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On the darkest of nights, far from the lights of cars and cities, something special appears
in the sky. A cloudy white stripe of light stretches across the heavens! Ancient peoples
thought it looked like a stream of milk spilled across the sky, and so it is called 'the Milky
Way.'
The Milky Way that we see in the sky is part of a galaxy, or a huge collection of stars,
gas, dust, and other material. Galaxies are held together by gravity, much the same way
that our solar system is held together by the sun's gravity. The Milky Way is our galaxy,
where our solar system is located, but the solar system is just a tiny piece of the galaxy.
The Milky Way is enormous, at least 100,000 light-years across. A light year is how far
light can travel in one year, nearly 6 trillion miles. In addition, the Milky Way has at least
200 billion stars and probably at least 100 billion planets in it.
Although it is massive, the Milky Way is not alone in the sky. It's part of a collection
of other galaxies called the local group, made of about 50 galaxies nearby to ours.
The local group is estimated to be at least 10 million light years across. Outside of
the local group, scientists estimate that at least 170 billion galaxies exist in the
part of the universe that can be seen from Earth, and there is no way to know how many
more there are beyond that.
Galaxies come in many different shapes and sizes. They may have only a few million stars
in them or over a trillion. They may be elliptical, spiral, or irregular in shape.
The Milky Way, like most known galaxies, is a spiral galaxy. Although no one knows what
the Milky Way actually looks like, because it is impossible to see all of it from the
inside, careful observations of it as well as other galaxies has given scientists a good
idea of its shape.
There are four main parts in the Milky Way. In the center is the 'bulge,' or Galactic
center. This is a round bump in the center of the galaxy, about 10,000 light years across,
densely packed with stars and other material. Scientists believe that at the very center
of the Milky Way is a giant black hole. The galactic center is the brightest part of the
galaxy.
Next is the disk, a flattened region surrounding the bulge in the center. The disk is about
100,000 light-years across. It is filled with young stars, gas and dust, concentrated in
the spiral arms. The spiral arms are part of the disk, and make the galaxy look a little
like a pinwheel. Our solar system is located in one of the Milky Way's spiral arms, where
star systems are spread much farther apart than in the crowded Galactic Center.
The last part of the Milky Way is the halo. The halo is a mostly spherical layer of old
stars and star clusters that surrounds the main disk of the galaxy. The halo is very
empty compared to the rest of the galaxy, with everything in it spread very far apart.
Ancient Greek philosophers thousands of years ago first suggested that the Milky Way might
be made of far away stars that were close together, but there was no way to prove this
until the 1600s when the astronomer Galileo Galilei observed the Milky Way with his telescope
and discovered that it was, in fact, made of many, many faint stars. If you can find
somewhere to go that is very dark and far from the lights of cities and cars, you, too,
can see the mysterious sight that inspired myths and legends for thousands of years - the
Milky Way.