A galaxy wound so precisely it became the textbook definition of a multi-armed spiral
NGC 488 is a face-on spiral galaxy in the constellation Pisces, classified as SA(r)b, an unbarred spiral with an inner ring structure and moderately tightly wound arms. It lies at a distance of approximately 94.6 million light-years, spans about 6.8 arcminutes across the sky and has a true diameter of roughly 186,000 light-years, making it considerably larger than the Milky Way. It shines at a visual magnitude of about 10.3
What sets NGC 488 apart is the extraordinary regularity of its disk. Rather than two dominant arms, it displays a dense series of thin, evenly spaced arm segments coiling outward from a large, smooth, yellow bulge, so orderly that the galaxy has long served as the prototype for the multi-armed spiral class. The bulge is warm and featureless, dominated by an old stellar population, while the arms carry a scattering of blue star-forming knots and fine threads of dust that trace the spiral pattern all the way into the inner ring. Beyond the bright disk a faint, extended outer envelope continues the spiral structure well past the visible arms. The nucleus is chemically decoupled from the surrounding bulge, roughly twice as metal-rich, a signature that points to a past accretion event of gas or a small companion
NGC 488 is the dominant member of a small, relatively isolated group in which it is accompanied only by minor companions, and its undisturbed symmetry reflects a long history free of major interactions
Galaxies like NGC 488 are important because the spacing and pitch of their arms are governed by the mass and rotation of the disk itself. With stars orbiting at about 330 kilometres per second, well above the Milky Way’s rotation speed, the disk is massive enough to hold its spiral pattern in a state of quiet equilibrium. The result is a galaxy that shows what a spiral looks like when left alone for billions of years, evolving slowly and without the distortions of collision or close encounter
Imaged in LRGB on ASA AZ 1500, Camera Moravian C5A 150M, at Observatorio El Sauce, Chile
Image Acquisition and Processing: Mike Selby