Sunday, 13 August 2017

NGC6960: The Western Veil

The Veil Nebula is undoubtedly the most breath-taking supernova remnant in the sky and - to my humble taste - it's the most spectacular object in the rich summer constellation of Cygnus, the swan. The hot clouds of gas that were blown into space at 30,000km/s (!) when a giant star became critically unstable and exploded, have expanded in the last 6,000 years to a frail bubble 110 light-years in diameter. It is still growing at a rate of 170km/s and eventually the filaments of ionised gas will dissolve into space.

A year ago I showed you the eastern part of this nebular complex that spans an area of six full moons in our sky. This time, I'll show you a detail of its western part. This area is notoriously famous for the extremely bright star that seems to lie right in the middle of it. 52 Cygni is a star of magnitude 4.22 and therefore easily visible to the naked eye, if there aren't too many useless street lights around. Again, you see how much appearances may deceive because this star lies at a distance of 210 light-years, whereas the Veil Nebula lies seven times further away from us. The star with its large "wings" of nebulosity are a lovely sight, but 52 Cygni shines so brightly that it tends to shade the delicate whiffs of the supernova remnant. For this reason I chose to increase telescope power to 190x and to concentrate on one of the "wings", leaving 52 Cygni just beyond the right border of the field of view. The details that emerged, left me with my mouth wide open. I hope that this sketch, albeit not a real telescope image, may have the same effect on you...

 

Friday, 11 August 2017

NGC6822: Barnard's Galaxy

In 1884, E. E. Barnard pointed his modest 6" refractor to one of Sagittarius' less-fashionable corners, slightly below the Milky Way. There, he discovered a faint nebula which he soon identified as a galaxy. Later, Edwin Hubble determined that this odd, irregular cloud of stars belongs to our Local Group, like the Andromeda and Triangulum galaxies. Indeed, it lies merely 1,6 million light-years away, which is quite close in astronomical terms. To compare, the Andromeda Galaxy lies 2,5 million light-years from our solar-system. 

If you want to observe it, I'd suggest high aperture and low power because Barnard's Galaxy has a very low surface brightness, yet all of its light is smeared out over a large area. To make things worse, a lot of its light is being absorbed by interstellar dust. And to round it off, it travels quite low in the sky to northern observers and easily disappears in the glow above the horizon. Therefore it can be a serious challenge and even with my binoscope it wasn't easy to identify and discover the many structures and star forming regions within it. In fact, even though this galaxy only has a central bar without any significant spiral arms, it exhibits no less than 150 star forming clouds and some of those appear quite brightly against the faint background of the galaxy itself, as you can see on my sketch. Undoubtedly NGC6822 experiences a lot of gravitational influences from the other Local Group members, in the first place from our Milky Way and Andromeda. 

In every aspect Barnard's Galaxy resembles the Small Magellanic Cloud a lot, which decorates southern skies. They're both 7,000 light-years in size and have comparable masses, but obviously the SMC lies a lot closer to us, at a distance of 200,000 light-years. 

So you see that there's a lot more to our Local Group than M31 and M33. In total, 54 member galaxies have already been discovered! 

 

Tuesday, 8 August 2017

M8: The Hourglass in the Lagoon

In my previous post I showed you the Lagoon Nebula, one of our Galaxy's largest star forming regions. Today I'm going to zoom right into its core, towards a feature John Herschel called the Hourglass Nebula for obvious reasons (not to be mistaken with the small planetary nebula in the southern hemisphere which bears the same nickname). This extremely bright structure's lit up by the very hot star just next to it.

The Hubble Space Telescope discovered several globules in it, which are dark knots of gas and dust that are contracting and which will eventually light up and become new stars. Each of these knots is 10,000 times the distance Earth-Sun across and mark the boundaries of future solar systems. Obviously I wasn't able to observe these little knots with my humble telescope, but nevertheless it's very interesting to know that they're there and that all of this is happening right now as we speak. The star that's causing the radiant glow of the hourglass appears to be surrounded by about a hundred baby stars that have ignited merely a million years ago. All of these little stars are also still invisible to amateur telescopes but soon the baby stars will shed the clouds of dust that envelop them and form a bright star cluster similar to the one on the eastern side of the Lagoon Nebula.

Saturday, 5 August 2017

M8: The Blue Lagoon

No, this post isn't about alcohol or eighties erotic cult movies, although there is some eroticism in what I'm about to write. Today I'd like to take you to one of the biggest baby star factories in our galaxy: M8 or the Lagoon Nebula. This gigantic gas cloud extends over 100 by 50 light-years across and is probably as deep as it is large, making it at least fifteen to twenty times the size of the mighty Orion Nebula. The latter looks bigger and brighter from our point of view but you have to bear in mind that the Lagoon Nebula lies four times further away from us, at a distance of 5,000 light-years. Yet, it still covers an area as large as three times the full Moon in our sky. Unfortunately northern viewers are eating their heart out because the Lagoon resides in Sagittarius, very low in the sky. Even from my observing spot in Northern Italy I had to point my telescope into the horizon glow in order to make this drawing. I can only dream about how this nebula would splendour from more southern latitudes.

As I said, this nebula is an enormous baby factory and the bright star cluster that illuminates its left half has only just emerged from it. With "only just" I mean two to three million years, the time when our first ancestors emerged from the African plains. A lane of foreground dust seems to cut the nebula in half and on its right-hand side we find the so-called "Hourglass Nebula", the brightest and most active region, heated up by the small but very hot star just next to it. Several nodules have been discovered here; clumps of contracting gas that will soon light up and become stars. 

To put things in perspective, the bright star halfway between the "hourglass" and the dark lane is 23,000 times brighter than our Sun in visual wavelengths and maybe 200,000 times brighter if we add Ultraviolet radiation!

Thursday, 3 August 2017

Her Royal Majesty M22

Number 22 on Messier's list of astronomical objects is the third-brightest globular cluster in the sky and the brightest visible from northern latitudes. On a clear night it will already reveal itself to the naked eye in the heart of one of the most stunning stellar landscapes that the Universe grants us. It lies just south of the densest part of our Milky Way, with the Sagittarius Stellar Cloud and the immense Lagoon Nebula nearby. 

M22's a fairly large globular cluster with its half a million stars, but nothing out of the ordinary. The reason why it's so bright is because it's one of the closest globulars, its distance estimated to be only 10,400 light-years. Considering that the centre of our galaxy lies more than twice as far away, that's pretty close. In any case, M22 certainly merits its reputation as one of the finest globular clusters as I hope my sketch illustrates.  

Interesting to note is that recent investigations with the Hubble Space Telescope discovered a large number of planet-sized objects which appear to roam in this cluster without belonging to any particular star. It seems only logical that planets, which form at a certain distance from their parent star, get severely disturbed by the multitude of extremely close neighbour stars and as a result these planets are torn away, destined to float from one star to the other. 

Monday, 31 July 2017

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.

Thursday, 27 July 2017

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.


Wednesday, 26 July 2017

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. 

 

Friday, 21 July 2017

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"...

  

Wednesday, 19 July 2017

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!  

Tuesday, 11 July 2017

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...

 

Wednesday, 5 July 2017

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 M├ęchain 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.


Friday, 30 June 2017

M11: The Wild Duck Cluster

I've already written about this object here, but it definitely deserves a more detailed blog post, so here we go. The 11th object on Messier's list, also referred to as the "Wild Duck Cluster", is one of the richest star clusters in our Milky Way. Its rather odd nickname was invented by Admiral Smyth in the 19th century, who saw a sort of V-shape in it, just like a flock of wild ducks. It contains some 2,900 stars, 500 of which are brighter than mag. 14, and various dark lanes which seem to divide the cluster. In spite of its considerable distance (6,100 light-years), it is one of summer's grandest objects and can already be spotted quite easily with ordinary binoculars in the inconspicuous constellation of Scutum, the shield. It's so amazingly compact that at low power it might be mistaken for a globular cluster. Increasing telescope power will reveal its true nature as a cluster of newborn stars. Being some 250 million years old, this cluster's middle-aged but considering its size and compactness there's no doubt that it will continue to resist our galaxy's gravitational pull for millions of years to come. Yet, some of the most massive stars in it have already evolved into red giant phase, meaning that they've depleted their hydrogen and are now fusing helium into heavier elements. As I've already explained, the bigger a star, the faster it will burn its hydrogen and the shorter it will live. 

The brightest star, right at the cluster's centre however... isn't a member at all. It lies 1,300 light-years closer to us! So once again you see that astronomical observations can be deceiving. 

Wednesday, 28 June 2017

NGC6445: Is it a box, a beetle or a crawling monster?

The peculiar planetary nebula NGC6445 goes by a lot of nicknames and most of those are not original. During the research I did for this post I came across "Box Nebula" (which it shares with NGC6309), "Crescent Nebula" (like NGC6888), "Beetle Nebula" (similar to NGC6302 - "Bug Nebula") and even "Crawling Monster Nebula". Personally, I find the first one the most convincing because rarely does nature create such angular-shaped objects.

The reason why its main ring of ionised gas and dust particles has taken on this broken rectangular form is still unclear. Planetary nebulae form under the violent radiation pressure from their dying central star and are therefore usually spherical or cylinder-shaped. Scientific research suggest that the sometimes odd shapes of older planetary nebulae are caused by the collision with slower particles that were already ejected during the star's red giant phase, long before it collapsed. The gas shell that was blown into space upon the star's death catches up with the formerly ejected shell and ploughs through it, creating a bright ring under the powerful stellar winds from the hot white dwarf. 

Our Box Nebula is, however, one of the largest and therefore one of the oldest planetaries and its central white dwarf's become extremely faint; beyond the reach of any amateur telescope. The combination of its enormous size and the cooling down of its central star have significantly reduced the impact of the stellar winds on the nebula and the ploughing has stopped. Imagine a small boat that's sailing very fast. As it's ploughing through the lake at high speed, it generates strong, geometrical ripples in the water. Now imagine that the boat suddenly stops and look at the ripples. All of a sudden they'll break up and disperse themselves randomly in all directions. Long-exposure photographs reveal a vast web of external structures and filaments, only partially visible through my telescope, which extend up to two light-years into space.

We can conclude that this nebula has entered the last phase of its life. As it slowly expands further and as the central star keeps cooling down, also the nebula will cool down and eventually dissolve completely.

 


 

Monday, 26 June 2017

M27: Her Royal Majesty the Dumbbell Nebula

M27 is the brightest of the so-called planetary nebulae in the sky. The classification "planetary" was derived from the fact that these nebulae often appear like little disks, much like a planet. In reality they're huge shells of gas that were expelled by a dying star.

Right at first glance it's obvious where this particular nebula got its "dumbbell" nickname from, isn't it? Actually, we see this nebula from its equatorial plane. When viewed from its poles it would probably appear ring-shaped, much like the Ring Nebula in Lyra. As I've explained before, it's best to imagine these older, more developed planetary nebulae as an apple without a core. In this case the bright, inner gas shells that were expelled during the collapse of the central star, have caught up with the thinner external shell that already formed before the star exhaled its dying breath. Together they're expanding at 31km/s until they'll dissolve into space. The age estimates vary greatly, between 4,000 and 15,000 years, but the most recent spectroscopic analysis suggests an age of somewhere in between: 9,800 years. There's also been a lot of controversy regarding the Dumbbell’s distance but 1,300 light-years seems to be the most recent consensus. From this distance, the nebula’s a full light-year across.

In order to find the reason why this nebula became so big and bright, we have to examine its central star which is already visible in small telescopes. This star used to be a giant containing as much as ten solar masses. It was big, but not quite big enough to explode as a supernova. Even now, the dying stellar core has a diameter of 70,000km and it still contains 60% of our Sun’s mass, making it the biggest white dwarf known. With an extremely hot surface temperature of 85,000°C, it heats up the vast gas clouds so much that they start to emit a bluish-green light.

Today, the Dumbbell’s one of the brightest and most popular objects in the northern skies, extending almost half of the diameter of a full Moon and easily visible through a pair of binoculars. It’s a privilege to be able to observe it with an 18” binoscope under a reasonably dark sky.

Thursday, 22 June 2017

Me 1-1: A difficult name doesn't necessarily mean a difficult object



When I was a kid, I had this big poster of the stars above my bed. It was my perennial source of inspiration and I was constantly scanning it in search for new objects. Being the proud owner of a modest 60mm refractor, which nevertheless was considered a medium-type telescope at the time, I knew that I didn’t have to put the more daring objects on my observing list. So I stuck to the classic Messiers and I felt proud to have seen a great many of them. But on the chart there were also other objects, which had the scary “NGC” denominator, followed by a four-digit number. At first I sort of neglected them in the knowledge that they would turn out to be invisible in my telescope. Also my friends at the Antwerp observatory talked with great apprehension about the NGCs and so I never even bothered to try them, apart from those which I didn’t know were NGCs such as the Double Cluster in Perseus, which is already visible to the naked eye. 

And still... there was one that fascinated me. Right at the heart of my stellar map, near the celestial North Pole, there was this tiny little nebula denominated NGC6543. Was it the number that sounded like a countdown? I don’t know what it was exactly that eventually pushed me to give it a try, regardless of its so-called impossibility status. Surprisingly I didn’t have to look very long in order to find it. Yes, it was small, very small in my humble telescope, but it was unmistakingly there and quite bright too! The next Saturday I couldn’t wait to go back to the observatory to tell all of my astronomy-friends about my observation and for many years to come I felt so proud of this achievement. It was the start of my unstoppable passion for the real faint fuzzies, the ones which others deem impossible, the weird ones, the never-heard-of ones. Many of those reveal themselves to be extremely difficult indeed. Others, like exotic NGC6543 in my 60mm scope, turn out to be remarkably easy. 

Take this little bugger for example. It carries the name Merrill 1-1 (or Me 1-1 in short) after its discoverer. I have to admit that I only came across this one by accident and I suppose that only freaks will know about its existence. But when I pointed my binoscope at it, it appeared surprisingly bright. Yes, it’s tiny and I advise you to use an OIII filter when trying to locate it because in small telescopes or at low power it will look almost stellar. But once you’ve found it, try to push magnifications as much as you can and perhaps this little planetary nebula will reveal as much to you as it did to me. At first glance it’s just a tiny patch and not very interesting as such. But the reason why it’s so small is because it lies incredibly far away from us: 19,000 light-years if you please! Imagine that! This almost certainly means that at the time of writing this planetary nebula won’t be there anymore. It will have completely dissolved into space! It’s just that the light from it being blown up and scattered hasn’t reached us yet. 

Another puzzling thing to which scientists have only recently found the answer is its central star (which was completely invisible to me by the way). This central star’s quite cold with a surface temperature of only 4,300°C (against e.g. 5,500°C for our Sun) and therefore it remained a mystery how such a cold star could heat up the nebula enough for it to emit light. The central star’s also displaced a great distance from the nebula’s centre, by over 10 times the distance to Pluto. A spectroscopic analysis in 2008 revealed, however, that this star isn’t the cause but that a small companion star is. It is a double star, very much like Ras Algethi, the smaller member of which died and released its atmosphere into space.

From our point of view, we see this nebula edge-on and it’s supposed to have a ring-like shape when seen top-down. All that I could see was an elongated sort of box, but what struck me in particular were two cavities on opposite sides. I don’t know if I’ve observed this correctly since it was so small, so any confirmation would be greatly appreciated.    

 

Wednesday, 21 June 2017

Aquila's double cluster (part 2)


About half a degree to the west of NGC6756 lies this open cluster, called NGC6755. Both are definitely one of these very rare "real" double clusters, i.e. which share the same origin. They lie more or less at the same distance (5,000 lightyears) and are just as young (only 8 million years old). Intrinsically they look very similar i.e. most of the stars in both clusters are more or less of the same brightness. But that's where the likeness ends because NGC6755's a lot bigger and richer (80 confirmed members against 40 or so for NGC6756). Then again, NGC6756's a lot more compact and appears more like a faint globular in small to medium telescopes or at low power. For its bigger companion I obviously used lower power in order to fit all of it nicely in the field of view.

Another thing that struck me during my observation, was that NGC6755 appeared somewhat split in two. After doing some research there does seem to be a dark dustlane blocking the light of some of the stars, cutting across the field from top-right to bottom-left. 

Again I must warn you that appearances may be quite deceptive. Stars that seem to belong together from our point of view may be hundreds or even thousands of lightyears apart. The orangy star on the right, for example, is very old and most certainly isn't part of the cluster. The same goes for the somewhat yellowish star below the centre of the field. Unfortunately I wasn't able to find any distance data for these two stars, but I can confirm that the bright star near the top-left edge is a giant that lies a thousand lightyears further away from us. 

 

Tuesday, 20 June 2017

Aquila's double cluster (part 1)

Stars are born together in huge clouds of gas, sometimes by the hundreds such as in the Orion nebula. These young and hot stars burn fast and live a wild life, like adolescents, emitting violent radiation which not only illuminates the gas cloud from which they were born, but makes it emit light as well. Eventually the compound radiation from all these young stars will blow the nebula to pieces and the gas disperses into space, leaving only a cluster of stars. Scientists estimate that only 10% of the original gas cloud condenses into stars and that the rest blows away into space, possibly to coagulate into a nebula again when there's enough matter around to let gravity do its work. 

This little gem, denominated NGC6756, is a cluster of young stars in the constellation of Aquila, the eagle, not more than 8 million years old. It lies 5,000 lightyears away and so it isn't the brightest of objects. Actually, the brighter foreground stars, the brightest of which is of magnitude 11,9,  don't belong to the cluster at all but are dwelling much closer to us. The stars of this cluster are therefore quite faint, from mag. 14 down to mag. 18, and are difficult to resolve in smaller telescopes, also because the cluster's still fairly compact. Yet, this makes this particular cluster such an interesting object because even if it doesn't look stellar in a small instrument, it will appear as an attractive nebulous patch. With the binoscope however I had no trouble at all identifying all 40 or so members.

The most interesting thing about this little cluster is that it's one half of a rare double cluster. Although we've identified well over 1,100 of these star clusters in our Milky Way, only a handful of them are double, meaning that two separate clusters share the same origin. The most famous of those if of course the Double Cluster in Perseus, but also NGC1807-1817 in Taurus are two clusters born from the same cloud. As with most clusters, the stars of NGC6756 will eventually drift apart under the pull of our galaxy and most of them will lead solitary lives.

 

Wednesday, 14 June 2017

Very old, but still alive and kicking

Globular clusters are among the oldest entities in the universe. These strange and extremely dense balls of stars are generally older than the galaxies they accompany and some have emerged only just after the Big Bang. With "just" I intend hundreds of millions of years of course. At first sight they all seem alike, but when you take a closer look you'll notice that they're surprisingly different from one another. Take extremely compact M15, for example, and compare it to majestically large and irregular M5. There's NGC2419, so far away that it only marginally feels the gravitational pull of our Milky Way and NGC5466 that's literally torn to bits by it.  There are 150 to 160 of these globulars known to orbit our Milky Way but other galaxies such as giant M87 dominate over 12,000 of them!

And then there are the survivors, the oldest of them all. NGC6426 is, with a distance of almost 70,000 lightyears, quite far away from us. It's so far that you need a sizeable telescope and a good-quality sky in order to resolve some stars in it. Yet, it's a very interesting globular cluster because it's one of the oldest there is. We can tell because this cluster has an unusually low metal content. With "metal" astronomers mean everything heavier than hydrogen or helium. When the universe came to be 13.7 billion years ago, the first chemical element that arose was hydrogen of course, quickly followed by helium which was created through the fusion of hydrogen in the very first stars. But for anything heavier, we had to wait until these first stars ran out of hydrogen and started fusing helium into more complex elements such as carbon, nitrogen and oxygen. This took hundreds of millions to even billions of years. The stars in globular clusters, on the other hand, are extremely slow burners and although very old most of them are still in their hydrogen-fusing phase. The fact that the stars of NGC6426 contain such an extremely low level of "metals" indicates that this cluster was formed very early in the evolution of our Universe.  


Tuesday, 13 June 2017

Two old Suns

Next time you're observing the Turtle nebula in Hercules' belly, point your telescope slightly to the south and you'll bump into this lovely pair. STF2094 is a couple of greenish-yellow giant stars over 500 lightyears away. The term "giant" seems somewhat overrated at first sight since the biggest of the two's just 3,8 times the diameter of our Sun and contains only 1,3 solar masses. The giant classification in this case doesn't concern the star's mass or diameter, but rather its luminosity and the phase of stellar evolution. Originally these stars were not much unlike our Sun and they used to be much dimmer too. Then, suddenly, they ran out of hydrogen and it's exactly the change into a helium-fusing red giant that we're witnessing here, something that will also happen to our Sun in over 5 billion years. The transition into a cool, red giant isn't instantaneous and initially the star in question will not only brighten but also heat up slightly. The surface temperature of the two stars on my sketch measures almost 1.000°C hotter than our Sun. A famous example of a star in this phase of evolution is Capella, the brightest star of the winter constellation of Auriga and the sixth brightest star in our sky. When the star expands further, its surface will gradually cool down and the star becomes an old, red giant. 

The couple on my sketch is therefore quite interesting because they used to be so similar to our Sun and because they seem so close to one another. Well, you have to interpret the word "close" in astronomical terms because in reality these two stars are 171 times the distance between the Earth and the Sun apart, or almost six times the distance to Pluto! And yet, even from this distance, these stars would appear hundreds of times brighter than a full Moon to each other!

This double star represents a bit of a challenge for small to medium telescopes and you need a good-quality sky in order to separate them. Then again, that's precisely what double star fanatics are looking for or course. 

But... wait a minute! Before you go, focus on the upper-right corner of the field of view. Isn't there... something... ? Yes! The faint patch you may notice is a spiral galaxy (catalogued as UGC10525), quite similar to our Milky Way, but very distant. It lies 430 million lightyears away...

 

Monday, 12 June 2017

The Phantom Streak

In a recent post, I showed you an image of a dying star. Fusion of helium into heavier elements such as oxygen and carbon became critically unstable and the star collapsed under its own gravity, blowing away its entire atmosphere in the process. The expelled gas cloud's not become a so-called planetary nebula yet i this case because it isn't emitting light on its own. For this we have to fast-forward one or two thousand years until the remaining, super-hot nucleus of the dead star heats up the expanding gas bubble so much that it ionises. This is exactly what I'd like to show you here. NGC6741, aka the "Phantom Streak" nebula in Aquila, is an extremely young planetary, just marginally older than the Footprint of my former post. The difference is that the Phantom Streak has begun to emit light on its own and therefore it truly deserves to be categorised as a planetary nebula, whereas the Footprint isn't quite there yet. Being so young the Phantom Streak's incredibly tiny and needs a lot of telescope power to be recognisable as a nebula and not a star. On my sketch you can clearly see the tiny gas envelope with its rectangular shape. As I've explained before, stars are less dense at the poles, allowing the gas to escape more easily there. For this reason planetary nebulae often take on an elongated or even cylindrical shape such as the famous Ring nebula. If you look at the Saturn nebula, you'll also note the highly elongated internal structure. 

So how did planetary nebulae come by their "planetary" nickname you might ask? Yes, they got it because they usually do display a round, sort of planetary shape. This outer sphere, however, was already expelled earlier, thousands of years before the star exhaled its dying breath. Stars close to the end of their life swell significantly and become unstable. They grow under the pressure of the complex fusion process in their core until they reach the point that they've become too big and cool down. Gravity takes over and the star contracts again, heating up the star's surface until fusion shifts to a higher gear, and so on. In this unstable period the star already loses a large part of its atmosphere. Sometimes a very big star may lose so much of it that what remains contracts up to the point that fusion becomes stable again and the star's surface may even become extremely hot. The Thor's Helmet nebula is such an example, a giant star that's got rid of a large part of its atmosphere that's being hurled into space under the fierce radiation of the remaining very hot star. Eventually, in the case of a normal planetary nebula, the gas bubble that's expelled by the sudden death of the central star will catch up with the external gas shell and they'll dissolve into space together. 

As regards to our Phantom Streak, the external shell isn't visible to amateur telescopes for the time being since it hasn't been heated up sufficiently. But in another one to two thousand years it will look very much like the Saturn nebula or similar planetaries.


Friday, 9 June 2017

Going their own way

Stars are usually born together out of giant hydrogen clouds like the Orion Nebula. The gas cloud condenses under its own gravity and spawns dozens to even hundreds of stars until it's spent and the remaining gas dissolves into space. What's left is a cluster of stars that during their childhood remain together, bound by their mutual gravity. But after a while, the much stronger gravitational force of our galaxy will break their bond and disperse the stars, like going through a blob of paint with a thick brush. The stars will then each go their own way, just like children that've grown up will leave their parents' home to build a life of their own. A very fine example of this is probably the most famous constellation of them all: Ursa Major, the "great bear" or "big dipper". All of the stars in the "big dipper", apart from two (Dubhe and Alkaid), share the same origin and they were born together some 500 million years ago, together with a few dozen others among which Alpha Coronae Borealis (Gemma or Alphecca), Beta Aurigae (Menkalinan) and Delta Aquarii (Scheat).

What I'm showing you on this sketch is a star cluster containing about 30 members, NGC6633, that's breaking up. The young, hot stars are leaving the nest and will start their individual journeys through space very soon. They've already moved so far away from each other that with my big telescope I was almost looking through them. Definitely this lovely cluster's best enjoyed with smaller instruments at low power or even with a pair of binoculars... for as long as we still can. Within another 100-200 million years the stars will be scattered and the cluster will be no more.

Monday, 5 June 2017

Antennae

Spring is nearing its end and so we're saying goodbye to the galaxy season. In order to go out with a bang, I present you one of Spring's classics: the famous "Antennae" galaxies in the obscure constellation of Corvus, the crow. Frustration will be the share of observers in the more northern latitudes because this object appears not very high above the horizon to them. Even from my home in Northern Italy I had to point my binoscope fairly low, into the hazy glow above the mountains. Yet, I've never seen this pair of colliding galaxies like this before. Simply amazing! 

NGC4038 and NGC4039 are indeed two spiral galaxies that are crashing into each other right as we speak! Or make that 63 million year ago because that's how long it takes for their light to reach us. A billion years ago, they were still two separate galaxies, each minding its own business. But alas, gravity condemned them to a dramatic fate. 600 million years ago, they started to collide and NGC4039 (the somewhat smaller, upper half on my sketch) literally passed through its counterpart whereby both galaxies were severely disrupted. Both galaxies released long tails of stars in the process, which were invisible to me but which show well on long-exposure photographs - hence the nickname "Antennae". These star trails extend some 360,000 lightyears into space! 

Within another 400 million years the Antennae's nuclei will collide into a single, supermassive black hole. Simulations suggest that the galaxies will eventually melt into a big, elliptical galaxy, much like M87. For the time being, the collision's triggering an incredible burst of activity and star formation, as you can guess from the many complex and bright internal structures.

This is also the fate that awaits our own Milky Way in 5 billion years, when we'll crash into the Andromeda Galaxy. But don't worry, this doesn't mean the end. Even though galaxies contain hundreds of billions of stars, they're mostly void and it's not likely that the stars themselves crash into each other. Think of the enormous distance between our Solar System and the nearest stars! No, the Earth will die around approximately the same time - supposing that we, stupid human beings, haven't cocked up our planet before - but not because of the collision with Andromeda. In 5,4 billion years from now, our Sun will run out of hydrogen and will enter its red giant phase, fusing helium into heavier elements. This will cause our Sun to grow considerably; so much that Mercury and Venus will be swallowed up and the Sun's outer atmosphere might even reach the Earth. So we will get scorched or our planet may even evaporate altogether. Suddenly a collision with Andromeda seems somewhat less worrying, doesn't it?