Friday, 17 November 2017

Uranus

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.