Messier 73

History and Accurate Positions for the NGC/IC Objects (Corwin 2004)

NGC 6994 = M 73 is an asterism of four fairly bright stars. The Hipparcos data suggest that they are at different distances, so this is one of THE prototypical asterisms.

Remarks

Also catalogued as Collinder 426, this "open cluster" consists of only four stars! The brightest star is of 10th magnitude, and the combined magnitude is 9.0. The grouping spans 2.8' across.

Historical observations

Messier, Charles

It was discovered in October 1780 by Messier, who described it as "three or four small stars which look like a nebula at first sight; it contains a little nebulosity"

William Herschel (c.1784)

In the Appendix to the 1912 'Scientific Papers of Sir William Herschel' this object is described as "1783, September 28. Consists of a few stars arranged in a triangular form. No nebulosity among them. 10 ft, 150 power."

NGC/IC Dreyer (1888, 1895, 1908)

The NGC description for this cluster reads "extremely poor, very little compresed, no nebulosity."

Published comments

Burnham's Celestial Handbook

This non-object is merely a knot of four small stars which lies near M72.

Modern photographs show no signs of nebulosity, as Messier reported. However, as Burnham reflects, "it is a common experience that faint double or triple stars often appear fuzzy in small telescopes or with poor seeing" The overall diameter of the asterism is about 1.2 arc seconds, and the individual magnitudes, according to Burnham, are 10.5, 10.5, 11.0 and 12.0. Admiral Smythe only gives a brief mention: "A trio of 10th magnitude stars in a poor field - that is M73. I give it out of respect to Messier's memory."

Sulentic & Tifft (1973)

The RNGC (Sulentic and Tifft 1973) notes that this is a 9.0 mag open cluster.

Frommert, Hartmut (1994)

Subj: M73 - asterism or four component system ?

Date: Mon, 11 Jul 1994 15:48:19 +0100

From: Hartmut Frommert(phfrom@NYX.UNI-KONSTANZ.DE)

M73 (NGC 6994) is a group of 4 10th-mag stars in Aquarius. According to several catalogs, it is classified as asterism and not as physical system (mini cluster or multiple star). This was doubted e.g. by Paul Murdin and David Allen in their "Catalogue of the Universe" of 1979, where they state: Messier discovered this unspectacular little ``cluster'' of four stars in 1780. The cluster is in the constellation Aquarius, close to the globular cluster M 72, which Messier discovered on the same night. Perhaps M 72 is the real reason why Messier recorded M 73. Although M 73 found its way into Dreyer's compendium of star clusters and nebulae, the {New General Catalogue}, as NGC 6994, it has been regarded as an asterism, a chance coincidence of four stars along the line of sight. The authors of this book do not know if this is true; one day they plan to determine the types of stars concerned to see if they are related. They suspect in fact that M 73 might be a real little cluster, for the following reason. On average there are 60 stars per square degree which are brighter than magnitude 12, as are the four stars of M 73. The probability of finding four such stars by chance in a given area of sky one arc minute across (like M 73) is about two chances in a billion. However, there are 150 million such little areas on the sky, so the chances are only one in four that such random asterism exists on the sky. M 73 could be it, but we would gamble that it is a genuine multiple star of some kind.

My question now: Was this proposed investigation ever performed, i.e. did anyone take a closer look on the M73 stars in order to confirm or exclude a physical relationship ? Or are we still in the "Don't Know" situation of 1979 when this piece was written [and thus may gamble ;-) -- I'd bet -- with the authors of that book -- that it is a system ;-) ;-) ;-) ;-) ;-) ;-) ] ? Anyway, data exceeding those in Mallas' and Kreimer's "Messier Album" and Hirshfeld and Sinnott's "Sky Catalog 2000" would be appreciated.

Hartmut Frommert | Russia HAS a space station !

Modern observations

Harrington, Phil

Phil Harrington (1990, Touring the Universe through Binoculars) notes that NGC 6994 "is erroneously included as an open cluster in both the Messier and NGC catalogues. Now known to be merely a small asterism of four faint stars .. it was recorded by Messier as a cluster with nebulosity. This cloudlike effect is only an illusion due to low power. Higher magnified telescopic views and photographs fail to show any nebulous wisps."

Tom Lorenzin

Tom Lorenzin, in the e-version of "1000+ The Amateur Astronomers' Field Guide to Deep Sky Observing", notes: "8.9M; Very economical of stars (4) this "cluster"; only looks nebulous in scopes of <1-in. aperture, or in larger, mis-aligned or mis-figured light buckets; move on!."

Gramer, Lew (IAAC)

Observer: Lew Gramer; Your skills: Intermediate; Date and UT of Observation: 1997-07-25/26, 04:30 UT; Location: Medford, MA, USA (42N); Site classification: urban; Limiting magnitude: 5.8 (zenith), intermittent haze; Seeing: 5 (out of 10 highest) - mediocre; Moon up: yes, 50% (not visible at site); Instrument: 7x50mm Simmons binoculars

Visible to simple averted vision in the same binocular field with M72 (see observation for same night), M73 was about 1o ESE of the globular. Diffuse and without noticeable detail, but nonetheless clearly distinguishable in an otherwise near-empty field of view.

Steve Coe

Steve Coe, using a 17.5" f/4.5, notes: "a small asterism of four stars, looks like a miniature of the Water Jar at 60X."

Brian Skiff

15cm - not partic eye-catching *ism of four *s in Y-shape. SErn * is brtst,

Wmost fntst. two sim-br *s a few arcmin S, the closer/fntr of which is an

un= pair vis @ 80x. BS, 8Sep1988, Anderson Mesa.

25cm - nice *ism of four *s. two fntr *s are on W, leading to m10 * on S; fourth

m11 * on NE. BS, 14Oct1982, Anderson Mesa.

Contemporary observations

Magda Streicher

22 August 1998

22 August 1998, Farm site, Meade 8" with a 18mm wide angle & 26mm Plossl, 36' fov, Sky Conditions: Clear 7 to 8 magnitude

Four bright stars in a Y shape grouping with the faintest member to the end of the letter.