How Far to a Star?

Its the first day of school. Your teacher is passing out textbooks. Your
neighbor gets a shiny science book, with no rips or marks, and you get a book
with chewed corners and a scuffed, dirty cover. Hey! you say. Why does he
get a new book and I get an old one?

How can you tell your book is older than his? By how it looks, of course!

Just as we can guess a textbooks age by how it looks, astronomers can estimate
the age of light from a star or galaxy. This ability helps astronomers know
how far away the stars are.

What makes old starlight look different from young starlight?

Light travels in waves, similar to the way waves of energy move through the
ocean or sound waves move through the air. Next time a fire truck approaches
with its siren blasting, listen closely. You will notice that the sirens note
deepens a little just as the truck passes you. The note is lower because the
sound waves are being stretched as the truck continues to move away from you.

The same thing happens to starlight. The galaxies are all moving away from us
and each other because space itself is expanding. As the light waves move
through expanding space, they get stretched out. The longer the light waves
journey through space, the more stretched out they become. Astronomers say
this stretched out light is red-shifted.

NASAs GALEX space telescope sees very faint starlight that has been traveling
for most of the time the Universe has existed! These light waves are very,
very stretched out, or red-shifted. So GALEX is seeing galaxies as they looked
very long ago, when the stars gave off the light.

Decorate your room with printable mini-posters of beautiful GALEX images, as you
test your own skill at solving Whats Older puzzles at
http://spaceplace.jpl.nasa.gov/en/kids/galex/whats_older/ .

This article was written by Diane K. Fisher and provided by the Jet Propulsion
Laboratory, California Institute of Technology, under a contract with the
National Aeronautics and Space Administration.

The light from the Cartwheel Galaxy traveled for 500 million years to reach our
telescopes. This picture combines images from four NASA space telescopes:
GALEX, Hubble, Spitzer, and Chandra X-ray Observatory.