Thursday 29 September 2016

Sparkling glitterball

No, this blog post isn't about a wild seventies' disco night. It's about globular clusters.

On public star parties, there's usually a little professor around: a cute little boy or girl that already knows everything about the planets and stars and who'd been moaning all day until his (or her) parents finally gave in and took him on a 50-mile drive to an incredibly remote spot in the mountains in total darkness in order to meet us astronomy goons. Obviously he can't wait to cast an eye into my telescope and so for starters I point it at a globular cluster. Then, before I guide him up the ladder, I assign him with a very important scientific mission: "Please, this is incredibily important! You have to count exactly all the stars that you see!"

Now have a look at my sketch and smile. There are about a hundred thousand of them :-)

Globular clusters are more or less spherical objects that contain many thousands of stars and which accompany our Milky Way, or other galaxies as well. These clusters are among the most spectacular objects in the sky, especially when observed through a medium to large telescope which allows you to distinguish all of the individual stars right into the cluster's core. This particular one, M15, is 175 lightyears across and lies more than 33.000 lightyears away from us. It's also one of the most compact globulars that we know as it has undergone a "core collapse". Scientists believe that there may be a black hole in it which caused this sudden contraction. Another surprising fact about globulars is that they're not young objects at all. Originally, scientists believed that they were made up of stars which had been ejected from our galaxy. But in reality they were formed at about the same time as our Milky Way, some 12 billion years ago, very similarly to the the formation of planets around newborn stars. However, star formation within the globular clusters began much sooner than within the galaxy itself and so they contain some of the oldest stars in the universe. Given that the universe itself is estimated to be some 13,8 billion years old, these globulars and their stars have been around for most of that time. In comparison, our Sun and solar system are "only" 4,5 billion years old. 

Globular clusters are also incredibly dense, as you can see on the sketch. Whereas the nearest star to our solar system lies 4,2 lightyears away, the stars in the globular's core are at least a thousand times closer to one another. If you'd like to know what it would be like to live in a globular cluster, imagine that the whole sky's filled with stars that are 10 to 20 times brighter than the full moon! Such an environment would be most unfavourable for planets because of the constant gravitational interaction with nearby stars and therefore it's highly unlikely that we'd find life there. But they remain unparallelled in beauty when observed from good old Earth.

How to make astronomical sketches - Part 04

Here's the fourth episode of my video series about astronomical sketches: preparing the sketch on the PC.


Tuesday 20 September 2016

Saturn's mirror image

Like I already explained in other posts, every planetary nebula has its own little character and that's what makes them so fascinating to observe. The Saturn Nebula, or NGC7009 in scientific language, got its nickname from its obvious resemblance to the ringend planet. But there's much more. It's a fairly young nebula, estimated to be no more than 6.000 years old, and it's in full expansion phase (phase III - see my explanation here). The inner shell's incredibly complex and shows structures in all three dimensions, which I was able to distinguish quite well. Stellar winds are extremely high, even up to the point that we see "handles" (ansae) appearing at the poles, where the gas manages to escape more easily. Gas and dust build up near the dying star's equator, making it more difficult for the gas to flow out from there. This is why many planetary nebulae have an elongated or even hourglass appearance and why the famous Ring Nebula's in reality cylindrical. The white dwarf at its centre, the dying remnant of what once a medium-sized star and now not larger than a planet, is currently emitting 20 times more light than our Sun.

Planetary nebulae are generally small but very bright objects, making them stunning to look at, even with small telescopes and under light-polluted skies. So what are you waiting for to discover this one? It's currently at its highest position in the sky and eagerly awaiting for your visit.

Sunday 18 September 2016

The ring is fake... and a little surprise...

The most famous of all planetary nebula is probably the object that Charles Messier catalogued as number 57 back in 1779, more commonly known as the Ring Nebula. A small telescope will already reveal the doughnut shape consisting of ionised gas that's being expelled by what once was a star slightly bigger than our Sun. Some 7.000 years ago this star ran out of fuel and began to die slowly, ejecting its atmosphere into space whereas the star's hot nucleus will cool down and extinguish over the next billions of years. It was actually the first time that I managed to see this particular central star with my own eyes. Being of the 15th magnitude, the star isn't that impossible to see through a telescope as such, but the still fairly bright nebulosity in the "hole" of the doughnut tends to hide it. The ring itself's currently reached a diameter of almost 1 lightyear and is expanding at a rate of 20 to 30 km/s, some 70.000 to 100.000 km/h! Comparing old photographs to recent ones, the difference in size's quite noticeable! The nebula's also headed straight towards us, but being at a distance of 2.000 lightyears it would need more than 21 million years to reach us, by which time it will have dissolved completely.

The most recent observations with the Hubble space telescope revealed an even bigger surprise. The Ring Nebula's not a doughnut at all, but it's a cylinder! Astronomers didn't notice it at first because we see the cylinder end-on, as you'd look through the hole of a roll of toilet paper. The Hubble's high resolution snapshots, however, clearly showed clouds of dust flowing out of the central star which are silhouetted against the outer portions of the ring. Such barrel or hourglass shapes are not uncommon at all among planetary nebulae. Thick layers of gas and dust around the waist of the star often slow down the expansion in that direction, leaving the gas free to flow out from the poles. 

But there's more to see in my sketch apart from this spectacular nebula and I intentionally didn't place it at the centre. On the right and slightly up you may see a small, fuzzy patch, even with a sort of irregular shape. This is a galaxy, a bit smaller than our Milky Way, which lies at the incredible distance of 230 million lightyears! It's of the barred spiral type, which means that the spiral arms do not originate from the core itself, but that there's a bar-like structure that goes through the core. The spiral arms originate from the outer edges of that bar. Obviously this is all but impossible to see on my sketch. This little galaxy's so remote that you can't even see its nucleus unless you've got a sizeable telescope and an almost perfect sky. So you can imagine how happy I was that I could still make out some structure in it.   


Friday 16 September 2016

How to make astronomical sketches - Part 3

Here's the third episode of my video series about astronomical sketching, in which I explain how to sketch the object itself. I hope you that enjoy it!

Tuesday 13 September 2016

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.

Saturday 3 September 2016

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?