NGC 884 and NGC 869: The Double Cluster...

Objects: NGC 884 and NGC 869 (The "Double Cluster", Caldwell 14)
Type: Open Clusters
Constellation: Perseus
Distance: 7500 light years

Date: February 14th. 2019
Equipment: ATIK 460EX, Vixen 114mm f5.3 ED114 refractor, NEQ6 mount, guiding with Lodestar X2/PHD

Subframes: 20 x 300s luminance, 12 x 200s each for RGB (2x2 binned), no flats, hot pixel removal in Astroart (no main frame darks).

Known since antiquity, the “Double Cluster” was catalogued in 130 BC by Greek astronomer Hipparchus, who referred to it as a “nebula” or “cloudy spot”, one of the half a dozen then recognized.  The starry nature of the clusters remained a mystery until the invention of the telescope in the early 1600’s.

Now known respectively as h and chi Persei, or NGC 884 (to the east, or left, of the image) and NGC 869 (the cluster to the west, or right, of the image), the clusters themselves are separated by only a few hundred light-years and contain stars much younger and hotter than the Sun. 

In addition to being physically close together, the clusters' ages based on their individual stars are similar - evidence that both clusters were likely a product of the same star-forming region.  NGC 869 has a mass of 3700 solar masses and NGC 884 weighs in at 2800 solar masses; however, later research has shown both clusters are surrounded with a very extensive halo of stars, with a total mass for the complex of at least 20,000 solar masses. 

Based on their individual stars, the clusters are relatively young, both 12.8 million years old.  There are more than 300 blue-white super-giant stars in each of the clusters.

Discernible with the naked eye under dark skies, NGC 869 and NGC 884 occupy an area of sky approximately one degree across, each cluster being physically around 35 light years across. Binoculars are required to see the two clusters as separate entities from suburban skies. Telescopically, this is one of the most popular of objects for visual astronomers. It forms the “sword handle” of Perseus, as shown in the Stellarium star chart below:

This particular evening was clear but a bright gibbous moon was parked high in the sky, thus ruling out any attempt at faint nebulous objects. Processing was straightforward, with the RGB stacks combined in Astroart to give a colour image with pleasing and contrasting star colours. Some of the brighter stars have haloes, possibly caused by reflections from the RGB filters: Pixinsight acolytes would no doubt sneer at such things, but I quite like the effect.

Addition of the stacked luminance data as a luminance layer in PaintShop gave the final image above.  I used a star mask to just slightly drop the luminance and sharpen the stars surrounding the two clusters, just to get the clusters to stand out from the starry background a bit more.