The Galaxy Evolution Explorer (GALEX) is an orbiting space telescope launched on April 28, 2003 to observe galaxies in ultraviolet light. How can this satellite see light that our eyes cannot?

On the electromagnetic spectrum, UV light lies between violet (right next to blue) visible light and x-rays. Ultraviolet light is "invisible" to our eyes, but we can certainly feel its effects. UV rays break apart atoms and molecules like water, which is why it is a very good thing we have an atmosphere which blocks much of that radiation from reaching the ground.

The electromagnetic spectrum

Light collected by the GALEX telescope is directed toward the ultraviolet-sensitive detectors. The light first passes through a "dichroic beam-splitter," which separates the UV light into shorter wavelength Far UV, and slightly longer wavelength Near UV (nearer to visible violet). This beam-splitter is the first ever of its kind flown in space. Both the Far UV and Near UV detectors have the same basic design. Light entering the detector hits a thin layer of UV light-absorbing material. Every time a photon falls on the surface of either detector, extremely fast electronics record the position of the UV photon on the detector, along with the precise time the photon hit the detector. Eventually, astronomers can re-construct an image of the sky seen in UV light. We can view these images after they have been color-coded to indicate how many UV photons struck a particular part of the detector.

Here is a photo of GALEX before it was launched into space.

During its 29 month mission, GALEX will use its ultraviolet eyes to survey the sky.

To learn more about GALEX visit: http://www.galex.caltech.edu/

This week's question is provided by Dr. Beth Brown. Dr. Brown is an astrophysicist in the National Space Science Data Center at NASA Goddard Space Flight Center. She is involved with data acquisition and archiving, and her research focuses on the x-ray emission from elliptical galaxies.