Friday, 24 February 2017

Near but so easily overlooked

Some astronomical objects receive far less attention than they rightfully deserve. There may be many reasons for this, as for example that the object in question's too challenging. But that's a real pity because sometimes even the more difficult objects can turn out to be extremely rewarding. 

NGC4236 is a galaxy that resides in the tail of Draco, the dragon. It's a member of the M81-M82 group and therefore it lies only 11,7 million lightyears away from us. In astronomical terms, that's our backyard. Actually, NGC4236's only slightly smaller than our Milky Way and therefore it's by far the largest group member, significantly bigger than it's two famous neighbours. We also see this galaxy under a shallow angle, meaning that its light isn't dispersed over such a large area as is the case with galaxies we see face-on, like M33 for example. In short, NGC4236 has all that it takes to become one of the most popular astronomical objects. And yet, it isn't as it's surprisingly dim and contrary to smaller M81 and M82 it's unsuitable for small telescopes. The reason for this is unknown to me... I suspect that much of its light must be absorbed by a cloud of interstellar dust. Strangely enough NGC4236's located well above our galactic plane so its light doesn't have to plough through a lot of Milky Way matter, but it's the only explanation I can think of. All I can say is that it's a real pity because NGC4236 has so much to offer if you take the time for it. 

As I expected, this galaxy didn't "leap out" of the eyepiece at all when I turned my binoscope at it. The central bar was easily visible, but not more than that. Quite disappointing, but I didn't give up so easily. I tried to focus, used a bit of averted vision and let my eyes adjust fully to the telescope view. Little by little... I started to see more. The central bar wasn't homogeneous at all, but showed some interesting brighter knots. Eventually also most of the faint spiral structures slowly revealed themselves to me. What a beauty it was! A perfect barred-spiral galaxy under a perfect viewing angle... 

So please, next time that you're out and the sky's dark enough, give this one a try. I'm sure that you won't be disappointed!


But... wait a minute! I'm not finished yet! I didn't know this at the time of observation and discovered it only when I was doing some research for this blog post. But look on the right-hand side of the bright central bar. You'll notice that above the end of this bar there are two small, brighter patches. The middle one of which is... another galaxy!!! Yes! It's a dwarf galaxy that accompanies its big sister, just like M32 and M110 accompany Andromeda. But in this case PGC39369 lies right in front of the main galaxy and this makes it a thousand times harder to see. But it's there!!! I saw it, even without knowing it existed! 

How about that for a satisfactory emotion? Oh well... I know, I'm just a geek... :-)



Monday, 20 February 2017

The helmet of mighty Thor

I've already written about this spectacular nebula in Canis Major here, but it's so beautiful that I couldn't resist dedicating a second blog post to it. NGC2359 or "Thor's Helmet" is another astronomical showpiece that leaves anyone who has the chance of seeing it through a big telescope in awe. As I explained, the cause of this incredibly complex nebula is the star at its centre. It's one of those rare Wolf-Rayet stars, just like the one in the Crescent Nebula. They contain at least twenty times the mass of our Sun and burn their hydrogen much faster. Once they've run out of sufficient fuel to keep the fusion process stable, they start fusing helium into heavier elements such as carbon, nitrogen and oxygen. The consequence is that the star's blown up to gigantic proportions, so big that even Jupiter might be swallowed by it if that star were at the centre of our solar system! The star's surface cools down and turns orangy-red. A famous example of a supermassive star in this phase is Betelgeuse, Orion's left shoulder (from our perspective). The star becomes highly unstable, gravity fighting with the outward energy generated by the nuclear fusion in its core. It expands, cools down, contracts, heats up and expands again... until the critical limit's breached and the star's outer layers are expelled into space. But contrary to normally sized stars, these supergiants will not simply let their atmospheres dissipate into a planetary nebula while their cores slowly extinguish as a white dwarf. Oh no! They're simply too big for that!

By shedding a part of their atmosphere, a supergiant can regain a certain amount of stability. It contracts so much that its surface reaches a temperature of 200.000°C or more and fusion's re-ignited, only this time carbon and oxygen are being fused into heavier elements still, like iron for example. Imagine the power behind all this! Imagine the fierce stellar winds that such an extremely hot star generates! The previously expelled bubble of atmosphere's blown up into space at incredible speed, like a balloon. That's exactly what we can see here at the nebula's centre. 

Soon the central star will collapse under its own gravity and explode as a supernova, which will feed the universe with heavy molecules that are vital for the creation of planets and life.

For the record, the previous sketch of Thor's Helmet was made with my old home-built 18" Dobsonian, whereas this was observed through my 18" binoscope. 

Sunday, 12 February 2017

Almost invisible, but that's the challenge

What is it that pushes us astronomers to go hunting those little blobs so faint that you almost need a healthy dose of imagination in order to see them? Why are we so passionate about staring through a small hole for hours in the biting cold until we eventually think that we've seen "something"? The answers's very simple: because it's a challenge! Because we want to be able to raise our heads and say to the world that we've seen it. Or because we want to discover something that no-one has ever discovered before. Of course, in these days of Hubble and high-definition photography there's anything left for us amateurs to discover with our limited telescopes and poor human eyes. But we don't want to give up and stubbornly try to push our observations to the limits. There's barely a challenge in observing the Orion Nebula or a bright star cluster. But there is a challenge in observing... ARO 215 for example. What the heck is ARO 215 (also known as Abell 7) anyway? It's an old planetary nebula in the constellation of Lepus, the hare, that's become so large that it's almost completely dissipated into space. This is an object that's reserved for the largest of telescopes or, better still, a big binoscope because this instrument allows you to observe at lower magnifications for the same light-gathering power. Since ARO 215 is quite large with its 13 arc minutes diameter, you need as low a magnification as you can get in order to observe it. The sketch below represents very well how I saw it with my 18" binoscope. Are you getting the idea of how faint it is? The Medusa was faint, but this... must've been the faintest object I've observed so far! 

Aren't you seeing anything? I advise you to turn off the lights and to have another look. Go on... concentrate, as I had to do behind the telescope. You didn't think that I was going to make things easy, did you? Use averted vision because your eyes are more sensitive to light next to the point where they focus. Now are you seeing something? You may even discover that I saw two distinct lobes in this planetary with the left one marginally brighter than the one on the right. And that... dear readers... is the kick that we astronomy-weirdoes are so addicted to!

Wednesday, 8 February 2017

Nothing to do with it!

Last month I explained how deceiving appearances may be. Things that seem to belong together from our point of view may not be related to one another at all. In this post I'd like to take you on a trip towards one of the most spectacular star clusters in our sky: M46. One of the peculiar things about this star cluster is that it seems to lie right next to another and even brighter star cluster: M47. They're so close that you can easily catch them together in the same field of view with a pair of binoculars:


M46, the richer but fainter star cluster lies on the left, whereas you can see the younger and brighter M47 on the right. They look like a couple don't they? With a bit of fantasy you could even swear that M47's spitting stars in M46's direction. But don't be fooled! M47's not only brighter because it contains younger and hotter stars, but most of all because it lies thousands of lightyears closer to us. It lies at a distance of 1.600 lightyears whereas M46's 5.400 lightyears away.

Now concentrate on far-away M46. Towards its top edge you might see a little patch. It's extremely difficult to make out, but if you focus well enough (and perhaps turn off the lights) you may spot it. Now let's take my old home-built 18" telescope and zoom in at 85x:


On this sketch which I've made 4 years ago you can see the little "patch" on the right-hand side of the cluster's centre. Keen readers of my blog will already have guessed what it is: a planetary nebula! Yes, this is an enormous shell of gas that was blown away by a dying star. You can already see a lovely doughnut shape in it, can't you? Indeed, in every aspect this little planetary nebula resembles the famous Ring Nebula a lot. Now let's take my new binoscope and zoom into it at 504x:

 

Here you can see our little planetary, called NGC2438, in all its glory. Also its central star's quite obvious. Much more so than with the Ring Nebula you can see that the gas cloud's not really a ring but more a sort of a cylinder, or something halfway a cylinder and the apple without core like the Owl Nebula. See how it's surrounded by the cluster's many twinkling stars that seem to say farewell to the dying companion in their midst. 

But... wait a minute... the nebula seems to be travelling at a much higher speed than the stars in the cluster and in a slightly different direction too. Actually, the gravitational force of the cluster wouldn't nearly be strong enough to be able to hold the dying star and its nebula if they're travelling at that speed. What's more, M46's a cluster of young stars that are hardly 300.000 years old. The star that caused the planetary nebula on the other hand must be billions of years old. It simply can't be a member of the cluster! Recent distance measurements revealed that it is in fact more than 2.000 lightyears closer to us. So it has... nothing to do with it.

Tuesday, 7 February 2017

The eyes of the owl

In the constellation of Ursa Major - popularly known as the "big dipper" or "chariot" - resides a fascinating planetary nebula which astronomers refer to as M97. It's one of the faintest objects on Messier's list and being 3,4 arc minutes across it's pretty large for a planetary, meaning that its frail light's dispersed over a large surface. But already with an 8" telescope you'll be able to see why this nebula bears the nickname "Owl Nebula". If you look very carefully, you'll see two big, dark "eyes" in it. 

Actually, these "eyes" are only coincidental due to the angle at which we see this nebula. As I explained in many earlier posts, planetary nebula are caused by dying stars of normal size which become critically unstable and shed their atmosphere into space. The radiation from the extremely hot stellar core that's left over (surface temperatures can easily reach 100.000°C or more!) makes the gas bubble glow and so we can easily observe it. Often this gas bubble takes an hourglass or even cylindrical shape because the star's atmosphere's much thicker around its waist and therefore the gas can escape much easier from the poles. One fine example is the Ring Nebula, which isn't a ring at all but a cylinder we see face-on. In the Owl Nebula's case the nebula's quite spherical but it has two big holes, one on either pole, where gas ejected at much greater speed. An easy way to imagine this nebula is to see it as an apple from which the core's been removed with a corer (top-right to bottom-left). But Owl Nebula sounds much more intriguing than Apple-Without-Core Nebula and it does seem to stare at you, doesn't it? 

It's 8.000 years old, lies 2.000 lightyears away from us and is expanding at 30km per second. In 10.000 years it'll be gone.

Sunday, 5 February 2017

And now for something completely different...

I thought, after the spectacle of the Orion Nebula: "Now let's show you something incredibly difficult". Minkowski 1-18 (M1-18) is a planetary nebula. Being of the 14th magnitude it's also an incredibly faint one and you need quite a bit of telescope and a very good sky in order to see it. I've read about someone who claims to have seen it in a 10" scope but that doesn't sound very credible to me since it was as close to invisible in my 18" binoscope as it could get. The reason for that is that it must be very distant. I couldn't find any details about this little nebula anywhere but considering its location in the southern constellation of Puppis (the stern) and its faintness I assume it must reside somewhere in the furthest extension of the Perseus arm of our Milky Way. This means that we're talking about a distance of some 12.000 to 18.000 lightyears. This also implies that we're still seeing it because light's travelling so "slowly". The light that we're seeing today of this nebula started its voyage some 12.000 to 18.000 years ago. But in reality this nebula's already dissipated into space much more by now, probably beyond the point that it would still be observable visibly through amateur telescopes. It's one of these odd effects when considering the size of the universe and the speed of light. Probably we're seeing many things in out sky that in reality are no longer there, but the light of the events that caused their disappearance hasn't reached us yet. Observing the stars is observing the past and the further you look, the further you're looking back in time. But for now, let's still enjoy little Minkowski 1-18 and the challenge that it takes to find it. In the end, that's also one of the things that makes astronomical observation so rewarding. Often you can spend hours trying to find something. And when you eventually do find it, it fills you with a deep satisfaction and a gratifying sense of achievement. 

So on you go... try to find it on my sketch! :-)

Friday, 3 February 2017

The Orion Nebula in all its splendour

I've already written many times about the Orion Nebula (see e.g. here and here) and I've also created a video about what it would be like to fly through it with a spaceship. The Orion Nebula, or M42, is of course the brightest nebula in the sky and can already be seen quite easily with the naked eye, even under less than perfect conditions. It's a giant stellar nursery, where hundreds of baby stars are born out of the vast, glowing hydrogen clouds.

I wasn't actually planning on making another sketch of it for the moment because it's such an impressive object that it always takes me many hours of work to put it all on paper (or on the pc). But when I had another peek at it last week, I was so shocked by what I saw that I just had to sketch it. It was in fact only the second time that I'd looked at the Orion Nebula with my new binoscope and as you may recall the first time I concentrated on zooming into it at high magnifications. This time I "only" used 104x in order to see it in all of its glory. And what a glory it was! The bluish colour was more than obvious and the view was filled with its delicate filaments. Nothing out of the ordinary when you've got a big telescope and reasonably good sky. The extraordinary thing was the black cloud of dust that covers the brightest region in part. Really... it was pitch-black, or in any case much darker than the background! Usually dark nebula are hardly visible because they disappear against the dark background and can only be spotted if there's something bright behind them that makes them stand out. That's why scientists are having such a hard time to determine how many dark matter there actually is, and hence to calculate if the overall gravity of all the matter in our universe will stop its current expansion or not. But in this case, I observed how the dark dustlanes continued into the background and what's more, I "felt" that they were well in front of the nebula as if I saw them in 3D.

It's theoretically impossible to see objects as distant as stars in 3D, simply because they're too far away and the light rays come from "infinity", meaning that they don't reach our eyes under different angles. There is however a theory that explains why observing with both eyes such as with a binoviewer or binoculars can still give a 3D feeling, and that's because the light's diffracted differently in both eyes. But here I was far beyond a mere superficial feeling of three dimentions. This was like seeing the Orion Nebula through a Viewmaster (those who've grown up in the 70ies or 80ies will know what I'm talking about :-) ). I had the impression that slightly changing the position in front of the eyepieces also made the view change, as if the dark cloud would move a bit to one side or the other! This is of course complete nonsense, theoretically speaking. The whole idea simply must be a distortion of my human brain. Well, I don't care because astronomical observing is all about sensations. This was perhaps the strongest sensation I've had... ever.