Date:May 1, 2011
Title: Encore: Carolyn Herschel’s White Rose – NGC 7789
Organization: RapidEye Observatory – a private observatory in rural Lee County, NC
Links: http://www.rapideye.us/astro/RapidEye-ClearSky.html
Description: A brief overview of Open Clusters, discussing Carolyn Herschel and her unique finds while working with her brother, and observing one of Carolyn’s prettiest finds, NGC 7789. This podcast originally aired on Nov. 16, 2009: http://365daysofastronomy.org/2009/11/16/november-16th-carolyn-herschels-white-rose-ngc-7789/
Bio: I’ve been captivated by astronomy ever since I was a kid, living in NW Colorado where the Milky Way was bright enough to read by. I can be found most clear nights in my pasture with either my 4.5″ Dob, 10″ Dob, or my binoculars.
Sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
Caroline’s Rose: NGC 7789
Many astronomers are familiar with William Herschel, his discovery of Uranus and his systematic efforts to catalog the numerous deep sky objects he saw through his personally built telescopes. Many are also familiar with William’s son, John his continuation of his father’s work and his journey to South Africa to complete the catalog for objects deep in the southern skies. But not many people know that the Herschel family also had a third astronomer that not only provided support for William and John efforts, but also made many observations and numerous contributions to the family’s catalog of objects. This was William’s sister and John’s Aunt, Caroline Herschel.
Caroline moved from Germany to live with her brother in England when she was 22 where she assisted him not only with domestic responsibilities but also in his astronomical endeavors: grinding mirrors, recording observations, and even logging extensive eyepiece time on her own. She is credited with not only independently discovering dozens of deep sky objects that were included in her family’s catalog, but also discovered 8 comets, including the return of the periodic comet Encke in 1795. She was by all measures a very accomplished observer on her own accord.
Many of Caroline’s nicest deep sky discoveries were open clusters in the northern Milky Way, and easily one of the finest of not only her discoveries, but of any open cluster in the sky is NGC 7789 in Cassiopeia. Caroline discovered this cluster on October 30th, 1783 and included it as object number 14 in her own personal catalog. NGC 7789 is classified as a Trumpler type II 2 r Open Cluster, meaning, it has a strong central concentration, has a medium range in brightness of member stars, and is considered a “Rich” cluster with more than 100 members. It is about 16 arc minutes in diameter, has a visual magnitude of 6.7, and is estimated to be between 7,500 and 8,000 light years away from us in the Milky Way.
While those are interesting numbers, what really sets NGC 7789 apart is how it appears at the eyepiece. When most amateur astronomers think of showpiece open clusters they typically think of large loose arraignments of stars like the Pleiades’s and Hyades in Taurus or the Butterfly or Ptolemy’s cluster in Scorpius. In a telescope, those open clusters appear best at low power or even better through binoculars in some cases. NGC 7789 isn’t like those clusters. At magnitude 6.7 its easily visible in binoculars or even a moderate finderscope from dark skies. But through a telescope, NGC 7789 behaves more like a typical globular cluster, meaning, the more aperture and the more power you can observe it with, the better it appears.
Lets start by finding it in the sky. Here in the northern hemisphere, NGC 7789 is well positioned this time of year for early evening observing. Tonight, around 8 PM local time with a lightly illuminated moon will have the cluster under almost perfect conditions. Start by locating Caph, or Beta Cassiopeia. This time of year, Caph will be the western most member of the Cassiopeia “W” or “M” depending on how you see the asterism. If you drew an imaginary straight line from Alpha Cassiopeia, or Schedar, to Caph, then made a 90 degree turn back to the west almost the same distance, through a finder scope or binoculars you should easily see a small group of 3 stars with the brightest one being about 6th magnitude. This time of year, almost 2 degrees straight down you’ll see a pair of similarly bright variables stars between 4th and 6th magnitude. Look right between the three star group and two star group and you should see a small hazy patch, about half the size as a full moon. This is NGC 7789.
While its visible through 7×50 or 10×50 binoculars, most of the component stars are too dim and too tightly packed to resolve. Under good conditions NGC 7789 takes on a granular appearance and with averted vision, variations in the clusters brightness can be made out, but no real structure or shape is apparent. It appears to be a concentrated bright patch in a somewhat rich part of the Winter Milky Way – fairly easy to overlook with all of those other stars and clusters in the region.
When observing NGC 7789 through a 4″ class telescope, the cluster becomes more obvious and brighter than it does through binoculars, and with increases in power, hints of structure start to come into view. Probably the best view with this sized instrument is at modest powers where you can take in the cluster and its surrounding rich field of stars at the same time while being able to start to resolve some of the outer parts of the cluster – a very pretty sight.
When observing NGC 7789 through a 10″ to 12″ telescope at over 100X, the cluster really starts to come alive. Long arches of stars loop around in spiral patterns with dark lanes looping in between them making it appear like you are looking down on a single white rose. Like a rose, as you get closer to the center, the stars and loops get tighter and fade into a misty haze like a mass of pollen. Like a loose globular cluster, there seems to be a core of stars that are just beyond the grasp of the instrument. Averted vision shows that there is a granular misty glow in the center, but with direct vision, it seems to fall away. M11, the Wild Duck Cluster in Scutum has a very similar effect and is another great Open Cluster to compare NGC 7789 too.
Using my new 18″ F/4.5 newtonian, Caroline’s Rose cluster just explodes with stars. Being relatively small, it is easy to frame the cluster at higher powers with a wide view eyepiece and really soak in the beauty of the cluster. Even at 300X, the center mass of stars still appears like a fine mist that pops in and out with direct vision.
I spent many hours sharing views of Caroline’s Rose Cluster with friends at the MidAtlantic Star Party and East Coast Star Party last month. We used many different sized instruments and different combinations of eyepieces. Under almost every combination everyone considered it to be one of the finest objects of the season. Go take a look from moderately dark skies and let me know what you think!
This is my fourth and final podcast for the 365 Days of Astronomy this year and I’d like to thank the coordinators, editors, and behind the scenes staff that have helped to make this effort a success that I’ve been proud to be a part of. I’d also like to thank the coordinators of another IYA2009 project: “She is an Astronomer”. This cornerstone project is working to highlight and address gender equality problems in science. Like the 365 Days of Astronomy Podcast, they also have their own website where people can find more information, submit questions, and get answers. They have highlighted many historically important women astronomers and Caroline Herschel is of course profiled for her contributions and numerous international awards. I’ve provided a link to their website in this podcast’s show notes so you can find more information about Carline Herschel and other ground breaking women in astronomy.
Finally, my 9 year old daughter loves astronomy and spending time with me at the telescope in our pasture and out at star parties with our friends. Through outreach efforts like the “100 Hours of Astronomy”, “Galilean Nights”, “365 Days of Astronomy” and “She is and Astronomer” I hope that she continues to find the passion and knowledge that will compel her to continue learning about the universe around us and the mechanisms that make it all work and will be able to pursue her curiosity to whatever ends it takes her without any gender barriers. It is to her and all the other 9 year old girls that I dedicate this podcast, and through an eyepiece, give them a single white rose.
All Sky Map – Chart to find Cassiopeia: http://skymaps.com/downloads.html
Cassiopeia Major Detail:
http://en.wikipedia.org/wiki/File:Cassiopeia_constellation_map.png
Wikipedia Picture of NGC 7789:
http://en.wikipedia.org/wiki/File:NGC7789HunterWilson.jpg
The Official “She is an Astronomer” Website: http://www.sheisanastronomer.org/
More Details on Caroline Herschel: http://www.sheisanastronomer.org/index.php/history/carolineherschel
End of podcast:
365 Days of Astronomy
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