Universe
The Universe is a huge wide-open space that holds everything from the smallest particle to the biggest galaxy. No one knows just how big the Universe is. Astronomers try to measure it all the time. They use a special instrument called a spectroscope to tell whether an object is moving away from Earth or toward Earth. Based on the information from this instrument, scientists have learned that the Universe is still growing outward in every direction.
Scientists believe that about 13.7 billion years ago, a powerful explosion called the Big Bang happened. This powerful explosion set the Universe into motion and this motion continues today. Scientists are not yet sure if the motion will stop, change direction, or keep going forever.
Light Year
A light year is used by scientists to measure distances in the universe. A light-year is the distance light travels in one year. It is 9.5 trillion (9,500,000,000,000) kilometers. The size of a galaxy may be as little as a thousand light-years across or as much as a million light-years across.
Galaxies
A galaxy contains stars, gas, and dust which are held together as a group by gravity. There may be millions, or even billions, of stars in one galaxy. There are billions of galaxies in the Universe.
Galaxies are labeled according to their shape. Some galaxies are called "spiral", because they look like giant pinwheels in the sky. The galaxy we live in, the Milky Way, is a spiral galaxy. Some galaxies are called "elliptical", because they look like flat balls. A galaxy may be called "irregular" if it doesn't really have a shape. A new type of galaxy was discovered recently, called a "starburst" galaxy. In this type of galaxy, new stars just seem to 'burst out' very quickly.
The Milky Way
There are over 100 billion stars in the Milky Way galaxy. If you tried to count them one by one, it would take you over 3000 years!
The Milky Way is over 100,000 light-years wide. It is called a spiral galaxy because it has long arms which spin around like a giant pinwheel. Our Sun is a star in one of the arms. When you look up at the night sky, most of the stars you see are in one of the Milky Way arms.
Before we had telescopes, people could not see many of the stars very clearly. They blurred together in a white streak across the sky. A myth by the ancient Greeks said this white streak was a "river of milk". The ancient Romans called it the Via Galactica, or "road made of milk". This is how our galaxy became known as the Milky Way.
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