The universe contains at least 100 billion galaxies, each
with billions of stars—most of which probably have
orbiting planets. Our own galaxy, the Milky Way,
consists of about 500 billion stars, including all
the ones that we can see in the night sky, as
well as large clouds of gas and dust, some
of which form new stars. The Milky Way is
a flat disk with a central bulge and bright
spiral arms. Our Sun is a mediumsized
star in one of the spiral arms,
about two-thirds of the way out
from the center. From Earth, we
look out across the galaxy’s disk,
so the densely packed stars at its
center look like a milky band of
light across the night sky.