The Milky Way Galaxy and Beyond
All the stars that can be seen in the night sky are part of our galaxy, the Milky
Way. It contains more than 200 billion stars. Great distances separate these stars.
The diameter of the Milky Way is about 100,000 light-years. A light-year is the
distance light travels in one year. To appreciate how far this is, consider that
light can travel from the Sun to Earth in about eight minutes, and from the Sun
to distant Pluto in about five and a half hours. So the Sun and Earth are about
eight “light-minutes” apart, and the Sun and Pluto are about five and a half “lighthours”
apart. The closest star to the Sun is about four light-years away from it.
Stars produce light and make their galaxy bright. Galaxies are so far away that
they look like fuzzy patches in the sky when viewed with the unaided eye or even
with amateur telescopes. Only three galaxies can be seen with the unaided eye:
the Small and Large Magellanic Clouds in the Southern Hemisphere, and the
Andromeda Galaxy in the Northern Hemisphere.
Modern telescopes, including the Hubble Space Telescope, have made it possible to
make detailed observations of other galaxies. When astronomers started studying
galaxies, they discovered that these systems of stars also contain gas and dust and
come in a variety of shapes, colors, sizes, and orientations. Some galaxies have a
lot of gas and dust, while others have very little. Some have recognizable shapes,
while others do not.