3 Attachment(s)
Object of the Week, May 17th Palomar 5
Palomar 5
Serpens Dwarf
R.A.: 15h16m05.0s
Dec.: -00°06'42" (2000)
Magnitude: 11.80
Size: 3.2'
Concentration class XII
By mid-May astronomical twilight no longer occurs here in northern latitudes and won’t now until mid-August. However,
for those that do still get some astronomical darkness this week’s object should be an interesting one. As is well known
to this group researchers studying the newly completed POSS survey in the1950’s as well as finding lots of new
planetary nebulae and galaxy clusters there were also some 15 faint globular clusters found. Lack of checking meant
that some of these, such as Pal 7 and Pal 9, had already been found by the NGC/IC observers but the majority were
new. Pal 5 in Serpens Cauda, sometimes also known as the Serpens Dwarf as it was earlier thought it might be a
galaxy and given a UGC number, was first discovered by Walter Baade in 1950 and independently rediscovered by
Wilson in 1955. It has proved to be of considerable interest because it is a typical metal poor halo cluster that appears
to be in the process of tidal disintegration. Deep images show trails of stars both leading and trailing the cluster and
which form a stream perhaps 30000 light years long containing perhaps 5000 solar masses worth of stars.
Attachment 3881
Pal 5 is perhaps 21 kpc from us and maybe 17kpc from the galactic centre and is currently high above the galactic plane.
It may well be currently at its further point in its orbit from the Milky Way (apocenter?) and has obviously undergone
many disk passages in its life time, and its next one may be its last before it is totally disrupted.
Attachment 3882
It has been suggested that Pal 5 may have lost up to 90% of its mass during these crossings and now the tidal tails
stretch 10 degrees across the sky. Unlike most globular clusters Pal 5 does show a considerable amount of flattening.
Palomar 5 is not going to be an easy observational target as, with a concentration class of XII, it will appear like a very
faint open cluster.
Attachment 3883
However as always with these objects observing conditions and object altitude may make what is hard from one
location easy from another. This is also a plea that when reporting observations please give some account of the
observing location and sky conditions as what may be easy from 2000m in mountain skies is probably not going
to be visible from sea level. This can be seen in the fact that that the NSOG Vol 2 suggests it is a target for 20/22 inch
telescopes whereas the Interstellarum Field Guide suggests it may be visible in a 12” and there are reports of seeing
it with a 16”.
