Saturday 23 December 2017

IC2087: When dust lights up

This may sound a bit odd, but most of our Universe is actually invisible to our eyes. When we look up at the night's sky and admire that mesmerising blanket of millions of stars, we only see things that emit light, i.e. stars and gas clouds that are heated up so much that they start glowing. But this is only a small percentage of all matter that surrounds us. The so-called dark matter is, indeed, invisible because usually we can't see it in a Universe that is essentially dark. 

And yet, every now and then we can catch a glimpse of all the dark dust and gas that's floating through space. When a cloud of this dark matter drifts in front of a bright stellar background, for instance, or when it reflects the light of nearby stars. In the case of IC2087, we observe both.

On the border between Taurus and Auriga there's a gigantic dust cloud, merely 700 light-years distant and denominated as Barnard 22, which blocks the light of all the stars that lie behind it. Just point your telescope in that direction and you'll agree that there are only very few stars to be seen. Even with my binoscope the field of view looked strangely empty. In the middle of all that darkness, however, you may find the bright patch which is the protagonist of this blog post. IC2087 is a reflection nebula, i.e. that this cloud merely reflects the light of stars that are embedded in it. Careful study of this cloud with infrared telescopes has not only revealed its main light source, but also that this nebula contains a lot of embryonic stars. These baby stars haven't really lit up yet and therefore the nebula remains difficult to see, for the moment. But soon fusion will kick in in the star's cores and the gas and dust will be heated up. Probably, within a couple of hundreds of thousands of years, this nebula will become a spectacular stellar nursery, brighter than the Orion Nebula. 

Tuesday 19 December 2017

PuWe1: Mission Impossible?

There are certain objects which tickle the imagination. You know that they're supposed to be virtually impossible to see with any amateur telescope, and yet the temptation's too strong. No matter how cold it is; no matter having to look for it for over an hour, you simply give in to this nagging crave in your chest and you point your telescope towards a remote corner of the obscure constellation of Lynx. There, in 1980, two Austrian astronomers called Purgathofer and Weinberger, discovered an extremely large, ethereal planetary nebula. It appeared so faint on their original photographic plates that they were unable to present it for printing along with the report of their discovery. Nowadays it has become a challenging object for skilled photographers who manage to capture its delicate structures after many hours of exposure time.

With its extremely low surface brightness of 23.7 mag/arcsec², it has always been deemed impossible for visual observations. But, as loyal readers of my blog will know, impossible is my middle name. What's more, I've got just the perfect instrument for large and extremely faint objects at my disposal. A binoscope effectively collects the same amount of light as a telescope 1,42 times its diameter, but... it offers that amount of light at a much lower magnification. I won't bother you with the technical explanation of all this, but suffice to say that the lower magnification allows to concentrate the frail light of the object on a smaller surface. In other words, you see it more clearly. How clearly? Well, don't get overexcited. After having stared at the right spot for at least fifteen minutes, I daresay that I did see "something". Yes, there was a broken circle of extremely faint nebulosity. It must be the faintest and most difficult object I've ever observed, even more so than oddities like ARO215. Yet, that's exactly the thrill that we faint-fuzzies lovers get from it. You know that it's on the edge of impossible. You're freezing, your limbs are giving in, your eyes are having difficulties to focus. But then... suddenly... that extremely faint arc reveals itself. And it was all worth it.

PuWe1 is one of the nearest planetary nebulae, its distance estimated at under 1,200 light-years. As you might have guessed, it's also very old and has extended over 6 light-years across in the last 20,000 years. Let's enjoy the show while we still can, because it won't be long before this nebula will be gone forever.

Friday 15 December 2017

IC417: the yellow spider

Rounding off my series about vast and faint nebulae in Auriga, I present you with IC417, otherwise known as the Spider Nebula. Again, it's not an easy object for visual astronomers but with a sufficiently dark sky and generous aperture you should be able to spot it. Although the nebula is a hot star forming region and would therefore benefit from the use of filters, I decided to make my observation without as I shall explain. 

The nebula complex lies somewhere around 7,500 light-years away, in the outlying Perseus arm of our galaxy. As I said, it is another giant stellar nursery in the heart of which we find a lovely, somewhat elongated cluster of young stars, denominated Stock 8. Interesting to note is that these young stars seem to have different ages, ranging from 1 to 5 million years, which indicates that stellar formation has continued over a long period of time here. 

Now let's turn our attention to the bright, yellow star which appear to be the protagonist of this sketch. As you might have guessed, this star (Phi Aurigae) is much closer to us, at a distance of 450 light-years, and is already visible to the naked eye under suburban skies (mag. 5.05). It is an orange giant (although I saw it rather as bright yellow), meaning that it has evolved off the main sequence and is now fusing helium into carbon and oxygen. Although nearly 300 times as bright as our Sun and 31 times its diameter, it does bear a lot of similarities to our star. For starters, it weighs in at 1.2 solar masses and it has a very similar chemical composition. In fact, it's a perfect example of how our Sun will look like in about 5 billion years, after it'll have consumed all of its core hydrogen.

I guess this is just an optical illusion, caused by the brightness and deep colour of Phi Aurigae, but I had the impression that the nebulosity in its immediate vicinity also had a slight yellowish hue. The sight was so lovely that I wanted to capture it this way, rather than to use nebula filters which make all stars appear blue.

Friday 1 December 2017

NGC1893 and IC410: surrounded by tadpoles

Just one and a half degrees south-west of IC405, we find NGC1893, a young star cluster that lies embedded in the nebula from whence it originated some four million years ago. Even from its respectable distance of 12,000 light-years it shines with a magnitude of 7.5 and is therefore an easy target for binoculars. 

The nebula itself is quite a different matter. With its petals whirling around a dark centre, it closely resembles the Rosette Nebula, even though it appears much fainter and smaller due to its distance. In reality, this nebula spans over a hundred light-years across, four times the size of the Orion Nebula! From Earth, you need a medium to large telescope in order to see it and special nebula filters help as well. These filters block all light, apart from the very specific frequencies which these kinds of nebulae emit. The result is that the background and the stars significantly darken but the nebula doesn't. Therefore it becomes more visible because you get a lot more contrast. 

With my binoscope I was able to see some very interesting structures around the central void. The reason why the nebulosity disappears at the centre is because it's being blown away by the radiation of the hot, young stars that have just emerged from it. So here you're looking at a star-forming nebula in a somewhat advanced state of its evolution. As more stars are born, radiation and stellar winds increase, expediting the nebula's evaporation into space. 

Another interesting feature about this nebula is that it contains "tadpoles". They're extremely difficult to see with amateur telescopes and also I was only able to see two of the "heads" (scientifically referred to as "Simeis 129" (top) and "Simeis 130" (bottom)). Just left of the cluster's central stars you'll see two little knots in the nebulosity. The "tails", gas plumes that are blown away from these "heads" and eroded by a powerful stellar wind, were unfortunately invisible to me.

Monday 20 November 2017

IC405 and the runaway star

AE Auriga is a very peculiar giant star. It's radius 7 times solar is quite impressive and its 23 solar masses make it definitely a member of the most massive stars club. But this is not the main reason why this star's so out of the ordinary. What does make it so special is that it travels through our corner of the galaxy (well, 1,700 light-years away) at the breakneck speed of 200km/s! Now, if we take the star's estimated age of 2,2 million years and trace its movement all the way back, we find that it originated somewhere in the vicinity of the Orion Nebula! Until recently it was believed that AE Auriga, with Mu Columbae and 53 Arietis, were hurled out of the Orion Nebula together by some sort of cataclysm, like a supernova explosion or a near collision. However, recent measurements by the Hipparchos satellite reveal this to be highly unlikely and the three runaway stars have most probably different origins, albeit still within the Orion complex of star forming regions. 

Currently, our giant star's travelling through a cloud of gas and dust in the constellation of Auriga, the charioteer, which is reflecting its bluish light. The star's high velocity's causing a bow shock and leaves a trail of hot gas in its wake: IC405, better known as the "Flaming Star Nebula". It does seem as if the star's on fire and emitting fumes, isn't it? 

Friday 17 November 2017


Uranus is the seventh and before-last known planet of our Solar System and was the first to be discovered in recent history. Although it is just visible to the naked eye under dark skies, it has always been mistaken for a star until William Herschel realised it was a planet, back in 1781. 

Uranus is the third-largest planet of our Solar System, being only slightly larger than Neptune, but contrary to its more distant cousin it has no internal heat source. This makes it an extremely cold planet, with temperatures varying around -220°C in its outer layers (being a gas giant it doesn't have a solid surface). Its featureless, blue-greenish colour is caused by the large presence of methane in its atmosphere.

As dull as Uranus may look through earthbound telescopes, it is definitely very interesting in a lot of ways. First of all, it is the only planet that's severely tilted with respect to its orbit. Scientists believe that it may have been knocked over in a collision with another planet, early in its history. Also the orbits of the 27 known moons exhibit the same 97° tilt as the planet, compared to the system's orbit around the Sun. 

In 1977 a very faint system of rings was discovered, which is of course way beyond reach of amateur telescopes. However, in infrared they appear surprisingly prominent.

During my observation yesterday I was able to make out three of its moons: Titania on the top-left, Oberon on the left and probably Ariel (very faint) nearer to the planet on its bottom-right. Titania is with its diameter of 1,580km the eight-largest moon in our Solar System. Its surface has been dramatically carved in the past and one of its canyons is over 1,600km long, dwarfing the Grand Canyon! Oberon's slightly smaller and contrary to its sister shows little evidence of interaction, apart from being covered in craters. It's also the most distant moon to Uranus. Ariel is number four in size, but usually appears as brightest because of the high reflectivity of its surface. It looks covered in river beds, probably caused by a mixture of liquid ammonia, methane and carbon monoxide that shaped them during the moon's primeval history. There is a lot of water on Ariel but this couldn't have contributed because at the extremely low temperatures on its surface water ice is as hard as steel.

I did "believe" to have seen a sort of darker band on the planet during my observation. At first I wasn't sure if it was real or just the fruit of my imagination after having stared through the binoscope for a considerable time. But when I did some research afterwards, it appeared that the "band" indeed followed the planet's rotational axis as it should have been through my binoscope's view. Therefore the observation may have been correct. I've included a more detailed (and perhaps slightly exaggerated) enlarged inset to give you a better idea.

Uranus lies 19 times further away from the Sun than the Earth, at a distance of 2.9 billion kilometres, and completes its orbit in 84 Earth years. And... no, dear astrology believers, you won't find it in Aries. It's in Pisces at the moment. :-)


Thursday 9 November 2017

NGC133, NGC146 and K14: The Cassiopeia Triple Cluster

Everybody knows the glorious Double Cluster in Perseus. In spite of their considerable distance of 7,500 light-years, these two clusters are easily visible to the naked eye and offer an unforgettable spectacle in binoculars or low-power telescopes. Few people are aware, however, that in nearby Cassiopeia you can find a lovely triple cluster, although unlike their famous peers in Perseus its three members are physically unrelated. 

Let's start with NGC133 on the bottom-left (also see the bottom image with labels). It's a fairly poor cluster with only 13 identified members, based on the stars' proper motions and properties. Yet, it's an extremely young cluster with an estimated age of just about ten million years. The bright and hot youngsters are shining at us from a distance of 2,000 light-years.

Next, we movea bit up and towards the centre of the field of view, to find NGC146. Clearly this is a much richer cluster which may contain up to 200 members. The relative faintness of its stars, compared to NGC133, suggests that it lies a lot further away from us, and this is indeed what we find. The average of the various distance measurements places it some 9,300 light-years away from us. Abeit still very young, with an estimated age of 63 million years it's much older than its apparent neighbour. When I come to think of it, this cluster was formed at the time of the cataclism that caused the extinction of the dinosaurs!  

Now let's move down again, and even more towards the centre of the field of view. There lies the most obscure member of the three which doesn't have a New General Catalogue number but just bears the name of its discoverer: King 14. Being the least known of the three, it appeared as the most intriguing and perhaps even the most beautiful of the three, to my humble opinion. Fairly rich and slightly denser than NGC146, it offers a myriad of lanes and structures which are a lovely sight in any telescope. It lies somewhat closer to us as regards to NGC146, with an average estimate of 8,300 light-years. Still considerable I would say. Its estimated age of 32 million years also lies almost halfway between those of NGC133 and NGC146. 

Also note the bright star Kappa Cassiopeiae on the bottom-right. It looks like an ordinary star, just one of millions, barely visible to the naked eye under suburban skies. But what if I tell you that this star still still shines so relatively brightly from a distance of... 4,700 light-years? It looks brighter in our sky from its considerable distance than our Sun would shine from a distance of merely 30 light-years! What if I tell you that this star is one of the heaviest giants known, yielding a whopping 23 solar masses, and that its surface temperature's 4 times hotter than our Sun's? For the moment, this star's still very young, but being such an enormous giant it will burn its fuel in just a few million years and not much after that it will most certainly explode as a supernova!

So here you go. Three clusters and an extroardinary star that have nothing to do with each other whatsoever, but which... as seen from our little corner of out galaxy, make a wonderful sight together.


Sunday 29 October 2017

Abell 72: For those who love a real challenge

I already told you about Abell objects in a recent post and explained that usually they're extremely difficult and even at the limit of large telescopes. Abell 82 is one of the exceptions and even showed some remarkable detail in my binoscope. Number 72 on Abell's list, on the other hand, is another pair of shoes. It's a very difficult planetary nebula that resides in the lovely summer constellation of Delphinus. Its  considerable distance of 4,200 light-years explains its faintness in part, but more important is that this nebula has grown over two and a half light-years in diameter and therefore its feeble light is distributed over a large surface. If you'd like to take on this challenge, approach it with as much aperture as you can get your hands on, low power and an OIII filter to maximise contrast. Did I mention that you're going to need a very good sky too? Well, I guess that was already obvious.

Not a lot is known about this obscure nebula. Given its size, it must be quite old, probably more than 10,000 years, and nearing the end of its existence. Its central star (invisible to me) still appears to be quite hot, at least 100,000°C on its surface, but it's no longer able to fully ionise the enormous gas bubble it ejected and blew up to its current proportions. Soon, interstellar winds will tear the bubble apart and the nebula will completely dissipate into space whereas the remaining white dwarf will extinguish very slowly.  

Look very carefully!

Sunday 22 October 2017

IC10: Starburst in our Local Group

As I already explained in this post, our Local Group of galaxies has a lot more to offer than the Andromeda and Triangulum galaxies. Actually, its smaller and more obscure members often seem to hide the biggest surprises. IC10 in Cassiopeia is another dwarf galaxy that only recently has been confirmed as a Local Group member. It's a bit larger than Barnard's Galaxy (13,000 light-years across compared to 7,000) and resembles the Large Magellanic Cloud a lot. Unfortunately it lies 2 million light-years away and appearing close to our galaxy's plane a lot of its light is absorbed by interstellar dust. All of this means that it's a challenging object for visual astronomers, as you can also see on my sketch. This is really a pity because IC10 seems to be a mild starburst galaxy, perhaps generating the most active star formation in our Local Group. If it continues at this rate, it will have spent its entire gas supply in the next one to two billion years! 

This radiant activity shows very well on long-exposure photos, where one can identify dozens of star- forming regions. Through the eyepiece of an amateur telescope, on the other hand, this little galaxy only appears as a very faint blob and even with my 18" binoscope I could only imagine a hint of these massive star-forming clouds.

Another interesting feature of this galaxy is that it's enveloped by a huge bubble of hydrogen gas which appears to be rotating in the opposite direction as the galaxy itself!

It's approaching us at 350km/s, which means that it's bound to collide with our Milky Way in 1.7 billion years...

Wednesday 18 October 2017

IC59 & IC63: The Gamma Cassiopeiae Nebulae

Gamma Cassiopeiae, the middle star of the famous W-shaped constellation, is a hot giant with a surface temperature of 25,000°C (compared to 5,500°C for our Sun) and a mass fifteen times our Sun's. The star shines at us with an incredible luminosity of 40,000 Suns and only looks ordinary from our point of view because it lies at the respectable distance of 610 light-years. It's burning its core hydrogen at an excruciatingly high rate and will most certainly explode as a supernova one day. Already now this star's known as an eruptive variable that may lighten up abruptly to even become the brightest star in its constellation.

Such a big and powerful star must have a tremendous impact on its environment and that's indeed what we find. If you are the proud owner of a ten-inch or larger telescope and have access to a nice and dark sky, point it towards this star and then slightly towards north-east to put Gamma Cassiopeiae just outside of the field of view. This way this bright star will not impair your search for the faint nebulae that surround it too much. These may not be the most spectacular objects you've ever come across and even with my binoscope they were not so easy to detect. But if you try to grasp what it is exactly that you're seeing, you'll surely be impressed. The extreme radiation from Gamma Cassiopeiae is literally tearing neighbouring dust clouds to pieces. The leading edges of these clouds are glowing because of the immense heat and are evaporating as we speak! In a few thousands of years these clouds will have been blown away completely. 

Sunday 15 October 2017

Abell 82: the miniature Dumbbell

George O. Abell was a famous American astronomer and science professor at UCLA, who became notably renowned for two reasons:

  • His outstanding work combatting superstition, astrology and other forms of pseudoscience, inciting people to embrace reasoning and enlightenment.
  • The catalogue he compiled in the 1950's of over 4,000 astronomical objects, mostly clusters of galaxies but also some faint nebulae.
Abell objects are notorious for being incredibly difficult and often at the limit of sizeable telescopes, even under dark skies. However, there are a few exceptions which are also within reach of more modest apertures. One of them is Abell 82, a planetary nebula in Cassiopeia, very close to Caroline's Rose. Don't be fooled, because it remains one of Abell's faint fuzzies and therefore don't expect anything spectacular. That being said, the planetary was quite evident in my binoscope and I was able to distiguish some interesting detail at 190x. Some have described this little nebula as a miniature Dumbbell and I have to agree. Both "wings" of the dumbbell structure were quite visible. 
A peculiar thing about this planetary is that no-one still knows for sure where its central star's got to. The reasonably bright star, which appears within the nebula's boundaries, matches a typical central star's hot surface temperature but is too far off-centre to be a suitable candidate. The faint star near the nebula's centre, on the other hand, is a cool K-class star and is therefore most unlikely the culprit. Perhaps this cool star has a close companion that recently kicked the bucket, shedding its atmosphere in the process, but it has yet to be discovered. 

Saturday 14 October 2017

NGC7789: Caroline's Rose

Discovered by Caroline Herschel in 1783, NGC7789 is one of autumn's finest jewels. Notwithstanding its considerable distance of 7,600 light-years, this open cluster appears so brightly in the constellation of Cassiopeia that it can almost be seen with the naked eye under a perfect sky. Being extremely rich and dense, the cluster's gravity has been able so far to keep most of its stars together during its already fairly long existence of 1.6 billion years. Since more massive stars burn their fuel more quickly, many of its bigger members have already left their main-sequence, "adult" life and have evolved into helium-fusing red giants. 

It was quite a challenge to sketch and it took me two full observing nights and a week behind the pc to obtain this result. So I hope that you'll appreciate the effort. The cluster's nickname "Caroline's Rose" comes from the many dark dust-lanes which cut through the cluster and give the impression of flower petals, or of indeed a rose. 

Thursday 5 October 2017

NGC7538: Super-massive stars in an exciting nebula

Last week I showed you the famous Bubble Nebula. Now let's move the telescope slightly northwest and you're going to bump into this exceptional beauty. NGC7538's not nearly as popular as it's larger neighbour and this is a disgrace I'd like to rectify with this post. 

"Dreyer's Object", named after the great Danish-Irish astronomer John Dreyer who compiled the New General Catalogue (NGC) in the 19th century, is a very active star forming region that lies approximately 9,100 light-years away from us, in the same spiral arm as the Bubble. Many of the stars within it are still very young, "only" 1 to 4 million years old, or are still in the process of forming. What makes this nebula truly special is that it's home to the most massive protostar known to day. On my sketch, it's the lower one of the pair in the nebula's centre, denominated NGC7538S. This star under construction is currently 300 times the size of our Solar System (!), and has a contracting core of anything between 85 and 115 solar masses! Mass accretion is still on-going at an astonishing rate of 1/1,000th of a solar mass per year! The star's surrounded by an enormous disk of gas and dust in which perhaps one day planets will be formed. Unfortunately, the bigger the star, the shorter its lifespan and this extraordinary giant will probably not live longer than a few million years.

The upper star of the pair, MM1, is also one of the largest known giants, although with its "merely" 20 to 30 solar masses it's dwarfed by its incredible sister. This star is slightly more ahead in its formation process and exhibits powerful jets. At least 8 more protostars are currently being formed in MM1's vicinity, in an area only 1 light-year across. This is fairly odd because molecular density and temperature don't seem to be sufficient in that region to induce star formation. Scientists believe that in addition to ordinary mass collapse these stars are being created by shock waves and/or strong magnetic fields.   

Sunday 1 October 2017

NGC7635: The Bubble

When I showed you Thor's Helmet, I told you about the rare, extraordinary Wolf-Rayet stars. These are super-massive stars, easily containing 20 solar masses or more, which at the end of their very short lives shed a part of their atmosphere and thereby regain a certain stability, re-igniting fusion in their cores. Their surfaces become extremely hot with temperatures reaching 200,000°C and their immense radiation blows the previously ejected matter away, which appears to form a sort of bubble. Eventually these stars will explode as supernovae.

In Cassiopeia we find another beautiful example within NGC7635 or the Bubble Nebula. It's one of every photographer's favourites, but unlike Thor's Helmet it doesn't do as well for visual observers. They both lie at approximately the same distance (around 11,000 light-years), but the Bubble Nebula appears decisively fainter and you need a sizeable telescope in order to see the bubble itself within the faint, nebulous patch. Then again, the bright central star really leaps out at you, even from its incredible distance. Don't be confused by the brightest star in the field of view, by the way, which lies ten times closer to us and doesn't have anything to do with the nebula.

SAO20575, the central star's scientific name, coincidentally lies near a big cloud of gas and dust and the bubble, consisting of ejected stellar matter, is currently ploughing through it. The surrounding cloud itself is also excited by the fierce radiation of SAO20575, up to the point that it's beginning to glow, offering us an unforgettable spectacle...

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 voilà: 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.

Sunday 13 August 2017

NGC6960: The Western Veil

The Veil Nebula is undoubtedly the most breath-taking supernova remnant in the sky and - to my humble taste - it's the most spectacular object in the rich summer constellation of Cygnus, the swan. The hot clouds of gas that were blown into space at 30,000km/s (!) when a giant star became critically unstable and exploded, have expanded in the last 6,000 years to a frail bubble 110 light-years in diameter. It is still growing at a rate of 170km/s and eventually the filaments of ionised gas will dissolve into space.

A year ago I showed you the eastern part of this nebular complex that spans an area of six full moons in our sky. This time, I'll show you a detail of its western part. This area is notoriously famous for the extremely bright star that seems to lie right in the middle of it. 52 Cygni is a star of magnitude 4.22 and therefore easily visible to the naked eye, if there aren't too many useless street lights around. Again, you see how much appearances may deceive because this star lies at a distance of 210 light-years, whereas the Veil Nebula lies seven times further away from us. The star with its large "wings" of nebulosity are a lovely sight, but 52 Cygni shines so brightly that it tends to shade the delicate whiffs of the supernova remnant. For this reason I chose to increase telescope power to 190x and to concentrate on one of the "wings", leaving 52 Cygni just beyond the right border of the field of view. The details that emerged, left me with my mouth wide open. I hope that this sketch, albeit not a real telescope image, may have the same effect on you...


Friday 11 August 2017

NGC6822: Barnard's Galaxy

In 1884, E. E. Barnard pointed his modest 6" refractor to one of Sagittarius' less-fashionable corners, slightly below the Milky Way. There, he discovered a faint nebula which he soon identified as a galaxy. Later, Edwin Hubble determined that this odd, irregular cloud of stars belongs to our Local Group, like the Andromeda and Triangulum galaxies. Indeed, it lies merely 1,6 million light-years away, which is quite close in astronomical terms. To compare, the Andromeda Galaxy lies 2,5 million light-years from our solar-system. 

If you want to observe it, I'd suggest high aperture and low power because Barnard's Galaxy has a very low surface brightness, yet all of its light is smeared out over a large area. To make things worse, a lot of its light is being absorbed by interstellar dust. And to round it off, it travels quite low in the sky to northern observers and easily disappears in the glow above the horizon. Therefore it can be a serious challenge and even with my binoscope it wasn't easy to identify and discover the many structures and star forming regions within it. In fact, even though this galaxy only has a central bar without any significant spiral arms, it exhibits no less than 150 star forming clouds and some of those appear quite brightly against the faint background of the galaxy itself, as you can see on my sketch. Undoubtedly NGC6822 experiences a lot of gravitational influences from the other Local Group members, in the first place from our Milky Way and Andromeda. 

In every aspect Barnard's Galaxy resembles the Small Magellanic Cloud a lot, which decorates southern skies. They're both 7,000 light-years in size and have comparable masses, but obviously the SMC lies a lot closer to us, at a distance of 200,000 light-years. 

So you see that there's a lot more to our Local Group than M31 and M33. In total, 54 member galaxies have already been discovered! 


Tuesday 8 August 2017

M8: The Hourglass in the Lagoon

In my previous post I showed you the Lagoon Nebula, one of our Galaxy's largest star forming regions. Today I'm going to zoom right into its core, towards a feature John Herschel called the Hourglass Nebula for obvious reasons (not to be mistaken with the small planetary nebula in the southern hemisphere which bears the same nickname). This extremely bright structure's lit up by the very hot star just next to it.

The Hubble Space Telescope discovered several globules in it, which are dark knots of gas and dust that are contracting and which will eventually light up and become new stars. Each of these knots is 10,000 times the distance Earth-Sun across and mark the boundaries of future solar systems. Obviously I wasn't able to observe these little knots with my humble telescope, but nevertheless it's very interesting to know that they're there and that all of this is happening right now as we speak. The star that's causing the radiant glow of the hourglass appears to be surrounded by about a hundred baby stars that have ignited merely a million years ago. All of these little stars are also still invisible to amateur telescopes but soon the baby stars will shed the clouds of dust that envelop them and form a bright star cluster similar to the one on the eastern side of the Lagoon Nebula.

Saturday 5 August 2017

M8: The Blue Lagoon

No, this post isn't about alcohol or eighties erotic cult movies, although there is some eroticism in what I'm about to write. Today I'd like to take you to one of the biggest baby star factories in our galaxy: M8 or the Lagoon Nebula. This gigantic gas cloud extends over 100 by 50 light-years across and is probably as deep as it is large, making it at least fifteen to twenty times the size of the mighty Orion Nebula. The latter looks bigger and brighter from our point of view but you have to bear in mind that the Lagoon Nebula lies four times further away from us, at a distance of 5,000 light-years. Yet, it still covers an area as large as three times the full Moon in our sky. Unfortunately northern viewers are eating their heart out because the Lagoon resides in Sagittarius, very low in the sky. Even from my observing spot in Northern Italy I had to point my telescope into the horizon glow in order to make this drawing. I can only dream about how this nebula would splendour from more southern latitudes.

As I said, this nebula is an enormous baby factory and the bright star cluster that illuminates its left half has only just emerged from it. With "only just" I mean two to three million years, the time when our first ancestors emerged from the African plains. A lane of foreground dust seems to cut the nebula in half and on its right-hand side we find the so-called "Hourglass Nebula", the brightest and most active region, heated up by the small but very hot star just next to it. Several nodules have been discovered here; clumps of contracting gas that will soon light up and become stars. 

To put things in perspective, the bright star halfway between the "hourglass" and the dark lane is 23,000 times brighter than our Sun in visual wavelengths and maybe 200,000 times brighter if we add Ultraviolet radiation!

Thursday 3 August 2017

Her Royal Majesty M22

Number 22 on Messier's list of astronomical objects is the third-brightest globular cluster in the sky and the brightest visible from northern latitudes. On a clear night it will already reveal itself to the naked eye in the heart of one of the most stunning stellar landscapes that the Universe grants us. It lies just south of the densest part of our Milky Way, with the Sagittarius Stellar Cloud and the immense Lagoon Nebula nearby. 

M22's a fairly large globular cluster with its half a million stars, but nothing out of the ordinary. The reason why it's so bright is because it's one of the closest globulars, its distance estimated to be only 10,400 light-years. Considering that the centre of our galaxy lies more than twice as far away, that's pretty close. In any case, M22 certainly merits its reputation as one of the finest globular clusters as I hope my sketch illustrates.  

Interesting to note is that recent investigations with the Hubble Space Telescope discovered a large number of planet-sized objects which appear to roam in this cluster without belonging to any particular star. It seems only logical that planets, which form at a certain distance from their parent star, get severely disturbed by the multitude of extremely close neighbour stars and as a result these planets are torn away, destined to float from one star to the other. 

Monday 31 July 2017

NGC6772: Ploughing through interstellar space

I have already elaborated very often on the death of ordinary stars and the evolution of the planetary nebulae that form when these stars eventually collapse. This time I'd like to highlight a very interesting phenomenon that happens when these planetary nebulae expand way beyond the original boundaries of their "solar system" and into intergalactic space.

NGC6772 is a planetary of a certain age in the constellation of Aquila, the eagle. The gas shell that was violently expelled during the star's collapse has smashed through the faint outer shell which had already formed many thousands of years earlier, during the last stage of the star's life. Now, it has reached interstellar space and the giant gas bubble that's blowing up at a rate of 30 km/s crashes into a medium with completely different mechanics. Interstellar dust is moving in other directions than matter within the gravitational influence of the former star, driven by the gravitational pull of our galaxy, and the expanding planetary nebula finds it ever harder to plough through it. Gas at the border of the nebula's building up, as if it were hitting a brick wall, and we observe a significant brightening there. The nebula's not spherical anymore, deformed as it is by areas of less or more interstellar resistance. Its central star's cooling down and losing brightness quickly, up to the point that it isn't visible through amateur telescopes anymore, and only shows itself on long-exposure photographs. 

Yet, the nebula will continue to plough through interstellar space, at an ever decreasing rate, until it completely dissolves.

Thursday 27 July 2017

M16: the pillars of creation

M16, otherwise known as the Eagle Nebula, is another one of summer's highlights. Though less bright than nearby M17, it only takes a pair of binoculars to discover this very young star cluster surrounded by their maternal gas cloud. The nebula became especially famous when astronomers discovered active star formation for the first time, right in its central region. There lie the so-called "Pillars of Creation" which are obvious on long-exposure photographs but extremely difficult to make out visually. When I come to think of it, I've never seen them through a telescope before, until I got my new binos that is. These "pillars" are in fact long and very dense clouds of gas and matter that are creating hundreds of new stars as we speak. I was able to make out three "fingers" on top of them, which in reality are at least four light-years long! Within these "fingers", the Hubble space telescope discovered hundreds of globules and proto-stars. A theory goes that a nearby supernova explosion, which happened some 7,000 years ago, has blown away most of the surrounding matter and that the "pillars" are currently the only thing left. Given that the Eagle Nebula lies between 6,500 and 7,000 light-years away, the "pillars" have probably dissipated completely as well by now; it's only that the light of that event hasn't reached us yet. What we can already see very clearly is that the intense radiation of the hot, newborn stars is eroding the nebula very quickly. The "fingers", for example, are being torn apart by the stellar winds of the bright little star in the middle of them. Eventually the whole nebula will disappear, as will the cluster of young stars that has formed within it.

Wednesday 26 July 2017

M20: the fascinating Trifid Nebula

The Universe is awash with objects of rare beauty. Such marvels as to leave you with your mouth wide open every time you look at them, in the knowledge that we humans are so insignificant in comparison. One object in particular that rates very high on the beauty scale, is undoubtedly the bewildering Trifid Nebula (M20). Not only is it an incredible spectacle to behold, but it's also a stunning combination of three different kinds of nebulae. The brightest part, surrounding the conspicuous double star, is an emission nebula. It's a region of intense star formation that's being heated up by the radiation of the hot, young stars embedded in it, up to the point that it starts emitting light on its own. The central star is in fact a sextuple system, of which I was able to discern four members at the "modest" magnification of 190x I used here. No star formation's going on anymore in the immediate vicinity of these stars because they've literally scorched away the surrounding gas cloud. The less bright region below is an enormous reflection nebula, merely reflecting the light of the giant star in its centre. And then there are of course the fascinating dark lanes, which are clouds of dust drifting in the foreground. Interesting to note is that these dark lanes show a bright rim on the side which is illuminated by nearby stars.

The whole complex is estimated to be some 21 light-years across, which is five times the distance from our Sun to Proxima Centauri, the nearest other star, and almost the diameter of the mighty Orion Nebula. The reason why the Trifid appears a lot smaller and less bright is because it lies much further away from us: 5,200 light-years as opposed to merely 1,300 for the Orion Nebula. In all, you could say that the two nebular complexes are very similar in size and they're both giant stellar nurseries. Also in the Trifid Nebula dozens of embryonic stars ("proto-stars") have already been discovered.

Another interesting fact about the Trifid is that it's very young, estimated to be no more than 300.000 years old. This makes it one of the youngest emission nebulae known. 


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 Méchain 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.