| Moondark for August: Standup Cluster NGC 6791 | |
| Draw a line between Vega and Deneb along the northern edge of the Summer Triangle. A third of the way from Vega, star hop 4° to the south to just beyond two 4th magnitude stars. There you have to look closely for a large, faint smudge of an open cluster. Visually, NGC 6791 doesn’t stand a chance. Nearby, along the Milky Way are numerous open clusters, globulars and planetaries, many of them Messier objects. Globular clusters like M56, M13 and M71, assorted open clusters including M39, and the Ring Nebula M57, for example, are the marquee objects for summertime cruising along the Milky Way. While 6791 gets “no respect” from amateurs, this is one cluster that has not been overlooked by professional astronomers. In fact, it may be the best-studied open cluster in the Universe. The stars you glimpse in 6791 are old: the real question is just how old. Because cluster stars form at the same time from the same interstellar gas cloud, they represent the ideal opportunity to study stellar lifetimes. Since all stars are essentially at the same distance from us, their apparent magnitude reflects their true luminosity. Using brightness and spectral color data, there are reliable ways to determine a star’s age. Stars change in color and brightness in predictable ways depending on their age and initial mass of stellar fuel. Using the main sequence “turn-off” to age clusters is a well-tested astrophysical technique. But the stars in 6791 seem to have three different ages. Analysis of main sequence stars yields an age of 8 billion years. This method adjusted specifically for white dwarfs gives mixed results of 6 and 4 billion years. So how old is it, or what makes this cluster so unusual? Add to the puzzle with the observation that these stars appear “dirty” in the sense of having more heavy elements (called metals by astronomers) than expected for open clusters. NGC 6791 made a bit of a news splash recently when these discrepancies were at least partly resolved. The anomalously young white dwarfs appeared about twice as bright as expected. If they are doubles, pairs of white dwarfs whose brightness is combined, the older age seems more likely, though admittedly still 2 billion years younger than the main sequence result. So while it has been a tricky question to answer, results seem to be converging on older rather than younger: 6 or 8 billion years is still very old for an open cluster. There remains the other question: why are NGC 6791 stars relatively rich in metals, elements heavier than helium? Only relatively newer stars, the so-called second generation like our Sun are supposed to be enriched in these elements formed in the first generation of stars. It’s kind of neat to see science in action. Like all scientists, astronomers never have all the answers, sometimes only a few inklings and not infrequently, they miss the mark altogether. Add some new data to freshen the scholarly dialog and maybe an outstanding question will be resolved as in this case. Or maybe not, but everyone agrees the iteration of data and models, open discussion and communication among professional peers and the public are both key to moving forward. Strong science certainly adds to the beautiful images of 6791 from the Hubble Space Telescope. But you don’t need to wait until this fall’s HST servicing mission or the next round of press releases and journal articles. When hopping about the Summer Triangle, ideally placed for viewing on evenings this month, move about 8° degrees due north from the globular M56. NGC 6791, the "Rodney Dangerfield" of open clusters, is close to two stars visible to the unaided eye and is nicely framed in a triangle of 6-8th magnitude field stars. In comparison to M56, NGC 6791 is a decidedly more difficult target and challenging for even large binoculars. Through a 6” reflector, it appeared much larger and far less condensed than M56, with few if any stars resolved. It is listed in Burnham’s Volume 2 and described for larger telescopes in Kepple & Sanner’s Night Sky Observer’s Guide. What’s your take? NGC 6791 lies near Eta and Theta Lyra at RA 19 h 20.7 m and Dec 37° 51’. At visual magnitude 9.5, this cluster isn’t plotted on the Bright Star Atlas, but is shown on the Sky Atlas 2000 Chart 8 and Uranometria 2000 Chart 118. The cluster’s size is listed as 15’ in the Night Sky Observer’s Guide, so dark skies and large aperture are needed to resolve stars against a hazy background glow. Moondark is written by Douglas C. Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 26 July 2008. Text on this web page is free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses. |
 NGC 6791 (circled) within the Summer Triangle in this view from the WorldWideTelescope. Then star-hop using this finder chart from Google Sky.  Credits: WorldWideTelescope.org, Google Earth;and DSS; see image links  |
