Friday 27 July 2018

Berkeley 81: Into the mist of our galaxy

Here we go again... Once more a sketch that'll rank high on the least-popular list because, let's be honest, it almost looks like a blank sheet, apart from a couple of small stray stars. Yes, I know that this won't be the easiest or most spectacular sketch and that Blogger doesn't allow you to open the image real size, but please give it a try. 

No, don't look at the sort of framework of brighter stars that seem to form two parallel lines from top to bottom because they lie a lot closer to us than the object I'm trying to tell you about here. Look at the two stars at the centre and then just below them. Look very carefully; turn off the lights if you have to. Don't you see a small group of incredibly faint stars appearing? I've tried to represent them just as faint as they were for me at the telescope and also I had to glance for a bit before these stars suddenly revealed themselves in the dark grey mist. 

Berkeley 81 is a remote cluster that lies 10,000 light-years away in the direction of our Milky Way's core. With all the dense clouds of dust and gas in the area, it's not surprising that this cluster appears so faint because a lot of its light's being absorbed. It's like looking at a distant light on a foggy night. The cluster's intriguing because of its position and its high age, estimated between 750 million and 1 billion years. Usually galactic star clusters get torn apart quite soon by our Milky Way's gravity, especially when they're so close to the core. Yet, this one's still resisting. Perhaps because it's lying somewhat above the plane of our galaxy? Or because it's not an insignificant cluster, containing about 1,500 solar masses? 

Oh, how I just love these off the beaten track objects... :-)

Thursday 26 July 2018

NGC7026: The cheeseburger

Some 5,700 light-years away in the constellation of Cygnus, the swan, lies this small but highly interesting planetary nebula. At first glance you'll notice its two bright equatorial lobes and the sort of dark lane that runs across it, effectively giving it the appearance of a cheeseburger. 

The progenitor of this nebula is a very hot (80.000°C) white dwarf star in its centre, which was invisible to me at the telescope. This central star was once a star very much like our own Sun, with an estimated mass of 1.1 times solar. Having finished its hydrogen supply long ago and with helium fusion becoming unstable, the star blew off its entire atmosphere merely 1,500 years ago. Since the atmosphere's much thicker at the equator due to centrifugal forces, the gas can't escape as quickly there and builds up in these bright lobes, expanding at 50 km/s. At the poles, on the other hand, the gas blasts away much more easily and reaches speeds of 150 km/s, hence creating the somewhat cylindrical-shaped nebula. Beyond the polar gaps we see several emission knots which are in fact high speed parcels of gas related to the early stage of the nebula's formation and which are flung out at over 180 km/s. 

Scientists theorise that this nebula developed very fast, producing violent shockwaves and instabilities to its environment, unlike more common spherical-shaped planetaries. In a sense this nebula's quite similar to the older and better-known Butterfly (M76) which also exhibits a turbulent, bi-polar structure. Without any doubt the Cheeseburger will eventually evolve along the same lines before disappearing forever within the next ten to fifteen thousand years.

Tuesday 24 July 2018

NGC7008: Evidence of extrasolar planets

Every planetary nebula, the remnant of a small to medium-sized star, is unique. Just browse through all of my sketches and I'm sure that you'll agree. They all tell a different story and even reveal a lot about the nature of the star system that eventually got destroyed when the central star exhaled its dying breath. 

NGC7008, nicknamed the "Fetus Nebula", is one of my all-time favourites. Just look at its highly irregular structure, which is already obvious through a small telescope. After zooming in with the binoscope I had to pick up my jaw from the ground because I'd never seen anything like this before... and believe me, I've seen quite a few planetaries in my 36 years of being an astronomy enthusiast.

On either side of the central star there are bright patches of nebulosity. Remember the FLIERs I told you about when discussing NGC6826? These are exactly the same but much older and more developed. But obviously there's more... much more. Usually planetary nebulae form a spherical or ellipsoid sort of bubble. Here, the bubble's irregular and even appears ruptured. 

Scientists speculated that the big central star was in fact a binary and that the interaction with this companion disrupted the nebula's formation. More recent observations with the Hubble space telescope revealed dual layers of completely different content in them. Moreover, these layers appear near the edge of the nebula, where it meets the interstellar medium. This makes a companion star highly unlikely. A new theory suggests that the expanding nebula's interacting with planetary debris... bits and bobs of planets that were destroyed during the expulsion of the central star's atmosphere. This would account for the inhomogeneities in the nebula and, if massive enough, rings of matter would be formed that generate the sort of structures we see here when stellar gas crashes into them.

In short, it seems that this odd-looking nebula is providing us with key evidence that once a complex planetary system existed around the central star. 


Monday 23 July 2018

NGC6826: The blinking planetary

Visual astronomy is a very subjective matter because the human brain can easily be fooled. I'll be the first to state that my sketches are merely an impression of things that I think I've seen, without claiming any true scientific value. 

A popular example of such optical illusions is this bright planetary nebula in Cygnus, which bears the nickname "Blinking Planetary". The reason for this is that many people report that it seems to fade and reappear very quickly, as if someone's playing games with a light switch. Obviously this is wrong as planetary nebulae generally do not change brightness overnight, let alone in a matter of seconds. The illusion's caused by its very bright central star. When focusing on it with your eye, the surrounding - much fainter - nebula seems to disappear somewhat. When you then turn your gaze away from the star, the nebula reappears. No magic, just our eyes having difficulties adapting to different brightnesses when already observing under unusual (very dark) circumstances.  

What's more interesting though, are the bright patches at the nebula's border, on either side of the central star. Scientists call them "FLIERs" (Fast Low-Ionisation Emission Regions), the origins of which are still not well understood. One theory goes that they're gas that was hurled out from the star about a thousand years ago at supersonic speed, but in that case their bow-shock points in the wrong direction. Another theory suggests that these patches of gas are stationary and that the expanding gas bubble scrapes past them.  

Friday 20 July 2018

Kappa Herculis: Not what it seems

Did I already mention that appearances can be very deceiving when looking at the night's sky? Wait until I tell you about Kappa Herculis!

Kappa Herculis, also known as Marsic, is a fine double star that can easily be separated with small telescopes or even binoculars (separation 27"). Though their measured temperatures are almost identical (4990°K for K Her A and 4650°K for K Her B), most observers - including me - see them as bright yellow and orange. Their luminosities are 170 and 70 Suns respectively and they have radii of 3 and 2,5 times solar. This means that their respective ages are 400 and 700 million years. Nothing out of the ordinary so far... er...

What did I just say? Two components of a double star that don't share the same age? Highly suspicious! Now, if we measure their distances we find that A lies 388 light-years away from us, whereas B is almost 100 light-years further away! Okay, given the proximity of A, the measured distance to B has a large uncertainty so they still could be near to each other. On the other hand, in the 300 years that this double star has been observed, their separation has diminished from 57" to 27", much more than it should. 

Conclusion: This is no double star but merely an incredible line-of-sight coincidence!

Yet it does seem that A is a binary star with a small companion an arcminute away, which would be 7,500 times the distance from Earth to the Sun, and with an orbital period of 340,000 years. From A, this tiny companion would shine with the same brightness as Saturn.

To make things even more confusing, there's also 8 Her, the bright, white star above our "double" on my sketch. It appears that 8 Her is 367 light-years away, more or less the same measured distance as K Her A! Could they be a real pair? Unlikely, since in that case they would be 1.3 light-years apart, too far to maintain a gravitational bond. Their apparent motion across the sky also appears different. 

Even if 8 Her and K Her A are not related, the former doesn't seem to be ordinary as well because it's an extremely fast rotator. It spins at an amazing speed of 259 km/s, only slightly under the limit at which the star would tear itself apart, resulting in a rotational period of 10.3 hours. 

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.