Friday, 13 July 2018

M21: Young and bright

Another pretty and well-known star cluster in Sagittarius is M21, only half a degree away from the famous Trifid Nebula. Containing only 57 confirmed members, it's not as well populated as M23, but nevertheless it's a beautiful object in binoculars and small telescopes. As was the case with M23, the binoscope magnifies just a bit too much to frame this cluster nicely and you're almost getting the impression that you're looking though it. Still, it lies 4,200 light-years away from us, double the distance to M23 and in fact not all that far from the Trifid. Therefore most of its stars appear somewhat dim, although a bright blue supergiant, ten times our Sun's diameter and radiating 64,000 times as much energy, stands out centrally. Many cluster members also appear to be close binaries so in reality the total star count should be a bit higher. 

Interesting to note is that this cluster's still extremely young, hardly 4.6 million years old and therefore not much older than the human species.  

Tuesday, 10 July 2018

M23: comparison Nexus 100 binoculars vs. 18" binoscope

A week ago I showed you M23 as seen through an 18" binoscope, commenting that such a large instrument is probably too big for such an extensive object. So A few days later I re-observed this amazingly beautiful star cluster with my Nexus 100 binoculars at 24x. 

I have to tell you that I was somewhat disappointed because appearing so exaggeratively bright in the binoscope, I had expected something more spectacular in the 100mm binoculars. The cluster was, of course, clearly visible at first glance, but it took a while for all the tiny individual stars to reveal themselves and the overall appearance was slightly faint.

What can I say? I admit that I haven't used the binoculars for over two years, at least not since I got the gigantic binoscope, and therefore my mind's probably got used to quite another level of brightness and detail. It's very easy to get used to larger aperture, but downscaling's a different matter. 

Does this mean that I'll but the Nexus up for sale? NOT IN A MILLION YEARS! It's simply a different kind of observing, a kind that I've neglected for far too long and that I need to take up again some more. Big binoculars are not meant to bring out tiny details or to show you a star cluster scattered across the entire field of view. Contrary to telescopes, they're meant to take you on a sightseeing trip around the heavens, giving you a taste of every splendour that's up there. From M23 I quickly jumped to M20-M21 (nicely in the same field of view of course), then onto M8, M22, M24 (completely in the field of view), M25 and M16-M17 (almost in the same field of view). All of this without hardly having to move from my comfortable chair. That's the difference.

Anyway, here's my M23 sketch. To the bottom you'll find the sketch I made with the binoscope for comparison.



Thursday, 5 July 2018

IC4634: not going quietly

I got the question why the heck I'm so passionate about these tiny planetary nebulae. Indeed, most people only see a blurry little dot and what could possibly be interesting about that? Well, most people simply don't look very well. Even though usually bright and easily visible, it takes some time for your eyes to adjust to the image and to discover all possible details. Next time you're looking though a telescope, try staring at this little dot for at least two full minutes, let your eyes move around it, relax, take it all in. Suddenly there will be a point at which you can see more... structures... filaments... 

IC4634 is still a young planetary nebula, hardly a few thousand years old, and unfortunately it lies a respectable 7,500 light-years away. So yes, it looks tiny. This is a real pity because this is a truly spectacular little bugger. The dying central star (which I had difficulty making out) is not ejecting its atmosphere in one big blow, but in puffs whilst it keeps spinning rapidly. The result is that the gaseous shells form expanding waves in different directions. Difficult to see, I know, but it was definitely there. I also had the impression that I could see a faint halo around it, which may be material the star already ejected when it was in the last, unstable phase of its life and which now slowly begins to glow under the tremendous radiation from the remaining stellar core.

Wednesday, 4 July 2018

NGC6507: My cup of tea

A few days ago I showed you brilliant M23, an open star cluster in Sagittarius that's a real treat for binoculars and small telescopes. In a large telescope you'll have a hard time trying to fit everything in the telescope's eyepiece and therefore the view will become somewhat less pleasing. Now onto a small cluster in M23's vicinity: NGC6507. There's surprisingly little information to be found about it and that's a real shame because this is the sort of object I (personally) go nuts about. Being over 3,900 light-years away it lies almost twice as far from us as M23 and if it doesn't nearly appear as impressive it's mostly due to its much greater distance. Observe it with a sufficiently large telescope and you'll be surprised about the number of tiny little stars that appear in between the brighter ones. Well... brighter ones... even those are of mag. 11-12. 

Now try to focus on what this cluster's telling you. Look at how its stars are no longer contained in a more or less spherical shape but how they seem to be smeared out somewhat. Clearly, this cluster's already a bit older than M23 (400-450 million years) and the effect of our galaxy's gravity is taking its toll. Its stars are being pulled into a streak along our galaxy's plane and will then disperse to lead their adult lives in solitude. 

Again, the brightest star, towards the bottom-right, lies much closer to us and therefore doesn't belong to the cluster at all. 

Monday, 2 July 2018

Glorious M23

This may sound a little weird, but sometimes a telescope can be too big. Last night, for example, I observed M23, a bright and rich star cluster which lies 2,100 light-years away in the direction of the centre of our galaxy. The largest true field of view my binoscope can offer is less than a degree across, which is a tight fit for this big and relatively close cluster. The view in binoculars or small telescopes, albeit less bright, will certainly be more pleasing but hey... it wasn't all that bad in my binos after all. 

M23 contains about 150 confirmed members and came into existence some 300 million years ago, making it a not a very young cluster. In spite of its many members, it's now starting to break up slowly, after which all of the individual stars will go their own way. For many of them this journey will not be long because bright blue giant stars lead very short lives. The bigger a star, the faster it will burn its fuel and there are quite a few biggies in M23, some of which are already shutting down hydrogen fusion and are evolving towards red giant status. Through my binoscope on the other hand, most of the stars still appeared bright blue as I've tried to reflect in my sketch.

Once more I have to inform you that sometimes appearances can be deceiving because the very bright star near the bottom-right border (HR6679) doesn't belong to the cluster at all. Being only 320 light-years away it's a lot closer to us and with its 2.2 solar masses I wouldn't exactly call it a giant.