Messier 39

Right Ascension	21 : 32.2 (h:m)
Declination	+48 : 26 (deg:m)
Distance	0.825 (kly)
Visual Brightness	4.6 (mag)
Apparent Dimension	32.0 (arc min)

Discovered by Charles Messier in 1764.

Messier 39 (M39, NGC 7092) is a very large but very loose open cluster, situated some 9 degrees east and a bit north of Deneb (Alpha Cygni).

Its distance is only about 800 light years, and it is of intermediate age (estimates between 230 and 300 million years). 30 stars are proven members and contained in a volume of about 7 light years diameter. Its apparent visual brightness of 4.6 magnitudes (e.g., Sky Catalogue 2000.0, Uranometria 2000) corresponds to an absolute magnitude of -2.5, or an intrinsic luminosity of 830 suns. Kenneth Glyn Jones gives its apparent visual brightness as 5.2 mag only, while Don Machholz has estimated it at mag 5.4, in agreement with estimates quoted by Mallas/Kreimer, who also mention D.F. Gray's estimate of a total visual brightness of 6.0 magnitudes.

M39's brightest star is of magnitude 6.83 visually, and of spectral type A0. All stars were found to be main sequence stars in the Color-Magnitude Diagram (CMD), or Hertzsprung-Russell Diagram (HRD), with the brightest stars apparently just before the point of evolution toward the red giant phase. The Sky Catalogue 2000.0 gives an estimated age of 270 million years for this cluster - this is between the two determinations cited by Kenneth Glyn Jones of 300 million years by Lohmann and 230 million years by Van Hoerner. M39 is approaching us at 28 km/s; its proper motion was given as 0.024" per year in the direction of position angle 222 deg (by E.G. Ebbighausen 1940, according to Burnham).

Woldemar Götz gives the Trumpler type of this cluster as III,2,m: Detached from the surrounding field but not concentrated toward its center, the stars are in a moderate range in brightness, and it is moderately rich (50--100 members). The Sky Catalogue 2000.0 has III,2,p (i.e., "poor", less than 50 member stars).

While Kenneth Glyn Jones counts M39 as one of Charles Messier's original discoveries, cataloged by him on October 24, 1764, Burnham claims that its discovery "is often credited to Le Gentil in 1750," but Kenneth Glyn Jones rates the identification of Le Gentil's observation (done by Bigourdan) as "extremely doubtful;" the present author also could not find any hint for this. Burnham also claims that P. Doig (1925) quoted a statement made by J.E. Gore that this cluster was noted by Aristotle as "a cometary appearing object about 325 BC," but the present author did not find any evidence for Doig's quote or Gore's statement; it seems that Burnham had confused it with Aristotle's probable sighting of M41.

This cluster is best observed with lowest powers because of its considerable angular size of 32 arc minutes, more than the Moon. Under good conditions, it can just be glimpsed with the unaided eye. It is well seen in opera glasses and smaller binoculars as a nebulous object, resolved in a 7x50, great at small powers, where its shape stands out: An equilateral triangle with a bright star at each corner, the southerly side aligned about East-West: a 9th mag star at its Northern corner, and one of 7th mag each at the SE and SW corners. About 25 fainter stars within. Many of its stars are grouped in pairs. The cluster is impressive and despites its looseness, well defined in and detached from a rich Milky Way star field. At higher magnifications it fills many fields and gets less impressive.

M39 is not difficult to find: From Deneb (Alpha Cygni) first locate Rho Cygni, a star of 4th magnitude which is about 9 deg East, possibly via intermediate Zeta Cygni. M39 is 3 deg N and 1/4 deg W. It is also 2 1/2 deg W and 1 deg S of 4.5-mag Pi2 Cygni.

NGC Online data for M39

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Hartmut Frommert
Christine Kronberg
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Last Modification: August 25, 2007