Castle Point Astronomy Club
The Double Cluster in Perseus by Jim Vincent
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The Double Cluster in Perseus by Jim Vincent
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LRGB image total time 25 mins, Orion Ed 80 fl 600mm & SXV H9ccd

You can see more of Jim Vincent's photographs on his website by going to our Links page.

Visually the Double Cluster is undoubtedly one of the most amazing and beautiful sights in astronomy. Probably few would disagree with the notion that they appear at their best when viewed through a telescope and eyepiece combination that allows the central regions of both clusters to be accommodated in a single field of view. Furthermore, the view seems to become ever more amazing and beautiful with increasing aperture.

The two clusters, which are part of the Perseus OB1 Association, are at very similar distances. The more Westerly of the two, designated h Persei or NGC 869, is probably around 7,100 light years away, whilst chi Persei or NGC 884 is reckoned to be a little further away at around 7,400 light years. Their almost intimate apparent proximity to one another is in part a line of sight effect. However, this of course means that they are actually quite close to one another, there being only around 300 light years between them. With regard to their distances from us, they are in fact getting closer all the time, but donít expect them to arrive any time soon because their radial velocities are only a shade over 20 kms per second.

They are both reckoned to be very young clusters. For many years they have been thought to be less than 10 million years old. However, a number of sources nowadays seem to put them at as young as 5.6 and 3.2 million years respectively.

One of the ways by which we can get a handle on the ages of star clusters such as these is by creating a Hertzsprung-Russell diagram for the cluster in question. If the cluster is really young, all the stars will be found to lie on the main sequence. However, in the case of older clusters, the most massive and luminous stars will have begun to evolve off the main sequence and so the star plots will follow the main sequence up to some point and then turn off to the right, where lies the red giant region of the diagram. For clusters such as these, there is a very clear turn-off point above which there are no main sequence stars.

By applying theoretical physics to the stars that are at the main sequence turn off point and beginning to evolve into red giants, an estimate can be made as to how old these stars must be in order to have evolved to this stage. This can then be taken as the age of the cluster as a whole. This is of course dependant upon the supposition that all the stars within the cluster formed at pretty well the same time, but there are good reasons for supposing that that will be the case with most such clusters.

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