World Builders™                                                                          Session One  --  Astronomy 


 Distances in Space

Where Will You Put Your Solar System?

Read this page carefully to find out how you can describe the location of your star.

    Distances in the universe are vast beyond our ability to even imagine. Astronomers have created a measurement system which makes it possible to think about, and compare, these huge distances. The unit that they use is called a Light Year. A light year is equal to how far light can travel in a year -- about 6,000,000,000,000 miles.

     Nothing in the universe travels faster than light. Light travels about 186,000 miles a second. It travels between the earth and the moon in a little over a second, and takes eight and a half minutes to get from the sun to us. When light leaves the sun it takes 5 hours and 40 minutes to get to Pluto, the outermost of our planets.

     In a year, light travels 6,000,000,000,000 miles. That is a huge number! But out in space it is not much. The star that is closest to us, Proxima Centauri, is 4.3 light years away. Other stars spread out farther and farther from earth. Some stars are single, like our sun, and some are double, where two stars rotate around a common center of mass. Triple, quadruple, and even quintuple star groups have been observed, but the gravitational forces around them often make planetary systems unstable. Check this nice diagram of a binary system that might have planets. Look at this site if you want to work with a multiple star system.

     Stars in space are grouped in huge systems called galaxies. Our galaxy, the Milky Way, is about 100,000 light years across and 3000 light years deep. When you place your planet, it could orbit a star somewhere in the galaxy, perhaps between 100 and 70,000 light years from earth. There are over a hundred billion stars in our galaxy, so you have plenty of choice! Our own sun is about 30,000 light years from the center of the Milky Way galaxy. The picture above is a spiral galaxy like our own. These galaxies are disk-shaped, and thicker in the center.

     Galaxies, although surrounded by a lot of empty space, are found in clusters. Between the clusters there are enormous stretches of emptiness. The Andromeda Galaxy is 2 million light years away. The M105 Galaxy is 38 million light years away. The Cartwheel Galaxy is 500 million light years away. Abell 2256, a collection of more than 500 galaxies, is a billion light years away. Your world might be located in a different galaxy. Again, you have plenty of choices!

     We do not know how large the universe is, as our telescopes cannot see out to the edge of it -- if there is an edge. The Hubble Telescope has yielded images of galaxies believed to be 10 billion light years away.   Astronomers hope to learn more about the development of the universe by studying these pictures.

     Scientists assume that wherever there is matter there will be gravity, the same elements as we see on earth, and the same physical laws. However, there are still many surprises in space: beautiful, glowing clouds of stellar dust, puzzling geology, planets glimpsed in orbit around other stars. Now that we are able to put telescopes above our planet's atmosphere, new and exciting information is being uncovered.

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© 1998, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at