Galaxies

There are over 100 billion galaxies in the known Universe. When one considers that the Milky Way, a very average galaxy, contains over 100 billion stars held together by gravity one starts to appreciate the scale of the Universe. they vary greatly in size, shape and colour. Bear in mind also that the appearance will depend on the orientation relative to us, i.e. a spiral galaxy will look very different if viewed sideways.

Charles Messier (1730-1817) catalogued a number of objects in the sky including many galaxies, e.g. the Whirlpool galaxy is M51. From 1888 galaxies, and other celestial objects, were catalogued with the NGC (New general catalogue) system, e.g. NGC891 is a spiral galaxy in the Andromeda system.

The appearance of galaxies varies greatly though Hubble, after studying many, classified them into three main groups.

Elliptical (E)	Spiral (S)	Spiral Barred (SB)
all pics courtesy of NASA
Most galaxies are elliptical. They may be circular in shape (E0) or extremely stretched out (E7) with degrees of elliptically in between.	A bright centre, or nucleus, with spiral arms. The spiral arms may be tightly wound (Sa) or more loosely wound (Sb and Sc)	Spiral but with a bar across the centre. SBa, SBb and SBc have arms which are increasingly open from the centre.

Hubble's Tuning Fork diagram is a useful way of remembering the galaxy types.

There are many galaxies that do not fit neatly into any of these categories. They are just referred to as being irregular in shape.

Galaxies are not distributed evenly throughout the Universe. There are regions where there are many more galaxies than in others. Many galaxies are found in clusters.

Our own galaxy is found in The Local Group which is part of the local Virgo supercluster.

The image below, take by the Hubble Space Telescope, is of a very small part of the sky where there is known to be a large concentration of galaxies.

for more fantastic Hubble pictures of galaxies go to  http://heritage.stsci.edu/gallery/galindex.html