This image from NASA's Galaxy Evolution Explorer shows galaxy NGC 4522 being stripped of its star-forming material. Astronomers refer to this process as "ram-pressure stripping" - where surrounding hot-gas shoves star-forming gas out of the galaxy's outer disk. NGC 4522 is located approximately 50 million light-years away in the Virgo galaxy cluster.
In this image, the galaxy's older stellar population is marked by tints of yellow. Meanwhile, the bluish-white ridge near NGC 4522's center reveals a population of very young stars - most likely less than a million years old. The bluish-white coloration also indicates that new stars are actively forming in this area.
The light-blue haze beyond the white-ridge represents the galaxy's outer disk. This area is currently not a site of active star formation, but is still very bright in the ultraviolet, indicating that some stars in this region are very young and likely formed very recently, within the last 50-100 million years.
This image is a three color composite where yellow represents R-band visible-light data from the Kitt Peak National Observatory's 0.9-meter telescope. Green shows near-ultraviolet light data, and blue reveals far-ultraviolet light data from GALEX.
Photo credit: NASA/JPL-Caltech/Yale University/H. Crowl (Yale University)
Note: This image was used as an example in a recent Science@NASA article entitled Stellar Extremophiles.