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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.

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