Autumn and NGC 7789
Open clusters like NGC 7789 often contain hundreds of stars that formed in the same cloud of gas and dust at about the same time.
As the summer Milky Way swings into the west, the autumn portion of the Milky Way rises high overhead. Scattered along this section of our galaxy's spiral arms are dozens of open star clusters. These are less populous than globulars, containing a few dozen to several hundred stars. Open clusters have a different history of formation. Globulars are ancient clusters left over from the formation of our galaxy. Open clusters are left when a nebula such as Orion collapses, forms stars, then dissipates into space.
Most open clusters are made of young stars that formed in the active star-making regions of our galaxy's spiral arms. That's why we find most of the sky's open clusters confined to the band of the Milky Way. The autumn Milky Way through Cassiopeia is rich in clusters. To find NGC 7789, one of the best of these groups, first look high overhead and locate the distinctive W-shaped grouping of five stars that marks
Cassiopeia, the Queen.
The distinctive W-shape of Cassiopeia is easily seen in the north.
Courtesy Akira Fujii.
Using binoculars, locate the two stars, Alpha and Beta, on the right side of the W
. (See the image opposite Beta is the uppermost star and Alpha is just below it.) They should both fit in the field of view of most binoculars. Center the top star, Beta. Now come off Beta, moving away from the W
to the right at a right angle to a line between Alpha and Beta. About half a binocular field away from Beta you should see the cluster a nebulous, hazy area.
Binoculars may not resolve NGC 7789 well. It takes the added power of a telescope to make it a rich eyepiece full of stars. Just remember to keep the magnification low for the best view. With 300 faint stars packed into an area smaller than the apparent diameter of the Moon, this is one of the richest open clusters in the sky.