Although M73 is apparently consisted of 4 stars, 3 of them being of about
10th to 11th magnitude (Burnham and Kenneth Glyn Jones give A:10.5, B:10.5,
and C:11.0), the fourth (D) being of mag 12.0, it is obviously a true Messier
object, as Charles Messier described it as
"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 ! And as the information belongs here, please email me!
Right ascension | 20 : 56.7 (hours : minutes) |
---|---|
Declination | -12 : 50 (degrees : minutes) |
Distance | ??? (light-years*10^3) |
Visual magnitude | 9.0 |