than even the most distant stars you can see. The closest major galaxy—the Andromeda
Galaxy, shown inFigure26.10—looks like only a dim, fuzzy spot to the naked eye. But
that fuzzy spot contains one trillion stars; that is a thousand billion, or 1,000,000,000,000
stars!
Figure 26.10: The Andromeda Galaxy is the closest major galaxy to our own. Andromeda
is a large spiral galaxy that contains about a trillion stars. ( 3 )
Spiral Galaxies
Galaxies are divided into three types according to shape: spiral galaxies, elliptical galaxies,
and irregular galaxies. Spiral galaxiesrotate or spin, so they have a rotating disk of stars
and dust, a bulge in the middle, and several arms spiraling out from the center. Spiral
galaxies have lots of gas and dust and lots of young stars. Figure26.11shows a spiral
galaxy from the side, so you can see the disk and central bulge.
Figure26.12shows a spiral galaxy face-on, so you can see the spiral arms. The spiral arms
of a galaxy contain lots of dust. New stars form from this dust. Because they contain lots
of young stars, spiral arms tend to be blue.
Elliptical Galaxies
Figure 26.13shows a typical elliptical galaxy. As you might have guessed, elliptical
galaxies are elliptical, or egg-shaped. The smallest elliptical galaxies are as small as some
globular clusters. Giant elliptical galaxies, on the other hand, can contain over a trillion
stars. Elliptical galaxies are reddish to yellowish in color because they contain mostly old
stars.