M3 (NGC 5272)

All Images Copyright Steve Cannistra

Please click here for a medium sized view (35%)
Please click here for a larger sized view (60%)
Please click here for a partial light curve and distance calculation using an RR Lyrae star
Please click here for a Herztsprung-Russell diagram generated using the data from this image

Distance: 33,900 light years
RA: 13h 42m 09s
Dec: +28 degrees 22' 09"
PA: plus 90 degrees

M3 is one of the most beautiful and extensively studied globulars in the northern hemisphere.  Like the majority of globular clusters, most of the stars in M3 are old and metal poor (representing some of the earliest stars formed in the galaxy, at a time when supernovae explosions were not common enough to give rise to heavier elements like iron, oxygen, silicon etc. that became incorporated into later, metal rich stars).  The Hertzsprung-Russell diagram of M3 (found half way down the page) shows that most of its stars have left the main sequence and are either red giants or bluer horizontal branch stars (the latter mostly comprised of stars that are burning helium to carbon in their cores).  For anyone interested in learning more about HR diagrams, I described this in greater detail 5 years ago when I took a rather modest image of M13

The interesting feature of M3 that was most impressive while processing this image is the abundance of blue stars, due to hot stars on the horizontal branch of the HR diagram, as well as stars called "blue stragglers" (because they are still on the main sequence when they should have long since moved on to become red giants).  The mechanism by which blue stragglers are formed is thought to involve interaction with other stars, especially those in the dense central portion of the cluster, during which hydrogen is replenished and the star can continue to burn hydrogen to helium long after other stars have left the main sequence.

Another source of blue stars are the RR Lyrae variables, which are very abundant in M3.  These stars have shorter periods then Cepheids and have a relatively consistent absolute magnitude in the range of 0.75.  This means that once you identify a star as an RR Lyrae class variable, and once you determine its average relative magnitude from its light curve, you can calculate the distance to the cluster.  I illustrate this principle using the above image here.

A nice description of M3 may be found on Rob Gendler's website.

Photographic Details:
Date:  April 11, 2010
Scope:  Vixen VC200L at f6.4 on the Takahashi NJP Mount
Autoguider:  SBIG ST-402
Camera:  ST8300 -20C

Astronomik Clear (unblocked), R, G, B filters
Exposures:  Clear- 60 x 2' = 120'; R- 10 x 7'; G- 6 x 10'; B- 6 x 15'. Total exposure 5.7 hours
Post-processing:  Calibrated in Maxim, aligned and stacked using DeepSkyStacker, followed by DDP in ImagesPlus (IP).  Further processing in Photoshop CS4. 

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