NGC6772: Ploughing through interstellar space

I have already elaborated very often on the death of ordinary stars and the evolution of the planetary nebulae that form when these stars eventually collapse. This time I'd like to highlight a very interesting phenomenon that happens when these planetary nebulae expand way beyond the original boundaries of their "solar system" and into intergalactic space.

NGC6772 is a planetary of a certain age in the constellation of Aquila, the eagle. The gas shell that was violently expelled during the star's collapse has smashed through the faint outer shell which had already formed many thousands of years earlier, during the last stage of the star's life. Now, it has reached interstellar space and the giant gas bubble that's blowing up at a rate of 30 km/s crashes into a medium with completely different mechanics. Interstellar dust is moving in other directions than matter within the gravitational influence of the former star, driven by the gravitational pull of our galaxy, and the expanding planetary nebula finds it ever harder to plough through it. Gas at the border of the nebula's building up, as if it were hitting a brick wall, and we observe a significant brightening there. The nebula's not spherical anymore, deformed as it is by areas of less or more interstellar resistance. Its central star's cooling down and losing brightness quickly, up to the point that it isn't visible through amateur telescopes anymore, and only shows itself on long-exposure photographs. 

Yet, the nebula will continue to plough through interstellar space, at an ever decreasing rate, until it completely dissolves.

M16: the pillars of creation

M16, otherwise known as the Eagle Nebula, is another one of summer's highlights. Though less bright than nearby M17, it only takes a pair of binoculars to discover this very young star cluster surrounded by their maternal gas cloud. The nebula became especially famous when astronomers discovered active star formation for the first time, right in its central region. There lie the so-called "Pillars of Creation" which are obvious on long-exposure photographs but extremely difficult to make out visually. When I come to think of it, I've never seen them through a telescope before, until I got my new binos that is. These "pillars" are in fact long and very dense clouds of gas and matter that are creating hundreds of new stars as we speak. I was able to make out three "fingers" on top of them, which in reality are at least four light-years long! Within these "fingers", the Hubble space telescope discovered hundreds of globules and proto-stars. A theory goes that a nearby supernova explosion, which happened some 7,000 years ago, has blown away most of the surrounding matter and that the "pillars" are currently the only thing left. Given that the Eagle Nebula lies between 6,500 and 7,000 light-years away, the "pillars" have probably dissipated completely as well by now; it's only that the light of that event hasn't reached us yet. What we can already see very clearly is that the intense radiation of the hot, newborn stars is eroding the nebula very quickly. The "fingers", for example, are being torn apart by the stellar winds of the bright little star in the middle of them. Eventually the whole nebula will disappear, as will the cluster of young stars that has formed within it.

M20: the fascinating Trifid Nebula

The Universe is awash with objects of rare beauty. Such marvels as to leave you with your mouth wide open every time you look at them, in the knowledge that we humans are so insignificant in comparison. One object in particular that rates very high on the beauty scale, is undoubtedly the bewildering Trifid Nebula (M20). Not only is it an incredible spectacle to behold, but it's also a stunning combination of three different kinds of nebulae. The brightest part, surrounding the conspicuous double star, is an emission nebula. It's a region of intense star formation that's being heated up by the radiation of the hot, young stars embedded in it, up to the point that it starts emitting light on its own. The central star is in fact a sextuple system, of which I was able to discern four members at the "modest" magnification of 190x I used here. No star formation's going on anymore in the immediate vicinity of these stars because they've literally scorched away the surrounding gas cloud. The less bright region below is an enormous reflection nebula, merely reflecting the light of the giant star in its centre. And then there are of course the fascinating dark lanes, which are clouds of dust drifting in the foreground. Interesting to note is that these dark lanes show a bright rim on the side which is illuminated by nearby stars.

The whole complex is estimated to be some 21 light-years across, which is five times the distance from our Sun to Proxima Centauri, the nearest other star, and almost the diameter of the mighty Orion Nebula. The reason why the Trifid appears a lot smaller and less bright is because it lies much further away from us: 5,200 light-years as opposed to merely 1,300 for the Orion Nebula. In all, you could say that the two nebular complexes are very similar in size and they're both giant stellar nurseries. Also in the Trifid Nebula dozens of embryonic stars ("proto-stars") have already been discovered.

Another interesting fact about the Trifid is that it's very young, estimated to be no more than 300.000 years old. This makes it one of the youngest emission nebulae known. 

 

NGC6537: the Red Spider

Planetary nebulae are fascinating objects and exist in a seemingly infinite number of varieties, as many as there are dying stars. The Red Spider Nebula (NGC6537), for instance, is definitely one of the more eccentric planetaries but unfortunately requires quite a bit of telescope and very good skies to be admired fully. Photographs clearly show its bipolar structure and four "legs" that extend up to a hundred billion kilometres away from the central star. A bipolar structure is certainly not uncommon in planetary nebulae because often the matter outflow is obstructed at the star's equator by its greater density there, forcing the gas to blow out via the poles. The peculiar shape of the Red Spider, however, leads scientists to believe that there must be a small companion star nearby which distorts the nebula formation. 

It's clearly still a very young nebula in full expansion, with complex and turbulent gas structures that are being hurled into space by stellar winds up to 300 km/s. Another odd thing is that it appears red in stead of greenish-blue, an indication of a high presence of ionised nitrogen, although there's also a second explanation. The Red Spider lies in one of the densest parts of our Milky Way, towards its nucleus, and is surrounded by thick clouds of interstellar dust. These clouds work like sunglasses, i.e. they not only dim the light from the nebula considerably, but also change its colour towards the lower end of the frequency range (red). Without those clouds the Red Spider would shine 40 times brighter and would easily be within reach of small telescopes. Unfortunately, even with my big binoscope I was only able to distinguish a hint of its four famous "legs"...

  

A window towards the centre of our galaxy

In summer, the night side of our humble planet's turned towards the centre of our galaxy which we can find in Sagittarius. Although the Milky Way's largest and brightest in that area, it's not all that easy to take a peek into the depths of our galaxy because its core remains mostly hidden behind dark clouds of interstellar dust. And yet, here and there these clouds show holes which offer invaluable observing windows towards the nucleus. One of these "holes" we call M24, which we can already identify with the naked eye as a bright patch in the Milky Way. This object's a delight in binoculars and small telescopes with its thousands of stars that shine towards us from at least 10,000 to 16,000 light-years distance. Larger telescopes, on the other hand, magnify a bit too much to appreciate this stellar cloud fully so in this blog post I will concentrate on a detail which lies near its soutwestern edge. 

You'll immediately notice two bright reflection nebulae at the centre, denominated NGC6590/5 (top) and NGC6589 (bottom). The term "reflection nebula" implies that these nebulae do not emit light on their own - they're not hot enough for that - but that they merely reflect the light of the stars that are imbedded in them. These nebulae lie a lot closer to us, at a distance of roughly 2,000 light-years. 

To the left of my drawing you can see a large but faint nebula, which is in fact a part of a gigantic hydrogen cloud (IC1283/4) in which new stars are born at a distance of some 10,000 light-years. 

A dark dustlane cuts the background in two and on the right we have the edge of the window I was talking about earlier. It rather looked like another "cloud" through my telescope and it was impossible to identify any individual stars in it, but I "had the impression" that it was made up of millions of stars, so that's what I've tried to reflect in this sketch. 

It may not appear as such at first sight, but this is probably my most demanding sketch so far, with at least 150-200 stars identified and drawn!  

NGC6802: When star clusters come of age...

Stars are born together in vast hydrogen clouds. Eventually the immense radiation from all of those hot, new-born stars blows the gas cloud away and all that's left is a cluster of stars. Unfortunately in most cases these stars are not destined to remain together. Our galaxy's mighty gravity exerts such a tremendous pull that the mutual gravity of the cluster members is insufficient to keep them united. Slowly, after hundreds of thousands of years, the cluster will start to fall apart and the individual stars will be smeared out into space where they will start their solitary adult lives.

NGC6802's a large but quite distant cluster (5,400 light-years away) in the surprising summer constellation of Vulpecula, the fox. You need to look carefully because it appears small and faint in amateur telescopes, but you'll unmistakingly notice that this cluster's not spherical at all. As you can see on my sketch, our galaxy's gravity's currently tearing it apart! Within a few tens of thousands of years this cluster will be no more and all of its stars will be dispersed into space...

 

M107: A globular with some dark patches

Globular clusters are extremely dense balls of stars. They may contain hundreds of thousands of stars in a volume that's only twenty times the distance from Earth to the nearest star across. Imagine Venus, Jupiter and Sirius and how brightly they're shining in the sky. To anyone living in a globular cluster the sky would be filled with such very bright stars, as many as there are camera flashes during a football World Cup penalty shootout. Some would even be brighter than the full Moon! So after all it's very unlikely that these globulars contain life because any planetary system would be severely disrupted by the tidal forces from nearby stars. Similar as these fascinating objects may seem at first sight, careful observation through a telescope will show you that they all have a character of their own.

When speaking of M107's character, it's an odd globular in many ways. First of all, it's unusually loose: it ranks X on a density scale from I to XII. Second, it lies almost right above the centre of our Milky Way. And third, it's one of the few globular clusters that show dark patches in them, as you can also see on my sketch. Before you get carried away, these dark patches have nothing to do with M107 whatsoever. Since this globular hovers only slightly above our galactic plane, some of its light is being blocked by our Milky Way's interstellar dust. Infrared images, on the other hand, reveal that it's just as round and regular as most other globulars.

M107 was only posthumously added to Messier's catalogue and, as it turns out, it was also the last astronomical object that Charles Messier and his assistant Pierre Méchain discovered, despite the Messier list containing 110 objects. Messier was an 18th century comet hunter and for years he was browsing the night's sky. When he accidentally stumbled upon a fuzzy patch which he identified as not being a comet (since it always remained immobile at the same position in the sky), he wrote its coordinates and description down in a list so he wouldn't confuse it with a real comet. This list became the first catalogue of astronomical objects and is still the most popular reference among amateur astronomers.

This particular globular cluster's one of the least-known Messier objects and it's also reasonably faint. You may already spot it with a pair of binoculars under a sufficiently dark sky, but it will remain difficult to see. In order to resolve some stars in it, you're going to need at least an 8" telescope. With my binoscope this cluster appeared completely resolved but remained fairly dim. Yet, it's a fascinating object.