The Double Cluster
Open Star Clusters NGC 869 and NGC 884, plus NGC 957

The image is a color photograph taken with Kopernik's 8-inch F/1.5 Schmidt Camera. The field of view is about 5x7 degrees; about what can be seen thru 7x50 binoculars. The smaller version has the two open (or galactic) star clusters marked, along with the position of the more distant star cluster NGC 957.

The “Double Cluster” in the Constellation Perseus is justly famous as one of the most beautiful objects for small amateur sized telescopes. This object can be seen as a small misty patch even with the naked eye, and it was recorded as a “nebula” as far back as 150 BC by Greek astronomers Hipparchus and Ptolemy. It is best viewed in a small telescope at 50 power or less, and in giant 20x80 binoculars it appears as W. Olcott described it: “The field is simply sown with scintillating stars, and the contrasting colors are very beautiful”.

These two are among the youngest star clusters known. The best estimate for the age of NGC 869 is 6.4 million years, while NGC 884 is about 11.5 million years old. NGC 884 is somewhat farther away, at 8,150 light years. NGC 869 is estimated to be 7,000 light years distant. Most of the brightest stars are blue giants, as would be expected of such young stars. However, a few have already evolved into red giants.

This area in Perseus marks our galaxy’s next spiral arm outward from the sun’s position. It is filled with young open star clusters, one of which, NGC 957, also appears in our photo. The region is dominated by bright, hot, and young blue stars, and is laced with dark and bright nebulae. These are clouds of gas, mostly hydrogen, that have formed the new stars in the open clusters.

NGC 869 Magnitude: 4.0 Constellation: Perseus RA: 02h 19m 0.0s Dec: +57d 09' 00" Epoch 2000 Size (mins): 30.0 Description: Open star cluster; Rich in stars; large brightness range; strong central condensation; detached. 200+ stars.

NGC 884 Magnitude: 4.0 Constellation: Perseus RA: 02h 22m 24.0s Dec: +57d 07' 01" Epoch 2000 Size (mins): 30.0 Description: Open star cluster; Rich in stars; large brightness range; strong central condensation; detached. 200+ stars.

NGC 957 Magnitude: 7.6 Constellation: Perseus RA: 02h 28m 11.9s Dec: +19d 34' 23" Epoch 2000 Size (mins): 11.0 Description: Open star cluster; Moderately rich in stars; moderate brightness range; no central condensation; detached. 30 stars.

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George Normandin, KAS

February 15th, 1998