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Object of the Week November 21, 2021 - NGC 7789 Caroline’s Rose
NGC 7789, Melotte 245
Caroline’s Rose, Caroline’s Haystack, Herschel’s Spiral Cluster, Crab Cluster, Screaming Skull Cluster
Constellation: Cassiopeia
Type: Open Cluster
Class: Trumpler type II 2 r
RA: 23h 57.7m
Dec: +56° 43’ 00”
Mag: 6.7v
Diameter: 25’
Some sequestered star
That rolls in its Creator’s beam afar,
Unseen by man; till telescopic eye,
Sounding the blue abysses of the sky,
Draws forth its hidden beauty into the light,
And adds a jewel to the crown of night.
James Montgomery 1771-1854
NGC 7789 is one of the finest and richest open star clusters in the sky. The Rev. T.W. Webb, in his Celestial Objects for Common Telescopes, calls it a “…beautiful large faint cloud of minute stars,” while John Herschel called it a “most superb” cluster. Adm. William Henry Smyth was also quite impressed with the cluster, which he said was comprised of “minute stars, on a ground of stardust.” He also saw the cluster as “having spangled rays of stars which give it a remote resemblance to a crab…The crab itself is but a mere condensed patch in the vast region of inexpressible splendor, spreading over many fields.”
Discovered by William Herschel’s sister Caroline on October 30, 1783. She included it in her small private list as entry #14. Her brother William included it in his catalog as H VI.30.
NGC 7789 is one of those rare objects that is impressive in any size instrument. The cluster is commonly known as Caroline's Rose. Its flowery visual appearance in small telescopes is created by the cluster's nestled complex of stars and voids. Looking at it, one gains the impression of a knot of bright stars superimposed on a larger mass of faint points of light.
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The cluster is made up of nearly 600 stars, the brightest shining at magnitude 10.0 down and ranging down to around magnitude 18.0. At a distance of 7,600 light-years and over 50 light-years across, Caroline's Rose spans about half a degree (the same apparent diameter as that of the Moon). Estimated to be 1.6 billion years young, the open cluster of stars also shows its age. All the stars in the cluster were likely born at the same time, but the brighter and more massive ones have more rapidly exhausted the hydrogen fuel in their cores. These have evolved from main sequence stars like the Sun into the many red giant stars shown with a yellowish cast.
NGC 7789 has a very rich blue-straggler population. What are blue stragglers? They are a class of star observed in old, dense stellar systems. They stand out because old stellar populations are expected to be devoid of blue (high-mass) stars which possess very short lifespans. For many decades, the most likely explanation for blue stragglers for many decades was that they were the result of stars that come too close to another star or similar mass object and collide. The newly formed star has a higher mass, and occupies a position on the HR diagram which would be populated by genuinely young stars. A study published in 2000 concluded that the existence of blue stragglers in NGC 7789 does not result from collisions and internal mixing processes, but that they evolved like normal stars. However they show a remarkable surface magnesium deficiency which is quite unusual and all confirmed blue stragglers in the cluster have rather low rotational velocities (with one exception).
The cluster is also rich in variable stars. At least fifteen variable stars have been discovered in NGC 7789. One is a blue straggler in the cluster. Another is an eclipsing system that shows brightness changes characteristic for a total eclipse.
To whet your appetite before you give it a go I share this selection of observational notes.
Stephen James O’Meara notes that 60 some stars are visible in a 4-inch at 23x and about 150 stars visible at 72x.
- Walter Scott Houston noted that through a 16-inch scope the view is spectacular, and the whole field is scattered with diamond dust. And in a 22-inch scope gave the most impressive view with countless sparkling points filling an entire 60x field.
- Phil Harrington notes that the cluster is famous as one of the richest open clusters north of the celestial equator. Current estimates put the total population at 300 suns crammed into a 16' circle. "This great density, coupled with the fact that none of the cluster stars shine brighter than magnitude 10.7, makes NGC 7789 difficult to resolve in small backyard telescopes. A 6-inch instrument reveals about five dozen stars sprinkled across a moderately bright background glow. An 8-inch increases the star count to about 100. Resolution steadily increases as aperture grows, with close to 200 stars visible in 12- to 14-inch instruments. My 13.1 inch Newtonian shows the cluster's few bright stars set in a loose arc. Curiously, I didn't notice the effect when viewing with my 8-inch reflector."
- J. Mullaney calls it a "ghostly open cluster…glowing softly at about 7th mag and spanning some 30', this cosmic jewel box contains at least 300 suns (some estimates put the count as high as 900), making it among the richest objects of its class. Users of small telescopes often pass over this object at first glance since the individual stars are relatively faint and blend in with the rich Milky Way background. In 6-inch and larger instruments having at least a one degree field of view, it is a fascinating sight. Most of the members of this distant stellar commune are remarkably uniform in luster, looking like diamond dust against a black velvet sky."
- (Steve Coe) This magnificent object is obviously a cluster, even in the 11X80 finder. Only 2 stars are resolved but the cluster is bright, large and brighter in the middle. With the 13" at 60X it is rich and much compressed, I can resolve 32 stars even at the lowest power and there are 2 dark lanes winding through the cluster. 100X with a 22mm Panoptic is a great view, I counted 76 stars of mags 10 and fainter. The cluster is about one third of the field of view. There are lots of curved chains of stars within the cluster. Going to 150X with the 14mm Meade UWA provides what I considered to be the best view, the cluster is about half the field and so many stars are resolved that I counted 63 in the NE quadrant alone, so I estimate over 250 member stars at this power. There are many dark lanes now seen within this compressed cluster and the lovely winding chains of stars are very prominent. At this power and at 220X there are 9 faint, delicate binary stars that are seen within the cluster. All the stars are white or very light yellow, little color seen; but wow, what a field full of stars. A long-time favorite.
Now it is your turn. Give it a go and let us know.
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