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.