IC 166: a billion year old open cluster

[Martin Meredith]

IC 166 is a relatively faint open cluster set amongst a rich star field in Cassiopeia, a couple of degrees almost due south of Segin. At times the dense foreground can make such clusters hard to spot, but on this occasion the profusion of fainter stars makes the cluster stand out even in mono. But what makes this cluster interesting for me is the uniform reddening of its members in contrast to the range of stellar colours on offer in the foreground. This is reminiscent of the best of the Berkeley clusters which often have a similar configuration -- lots of faint red stars tightly clustered, giving it a kind of 'gritty' texture. With a diameter of just over 7 arcmin it fits nicely in a small FOV.

IC 166 is about 1 billion years old (give or take 200 million years) and while this makes it quite a lot older than most, it is classified as an 'intermediate' age cluster. Due to disruption to members as the cluster orbits the galaxy, not many clusters reach the grand old age of a billion. However,  IC 166 sits beyond the Perseus arm of the galaxy at a distance of 4.3 kpc from us, or 12.7 kpc from the centre of the galaxy, and it is hypothesised that disruptive effects are lower in the outer disc so chances of survival are higher. The dense 'nearby' star field set against the more distant cluster I think adds some depth to the shot.

Ref [1] gives some optically-derived information while [2] contains a spectrographic study, complete with the slightly deflating conclusion that IC 166 is an unremarkable cluster. For me it was probably the highlight of the night...

 

This is a live LRGB combination with manipulations in LAB colour space. The camera is not mentioned on the image -- it was my usual Lodestar X2 mono.

cheers

Martin

 

[1] https://arxiv.org/pdf/astro-ph/0703075.pdf

[2] https://arxiv.org/pdf/1806.09575.pdf

Edited October 24, 2021 by Martin Meredith
typo

[Mike JW]

Hi Martin,

Always enjoy your colour views of clusters. Surprised me just how many yellow (orange) stars there are in this one - function of the clusters age?  I shall note this gem for a visual look at the "diamonds sparkling away against the black background" with the 15 (wish I still had the 20!).

Below is the Aladin/SDSS9 view.

Mike

[Martin Meredith]

Hi Mike

I think it is more a function of distance (reddening with all the intermediate dust).

As a bit of fun I turned saturation up to try to match the SDSS view. Its always interesting to match the star colours to an external reference (though I do find the SDSS a bit over-the-top).

Martin

 

[ONIKKINEN]

All the blue stars have died long ago, leading to a redder than expected overall colour.

If a star is massive enough to shine blue or white it has a short lifespan, blowing up early and leaving the smaller stars behind. Open clusters tend to have very similar stars in them because they are all born from the same molecular clouds and in this case it just so happens that a lot of them have reached their red giant phases now, leading to the red colour.

Looks like there are blue stars too, but they are overpowered by the red giants.

Edited October 25, 2021 by ONIKKINEN

[Sunshine]

A wonderful cluster image, thanks for the info!