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.