Galaxies
A galaxy is a huge group of individual stars, star systems, star clusters, dust, and gas bound together by gravity. Our own galaxy is called the Milky Way. In the 1800s and early 1900s, many astronomers thought that the universe did not extend beyond the Milky Way. However, observations of odd spiral-shaped objects sparked a spirited scientific debate. The debate focused on whether these so-called “spiral nebulae” were within or outside the Milky Way. The question was settled only in the 1920s, after a large new telescope was built on Mt. Wilson in California. Astronomers could see individual stars, proving that spiral nebulae were actually distant galaxies consisting of billions of individual stars.
Figure 22 Galaxies have a variety of shapes. A A spiral galaxy in the constellation Coma Berenices B A barred-spiral galaxy in the Fornax cluster C Elliptical galaxy M87 D An irregular galaxy with many areas of star formation
Astronomers now know that there are billions of galaxies in the universe. The largest galaxies consist of more than a trillion stars. Galaxies vary widely in size and shape. 
Astronomers classify galaxies into four main types: spiral, barred-spiral, elliptical, and irregular.
Spiral and Barred-Spiral Galaxies
Galaxies like the Milky Way are called spiral galaxies. Spiral galaxies have a bulge of stars at the center, with arms extending outward like a pinwheel. These spiral arms contain gas, dust, and many bright young stars. Figure 22A shows the spiral galaxy M100. It belongs to the Virgo Cluster of galaxies, which is the nearest major cluster to our own Local Group of galaxies.
Some spiral galaxies, like the one in Figure 22B, have a bar through the center with the arms extending outward from the bar on either side. These are called barred-spiral galaxies.
Elliptical Galaxies
Elliptical galaxies are spherical or oval, with no trace of spiral arms. M87, shown in Figure 22C, is one of the largest and brightest of all galaxies. Elliptical galaxies come in a wide range of sizes. They have very little gas or dust between stars. For this reason, new stars are not forming in older elliptical galaxies. They contain only old stars.
Irregular Galaxies
A small fraction of all galaxies are known as irregular galaxies. As shown in Figure 22D, irregular galaxies have a disorganized appearance. They have many young stars and large amounts of gas and dust. Irregular galaxies come in many shapes, but are typically smaller than other types of galaxies. They are often located near larger galaxies, whose strong gravity may be causing their shape to become irregular. One of the closest galaxies to our own, the Large Magellanic Cloud, is an irregular galaxy.
What are some characteristics of irregular galaxies?
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