A galaxy is a system of many thousands of millions of stars, together with interstellar gas and dust. Galaxies may be spiral, spherical, elliptical or irregular in shape. Telescopes have revealed the existence of about 1000 million galaxies, although only very few of those can be seen with the naked eye.
The system for classifying galaxies was designed by the American astronomer
Edwin Hubble and is still used today, despite it's artificial attributes.
Galaxies are grouped into three basic categories: elliptical, spiral and
Our own galaxy - still known by it's ancient title the Milky Way - is a fairly ordinary spiral galaxy. Nevertheless it is unimaginatively huge; about 100 000 light years in diameter and containing some ten billion (10,000,000,000) stars. Our Sun is situated in one of the spiral arms, about 30,000 light years from the galactic centre. The Milky Way Galaxy is rotating and the Sun takes 225 million years to complete one revolution. This is sometimes called a cosmic year.