Two for the price of one! The Double Cluster consists of two open star clusters, known as H and Chi Persei (also called NGC 884 and 869).
The Double Cluster rates among the most magnificent deep-sky objects that was not included in the famous Messier catalog. Of course, Charles Messier (1730-1817) was looking for deep-sky objects that could be mistaken for comets. He must have thought nobody would see this pair of glittery clusters as a comet in the sky.
Each cluster contains 300 to 400 stars. The westernmost cluster, NGC 869, appears slightly brighter, richer and more compact than NGC 884.
The Double Cluster is about 7000 light-years distant, and they are separated from each other by a few hundred light-years. It’s amazing that we can see these stars at all across this great span of space. We know they must be bright stars, intrinsically, or we wouldn’t be able to see them. Each cluster contains a few hundred stars, and, indeed, these stars are young, hot supergiant suns that are many thousands of times more luminous than our sun.
NGC 884 is on the left and NGC 869 is on the right in this image
(The Double Cluster was another of my favorite objects from sixty years ago - bright and easy to find with a small telescope - or even a good pair of binoculars. In general, when viewed through the eyepiece of a small telescope, open clusters look similar to their photographs. This is completely unlike galaxies, nebula, or even globular clusters, all of which tend to just look like smudges of light visually through a small telescope.)