NGC 6791, Berkeley (Be) 46
Platais’ Oddball
Constellation: Lyra
Type: Open Cluster
Class: Trumpler Type I 2 r
RA: 19h 20m 53.2s
Dec: +37° 46m 19s
Mag: 9.5v
Diameter: 8.2’ x 8.2’
Nothing like a good mystery to keep one occupied on a cold winter day; sitting in front of the fireplace with a stack of astronomy books and my laptop. It began quite innocently as I was researching several candidates for Object of the Week. What caught my eye was the name of this object - Platais’ Oddball. My initial reaction was “Wow, I have never heard of this one before, what is it, who is Platais, and what is so odd about it.”
So I present to you this week’s Object of the Week, Platais’ Oddball, better known as NGC 6791, an open cluster in the constellation Lyra. It was discovered by German astronomer (actually an astronomy student at the time) Friedrich August Theodor Winnecke in December 1853 using a 3-inch refractor.
Who is Platais? He is Imants Platais, a contemporary Latvian astrophysicist and an internationally recognized star cluster researcher. Platais did his postgraduate studies at the Pulkovo Observatory (1978–1983), and in 1984 his dissertation on open star clusters. Until 1999, he worked in the Department of Astronomy at Yale University, participated in the compilation of a catalog of special movements of stars in the Southern Hemisphere, and later created an astronomical catalog of open star clusters. Now he is a researcher in the Department of Physics and Astronomy at Johns Hopkins University.
NGC 6791 is one of the oldest and largest open clusters known, about 10 times larger than most open clusters and containing roughly 10,000 stars. At roughly 8 billion years old, and with an iron to hydrogen abundance ratio that is more than twice that of the Sun, and it is one of the most metal-rich clusters in the Milky Way. This is contrary to the typical rule-of-thumb where older means more metal-poor. Compounded with the fact that it has an unusually high population of stars, NGC 6791 is among the most studied clusters in the sky.
ngc6791 by Dean Glace.jpeg
Image courtesy of Dean Glace
Using NASA's Hubble Space Telescope to study the dimmest stars in the cluster, astronomers uncovered three different age groups. Two of the populations are burned-out stars called white dwarfs. One group of these low-wattage stellar remnants appears to be 6 billion years old, another appears to be 4 billion years old. The ages are out of sync with those of the cluster's normal stars, which are 8 billion years old.
The age discrepancy is a problem because stars in an open cluster should be the same age. They form at the same time within a large cloud of interstellar dust and gas. After extensive analysis, members of the research team realized how the two groups of white dwarfs can look different and yet have the same age. It is possible that the younger-looking group consists of the same type of stars, but the stars are paired off in binary-star systems. Because of the cluster's great distance, astronomers see the paired stars as a brighter single star. It is their brightness that makes them look younger. Binary systems are also a significant fraction of the normal stellar population in NGC 6791, and are also observed in many other clusters. This would be the first time they have been found in a white-dwarf population. The demonstration that binaries are the cause of the anomaly is an elegant resolution of a seemingly inexplicable enigma.
I was unable to find any specific reference that explained the alias tag of Platais’ Oddball. Platais led extensive studies of this open cluster and authored at least 15 papers specifically on NGC 6791. Given the discrepancies from “standard” open clusters noted in the previous 3 paragraphs, my hypothesis is Platais’ Oddball refers to the uniqueness of this open cluster.
I have never observed NGC 6791 and perhaps there is good reason. It is considered by many observers to be rather nondescript, a challenging object usually requiring larger aperture scopes and excellent sky conditions. Walter Scott Houston described it as "Sparse cluster, quarter degree in diameter, that is very difficult to distinguish from its rich milky way background. When I first hunted for it a few months ago, I decided it didn't exist. A later night and lower power finally produced this frail grouping of about 2 dozen members." While Archinal and Hynes’ in their seminal work, Star Clusters, don't even mention NGC 6791.
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Sketch by Ferenc Lovró
Ferenc Lovró of Zselic Hungary using a 4.5" f/8 Newtonian at 60x to sketch NGC 6791 (above). His description of the open cluster follows. "With my small telescope this is a pretty hard object despite the great dark sky we have in our second home in Zselic. With direct vision it's almost invisible, however by moving the scope all around you can notice a diffuse, silver glow. With averted vision I'm able to see even some details. Higher magnifications vanish the object."
Luginbuhl and Skiff in their Observing Handbook and Catalog of Deep-Sky Objects describe their observations as follows. “6 cm will show this cluster as a moderately faint and slightly concentrated glow with an overall brightness similar to that of oc 188 (cf. in Cepheus), though it is a little smaller. Three mag. 11 stars stand out in a line on the S side. In 25 cm the cluster stars just begin to be resolved. The hazy, partially resolved area is 10' diameter, overlain with about two dozen mag. 11-13 field stars.”
Kepple and Sanner in The Night Sky Observer’s Guide Volume 2 share this observation of NGC 6791. “16/18" Scopes-150x: NGC 6791 is a very faint granular concentration of 11th to 15th magnitude stars covering a 16' area. At 175x, in moments of good seeing, hundreds of cluster members may be partially resolved. At least three dozen 11th to 13th magnitude stars can be counted.”
Now it is your turn. Give it a go and let us know!
If you know or uncover anything further on the context of Platais’ Oddball be sure you share that with us also.
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