Monday, 25 September 2017

Pickering's... eh, no, Fleming's Triangle

Today I'd like to take you back to the remnant of a supernova that exploded somewhere between 6,000 to 10,000 years ago and which must have been a frightening spectacle for the people of the early human civilisations. Today, we know this remnant as the Veil Nebula, a vast web of gaseous filaments that span an area six times the full Moon in the constellation of Cygnus, the swan. Most of us know the Veil because of its spectacular eastern (NGC6992-5) and western (NGC6960) parts. And yet, there's so much more to discover. Try to point your telescope exactly in between those two and you'll find this strange, triangular-shaped cloud. Admittedly, this nebula appears much fainter than the other two because we see it face-on and not edge-on. Therefore its frail light's distributed over a much larger area and it doesn't come as a surprise that it was only discovered in 1904, more than a century after the discovery of the eastern and western Veil. About its discovery, it should be noted that it was Williamina Fleming who noticed the nebula when examining photographic plates, but unfortunately, as was customary at the time, credit went to Edward Charles Pickering, the director of her observatory. Forgive me if I personally prefer Fleming's Triangle (perhaps also because I'm a Fleming? :-) ) Because of its late discovery, the Triangle has no NGC number, although sometimes NGC6979 is erroneously used to refer to it. 

In spite of its relative faintness compared to the better-known parts of the Veil, Fleming's Triangle is an amazing object that truly deserves a bit more attention. In the binoscope at 104x it filled the field of view with gorgeous nebulous filaments that had me glued to the eyepieces for hours. So what are you still waiting for?

Friday, 22 September 2017

NGC6852: a rare, pulsating planetary

NGC6852 looks like a small, ordinary planetary nebula. It doesn't come as a surprise that it looks so small from our point of view because it lies some 10,000 light-years away from us. Yet, it still appears surprisingly bright in medium to large telescopes and its annular shape was quite evident to me.

Yet, this seemingly plain and uninteresting little nebula hides a very interesting secret. First of all, it's one of the rare planetaries that's hydrogen-deficient. In most planetary nebulae hydrogen is the most abundant element but in this case there doesn't seem to be a lot of it around. But what's even more odd, NGC6852 is one of only 13 known pulsating planetary nebulae. The mechanics behind these pulsations must be sought in its dying central star but are still poorly understood. In this particular case, the nebula exhibits a low-amplitude pulsation with a period of about an hour and a half. These pulsations are irregular and may even change rapidly over time due to the central star's mass loss. In any case, I don't expect these pulsations to be visible in amateur telescopes, but it's a nice trivia.

A much more famous example of such a hydrogen-deficient, pulsating planetary is NGC246, the Skull Nebula, in Cetus (the whale). 

Wednesday, 20 September 2017

Cr401: What is it?

Per Collinder was a Swedish astronomy student who, in 1931, compiled a list of open clusters as part of his paper "On structural properties of open galactic clusters and their spatial distribution". Strangely enough, he didn't grab a telescope and browsed the night's sky but studied photographs in search for clusters. Most of the 471 clusters on his list are not original and already appeared in the much older Messier and NGC catalogues. Number 401, however, is. Actually, it's so original that no-one seems to know for sure what good old Per had in mind when adding it to his list. 

Some sources claim that Cr401 refers to the bright double star HD185297, which is surrounded by an asterism of seven or eight stars. This seventh magnitude star is actually a very interesting double, with a smaller companion only 0.8 arc-seconds apart. Other sources state that Cr401 is the loose cluster just south of it (up on my sketch). Not even Stellarium, Sky Safari or my telescope's Argo Navis seem to agree! So in order to content all, I've sketched them both in the same field of view. Enjoy!


Monday, 18 September 2017

Sh 2-71: of yet unknown origin

Sharpless 2-71 (or in short Sh 2-71) is another one of those objects with an unusual name. That's because it was only discovered in 1946, long after the Messier and NGC catalogues were compiled. Nonetheless it is a beautiful, yet unknown planetary nebula in Aquila (the eagle) that definitely deserves a bit more attention. It's not the brightest of objects and medium to large telescopes are required in order to see it clearly, and preferably also a sufficiently dark sky. But the first thing that you'll undoubtedly notice is its highly irregular shape. Unlike most planetaries that are round or at least symmetrical, this nebula looks like if a fifty-tonne truck has just run over it. 

The reason for its irregular shape is thought to be its fairly bright central star, which is in fact a binary (which I wasn't able to resolve at 190x). A companion of the dying star would undoubtedly distort the nebula's form. However, recent investigations have shed some doubt about this star's parental claim. It doesn't seem to emit enough (high energy) ultraviolet radiation and also its faint companion would not really fit the right profile. 

Possibly a more likely candidate would be the tiny star just below the brighter one. It fits the right sort of brightness which you could expect from the nebula's distance (3.200 light-years), but it is unknown at the moment if this is also a double star.

An even more tempting thought, would be that all three stars are involved. The brighter one does emit a large amount of broad hydrogen-alpha radiation, which also appears in some other planetary nebulae. The nebula's multi-polar structure, with its many lobes that formed at different times, suggests that a very complex formation process which at least requires a binary star to explain. Or... perhaps we've just discovered one of the Universe's threesomes?


Wednesday, 13 September 2017

The Binocular Summation Factor

How much more can you see with both eyes instead of only one? Or in case of astronomers, how much more can you see with a binoscope compared to a traditional monocular telescope?

It has been a heated debate for years, especially after someone published the idea on Cloudynights, the world's biggest astronomy forum, that a binoscope performs only 1.18 times the diameter of a telescope with one identical lens or mirror. In other words, the difference between a traditional telescope and a binoscope would be insignificantly small. Compare it to a C8 and a (hypothetical) C9.5.

The reasoning seems logical at first sight, because it's based on the old binocular summation factor that was established by Campbell and Green in 1965. Their study revealed that people have a 1.41x better light signal perception with both eyes, compared to only one. A 41% increase in perception matches a telescope with a diameter only 1.18x larger and voilĂ : a binoscope is a ludicrous instrument that costs an awful lot of money and is impossibly big and complex for a miserable gain.

I found this idea rather strange because my experience, and that of everyone else who’s looked through a binoscope, showed that the difference between one and both eyes is quite significant.

So where do we go wrong? First of all, a binoscope captures twice the amount of light as a monoscope, hence it should be equal to a telescope 1.41 times the diameter. That's a significant difference that would match my observations. Critics, however, cite Campbell and Green and therefore state that our brain doesn't simply mix both images into one and that there's a performance loss, resulting in only 1.41x more light gathering power instead of 2x. How odd! If the light of both mirrors were transferred to one and the same eyepiece, they would undoubtedly agree that in that case the performance of a binoscope would double, compared to a monoscope.

The critics go wrong because they misinterpret the Campbell and Green study. This study was meant for medical purposes and has no bearing whatsoever on an astronomical environment, i.e. in the total dark when observing at the limits. Pirenne already demonstrated in 1949 that there's no such thing as a single binocular summation factor and that results may vary greatly with different circumstances. Meese et al. demonstrated in 2006 that the binocular summation factor may even increase to 1.7x in certain circumstances. Unfortunately, as I said, all of these studies were being conducted for medical purposes and no-one seems to be interested in doing a study for an astronomical audience. However, they all seem to agree on:

- There is no such thing as a single binocular summation factor and that summation improves when conditions worsen (Pirenne, 1949)
- The extent of summation depends on stimulus contrast and duration (Bearse and Freeman, 1994)
-  There is significant summation at low contrast (Banton and Levi, 1991)
- At low contrast, the level of summation is greater than could be expected by probability summation alone (Simmons and Kingdom, 1988)
- Summation depends on the complexity of the task, with simple tasks (detection) displaying far greater summation than complicated ones (pattern recognition) (Frisen and Lindblom, 1988)

In contrast, a lot has already been written on the subject on the popular astronomy fora, especially in endless and meaningless yes/no debates. But what does real experience tell us? Obviously, it would be impossible to do a comparative limiting magnitude test because which stars would you use as a reference? And what does “I've seen it” mean anyway? You've seen it or you think you’ve seen it? Such comparisons would never have any real scientific value unless you involve many people and then take an average. I know that "impressions" don't mean much, but the difference with closing one eye is simply too great and certainly much more than the hardly visible 1.19x. Faint stars for instance suddenly disappear or become very hard to see. With both eyes M104's dust lane suddenly appears full of structures, which fade to a dark band with monocular vision. With both eyes I see the Pillars of Creation in M16, whereas they're invisible with one eye (under my SQM20.9 sky and with my eyes). Nebulae appear so much brighter and richer in detail, faint galaxy clusters suddenly become easy, "impossible" objects such as the extremely faint planetary ARO215 suddenly become possible... An aperture increase of only 19%? Nah, I don't buy it.

Another, even more controversial statement is that a binoscope not only offers a significant light gain, it also increases resolution. Well,... yes and no. Unfortunately an amateur binoscope doesn't work like professional compound telescopes such as NASA's Large Binocular Telescope. Each of my 18" mirrors delivers the resolution of an 18" and our brain isn't capable of extracting a higher resolution from both images. However... it does work the same way as photographers stack various images in order to extract more detail, up to the technical resolution limit of their instrument.

In order to demonstrate this, I’ve conducted a small experiment. The other night the weather gods were in my favour because transparency was high and seeing was deliciously calm. I rolled out the binoscope and pointed it at a couple of double stars, some of which are very close to one another. First, I used monocular view, in order not to be biased, and then changed to binocular view. The results were astonishing. As you can see on my sketch, the difference between a single 18” telescope and an 18” binoscope became ever more important as the two components of the double star were closer to one another. 1 Del is pretty close with its separation of 0.9 arc-seconds, but binoscopic vision showed more black between both components than monocular view.

STF2696 was even more interesting. It is only 0.5 arc-seconds apart, which is very close to the 0.31 arc-second Rayleigh limit of an 18” telescope. With one eye, I could only see one, elongated star, suggesting that it is a double without really being able to resolve it. With both eyes, on the other hand, both stars were clearly resolved and appeared to be glued to each other. The technical resolution of an amateur binoscope may not increase compared to an instrument with a similar single mirror, but this observation nonetheless confirms that binocular vision makes it easier to observe up to this technical resolution limit and that it will cancel out a large part of inhibiting factors such as atmospheric turbulences.
Of course, this was only a small, personal experiment and I will never pretend that it has any scientific value because, again, for that you'd need a lot of people of which a vast majority would have to confirm the same thing. But as far as I'm concerned, I'm convinced and will now leave the debating to others while I have some  star gazing to do.


Thursday, 31 August 2017

NGC6638: old or not?

It is generally accepted that globular clusters are among the oldest entities in the Universe and that the stars of which they're composed are among the first that lit up after the Big Bang. The reason for this is that the stars in globular clusters have an unusually low metal content. In astronomical language, this means that they don't contain many chemical elements heavier than hydrogen. The first element that formed in the early Universe was hydrogen of course. Gigantic hydrogen clouds then contracted and gave birth to the first stars, which began to fuse the hydrogen in their cores into helium. When they ran out of hydrogen, they started fusing helium into oxygen, carbon and other heavier elements, which in turn got dispersed into the Universe when these stars eventually died.

In the hippie-age, people liked to believe that we humans are made of stardust and, as surprising as this may sound today, that statement is actually correct. No, I haven't been smoking anything weird lately! The calcium in our bones, the iron in our blood and the magnesium in our brains were all created in the cores of massive stars that exploded and fed the Universe with these heavy elements. 

Now let's get back to our globular clusters and their overall lack of anything heavier than hydrogen and helium. Some of them appear to have a metal content much higher than average, although still much lover than the average metal content of a galaxy. This may be because the clusters in question are relatively young, but recent scientific studies also focus on the effect that young, big straggler stars may have on the population of a globular. It may happen that such stragglers are being hurled out of our galaxy and get caught by the gravitational pull of a nearby globular. Such stars are in general metal-rich and disperse these elements in the globular through collisions or matter transfer from one star to the other. As you know, the stellar density in a globular is unusually high and they may contain hundreds of thousands of stars in a volume with a radius of merely a few tens of light-years. Stellar interference and even collisions must therefore happen regularly. 

One such metal-rich globular is NGC6638 in Sagittarius. It's not a very spectacular one from our Earthly perspective. Actually it's quite faint because it's not very large and yet hovers at a distance of some 26,000 light-years. Even with my binoscope at 507x I could hardly resolve any star in it. The interesting thing about this particular cluster is that it contains a lot of metals, for a globular that is, and therefore the exact determination of its age is not easy because it's impossible to tell if this is due to a relatively young age (stars formed when there were already a lot of heavier elements around) or if it absorbed a lot of young straggler stars in its lifetime.

Tuesday, 29 August 2017

NGC6818: a Little Gem

When visiting Barnard's Galaxy, don't forget to turn slightly to the north because there you'll find a little surprise. NGC6818's popularly known as the "Little Gem Nebula" and with good reason too. It's a small but very bright planetary nebula which will reveal a lot of detail also in smaller telescopes. With my 18" binoscope at 507x the main outer halo was obvious and within I could see a large numbers of clouds that are currently being blown away by the dying central star. This central star was invisible to me, unfortunately enough, because it's not an ordinary star. In fact, it's a system of two stars, separated by 5 times the distance between our Sun and Neptune. Given that the Little Gem lies a whopping 6,000 light-years away, it would be impossible to separate those stars with an amateur telescope anyway because they're too close to one another. Yet, the effect of the binary system on the planetary nebula is more than obvious because the expelled gas clouds are severely distorted, if you compare them for instance with the Saturn Nebula. The nebula's also fairly young, no more than 3,500 years old, and has reached a size of about half a light-year.