July 22, 2004: Like a photographer clicking random snapshots of a crowd of people, NASA's Hubble Space Telescope has taken a view of an eclectic mix of galaxies. In taking this picture, Hubble's Advanced Camera for Surveys was not looking at any particular target. The camera was taking a picture of a typical patch of sky, while Hubble's infrared camera was viewing a target in an adjacent galaxy-rich region. The most peculiar-looking galaxy in the image — the dramatic blue arc in the center — is actually an optical illusion. The blue arc is an image of a distant galaxy that has been smeared into the odd shape by a phenomenon called gravitational lensing.
The gravity of a massive galaxy is so powerful that it distorts surrounding space. The gravitational forces of the massive galaxy deflects the light from a galaxy lined up behind it and amplifies it, like a glass lens bending and focusing starlight in a telescope.
Albert Einstein predicted this effect in his theory of general relativity. But he knew that it would be difficult to see this phenomenon with the telescopes of his time.
In a gravitational lensing event, a massive galaxy acts like a lens, amplifying and brightening a distant galaxy behind it. The effect is similar to using your camera's zoom lens to take photographs of distant objects. Astronomers can study more features in the distant objects than they would if they viewed the galaxy through a telescope. In many cases, astronomers would not be able to see the distant galaxies at all because they are too far away.
Galaxies come in basic shapes, such as round spirals and football-shaped ellipticals. But just like people, no two galaxies are completely alike. One galaxy may appear red because of its older population of stars. Another galaxy's central disk of gas and dust may be slightly warped. Still another galaxy's shape may be distorted due to an encounter with another galaxy. In fact, galaxy interactions account for many of the odd-looking galaxies. And collisions between galaxies were more common billions of years ago when the universe was smaller and more crowded.
The colors represent the intrinsic colors of the stars in the galaxies. They yield clues about the types and ages of stars in the galaxies. Hot, young stars, for example, are blue; cool, older stars are red.