The floating swan

Some astronomical objects are of such rare beauty that I, with my limited writing skills, can't find the right words to describe them. Therefore it's probably best that I let my sketch do the talking for me. M17, aka the Swan Nebula (or also Omega Nebula, although I still don't know why), is one of the brightest star forming regions in our sky and under very good conditions already visible to the naked eye as a brighter knot in the Milky Way. Its structure's also quite similar to that of the famous Orion Nebula, with the difference that we see the Swan edge-on rather than face-on. Buried inside this nebula lies a very young cluster of newborn stars, believed to be only 1 million years old, containing some 800 members. The radiation from these extremely hot baby stars causes the gas cloud, some 15 lightyears in diameter, to glow. The cloud of interstellar matter of which the Swan is but a part, however, is at least 40 lightyears in diameter and has a total mass of 30.000 Suns! Another 1.000 stars are being formed in these outer regions, which in turn are beginning to emit light as well. So we expect that this nebula will still significantly gain in visible size and brightness over the next millennia. Currently, the main nebula does look a bit like a swan, doesn't it? It appears to be floating on a lake of ethereal nebulosity, with its bright "eye" gazing at us. This particular star's often used as a reference by scientists to measure radiation, the distribution of hot gas and the expansion velocity of the nebula. The distance of this nebula complex's estimated to be between 5.000 and 6.000 lightyears.

I can still remember very well the first time that I've looked at this object! It was the end of August 1986 and my family and I were on holiday in the south of France. Obviously I'd brought my telescope with me, which was my loyal 68mm Vixen refractor. One evening I had finally convinced my parents to go for a drive away from the beach and somewhat up in the mountains, so that my brother and I could have a look at the incredibly dark sky that you could still find there. It may have been a coincidence, but one of the first things that I noticed, was this strange little knot in the Milky Way, just above Sagittarius. I immediately pointed my scope at it and there it was. I didn't really see a swan in it back then, more a sort of bright, elongated patch with a kind of a hook at one end. I also couldn't see any of the surrounding nebulosity of course, with my limited instrument and 20mm Kellner eyepiece. But the sketch I made that evening may perhaps still be lying around somewhere at the observatory of Urania, near Antwerp, Belgium. It was the most beautiful sketch I had ever made during my youth and I'll never forget it. It's definitly one of my favourite objects and nearly every summer night when I'm out under the stars I simply must pay it a visit. I hope that you will do so too.

A slightly blue snowball

At first sight, all planetary nebulae look more or less the same. They look like fuzzy little disks and nothing much else. But appearances may deceive as I shall explain in this post. I can't stress enough how important it is to observe properly. When you get the chance to look through a telescope, please, relax and take your time. Don't feel pressured; other people will wait. Let your eye (or eyes in the case of a binoscope) adjust to the image. It can take minutes before the really interesting details appear and it's exactly the detail that makes every planetary nebula unique.

Let's talk about NGC7662, for example, or in human language: the Blue Snowball. It's one of the brightest and most easily visible planetaries on the northern hemisphere and you can already spot it with a small telescope. The Snowball has the typical three-layer structure of a fairly young nebula, in scientific terms a phase II. In the first phase, the dying star ejects its atmosphere but the remaining central star's still too cool to ionise the gas bubble and hence make it glow. Phase II is the so-called compression phase. Gravity compresses the star so much that its temperature rises to 100 million °C! The gas bubble's heated up to 10.000°C and even 25.000°C nearer to its centre. Extreme stellar winds blow up the bubble, creating a cavity in the nebula's centre and different layers of gas around it. Imagine that you're ploughing snow with a shovel, pushing it in front of you. You'll notice that the snow will also form different "waves", the largest of which against the shovel, a smaller one in front of the first and an even smaller one in front of the second. This is exactly what we're seeing here: A very bright and thick internal bubble, a less dense outer bubble and a very faint halo around it. In the third phase, the central star reaches its maximum temperature and the density contrast between the inner and outer shells is the highest. Eventually we reach phase IV. The central star begins to cool down and the nebula's violent expansion process slows down. The inner shell catches up with the outer and the clear structures that you can observe in a phase II or III nebula fade. Eventually the nebula will dissolve into space and the central star will extinguish.

But there's more. The winds generated by the incredibly hot central star are certainly not uniform and at times sudden bursts may appear. This is exactly what we observe in the Blue Snowball, where the inner shell is ruptured at opposite sides by such a burst of high-energy particles. 

Finally, these nebulae feed the universe with heavier and complex elements which were formed in the star before it died. The Blue Snowball, for example, contains a large amount of iron in its outer shell. Perhaps one day these elements will form the planets around a newborn star?

The tadpole galaxy

Often you could accuse astronomers of being overly fanciful when they invent nicknames for the things they've discovered or the constellations they've assigned. You might even believe that the astronomy society's a less successful branch of alcoholics anonymous. :-) But at times you don't need a lot of fantasy to understand why a certain object was given a certain name. Such is the case with the distant galaxy that's the protagonist of this sketch.

I've taken my new binoscope for a five nights' holiday in the Dolomites, at an elevation of 2000 metres and under a sky of a rare darkness. The goal: hunt the faintest fuzzies! The "tadpole galaxy"(or scientifically UGC10214 or Arp188) is quite faint indeed, being of magnitude 14,4, but it's one of the most spectacular of them all. Most galaxies are round or elliptical but this one has an unusual straight tail that's 280.000 lightyears long. Scientists believe that a smaller and more compact galaxy's come too close and that their mutual gravitational forces slung it around the "tadpole". Gas, dust and millions of stars were torn out of the larger one and formed the striking tail. Over time, the "tadpole" will lose its tail, which will probably contract into dwarf galaxies that accompany their mother. The intruder's not visible on the sketch because it now lies somewhere behind the "tadpole", but it can be seen on high-resolution photos of the Hubble space telescope. My binoscope's quite powerful but certainly not that powerful. Bear in mind that this particular galaxy lies at a distance of a whopping 460 million lightyears and therefore it must be the most distant object that I've observed thusfar. 

For those with a keen eye... there's a second galaxy on my sketch, smaller and even fainter. Its scientific denominator's PGC57108, it's of magnitude 15,5 and lies at approximately the same distance. I dare you to find it! :-) 

The eagle's glowing eye

There are so many planetary nebulae out there that sketching all of them would be close to impossible. As you know, they're dying stars that've just shed their atmosphere into space. When I use the term "just", we're talking about only several thousands of years ago, which is a nanosecond compared to the age of the universe. Their incredibly hot core heats up the resulting gas cloud up to the point that it starts to emit light on its own. Slowly this cloud of gas will expand and dissolve into space whereas the core, the remaining white dwarf star, will cool down and eventually extinguish. 

Planetary nebulae are called this way because they truly look a bit like a planet, with their generally round shape. But if you zoom into them, they'll reveal a surprising amount of detail. Gas filaments, structures and different layers give every single planetary nebula a character of its own and make every new one that you observe also a new experience. Yesterday I showed you the "Blue Flash" nebula. Not that far from it you can find this little fellow: the "Glowing Eye" in the tail of the constellation of Aquila, the eagle. With its magnitude of 11,9 it's within reach of most telescopes but due to its tiny size it can be quite tricky to find. For my sketch I used a magnification of 507x, which brought out quite some detail. I could easily see the brighter rim and some filaments of the inner sphere, which does look a bit like an iris. Its central star was also a lot more prominent than the one of NGC6905. 

The distance of this little gem is estimated at 6.500 lightyears...   

The Blue Flash

NGC6905 is a wonderful planetary nebula in the small but remarkable constellation of Delphinus. Loyal readers of my blog will already have guessed that this nebula is what's left of a dying normal-sized star. The star's nucleus still survives and has turned into an extremely hot white dwarf star with a surface temperature around 150.000°C. In comparison, the temperature on our Sun's surface is only 5.500°C. A white dwarf's a very peculiar kind of star because its size is comparable to that of the Earth whereas its mass is not much different to our Sun's. Needless to say that it's extremely dense and "heavy". Unlike a normal star, no nuclear fusion takes place in it anymore; it's light and energy emission being solely the result of the remaining heat of what was once an active nuclear fusion reactor. With time, this star will therefore slowly cool down and fade until all that's left is a ball of mainly carbon. This cooling process takes a lot of time, many billions of years, and therefore there aren't such carbon balls or "black dwarves" around yet because the universe simply isn't old enough. 

This particular white dwarf was clearly visible in my binoscope. But perhaps more interesting for visual astronomers, the star's atmosphere was expelled into space and now forms a rapidly expanding bubble of gas filaments around the white dwarf. Also these filaments were more than evident in the binoscope at a magnification of 285x. Actually, I was amazed at the amount of detail that I was able to make out. This planetary also has two extremely faint "wings" just above and below the main nebula on this sketch. These were difficult to see and I've tried to represent just that. Some people therefore call it a mini-Dumbbell nebula because it does look a bit like a smaller and fainter version of the famous nebula in Vulpecula. 

The "Blue Flash" however lies a lot further away from us: 7.500 lightyears compared to 1.300 for the Dumbbell. But I hope that my sketch will encourage you to visit this fainter and more distant planetary as well because it really deserves it.   

A veil in summer sky

I've already talked about supernovae before, cataclisms that mark the death of a giant star. Nuclear fusion becomes unstable... the star collapses under its own gravity which in turn causes the violent expulsion of the entire star's atmosphere in a matter of seconds. The acute energy release may be as high as 10⁴⁴ Joules or the entire energy output of the Sun during its whole 10-billion year life! The expelled matter may reach velocities up to 30.000km/s or one tenth of the speed of light! 

But as dramatic and spectacular as they appear, the remains of the star fade quickly and after a couple of months all that's left is an incredibly dense core that consist of neutrons. Although perhaps only 10km in diameter,the neutron star's density is 10¹⁵ higher than that of normal matter and hence it's incredibly heavy. In some cases it may be heavy enough to continue to collapse under its own gravity until it has become a point. At which stage it becomes a black hole: an object with such a high mass that you'd need to travel faster than light in order to escape from it. That's why we can't observe anything within them because nothing, not even light, travels fast enough to escape. 

But supernovae are not just the end. The blast is so strong that heavier elements such as metals are formed and expelled into the universe. So in a sense a supernova feeds the universe with a lot of complex elements which one day may be needed for the creation of planets and... life. And not all's destroyed instantly. Almost 6.000 years ago a vehement supernova lit up 1.400 lightyears from us in the constellation of Cygnus. As far as I'm aware no observation reports from that day exist so we can only speculate how our ancient ancestors stared at the sky in awe when an insignificant star suddenly became brighter than the full Moon. Now, thousands of years later, the remains of that explosion are still visible in a small to medium telescope: the Veil nebula. What I've sketched here is just a part of the eastern region. The total Veil nebula complex is 110 lightyears in diameter, or in our sky this equals 6 full moons, and continues to expand at a breathtaking rate. Gas filaments that mainly consist of oxygen are heated up and ionised by the blastwave of the supernova explosion and start to emit light themselves. Exactly these frail filaments are what makes this nebula so jaw-droppingly lovely to look at and in a big instrument such as my binoscope the spectacle surpasses even the wildest imagination. I just had to share this with you and I sincerely hope that you enjoy it.

How to make astronomical sketches - Part 2

Earlier than planned, I'm releasing the second part of my video series about astronomical sketching techniques. In this episode I'm focusing on the sketching of the background stars, something which is often overlooked but which highly contributes to the overall result. 

Sketching stars is not just putting dots on paper. In order to make the drawing as accurate as possible you need to master an easy technique which I demonstrate in the video. 

I sincerely hope that you enjoy it!