We will show you five pictures of similar things, such as people, buildings, or cars. The things in the pictures are all different ages. In each group, arrange the pictures by age, oldest on the left, youngest or newest on the right.

When you solve a puzzle, you can start your collection of beautiful mini-posters from the GALEX space telescope mission.

Some people are good at telling other people’s ages. They can look at you and know you are 9 years old or 22 or 49 or 99. How? They read the clues: your size, shape, whether your hair is gray (or gone), wrinkles, how you talk, and how you act.

Astronomers can tell the ages of galaxies—or least the ages of the galaxies’ light. A galaxy is a grouping of stars bound together by gravity. All but a few stars in the universe live in galaxies. Our Sun is just one of at least 200 billion stars in our own Milky Way Galaxy.

What clues do astronomers use to tell the age of a galaxy?

Light travels in waves, like energy moves through the ocean in waves. Light waves change as they travel through space and time. That is because space itself is always expanding and stretching the distances between things. So the light waves traveling through this expanding space become themselves expanded.

No matter what, light always travels at the same speed in space: 300,000 kilometers (or 186,000 miles) per second (in round numbers). That means it takes some amount of time—a little or a lot—for light to get anywhere. The distance light can travel in one Earth year is called a light year. A light year is very long distance: around 9 trillion kilometers (6 trillion miles). That's a 9 (or a 6) with 12 zeroes after it!

Light waves from a very distant galaxy that have been traveling a long, long time (say, billions of years) starts looking very stretched out! Astronomers say that the light is red-shifted, because red light has the longest, most "stretched out" waves of all the colors of the light we can see with our eyes.

Artwork of GALEX in orbit above Earth.

GALEX (short for Galaxy Evolution Explorer) is a space telescope that was launched into orbit around Earth in 2003. GALEX sees ultraviolet light, a kind of light not visible to humans. To see ultraviolet light, GALEX must be in space, because very little of this kind of light can shine through Earth's atmosphere. GALEX is particularly good at seeing areas of galaxies where stars are forming, because young stars glow more brightly in ultraviolet light than do older stars.

GALEX is now looking at tens of millions of galaxies.

Click for larger view

GALEX image of great spiral galaxy M81. Stars in the center (yellow) are very, very old. Stars that look blue in the spiral arms have formed much more recently.

GALEX sees starlight that has been traveling for just a few years from stars that are “only” a few trillion kilometers away. But it also sees really old, red-shifted starlight. Light from the farthest of these has been traveling for most of the 13.7 billion years that the universe has existed! So GALEX is seeing galaxies as they were billions of years ago, as well as how the nearby galaxies looked just a few hundred thousand years ago.

For example, astronomers know that galaxy M81 (pictured at the left) is 10 million light years away because of how red-shifted the light is coming from M81.

Just as you look younger in a picture of you from several years ago, GALEX sees galaxies as they looked when they were much younger than they are now, because that is when the light we see now was emitted. By comparing these "far" pictures with the "near" pictures, they can see how galaxies and their stars are born, age, and die over time. They can learn how galaxies evolve.