NGC 7331 - The Deer Lick Group

Designations:	NGC 7331, Caldwell C30, Deer Lick Group
Object Type:	Spiral Galaxy
Constellation:	Velpecula
RA:	22 hrs 37.1 min	Dec:	+34° 25 min
Magnitude:	9.5	Size:	11.0 X 4 arcminutes
Distance:	50 million light years
Discoverer:	William Herschel in 1784

When the Lavender curtain of dusk falls toward the western horizon in mid-December, the mythical winged horse Pegasus vaults high across the meridian. Many observers find this an opportune time to hunt down the great globular cluster M15 within sniffing distance of Enif, the star marking the Horse's nose, or the Andromeda galaxy, M31, which immediately follows the Horse across the sky. But about 4½° north-northwest of Eta (η) Pegasi, "beneath" the flying Horse's front legs, lies another magnificent, though often overlooked, spiral galaxy: NGC 7331. From dark skies this galaxy can be glimpsed as a dim slash of light in 7X35 binoculars. NGC 7331 belongs to the Pegasus Spur of galaxies, a small gathering of 35 systems that includes one member of the famous Stephan's Quintet (described below). NGC 7331's spiral disk is tilted 22° from edge on. It whisks away from the solar system at a generous 820 km per second. In detailed photographs the galaxy looks like an oblique aerial view of a hurricane at night; the glowing eye and spiral arms of the "storm" seem poised to snuff out the lesser stellar metropolises surrounding it. The galaxy's modest apparent size and brightness belie its true enormity. In fact, NGC 7331 is among the largest galaxies known. If we accept a distance of 47 million light-years (a value that has been verified by the Hubble Space Telescope), NGC 7331's linear diameter becomes 130,000 light-years. NGC 7331 appears to have a total mass of 300 billion Suns. Thus it equals in size and mass the great Andromeda Galaxy and outrank's our Milky Way.As early as 1961, astronomers recognized that NGC 7331's structure is less like that of M31 and more like that of M63 in Canes Venatici or NGC 2841 in Ursa Major (though NGC 7331's spiral arms appear thicker and are more easily seen than those of the latter two galaxies). NGC 7331 is now classified as an Sbc galaxy, meaning it is a spiral with somewhat tightly coiled arms, a relatively small nucleus, and no bar. Spectroscopic data indicate that there has been large-scale star formation in NGC 7331's nucleus. spectroscopy and photography also have helped astronomers deter­mine which side of the giant spiral is closer to us. The data suggest that the northern end of the spindle is approaching us and that its western side - the one marked by the especially prominent dust lane - is closer; the spiral arms are trailing. The stars in the central 3" (650 light-years) of the galaxy's disk, while relatively young (about 2 billion years old), are very rich in " metals " (the chemical elements, heavier than helium that stars produce via nuclear fusion). This region might represent a circumnuclear ring formed during a recent starburst episode. NGC 7331's starlike nucleus also probably experienced a secondary star-formation burst because, at an age of about 5 billion years, it is two to three times younger than the surrounding bulge. Rosat made the first deep X-ray observations of NGC 7331, and in the process discovered a nuclear X-ray source - a finding that bolsters the notion of a massive black hole at the galaxy's heart. In turn, that black hole may somehow be related to NGC 7331's modest low- ionization nuclear emission-line region, or LINER. It is curious that William Herschel believed that he had resolved NGC 7331. But as Larry Mitchell points out in Appendix C, Herschel suspected that all nebulae would be resolved into clusters of stars, if only they could be viewed with a sufficiently large telescope. Mitchell says this belief came as a result of Herschel's examination of the Messier objects, which he found either to be made of nothing but stars or at least to contain stars and to give "every other indication of consisting of stars entirely." With their modest instruments, today's amateurs can understand why Herschel made the assumptions he did. For example, when viewing NGC 7331 through a 6-inch telescope, Christian Luginbuhl and Brian Skiff detected a stellar nucleus and grainy texture within the disk. Increasing the aperture to 10 inches, they report, brought out increased mottling, especially in the inner halo, and two 13.5- magnitude stars west of the northern tip of the disk. Herschel, with his much larger instruments and very high magnifications, must have detected these features, among others, and he may well have envisioned something akin to a distant cluster, perhaps one like M4 in Scorpius, whose center is a needle of starlight. NGC 7331 is haunted by several faint companions, four of which appear close to its western edge. I say haunted because, at 72x in the 4-inch, these companions seem to pop dimly into view with averted vision, only to vanish under a direct gaze. The brightest companion, NGC 7335, is listed in The Deep Sky Field Guide at magnitude 13.3, though I wonder if it is not brighter; I blindly (and correctly) positioned the tiny fuzz ball in my original sketch, which I made at the eyepiece, and wondered in the margin if it was really a galaxy. Luginbuhl and Skiff detected two more companions, NGC 7337 ( magnitude 14.4) and NGC 7340 ( magnitude 13.7), with a 10-inch telescope. The fourth, NGC 7336, apparently was too difficult for them. The Deep Sky Field Guide lists it at magnitude 15.8, but Sky & Telescope's Roger W. Sinnott notes that this galaxy may not be so faint; the Lyon-Meudon Extragalactic Database, he says, attributes a blue magnitude of 15.4 to NGC 7336, and it probably is a little brighter visually.

Telescope:	TMB 152 F8 APO refractor
Focal Length:	1200 mm
Mount:	Takahashi NJP 160
Camera	SBIG ST10XME
Guider:	SBIG ST402ME
Exposures:	13 7-minute exposures (Luminance) and 5 7-minute exposures for each color (red, green, blue)
Location:	Illinois Dark Sky Star Party
Software:	CCDSoft for image acquisition, processed with CCDStack and Photoshop CS2