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!

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...

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. 

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. 

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. 

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.   

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...