7. What's making these gamma rays?
Like the last surviving junkers in a demolition derby, the galaxies in Stephan's Quintet are battered and bruised, but still ready for action. Gravitational interaction among the galaxies has compressed hydrogen gas to form new stars, seen as bluish-white. Stars are forming along the spiral arms of the galaxy at center, near the pair of galaxies at top right, and most prominently, at top left.
The two upper galaxies collided about 20 million years ago, and yet some of the stars sparked by that collision formed just two million years ago, indicating that star-formation may be self-propagating. Perhaps effects generated by newborn stars cause more stars to form.
The view also includes dwarf galaxies that range in age from two million to one billion years. As many as 15 dwarf galaxies are in the long tail of the galaxy at right center, NGC 7319.
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