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NGC 7419: a nest of red supergiants

Martin Meredith

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A week or so ago I was out observing open clusters in Cepheus using live RGB combination imaging in StarlightLive v3, and came across NGC 7419 near the border with Cassiopeia. It is a remarkable sight. This is a 30s exposure in each of R, G and B.


It might look as though the colour balance of the whole shot is off, but the presence of white foreground stars (the brightest, the A-type HD 216721 just above the cluster has a B-V colour index of 0.2 i.e. slightly on the yellow side of white) suggests otherwise. In fact, this young cluster, which sits in the outer part of the Perseus Arm of our galaxy, is heavily reddened [1]. But what stands out in this cluster are the five red M-type supergiants [2] which are redder still. This is the largest number of such stars in an open cluster in our galaxy [3]. Their colour indices range from 1.5 to 4. For comparison, Herschel's Garnet Star has an index of 2.2. The cluster has also been well-studied due to its large number of hot Be-type stars, possibly the highest percentage of any cluster [4]. I'd like to find out more about these 'active, fast-rotators' [5]. All in all it seems like an astrophysical special case amongst open clusters.

The supergiants are not the reddest things in the shot. This distinction belongs to MZ Cep, a variable star which I initially took to be part of the cluster. MZ Cep is a carbon star with a B-V colour index of 4.5 and a magnitude range of 14.7 to 15.4.

Also shown in a fairly tight double (nearly 5") HEI 412 whose companion shows up as a slight bulge to the left. Nearby is yet another very red star but as yet I've been unable to find any information on this one... 



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