Messier 26

Messier 26, also known as NGC 6694, is an open cluster of stars in the southern constellation of Scutum. It was discovered by Charles Messier in 1764.^[a] This 8th magnitude cluster is a challenge to find in ideal skies with typical binoculars, where it can be, with any modern minimum 3-inch (76 mm) aperture device. It is south-southwest of the open cluster Messier 11 and is 14′ across.^[3] About 25 stars are visible in a telescope with a 150–200 mm (6–8 in) aperture.^[6]

Messier 26
Open cluster Messier 26 Credit: Hillary Mathis, Vanessa Harvey, REU program/NOIRLab/NSF/AURA
Observation data (J2000.0 epoch)
Right ascension	18h 45m 18.0s[1]
Declination	−09° 23′ 00″[1]
Distance	5,160 ly (1,582 pc)[2]
Apparent magnitude (V)	8.0[3]
Apparent dimensions (V)	14′[3]
Physical characteristics
Radius	11 ly[4]
Estimated age	85.3[1] million years
Other designations	Messier 26, NGC 6694,[5] Cr 389, C 1842-094
Associations
Constellation	Scutum
See also: Open cluster, List of open clusters

M26 spans a linear size of 22^[4] light years across with a tidal radius of 25 light-years,^[7] and is at a distance of 5,160^[2] light years from the Earth. The brightest star is of magnitude 11^[6] and the age of this cluster has been calculated to be 85.3^[1] million years. It includes one known spectroscopic binary system.^[8]

An interesting feature of M26 is a region of low star density near the nucleus. A hypothesis was that it was caused by an obscuring cloud of interstellar matter between us and the cluster, but a paper by James Cuffey suggested that this is not possible and that it really is a "shell of low stellar space density".^[9] In 2015, Michael Merrifield of the University of Nottingham said that there is, as yet, no clear explanation for the phenomenon.^[10]