COTM: Pegasus

Albert Brakel

At this time of year, the Great Square of Pegasus is the most prominent guidepost in the northern sky. Pegasus is also, for a change, one of the few northern constellations that appears "right way up" from our perspective; this is probably because it was directly overhead where it was invented in ancient times, so that any orientation was as good as another. In mythology, Pegasus the Winged Horse featured in several legends, probably the best known being as the horse ridden by Perseus when he rescued Princess Andromeda from the sea monster Cetus.

For small scope owners in suburbia, Pegasus offers only the globular cluster M15 and a couple of double stars. Under dark skies they might be able to glimpse 1-3 of the brightest galaxies as well. Moderate and large apertures are more fortunate, particularly in dark conditions, in having a wide range of galaxies to choose from, all but one of them fainter than magnitude 10.

Globular Cluster

M15 (position RA 21h 30.0m, Dec. +12 deg. 10') is a good object to start with, as it is the best and brightest deep sky object in the constellation. It lies near the head of Pegasus. The 12' diameter disk contains a bright , very condensed core -- in fact, the core is one of the most concentrated of any globular cluster, and is an intense X-ray source, probably because a black hole is located at the centre. The cluster is also the only globular known to contain a planetary nebula (Pease 1), but this cannot be seen with backyard telescopes. I found that with the C8 it was unresolved at 77x, but at 154x it was grainy except for the core. It was still grainy with 235x, but with some small pin-prick stars now visible in the halo.

Galaxies

There was no hope of picking out most of the galaxies from my backyard, so I took to observing them from a dark sky site. The brightest galaxy in Pegasus is NGC 7331 (22h 37.1m, +34 25'), about 4 deg. NNW of Eta Pegasi, in the north of the constellation. It is a 9.5 mag. elliptical patch of light about 5' long, rising broadly in brightness to a brighter centre, and resembling a miniature Andromeda Galaxy (but 20 times further away). There seemed to be the faintest suggestion of a dust band near its eastern side.

Half a degree to the SSW is NGC 7320 (22h 36m, +33 58'), the brightest member of a group called Stephan's Quintet, after an observer who discovered them in 1877. NGC 7320 is only mag. 12.7 (while the other 4 members are no brighter than mag. 13); its distance is 42 million l.y., and so is a foreground object to the rest of the group which lies at 295 million l.y. I couldn't see any of them, but larger scopes should have better luck, seeing a 10' oval with 4 nebulous spots close by. An easier target is NGC 7217 (22h 07.9m, +31 22'), a 10.2 mag. galaxy about 2 deg. south of Pi Pegasi. It stands out as an oval blob about 4' across, with a brighter centre.

NGC 7332 (22h 37.4m, +23 48'), 2 deg W of Lambda Peg, is fainter (mag.11.0) and very elongate and lenticular. Though visible with 77x, a higher power such as 154x shows it up better, especially the brighter centre. Near the top of the Great Square, about 1.5 deg. NW of Alpha Peg, NGC 7448 (23h 00.1m, +15 59’) shines even more dimly at mag. 11.7. It appeared as a very elongate streak, with two stellate points in the haze, one of which is the nucleus.

3 deg. south of Alpha Peg you can find NGC 7479 (23h 04.9m, +12 19'). A diffuse dim spot of mag. 11, I could see no structure in it, but it is supposed to be a good example of a barred spiral for large scopes. NGC 7619 and NGC 7626 (23h 20m, +08 12'), half-way between 59 Pegasi and Theta Piscium, are the two brightest members of the Pegasus 1 galaxy cluster. Both are ellipticals of about mag. 11, and look like two small roundish fuzzies set in a field of brighter stars. Diagonally across the Great Square, 6 deg. SW of Alpha Andromedae, is the 11.4 mag. NGC 7741 (23h 43.9m, +26 05'). It was very small, very faint, and hard to find, but once found, a nucleus could be seen in it with averted vision.

The last galaxy we'll look at in Pegasus is the diffuse edge-on spiral NGC 7814 (0h 03.3m, +16 09'), 3 deg. NW of Gamma Peg. Although a bit brighter at mag. 10.5 and measuring about 6' x 3', a 7th mag. star nearby interferes with observation and it helps to put it outside the field of view. I saw no structure, but some instruments should be able to show an equatorial dust lane that stands out well in photos and splits the galaxy into equal halves.

Double Stars

There are only two doubles in Pegasus that can be considered easy. Struve 2848 (21h 58.0m, +05 56') in the head of Pegasus is one of them. With magnitudes 7.2 and 7.5, and a separation of 10.7", they will appear as a fine pair even with small apertures. The stars are white, and to me the secondary seemed slightly yellowish. Struve 3007 (23h 22.8m, +20 34'), in the Great Square, is also readily separable at 7.0", although the mag. 9.6 seconary looks faint beside the mag. 6.7 yellow primary.

Xi Pegasi (22h 46.7m, +12 11'), in the horse's neck, is more widely separated at 11.5", but the companion is only mag. 12.2 beside the 4.2 magnitude of the yellowish A star, so your observing conditions have to be good enough to see 12th mag. star. It is believed to be an intrinsically dim red dwarf.

Kappa Peg (21h 44.6m, +25 39') lies in the SE of the constellation, and it also has a companion with a very generous separation (14.2") and a great difference in magnitude (4.1 and 10.6). The faint star in this case appears to be physically unconnected to Kappa Peg, which is itself an unresolvable quadruple system.