Star clusters are collections of stars in our galaxy and they are either open clusters or globular clusters. The open clusters consist normally of young stars that have been created recently in the same molecular cloud. These clusters are dispersed in the arms of the galaxy. The number of stars in each open cluster range from a few dozens to thousands of stars. Some clusters can be seen with the naked eye (like the Pleiades) and some are faint and need a telescope to be seen from Earth.
The globular clusters, on the other hands consist of hundreds of thousands old stars and they are scattered around the core of our galaxy. An absolute majority of these clusters need a pair of binoculars or a telescope to be seen at all. At low magnification, the stars are not discernable individually but as a dim cloud.
The Open cluster NGC 7789 in Cassiopeia consists of several hundred stars and is visible in a pair of binoculars. The above photo is taken with an amateur telescope and resembles what you would see in a telescope at medium magnification. Photo: Hunter Wilson
It was with the invention of the telescope that the open as well as the globular clusters were started to be detected. And as the telescopes become better, astronomers still find new star clusters in our galaxy.
Caroline Herschel was the first person to find today’s topic for Astro Info – NGC 7789. She discovered this cluster in fall 1783 and included it in her small private list as entry No. 14. One of the more popular names of the cluster is therefore Caroline’s cluster or Caroline’s rose.
Caroline’s cluster is located in the Cassiopeia constellation and can be seen in northwest, high above the horizon in the March evening. At around 9 pm in March, the characteristic “W” of the main stars of Cassiopeia, is “standing” on its side, as if it was rotated 90 degrees clockwise. If you hop 3 degrees left downwards from the lowest star in the “W” – there you got it! See the map.
NGC 7789 is in the constellation Cassiopeia. The map shows the orientation at 9 pm in March in Northern Europe. Map from Stellarium.
The apparent magnitude of the star cluster is 6,7, which means that it is so faint you cannot see it with your bare eyes. But with a pair of binoculars, you will see dozens of stars.
As you collect more light and higher magnification, with a spotting scope or a telescope, more and more stars become visible. In total, there are more than 1000 stars in NGC 7789 and many of them are actually red giants. This shows that the cluster is older than most other open clusters. It has been appreciated that the oldest stars are around 1,7 billion years.
We would recommend trying observing Caroline’s cluster with all instruments you got. Compare what you can see with a pair of binoculars with a telescope with different magnification levels.
Text: Claes Tunälv