M 73

Group or Asterism of 4 Stars M73 (NGC 6994)

Right Ascension	20 : 58.9 (h:m)
Declination	-12 : 38 (deg:m)
Distance	0 (kly)
Visual Brightness	9.0 (mag)
Apparent Dimension	2.8 (arc min)

“Cluster of three or four small stars, which resembles a nebula at first

glance, containing very little nebulosity; this cluster is located on the

parallel [of declination] of the preceding [M 72]; its position has been

determined from the same star [Nu Aquarii].”

Apparently, this group found its way into Messier’s catalog because its

position was auxiliary for measuring M72 which is

1.5 degrees to the west. Although it is clear from this description that this

group was what Messier had observed and measured, some versions of Messier’s

catalog omit it as an “obscure” object, however Dreyer has included it in the

NGC catalog.

What remains to clear up to now, at least to the knowledge of the present

author, is the check if the 4 stars in M73, or at least some of them, are

physically related. There was always a great fraction of astronomers who

believed that M73 is an asterism, a chance alignment of 4 stars at different

distances. The present author, however, tends to join the opinion of

P. Murdin, D. Allen, and D. Malin, expressed in their

Catalog of the Universe:

“[The authors] 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.”

As Kenneth Glyn Jones states: This issue is perhaps a minor one, but every

student of the Messier catalog would be much interested in the outcome.

So if anybody knows newer data of the star, or could check a database like

that of the Hipparcos data, please do ! It would even be of advantage to get

some basic data, as e.g. the spectral classes, of these four stars, as they

are not given in the references known to the present author.

And as the information belongs here, please