M67 (NGC 2682)

All Images Copyright Steve Cannistra

Please click here or on image for a larger size.

Magnitude:  6.9
Size: About 29'
Distance: 2700 light years
RA: 08h 50m 50.1s
Dec: 11 degrees 48' 44"

M67 is an open star cluster, also known as a galactic cluster.  Before describing the unique features of M67, it is worth briefly describing the more typical characteristics of open clusters.  Such clusters are loose stellar associations usually located within our galactic plane.  They are relatively young, in the age range of less than 200 million years, and contain stars that are generally metal rich, having formed from the remnants of previous supernovae explosions.  Examples of typical open clusters include the Pleiades, the Hyades, Ursa Major (yes, the entire constellation!), and the Beehive cluster.  The stars comprising open clusters generally disaggregate over time, being pulled apart by the influence of other stellar encounters as they orbit within the galactic plane.  This is why most open clusters are relatively young (they don't exist long enough to become old) and have stars that are still on the main sequence.  In a typical open cluster we would expect to see a few scattered red giants, but most of the stars will be blue to orange main sequence stars still burning hydrogen to helium in their cores.  Once the stars disaggregate and go off on their own, they will eventually grow old and die, just like stars everywhere else in the universe.  Feel free to explore my previous image of M13 for more details about the HR diagram and stellar evolution.

The features of open clusters can be compared to those of globular clusters, which are billions of years old and contain stars that formed during the early epoch of the universe
(i.e., at a time when supernovae explosions were relatively infrequent), prior to the availability of derivative elements.  This is why most stars contained within globular clusters are metal poor.  Given the advanced age of globular clusters, most of its member stars have evolved off the main sequence and are in the red giant or perhaps the horizontal branch phase of the HR diagram (in which case they would appear blue and would be burning helium to carbon in their cores).  Unlike open clusters, globular clusters tend to be located in orbits around the galactic center.

So based upon this information, we would expect the open cluster M67 to be relatively young, located within in the galactic plane, and full of stars located mostly on the main sequence.  And yet, every one of these assumptions turns out to be incorrect!  M67 is one of the oldest known open clusters, with an age of around 4 billion years (compare this to the age of the Pleiades- 65 million years, or the Beehive Cluster- 700 million years).  Instead of being located within the galactic plane, it is located about 1400 light years above the galactic plane (or below it, depending upon your point of view).  This fact alone may be responsible for the advanced age of this open cluster, since its stars have been able to avoid other stellar encounters that would result in disaggregation of the cluster.  Because of the advanced age of this open cluster, it follows that most of its stars have had a good life and have already evolved off of the main sequence (which is very different compared to most open clusters).  Please click here for an HR diagram showing the turn off point of stars contained within M67, adapted from the following source available in pdf format.

There are a few other interesting things to note about M67.  It has lots of red giants as shown in my image above.  This is not surprising given the advanced age of this cluster.  However, it has an unmistakable component of blue stars, but these do not represent the young, hot main sequence stars that would be observed in a typical open cluster.  Instead, many of these stars are Blue Stragglers, which are old stars that have curiously not yet left the main sequence (even though they should have, since the HR diagram shows a prominent turn off point).  It is possible that Blue Stragglers exist because they have received an extra "dose" of hydrogen fuel from collision with another member of the cluster, or from a binary partner, which then stimulates another round of hydrogen to helium fusion (thereby keeping it on the main sequence for a while longer).  In fact, there are several "contact binaries" present in M67 (also known as W Ursae Majoris eclipsing binaries), which are binary systems rotating so closely that their surfaces actually touch.  Whether Blue Stragglers are a consequence of interaction between W Uma binaries within M67 is unknown.

The image above represents a wide field view of M67, showing a relatively sparse surrounding star field which is due to its position well above (or below) the galactic plane.  More information about M67 may be found here.

Photographic Details:

Date:  April 2, 2006
Scope:  Takahashi FS102 at f6, on the G11 Losmandy Mount.
Autoguider:  SBIG STV with e-finder.
Camera:  Maxcam CM10.
Astronomik RGB type II filter set; IDAS LPS filter.
Exposures:  LRGB composite.  8 x 4' for luminance (unbinned); 4 x 3' each for R, G, and B (unbinned).  Two thin pieces of kite string attached to the dew shield with tape were used to create the diffraction spikes.  However, the use of thicker material (such as an elastic band) produces more pleasing diffraction spikes in my experience.
Conditions:  Temperature 50 degrees F; poor transparency; average seeing; calm.  Conditions deteriorated quickly as clouds moved in.
Subs were debloomed with Ron Wodaski's Debloomer software, calibrated and aligned in Maxim.  Sigma combined using RC Sigma Reject MaximDL, followed by DDP in ImagesPlus (IP).  Final processing in Photoshop CS (16 bit format).

Please note:  Graphics on this website may not be reproduced without author permission.

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