Category: Deepsky log
Rigel was observed with the 12 inch dobson. The double star did not show itself as a double initially, due to a lot of turbulence in the tube (the scope was just outside for a few minutes. However, after switching on the fan, the weaker B companion popped into view, just like that. The image was stable right from the moment the fan started to make a laminar flow in the 12-inch tube. Amazing. Betelgeuse was also observed with the twelve inch, and the deep orange color was simply overwhelming, especially at lowest power. However, in the 85mm Zeiss, the color looked even much deeper than in the 12-inch.
Collinder 65 is a large open cluster, which actually belongs to Taurus, but lies on the Orion-Taurus border. I observed and sketched this large open cluster (3.3 degrees) using the Skywindow and the 8x42 Orion binoculars, field of view 8.2 degrees. Until a few days ago I didn't know this cluster was actually there, but when scanning the area between Collinder 69 and Messier 1, you cannot miss it. You immediately will recognize it as a cluster.
In the next few weeks I hope to publish my observing reports and sketches on http://www.starobserver.eu On the map below you can see where the objects I observed can be found.
image from Voyager by Capellasoft
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.
That night, I started my observing session at 21.00 hours UT. The seeing from my backyard was only 5 on a scale of 10. Most bright naked-eye stars were blinking like crazy. The transparency and sky darkness weren’t too good either. I couldn’t see all seven stars of Ursa Minor, so the limiting magnitude was well below 5. However, with the 15x80 I quickly located the diamond shaped asterism, but the Mini Coathanger stayed invisible. After observing for a few minutes, I started to see a little bar of four or five stars, that form a part of the Mini Coathanger. The stars were very faint, and I could not see the complete asterism, not even with averted vision. Then I switched to the 85mm Zeiss. At 20x the bar of stars already looked a bit brighter, and seemed easier to identify. Zooming up to 60 times, using averted vision, I could see the whole asterism, the bar and hook. I could not detect any colour in the stars. After observing for 20 minutes from under a black hood, I could see the whole asterism with direct vision. I made the sketch below to using the 85mm refractor and the eyepiece at maximum zoom. The field of view at 60x is about 1.25 degrees, but I only sketched the central part of the field of view. Before I finished my observing session I switched back once again to the 15x80 binoculars. This time, using the black hood and averted vision, I still could not identify the whole asterism with the 15x80.
At 22.00 hours UT I packed up and went inside (had to work the next day), enjoyed my cup of coffee and put down a few notes. Looking back at the short observing session I can definitely say that the refractor with its 85mm aperture shows stars of the 11th magnitude much easier than the 15x80 binoculars. With the big binoculars, you use two eyes, which should compensate for the 5mm difference with the 85mm Zeiss. But in the end, I think that the higher magnification (you start at 20x with the Zeiss) makes the difference. The slightly higher magnification gives a little more contrast. If you zoom in to 60x, the 15x80 can in no way keep up with the views the refractor shows. However, I will try to hunt down all the asterisms on Harrington’s STAR list using both instruments. They are ideal for short observing sessions. Both instruments are mounted on video tripods permanently, so I only have to take them into the garden and I’m ready to go!
Image from "Where is M13?" by Think Astronomy
To observe NGC 7510 I used the 300mm Dob with the 21mm Denkmeier, the 12mm Nagler and the 7 and 5mm Pentax XW. After locating it with my Argo-Navis, I immediately recognized the small and rich open cluster, even with the lowest magnification (76x). The familiar arrowhead shape is something you will not forget, once you’ve seen it. At first glance, NGC 7510 looks very small indeed. This is in part because of its distance. NGC 7510 lies in the Perseus Arm of our galaxy, approximately at a distance of 10.200 light-years. This makes it one of the more distant open clusters. However, would you place NGC 7510 at 440 light-years, where we find the Pleiades, it’s apparent size would be 1.55 degrees! It would 30% larger than M45.
After increasing the power to 320x (5mm Pentax) I noticed that a lot of stars I’d seen on previous occasions stayed invisible. In fact, the transparency proved to be horrible. When comparing the sketch with a printed map from SkyTools2 the next day, the faintest star I plotted using my 12-inch scope was only magnitude 13.3. From the same location, my own backyard, I reached magnitude 13.5 with the 8-inch TAL 200K, on a very good night a few years ago.
Anyway, I made a sketch at the telescope with just a HB pencil, eraser and a piece of sketching paper. I did not plot all the field stars. I only concentrated on the possible cluster members. It is always very hard to recognize the real border of a cluster, and to determine which stars do belong to the cluster and which don’t. But I’m not the only one! When checking different entries in my observing guides I noticed that the values for size, numbers of stars and even magnitude vary a lot from one another. The eyepiece used for the sketch was the 5mm Pentax XW (320x). It took me 45 minutes to observe NGC 7510 and to complete the rough sketch. The next morning, with the help of my rough sketch and the notes I recorded on my Ipod, I made a new, final sketch to file in my observing log. For this final sketch (see image below) I used black paper and very a very soft white pencil. I only plotted the stars I had on my original sketch. I used a printed map from Skytools2 to check the positions of the stars and to the check the limiting magnitude for that night.
As you can see, I counted about 19 stars in the cluster. There was no real color visible in the stars, though I suspected a little color in the brightest. It seemed a bit yellowish, but I must have been wrong because when checking my observing library, I found out that its Lucida is a luminous blue B1.5 III giant (Sky Vistas, page 78, Crossen and Rhemann). The sketch is made with north up and east is to the left.
In the low light of dusk we started of with a one of my favourite double stars, Polaris. What struck me was how easy it was "to see" the double in both telescopes. The white B component was very easy to see, next to the yellowish A component. The separation is a generous 18", but I always have to take a good look to see the B-component. Later that night we revisited Polaris and it was a very different story. The 2nd magnitude A component almost completely outshone its magnitude 8.2 companion. Of course it was a lot darker later in the evening, during our second visit of Polaris, so the difference in brightness was much more apparent than earlier on.
16 Cygni and the blinking planetary (Caldwell 15)
The next stop was another nice double, 16 Cygni, which had a companion in the same field of view, NGC 6826 (The Blinking Planetary). 16 Cygni consists of two yellow-golden suns of almost equal magnitude, separated about 40" from each other. About 30' to the northeast of 16 Cygni I spotted the Blinking Planetary. In the 21mm Denkmeier with a field of view of 51' it was easy to get this bright planetary nebula into the same field of view with 16 Cygni. A nice couple of objects to look at. It is a very strange experience, watching the Blinking Planetary. If you look straight at the central star (which is visible even at the lowest magnification of 76x) the nebula seems to disappear. However, if you look a bit to the left of the Blinking Planetary, using averted vision, it pops into view.
I tried several magnifications (76, 133, 229, 320) but the blinking effect disappeared at 320x This was for me the optimum magnification to look at the nebula. It was perfectly round and I could detect no real areas of uneven brightness, and there is no well-defined sharp edge to the nebula. It showed only a hint of bluish-green colour tonight. It was very difficult to really "see" the colour. The OIII filter killed all the stars in the field, so I switched to the UHC narrowband filter. This enhanced the contrast a bit but I still like the view without filter most.
T Lyrae (Carbon Star)
Again a star that surprised us. Last year we have been looking for it with the 10-inch TAL but only after increasing the magnification to 200+ we could find it. It was very dark red and almost invisible at lower magnifications. Last Saturday I pointed the 12-inch dob at the location where I should see T Lyrae, and there it was. Not dark red and invisible but very bright and intensive orange. This is the first variable star that I have ever observed near its minimum and near its maximum, and I must admit, it's a great experience. I never imagined that two magnitudes could make such a huge difference in visibility.
Image from Skytools by CapellaSoft
Phil Harrington's STAR 22, the mini-coat hanger
Next on the list were three objects from Phil Harrington's STAR list (Small Telescope Asterism Roster). The first to visit was the Mini-Coat hanger in Ursa Minor. It looks like a smaller version of Collinder 399 in Vulpecula, the (big) Coat hanger. It is made up of 11 stars and the diameter is about 30'.
Phil Harrington's STAR 23, the backward S and STAR 24, a conspicuous ring
These two asterisms can be found in Hercules. I just had a quick look, but all three STAR's from Harrington's list will be revisited for a closer examination and a sketch. At this point we went indoors for a short break and some refreshments.
Double Cluster (Caldwell 14)
After the break I turned my telescope towards the Double Cluster in Perseus, NGC 869 and NGC 884. In the Dobson I immediately noticed a difference with my 8-inch TAL. The colour in some stars was much more obvious in the 300mm Dob. At lowest magnification, 76x, I noticed three bright yellow stars among all the blue/blue-white stars of NGC 884. When checking with my observing bible, "The Night Sky Observers Guide", I got confirmation of my observation. Three red super giants can be found in NGC 884.
The screaming owl
This is not some obscure asterism, but something that scared us like hell. Around 1.30 on Sunday morning a large bird, with a wingspan of two to three feet, flew over us screaming like the devil himself was on his heels (or tail actually). After little investigation on the Internet it proved to be a Barn Owl. Wow, what a sound! Turn up the volume real loud and .........Click on this link to get an idea......but be warned, this is not for the faint-hearted!
And here is an image of this wonderful bird.
Click to enlarge
NGC 7510 and Markarian 50
After we managed to get our heartbeat down to a normal rate again, we moved to our next stop, two open clusters on Perseus-Cassiopeia Border, NGC 7510 and Markarian 50. NGC 7510 is a lovely object, even at low power. It looks like a kind of arrowhead at 76x. I liked the cluster best with the 5mm Pentax, magnification 320x, FOV 13'. The cluster is only 4' in diameter so there's plenty of room left in the field of view. I counted about 25 brighter and weaker stars that where arranged in a kind of small circle with three legs dangling from the circle. In fact it looked like an octopus! I could detect no background nebulosity and no coloured stars. Next stop was Markarian 50, but this cluster proved to be almost invisible tonight, so we will try it on a night with better transparency and sky-darkness.
M52, Czernik 43, and NGC 7635 (Bubble Nebula, Caldwell 11)
From the invisible Markarian 50, it was only a small step to a very interesting trio, just across the border to Cassiopeia. You can get two open clusters, M52 and Czernik 43, and an emission nebula, NGC 7635, into one field of view. M52 is a very irregular cluster without a real shape, while Czernik 43 looks much more organized, a kind of large triangular shape with 4 bright stars on a row on one side of the cluster. The Bubble nebula was only visible with the highest magnification and averted vision. With direct vision it stayed invisible. This is an object which we have to revisit under better conditions (no Moon, better transparency and darker skies).
At 03:00 we ended our observing session. We saw quit a few wonderful objects tonight, but for me the one thing I will remember is that impressive large bird, breaking the silence with its very harsh call. It was very special to see him soaring through the night sky, only lit by the light of the Moon. It was the first time ever I saw a barn owl and I hope I will see one again somewhere in the future.
Our next stop was Phil Harrington's STAR 26 ( STAR=Small Telescope Asterism Roster), the Red Necked Emu in Cygnus, The Swan. I had seen it once before in my 4-inch refractor a few years ago, so I knew what I was looking for. The Red Necked Emu looks like a giant propeller with three blades, about 1 degree in diameter. Just start at Gamma Cygni, the orange star at the heart of Cygnus, and move towards Albireo along the neck of the Swan for 2.5 degrees to 34 Cygnus. From there move about 1.5 degrees in the same direction to arrive at 29 Cygnus. This star marks the tip of the Emu's tail. On the image below you can see how the Emu is oriented in the sky. Through my 15x80 it was very simple to find. I think this object is best in 80 to 100 mm instruments, with a large field of view. This strange bird is called Red-Necked Emu because all the stars he consists of are white-bluish, except one orange-reddish star in its neck. This is very easy to see.
Click to enlarge
After this I first freewheeled a little through the Milky Way in Cygnus, a wonderful experience with binoculars. M 29, Albireo, Omicron 1 and 2, are just a few of the stops I made. After that I also made a little tour through Vulpecula: The Coathanger (Collinder 399) The Dumbbell (M 27), and the wonderful open cluster NGC 6940. Although not very well resolved, NGC 6940 stood out well from its surroundings, however I like this object more in my 300mm Newtonian. From Vulpecula I turned back again into the Swan, passing M 39 and on to Mu Cepheus, the Garnet star. In this area we looked at a few open clusters, but because we had some problems identifying them, we will have to revisit this area.