Odprta kopica - Open Cluster M46 (NGC 2437), type 'f',
||07 : 41.8 (u:m)
||-14 : 49 (sto:m)
||5.4 (*1000 sv.l.)
||27.0 (loc min)
M46 was the first object Messier discovered after he had published the first
edition of his list
on February 19, 1771, three days after presenting it to the academy.
The cluster is very rich, with 150 stars of mag 10-13 and probably a total
population of over 500. The brightest of these stars are of spectral type A0,
and each about 100 times more luminous than the Sun (the brightest is of
apparent magnitude 8.7). This indicates an age of about 300 million years.
The members are scattered over an angular diameter
of about 27', corresponding to a linear extension of 30 light years at the
cluster's distance of 5,400 light years, and are receding from us at
41.4 km/sec, according to Baade. M46 is classified as of Trumpler type
As a special and famous feature which is also obvious in our photograph, a
planetary nebula (NGC 2438, also FC 87) appears within the apparent borders
of M46. However, this nebula is most probably not a true member but is
superimposed, or perhaps a passing "guest", because of three reasons:
- the radial velocity of NGC 2438 is about 77 km/sec recession, which is
43 km/sec different from the cluster's value and would not allow the cluster
to hold it, even if it were at the same distance.
Woldemar Götz, however, gives derived distances of 4,600 light years
for the cluster and only about 2,900 light years for the nebula, which would
mean that the planetary is a foreground object.
- planetary nebulae are only visible for short times and fade quickly; most
of them are visible only for a few 10,000s of years before their material
has volatilized into the surrounding interstellar space.
- planetary nebula are late states in stellar evolution, which occur only
for comparatively low mass stars of less then 3 solar masses. These stars,
however, need more than a billion years for their evolution until they eject
their envelope to form the planetary,
which is much longer than the age of M46 (more massive stars "go supernova").
However, this last argument is questionable, because some young clusters as
the Pleiades (M45) contain a significant number
of white dwarfs, which must have evolved from more massive stars; these stars
must have lost most of their mass during their evolution, probably in the
form of strong stellar winds in their Red Giant phase, and must have gone
through a planetary nebula phase.
Vec posnetkov M46