Babels of Designations

Everything grows more complex with time. As astronomy has expanded, many objects have acquired an abundance of different names. One way a familiar entity acquires a new name is to be included in a list of special objects. Thus the galaxy Arp 220, mentioned at the start of this article, is IC 4553 and IC 4554. Because of its bizarre shape (it's probably two spirals colliding and merging), Halton Arp included it in his Atlas of Peculiar Galaxies published in 1966.

More designations get added when an object is detected at wavelengths outside the visible portion of the electromagnetic spectrum. One of the brightest galaxies in the Virgo cluster is Messier 87 (NGC 4486), but it is also known as Virgo A, 3C 274, 1ES 1228+126, 87GB 122819.0+124029, and IRAS F12282+1240. These are among the 20 names of this one galaxy listed in the NASA Extragalactic Database (NED), a bibliographic computer catalog maintained by the Jet Propulsion Laboratory. "Virgo A" dates from the early days of radio astronomy when the resolution of radio telescopes was so poor that a source's location couldn't be determined much better than to within a whole constellation. The name 3C 274 comes from the Third Cambridge catalog of radio sources. Like Messier's optical list, these early radio surveys corralled most of the "best" objects. So even though vastly better radio catalogs are now available, the brightest quasar in the sky (optically a 13th-magnitude "star" in Virgo) will forever be known as 3C 273.

The last three names for M87 mentioned above are built from celestial coordinates, in this case equinox-1950 right ascension and declination. Look again at 1ES 1228+126. The two blocks of numbers mean right ascension 12h 28m, declination +12.6°. Positional names of this sort are useful and nearly inexhaustible but quite cumbersome. What do the prefixes mean? "1ES" is the name of a 1992 catalog of sources detected at X-ray wavelengths by the Einstein satellite. The "87GB" catalog resulted from a radio-continuum survey of the northern sky done at Green Bank, the U. S. national radio astronomy facility in West Virginia. Finally, IRAS was the Infrared Astronomical Satellite, launched in 1983, which made the first far-infrared survey of the sky. M87 showed up as a "faint" source in the IRAS data catalog published in 1990, earning it one more designation.