NGC 7789, Carolines Rose, Herschels Spiral Cluster

NGC 7789
NGC 7789: Carolines Rose; FFC 4.0/760; © 1992 Beat Kohler

History

NGC 7789, «Caroline's Rose» or «Herschel's Spiral Cluster», is an unusually rich open star cluster. It was discovered on November 1, 1783 by William Herschel's sister Caroline. Wilhelm Herschel entered the star cluster in his catalog on October 18, 1786 as VI 30 (VI = very crowded and rich star clusters). He noted: «A beautiful cluster of very compressed stars. Very rich. C. H. discovered it 1783.» [464]

Physical Properties

The entire group of stars covers an area of about half a degree and stands out sharply from the surrounding area. The approximately 1000 stars that may belong to the star cluster vary in brightness from 11 to 18 magnitudes. The distance is about 6000 to 6200 light-years, giving this cluster a true diameter of 50 light-years. All the stars together are about 3000 times brighter than the Sun.

NGC 7789 has been classified by some astronomers as a star cluster that is an intermediate between an open galactic star cluster and a less concentrated globular star cluster. A study by A. Sandage and EM Burbidge (1958) showed that it is indeed a Galactic-type cluster - but a rather unusual one: the stars are well advanced in their evolution, suggesting that the cluster is very old, older than any other well-known open star cluster. The brightest stars are orange giants of the type K4 III with an absolute magnitude of -2.3 mag. The majority of the other bright stars are giants and subgiants. All stars brighter than +2 mag appear to have evolved out of the main sequence of the H-R diagram. The calculated age of the cluster is about 1.5 billion years - older than most other galactic star clusters but not as old as the globular star clusters. [4]

«Revised New General Catalogue and Index Catalogue», «Historically Corrected New General Catalogue», Dr. Wolfgang Steinicke, 2021 [277]
NameRADecTypeBmagVmagDimDreyer DescriptionIdentification
NGC 7788 23 56 46.0+61 24 00OCL (I2p)9.44.00Cl, S, pRi, vC, st 10, 13…OCL 275
NGC 7790 23 58 24.2+61 12 30OCL (III2p)8.55.00Cl, pRi, pCOCL 276

How do you find Herschel's Spiral Cluster?

The open star cluster is easy to spot; it lies to the right of the «W» in the constellation Cassiopeia, exactly between the two stars σ and ρ Cassiopeiae, which are clearly visible to the naked eye. The best observation time is July to January.

Chart NGC 7789
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

References

4«Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System» by Robert Burnham; Dover Publications, Inc.; Voume I: ISBN 0-486-23567-X; Volume II: ISBN 0-486-23568-8; Volume III: ISBN 0-486-23673-0
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)
464«Catalogue of a second thousand of new nebulae and clusters of stars; with a few introductory remarks on the construction of the heavens» William Herschel, Philosophical Transactions of the Royal Society of London, 1 January 1789; DOI:10.1098/rstl.1789.0021