M67, or it's neighbour ???

[StevenA (Steven)]

I know that sometimes the obvious is not always obvious, so could someone enlighten me about M67?? On my software (Which I have several) M67 seems to be a very sparse cluster and just a little to the East there happens to be a cluster looking object which is denser than M67. Is their something wrong here? Do all these software maufactuers have it wrong? Were the spacial co-ordinates mis-calculated and never corrected?? It would seem to 'me' the cluster near M67 is of more visual significance than M67 itsellf! Please would someone shed some enlightening information on this personal quandry I have. Please don't just give a flipant response. How am I meant to learn if I get simplist off hand remarks, about something which is obvious to the more learned. Thankyou for your time Steven.

[Robh (Rob)]

Steven,
I can't comment on your software but M67 is a magnitude 6.9 open cluster about the size of the full moon (30 minutes across). It has over 200 stars so you can't miss it. Nearby, less than 2 degrees is alpha cancri, Vmag 4.3. There are some 12 stars Vmag 5.5 to 9.5 within one degree of alpha Cancri.
Regards, Rob.

[Terry B]

I also noticed this using skymap pro. It has the circle a bit offest from where the stars are plotted. Not sure why.

[glenc (Glen)]

M67's coordinates are RA 8h51.5m, dec +11d49m

[cucmbere (Bob)]

Have a look at the attachment. There is no cluster other than M67 nearby.

Bob

[astroron (Ron)]

If you look at the "Deep Sky Survey" images they show the cluster off center.
I think the planetarium programs are guided by the co-ordinates of the DSS.
I use "The Sky" myself and it also shows the denser cluster off center.
Maybe Messiers co-ordinates where slightly off for the main cluster.

[ngcles]

Hi All,

Well strangely enough,Megastar 5 shows it perfectly centred.

These are the coordinates it provides:

M 67
NGC 2682
Cr 204
Open Cluster
RA: 08h 51m 24.0s
Dec: +11° 49' 00"
Mag: 6.9
Size: 29.0'
Class: II 3 r
# of Stars: 200

And the star plots are centred almost perfectly (arc-second) on their DSS counterparts.

Hmmm ...


Best,

Les D

[glenc (Glen)]

Karchenko is probably the most accurate open cluster catalogue.
It gives M 67 at RA 08 51 18 dec +11 48.0
http://simbad.harvard.edu/simbad/sim...6A...438.1163K
http://adsabs.harvard.edu/cgi-bin/bi...6A...438.1163K

The main catalogue of open clusters is Dias.
It has the same coords for NGC 2682.
http://www.astro.iag.usp.br/~wilton/
http://www.astro.iag.usp.br/~wilton/clusters.txt

See also http://www.univie.ac.at/webda/cgi-bi...gi?cluster=m67

The old NGC catalogue had poor coords for open clusters.

[Robh (Rob)]

I'm going to throw my bit in here ...
It is very rare that the exact number of stars are quoted for an open cluster or globular cluster. I've seen numbers quoted for clusters varying in the hundreds or even thousands. Often it is hard to determine whether a star actually belongs to a cluster or the measurements have simply just not been done. You only have to look at the variations given for the size of a cluster.
The centre of a cluster is purely an approximation based on one source's reading of the data and/or visual assessment off an image.
With our current level of data, we cannot accurately give the centre of a cluster to the nearest second of arc when it may be disputed to the nearest minute of arc.
This not only applies to clusters but also to larger galaxies or planetary nebulae, where the outer boundaries are just approximations.
Don't be fooled into thinking these numbers are that accurate.
Most clusters are large enough to be centered visually to the nearest one or two minutes of arc.
Regards, Rob.

[StevenA (Steven)]

Thankyou for all the info. It does makes sense after all. M 67 is M67 and its just as stated, software 'then' is not always that accurate. All mine show a feild enclosed in a reticle which is slightly westward of the actual cluster. My question has been answered to my satisfaction. I will keep looking here at this thread just in case something else rears itself. Thanks. Steven

[ngcles]

Hi Rob & All

Originally Posted by Robh

I'm going to throw my bit in here ...
It is very rare that the exact number of stars are quoted for an open cluster or globular cluster. I've seen numbers quoted for clusters varying in the hundreds or even thousands. Often it is hard to determine whether a star actually belongs to a cluster or the measurements have simply just not been done. You only have to look at the variations given for the size of a cluster.
The centre of a cluster is purely an approximation based on one source's reading of the data and/or visual assessment off an image.
With our current level of data, we cannot accurately give the centre of a cluster to the nearest second of arc when it may be disputed to the nearest minute of arc.
This not only applies to clusters but also to larger galaxies or planetary nebulae, where the outer boundaries are just approximations.
Don't be fooled into thinking these numbers are that accurate.
Most clusters are large enough to be centered visually to the nearest one or two minutes of arc.
Regards, Rob.

Spot on !

Plotting positions has been a problem that has plagued catalogues for years. Most of that sort of work is repetitious "trained monkey" type work and it is easy to make errors that are either related to the pure size of the thing on the plate or as often as not, the motivation level of the person doing the work. As for star-counts, they are at very best a guesstimate -- particularly where the object is within a milky way field.

And these sorts of problems are not new to astronomy by any means. Even Johannes Bayer made some howlers -- well it was either Bayer, his plate engraver or the person who took the position for some of the stars, but one of the three had a shocker now and then.

Take a look at this Bayer plate from Uranometria 1603 of Corvus (attached) and note the bright star just behind the rear foot of the raven. Apart from the stars making up the quadrangle of Corvus, the closest 2nd or 3rd magnitude star to that position is a full 6 degrees away -- 3rd magnitude Gamma Hydrae!

And no, it's no nova or supernova.

Someone here has had a bad night on the turps!


Best,

Les D