Contents:
This is an image of the central region of the Whirlpool galaxy.
It was discovered in 1773 by the French comet hunter, Charles Messier,
who described it as a "very faint nebula".
(He listed it in his catalog of nebulae, from which comes its alternative
name, M51.)
He saw what we can see by eye in our own telescope, the relatively
bright (magnitude 8.4) fuzzy core of the galaxy.
The much fainter spiral structure was discovered by Lord Rosse in 1845, using his giant 6-foot reflector telescope, and was the first spiral nebula known. The brightest portions of the spirals are just 10% brighter than the light-polluted night sky of Grand Rapids under the best of conditions, and are therefore invisible to the eye. However, by exposing the camera for an extended time (74 minutes total) and subtracting out the uniform sky contribution, our photograph shows rich detail in the spiral arms.
As the galaxy core is at least ten times brighter than the brighter portions of the spiral arms, the grayscale levels of a computer monitor is unable to show the full range of intensities adequately. We therefore present our intrinsically monochrome image with a false color scale (the "heat" palette applied to logarithmic values). In this representation, the intense core is white, the much fainter outer core is yellow, and the much fainter still spiral arms are orange-red.
A consensus among astronomers was reached in the 1920s that spiral nebulae are in fact galaxies of hundreds of billions of stars similar to our own Milky Way. Most stars lie in a disk (seen in this instance face on) making circular orbits around the galactic core. The spiral structure is a wave propagating through the disk that is particularly prominent due to the narrow dust lanes and the luminous, short-lived stars that outline the arms. The low intensity of the arms despite the high intensity of its stellar constituents is a consequence of the vast empty spaces between stars in galaxies.
Processing: This monochrome image was made by Calvin student Philip Ammar. He combined 120 fifteen second exposures taken on 23 March 2002 (between 11:30 pm and 2:30 am) and 106 twenty-five second exposures taken on 26 March (between 11 pm and 1:30 am). All exposures were taken with a clear filter. He applied dark and flat field corrections.
Orientation and scale: North is up and East is to the left. The image is 7.2 by 5.2 arcminutes. The galaxy core has celestial coordinates 13h29m53s, +47d11'48" (epoch 2000), which is 3.6 degrees southwest of Alkaid, the star at the end of the handle of the Big Dipper.
The disk of the Whirlpool is some 40,000 light years across and approximately 35 million light years away from Earth.