IC 4756

IC 4756 is a large bright open cluster in the constellation Serpens. Known as Graff's Cluster, it is bright enough to be seen with the naked eye and considered a fine cluster for binoculars or small telescopes.^[2]

IC 4756
Observation data (J2000 epoch)
Right ascension	18h 38m 31.2s[1]
Declination	+05° 29′ 24″[1]
Distance	1.3 kly (400 pc)
Apparent magnitude (V)	4.6[1]
Physical characteristics
Other designations	Cr 386, Mel 210
Associations
Constellation	Serpens
See also: Open cluster, List of open clusters

IC 4756 is also known as the Tweedledee Cluster (paired with NGC 6633 as Tweedledum), also as the Secret Garden Cluster.^[3]

Metallicity of IC 4756 is similar to the Sun, at -0.02±0.01 dex.^[4]

Open star clusters IC 4756 (left) and NGC 6633. Original astrophoto.

Stars

There are some noteworthy stars in the cluster. HD 172365^[5] is a likely post-blue straggler in the IC 4756 that contains a large excess of lithium.^[6] HD 172189, also in IC 4756, is an Algol variable eclipsing binary^[7] with a 5.70 day period. The primary star in the system is also a Delta Scuti variable, undergoing multiple pulsation frequencies, which, combined with the eclipses, causes the system to vary by around a tenth of a magnitude.^[8]