Spiral galaxy NGC 4631. Combined Chandra X-ray Observatory and Hubble Space Telescope image of a halo of hot gas (blue) around the central part of the spiral galaxy NGC 4631. The hot gas has been revealed by Chandra, and is overlaid on an HST image of huge starbirth regions (white and red) in the galaxy. NGC 4631 is a spiral galaxy similar to our own Milky Way. It lies around 27 million light years away from Earth in the constellation Canes Venatici. It has been debated for some time whether the Milky Way is surrounded by a halo of hot gas, and studying similar galaxies can provide important clues. Photographed in 2000.