties, M51 is easy and a showpiece if the sky is dark. However, light pollution can ruin the view. With even small instruments and under very good skies, observers can detect the spiral nature of the galaxy. In a large scope, it is well apparent.

The Hubble Space Telescope has investigated the central region of M51. Its compact nucleus is now classified as of Seyfert type 2.5. ESA's ISO (Infrared Space Observatory) satellite has investigated the Whirlpool Galaxy in the infrared light.

M51 The Whirlpool Galaxy

M51 in the Constellation Canes Venatici