M51, whose name comes from being the 51st entry in Charles Messier's
catalog, is considered to be one of the classic examples of a spiral
galaxy. At a distance of about 30 million light-years from Earth, it is
also one of the brightest spirals in the night sky. A composite image of
M51, also known as the Whirlpool Galaxy, shows the majesty of its
structure in a dramatic new way through several of NASA's orbiting
observatories. X-ray data from NASA's Chandra X-ray Observatory reveals
point-like sources (purple) that are black holes and neutron stars in
binary star systems. Chandra also detects a diffuse glow of hot gas that
permeates the space between the stars. Optical data from the Hubble Space
Telescope (green) and infrared emission from the Spitzer Space Telescope
(red) both highlight long lanes in the spiral arms that consist of stars
and gas laced with dust. A view of M51 with the Galaxy Evolution Explorer
telescope shows hot, young stars that produce lots of ultraviolet energy
(blue).
The textbook spiral structure is thought be the result of an interaction
M51 is experiencing with its close galactic neighbor, NGC 5195, which is
seen just above. Some simulations suggest M51's sharp spiral shape was
partially caused when NGC 5195 passed through its main disk about 500
million years ago. This gravitational tug of war may also have triggered
an increased level of star formation in M51. The companion galaxy's pull
would be inducing extra starbirth by compressing gas, jump-starting the
process by which stars form.
