| NGC 6811 | |
| Epoch: | 2000.0 |
| Constellation: | Cygnus |
| Ra: | [1] |
| Appmag V: | 6.8 |
| Size V: | 13' |
| Dist Ly: | 3600 ly |
| Dist Pc: | 1107 pc |
| Age: | 1.00±0.17 Gyr |
| Names: | Cr 402 |
NGC 6811 is an open cluster in the constellation of Cygnus,[2] near the constellation of Lyra.[3] It has an angular size half that of the full Moon and includes about 1000 stars[4] of roughly similar magnitude.[2] It has also been called "The Hole in the Cluster" or "Hole in a Cluster"[5] because of its dark center.[6] [7]
NGC 6811 lies far away from the galactic plane, a feature it shares with many other old open clusters.[6] It is 1107 ± 90 parsecs (about 3,285 light years) distant[8] [9] and approximately 4-6 parsecs (14–20 light years) in diameter,[6] with a total luminosity of 2100 suns. Approximately 1.00 ± 0.17 billion years old, the cluster probably contained some 6000 stars at birth,[3] but gravitational interactions and stellar evolution have since reduced the number substantially. A recent study reported 377 confirmed member stars, with spectral types ranging from mid-F to early K, and surface temperatures relatively similar to the Sun's.[3] The same study argued that the original cluster population likely included 8 O-type stars and 125 B-type stars, but all have evolved off the main sequence and are undetectable.[3] Sixteen stars have been observed to vary in brightness, twelve of which are Delta Scuti variables.[10] The cluster's Trumpler classification is III 1r—it is "a rich cluster with equally bright stars with no noticeable central concentration".[6] The stars do, however, have an unusual (if not concentrated) distribution, with an apparent stellar corona surrounding the core, leaving the impression of a hole.[6]
NGC 6811 was first observed by John Herschel in 1829 and was added to his General Catalogue of Nebulae and Clusters in 1864.[11] The cluster has been the subject of study by the Kepler mission, with the aim of characterizing its stars' rotation rate, age, and distance to help the hunt for exoplanets.[3]
NGC 6811 is best observed from Earth in the Northern Hemisphere in summer. In these conditions it lies close to the zenith during the night, northeast of Delta Cygni.[2] It is considered an aesthetically pleasant object for amateur astronomers, even if the brightest members are just 10th magnitude objects. It appears as a hazy patch in 10x binoculars, but it is best seen at around 70x with a moderate-aperture telescope.[12] It has been described by amateur astronomers as a "smoke ring of stars" or "a jeweled mask a woman might wear at a masquerade ball".[6]
Two planets (Kepler 66b and Kepler 67b), orbiting Sun-like stars in the NGC 6811 cluster, have been discovered by the Kepler mission using the transit method.[8] Both planets are smaller than Neptune and are both the first sub-Jupiter planets and the first transiting planets discovered orbiting stars within an open cluster.[8] Given that the age and distance of the cluster have been accurately measured, the two planets are among the few of which age and distance are accurately known.[13] This finding suggests that the frequency of planets in clusters is similar to that in stars not belonging to clusters or associations[8] [13] and that planets can form and survive in environments more crowded and violent than the one of our own Sun.[4]