Image by Dr. Franz Gruber
Today I launched my new website, Star Observer, a website dedicated to the observation of stars and open clusters. I created this website to have a more structured way to present my observing reports from my favorite objects: stars and open clusters. At the core of Star Observer you will find a new and growing collection of visual observing reports. There is a separate page for every object observed. On an object's page you will find my personal observing report, a sketch (or image) and some notes and background information about the object. The observing reports can be found in the top bar menu under "Single and multiple stars" and "Open clusters". In the section "observing stars" you will find some information about the equipment and resources I use for planning, executing and evaluating my observations.
In the future new observing reports will be added on a regular bases. I am also planning to publish a few articles about stars and clusters, and the method I developed for planning, executing and evaluating / publishing my observations.
You can navigate through StarObserver.eu using the different menus or the sitemap. If you have any questions or suggestions, please feel free to contact me, using the contact form.
Updates on StarObserver.eu will be announced in this blog. Follow this link to go to the homepage of Star Observer.
It's been a long time since you heard from me, but during the last few months I've been working on a new website dedicated to the observing of stars and star clusters. It will take a little while before it will go online, but I will let you know! In the meantime I'm working on the design and content for this new site, and every now and again I look outside, and wow, in the last week of January Venus and the Moon were lined up in the evening sky. Hope you like the images. Please click to enlarge.
Last week I shot a few images with my Canon Powershot handheld at the 85mm Zeiss Diascope. Here's a nice sample!
Click to enlarge
I will inform you about the equipment set-up and the stacking/processing software in the near future. Right now I'm busy reading the software manuals and experimenting with the few movies I recorded. In the end this monochrome camera will be used to shoot detailed images of the Moon, the Sun in combination with the Baader Herschel Prism/Continuum filter and the Sun in H-Alpha with the Coronado PST. I will keep you updated!
I would like to thank Jim Kaler (Prof. Emeritus of Astronomy, University of Illinois), for helping me with my “spectral riddle” in NGC 6910, the Rocking Horse cluster. Here’s why the B1 supergiant looks yellow visually instead of bluish-white what you would expect from a B-type star.
As you know it is in the middle of Cygnus, near Gamma Cygni, not far off the galactic plane (you can see my wide angle picture at http://www.astro.uiuc.edu/~kaler/sow/sadr.html). As such it is subject to a great deal of interstellar extinction and reddening from interstellar dust. The dust selectively absorbs and scatters blue light (roughly 1/wavelength), so when you look at a star thru dust it will appear redder than it really is (basically the same reason the sun is reddened at sunset). The intrinsic B-V color of a B1.5 Ia supergiant is -0.2. The observed color is 0.83, about the color of a K0 dwarf or a G5 giant, so the star appears yellowish to the eye.
The whole cluster is highly reddened in fact by about the same amount. The “color excess” (E) is the observed minus true color, which is about 1.0 for this cluster. The absorption at V is usually taken as 3.2E, so Av (abs at V) is 3 magnitudes. If there were not dust, a 7th magnitude star would appear 4th magnitude, and the cluster would be visible to the naked eye. Good observing on your part to notice that. You can see the effect in many other distant clusters near the galactic plane.
Where can you find NGC 6910? The map below should give you a rough idea. Just center your telescope on Gamma Cyngi and move just about half a degree north-northeast. There you will find this nice clump of stars.
(Image from Voyager 4.5, http://www.carinasoft.com)
When you start looking for NGC 6910, bear in mind that it is a small cluster, only 7' in diameter, but then again, at lowest magnification it jumped out at me in my 12-inch scope. With the 22mm Nagler (fov 68') I already saw the complete outline of the little horse, and with the 7mm at 230x I could see a few dimmer, magnitude 12 stars. NGC 6910 is a Y-shaped cluster oriented northwest southeast. I counted between 20 and 25 stars but its always difficult to tell which do belong to the cluster and which are not included. The two brightest stars looked definitely yellow and are from the 7th magnitude.
There is however something that riddles me about these two yellow suns. According to all planetary programs and Internet databases, the Northernmost of the two stars is SAO 49556, a spectral type K1III, which explains the yellowish color. The southernmost of the two is SAO 49563 (or V2118 Cygni), a variable star of spectral type B1.5Ia. Normally I would think that a B1 star shines Bluish or at least mainly white, and not yellow. I checked other observing reports and I found that Sue French's (Celestial Sampler, page 132) reports:
At 87x, two yellowish stars of 7th magnitude and a pearly, split chain of eight 10th magnitude stars unite in a Y-shaped pattern about 5' long.
Sue sees two yellow stars as well, so I'm very curious why this B1 star seems to appear yellow instead of white. Does it have something to do with the Variable character of the star? Maybe one of you out there can help on this one.
Anyway, visually it is a very interesting group, so I made a sketch. The sketch below was made with the 300mm dob, a 7mm eyepiece (fov 18') giving a magnification of 230x. South is up and west is to the left. The next time when you're in the area observing, check out on NGC 6910 and let me know which colors you could see.