Sorting Out Stephan's Quintet
Jack Kramer
Here's a challenge for those of you who observe from dark sites: Stephan's Quintet. It's a group of five faint galaxies that easily fit in the same field of view. Because they're located near the large, bright galaxy NGC 7331 in Pegasus, it's fairly easy to find your way to where they're located.
Actually seeing the galaxies is another matter because the brightest (NGC 7320) is mag. 12.6. The others are 7317 (13.6 mag.), 7318A (13.4 mag.), 7318B (13.4 mag.), and 7319 (13.1 mag.) There are many more galaxies in this field than are shown in the chart above, but all are very faint. The brightest are NGC 7338 at about Magnitude 14.0 and 7343 at 13.5. The "tick" marks (crosses) on this chart are 30 arcminutes apart.
Assuming you can see more than just one of the galaxies in the Quintet, actually identifying which galaxy is which is another challenge. You're most likely to see NGC 7320, since it's the brightest. Using that as a starting point, plus some field stars, may help. Also, don't rely completely on the stated relative brightness of each galaxy. The figures I gave for each member came from the Night Sky Observer's Guide, which is a visual magnitude. However, other sources give different values. For example, NGC 7318A is shown as 14th magnitude in the Guide computer program and Burnham's Celestial Handbook shows 14.8. The Saguaro database agrees with the N.S.O.G., and my impression is that this is closest to the actual brightness. The fainter numbers cited in some references might be photographic magnitudes. The above diagram adapted from the Guide computer program, shows the galaxies' positions relative to each other and their approximate shapes.
Under very dark skies, a couple of the members should be visible in an 8-inch scope. I first viewed the Quintet with a 10-inch Newtonian from the Groezinger farm in far western Illinois, but found it more easily from Gran Quivira, New Mexico, and from Horsetooth Mountain in Colorado. It's certainly an object that tests the quality of the sky at your observing site. The group shows up best from a dry, high altitude site, with good transparency. If you can see the galaxies, they will appear as mere smudges, and may require the use of averted vision. I did catch NGC 7343 from our site at the Von Bergen farm near Hebron, but have been unable to locate the other field galaxy, NGC 7338.
At least part of the Quintet forms an interacting group. Three of the galaxies show nearly the same redshift, indicating that they reside at the same distance from us. These three are in the midst a collision, each ripping the others apart with gravitational tidal forces.
The NASA Astronomy Picture of the Day archive has a great HST image of Stephan's Quintet at:
http://antwrp.gsfc.nasa.gov/apod/ap001113.html
Of special interest to LCAS folk is that if you read the accompanying text on this NASA page and click on the second imbedded link for "Stephan's Quintet", you'll be taken to our own Michael Purcell's CCD image of this object on his web site. Congratulations, Michael!
