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A galaxy is a system of millions to trillions of stars, together with gas and dust, held together by gravitational attraction. Deep-space observing telescopes show distant field of galaxies—galaxies and clusters of galaxies can be seen in all directions in distant space. The distance to these objects are in the range of thousands to billions of light years away from Earth.
Figure 1.37 shows a field of galaxies observed in on small region in deep space. Using images like this, astronomers estimate there may be 100 billion galaxies within the Observable Universe.
Galaxies appear as many shapes and sizes, but there are three general classes: spiral, elliptical, and irregular galaxies, but each of these groups are subdivided into classes (Figures 1-38 to 1-40). Small elliptical galaxies are the most common, and unlike spiral galaxies their stars do not seem to revolve around their galactic centers in an organized way. The galactic center is where the greatest mass and concentration of stars exist in a galaxy. Irregular galaxies take on many shapes, and many are interpreted as galaxies that have collided or merged under gravitational attraction.
The Milky Way Galaxy is probably a spiral galaxy.