Clustered

This is the second in a planned short series of image-rich posts within which I hope to share my progress in astrophotography; the first, on binary star systems, is here. Although I’ve been a stargazer since my childhood, this more serious-minded hobby started two or three years after I retired and has progressed haltingly in the six or seven years since. Now, as I approach my mid-seventies, I’d like to take stock of what I’ve achieved and there’s nothing like writing about it to sort out one’s thoughts.

The advent of being able to capture images of star clusters dawned when I finally managed to use plate solving methods to find and identify specific targets and then employ guiding to lock onto them for long exposures. Plate solving is, in essence, a little like using the features on a map to help one navigate to a destination; in this case the map is of star positions and it’s my laptop doing all the work. The plate solving software uses an image of whatever the telescope-astrocam is ‘seeing’ and from the relative positions of the stars in that image calculates what bit of sky the telescope must be pointing at. From there it’s a simple task to work out what correction to that direction is needed to get to the designated target. Indeed, within the software I’m currently using (SharpCap – other packages do similar things) it’s then possible to use the correction identified automatically to slew the telescope to the right direction. However, the Earth is still rotating beneath the night sky so our target will progressively drift off centre unless the telescope is moved in the opposite sense. (A setting-up process called Polar Alignment is a prerequisite for all this; the telescope’s mount is tilted to the observer’s latitude and aligned to the Celestial Pole – near Polaris.) This is where guiding comes in. A small secondary telescope and astrocam is used with appropriate software (I use PHD2) that locks onto the position of stars in its field of view and controls the motion of the telescope’s mount such that they stay in the same position: in other words, it ensures that the telescope accurately tracks the stars. Now we’re in a position to capture objects for extended periods of time rather than rely solely on polar alignment and the need to add together (stack) multiple images of only a few seconds duration. I’ve written about these steps in an earlier post, here.

What follows is a set of images showing the star clusters I have imaged thus far. The quality is mixed, to say the least, which is exactly what you’d expect given the continuing – indeed, never-ending – learning curve I’m on. There are two very distinct types of star clusters: globular and open. The names say it all really, globular clusters comprise groups of stars in a spheroidal arrangement – a ball of stars if you will. Open clusters comprise a group of stars with a separation large enough that each of them can be resolved … assuming they are bright enough to be seen at all. The one thing they have in common is that they exist and orbit within the Milky Way as a group; they are gravitationally bound to one another.

On the left is the best I could do with my first telescope – as described in the previous post; it shows the brightest star in the Pleiades and its near-neighbours. I have inverted the image, which is akin to creating a negative of the sort used by astronomers before digital cameras; it can make it easier to pick out the fainter details. The numbers shown refer to the apparent magnitudes of the stars – a term I outlined in the previous post when discussing brightness. On the right is a more recent view of the entire visible open cluster, Alcyone is central and about 1/3rd down from the top. This is high on my list for a return visit when the conditions are right since it’s travelling through a cloud of dust and gas and the cluster’s members illuminate it at a faint level. I have written at length about the beguiling Pleiades in an earlier post, here.

There seems to be nothing astronomers like better than giving particular groups of stars a descriptive name. The tendency began, I suppose, with the classic constellations – most of which comprise stars not actually held together by gravity but merely ‘lined up’ in the view of the observer. Thus, the Pleiades or any of the clusters shown here would be a defined cluster from whatever viewpoint one might have within the Milky Way but the Plough, for example, only has that shape when viewed from our solar system – it is an asterism. Making out why the above open cluster is called the Pyramid is challenging, so I have included an image on the right which might help 😉.

My final open cluster is called the Beehive – a name that continues to evade explanation to my mind. Although not visible to my setup, there are probably about 1,000 stars in the cluster in total. Its more formal designations include M44 or NGC2632. The ‘M’ denotes the eighteenth century astronomical catalogue begun by Charles Messier, who devoted much time and effort in the compilation of a list of ‘fuzzy’ objects which were not comets. ‘NGC’ stands for New General Catalogue of Nebulæ and Clusters of Stars, although the term new is relative since the catalogue and its supplements were put together in the late nineteenth and early twentieth centuries.

M56 is an interesting globular cluster in theat the stars have a particulalrly low metallic content (compared to our Sun for example). This suggests that the stars are unusually old: few new stars incorporating the heavier elements released in the death of earlier-generation stars. In addition, the cluster is moving counter to the overall rotation of the Milky Way and that suggests that it might have form outside our galaxy and then been captured at some time in the past.

M92 is a globular cluster which has a mass equivalent to ~330,000 Suns. Like many globular clusters its metallicity is relatively low, leading to estimates of its age in the region of 11 billion years.

M3 has a similar morphology to M92 but a higher mass (~½ million Suns) and although still low by galactic standards, its metallicity is a little higher than M92’s. It sits quite a long way above the plane of the Milky Way, which sets it apart. M3 contains an unusually large number of variable stars.

M13, the Hercules cluster, is arguably the most stunning globular cluster visible from the northern hemisphere. It probably has as many stars as M3, but is far closer to us.

The next post I plan to write in this series considers parts of the Milky Way which require us to go up in scale. Indeed, the targets won't all fit into my expanded field-of-view so we'll be viewing specific regions of them. These are the nebulæ: vast clouds of dust and gas. However, the idiosyncratic naming beloved of amateur astrophotographers will continue unabated.

There are several earlier posts which will give you the 'backstory' to this current short series. They'll be obvious if you peruse the blog. The first one I wrote on this post-retirement hobby of mine was uploaded a little over five years ago and focused on imaging the Moon and planets, as were several that came after. However, an alternative to reading those would be to watch a recording of the talk I gave in January 2025 which summarises the earlier stages of my journey: there’s a link to the YouTube capture in the first paragraph of ‘Climbing over Failure’.

Binary

This is, if I can carve out the time, hang on to my present motivation and avoid the distraction of the many other ‘shiny things’ in my world, the first in a short series of image-rich posts. I’d like to share my progress in astrophotography – a hobby started two or three years after I retired – and in the process to try to order my own thoughts and plans as I approach my mid-seventies. I currently envisage a post on the binary star systems I’ve looked at (this one), a follow-on post focused on star clusters, then a look at stars either in the process of ‘dying’ or via their post-explosion remnants. Somewhere in that mix, most probably in the latter post, I’ll bring in my images of vast clouds of gas and dust, and the shockwaves one can occasionally see within them. All of these astro images relate to sights within our own galaxy, The Milky Way, but the final post I want to write and share will focus on my images of other galaxies ranging in distance from our nearest neighbour, Andromeda, to those at distances large enough that their light took hundreds of millions of years to reach the little telescope in my garden.

We’ll start with binary star systems simply because I have already said a lot about them in posts written early on in my astrophotography hobby. These systems were of the necessarily easy-to-find variety given that my equipment, at that stage, was relatively simple and therefore limited: the Mizar-Alcor system, which is part of the Plough, and the Castor system within the Gemini constellation.

Mizar is one of the stars in the ‘handle’ of the Plough. It’s dimmer companion Alcor can be spotted by the naked eye under suitable conditions (i.e. a relatively young eye and a sky without a lot of light pollution!) and is easily spotted using binoculars. This is arguably the easiest binary system to start with. Add a telescope and entry-level astro-camera into the mix and it’s possible to make out that there are three stars in the system: a closer binary, Mizar A and Mizar B, and Alcor. Indeed, this deceptively simple and easy-to-spot system is even more fascinating because each of those three stars is itself a binary – six stars in total dancing around as pairs within pairs within a pair. Although a teeny bit more difficult to locate, the Castor system has a very similar makeup: Castor A, B and C – each of which is itself a binary. (Almost all binary star systems comprise a larger, gravitationally dominant star and a smaller partner. They’ll rotate around a common point in space, which will be closer to the large star. This is driven by the same physics which defines the barycentre between Earth and Moon, and also the Sun and the rest of the solar system. In the case of the Earth-Moon system, both bodies orbit around a point approximately 4,670 km from the Earth’s centre. I wrote about it here and there’s a useful YouTube animation here if you’d like to know more.)

My next step involved looking for colourful binaries – typically binary systems with a blue, white or yellow partners. The perceived colours are affected by our brain’s interpretation of adjacent points of light, but that doesn’t detract from the beautiful sight. It is unfortunate that at this point in my journey I hadn’t realised just how much the mirrors in my Newtonian reflector telescope were in need of re-alignment and calibration. One day I ought to retrace my steps and image them again, but …

The yellow-blue binary Almach (in the constellation of Andromeda) and Cor Caroli (in Canes Venatici) are two such systems, and very pretty they are too.

There is a caution to offer at this point: not all pairs of near-neighbours in the night sky are binary star systems. Yellow Albireo (in Cygnus) and its blue companion, for example, is definitely a pretty sight. However, the jury is out in terms of whether they are actually a gravitationally linked binary system or simply an unconnected pair of stars moving independently within the galaxy which, for a time, appear as a double when observed from our viewpoint. If there is a gravitational link then it’s tenuous.

Albireo: binary or double?

Having an overwhelmingly bright star in a binary system can be a problem when trying to obtain an image of the pair. Regulus is a good example of this; in order to see Regulus B I had to over-expose Regulus A. This is a good juncture for a working definition of a star’s apparent magnitude (- actually, every celestial object has an apparent magnitude when viewed from the Earth). The magnitude represents the object’s brightness; it’s a logarithmic scale: each step of 1 in magnitude represents a change in brightness of approximately 2½ times (more accurately, 2.515). Although it’s not immediately obvious, the more negative the magnitude, the brighter the object. Thus, the Sun has an apparent magnitude of -27, the brightest star visible from the northern hemisphere, Sirius, has a magnitude of -1.5 and Polaris, the Pole Star, sits at +2. Under good conditions the human eye can see stars as faint as magnitude +5. Regulus A has an apparent magnitude of +1.35 (so brighter than Polaris) but Regulus B sits at 8.1 – the difference in brightness on this logarithmic scale is a factor of over 500!

Regulus A & B (the latter at the 7 0’clock position) in the constellation Leo.

One of the peripheral uses of these images was to estimate the ‘performance’ of my telescope-astrocam combination. All of the above images were captured through a 150 mm diameter Newtonian reflector having a focal length of 1200 mm (Skywatcher 150PL) and an ‘entry-level’ camera (Altair Astro 290c). It was a fun thing to do (certainly for a geek) and it gave me confidence that everything was behaving as it ought to be.

Using the Mizar-Alcor system it was possible to verify that my camera’s field-of-view was indeed very small: only ¼° on its long axis! However, it could resolve objects a mere 0.002° apart. The actual distances shown, 1 lightyear between Mizar and Alcor and 55 light-hours between Mizar A and B, are quite short on the cosmic scale – the nearest star to the Sun, for instance, is over 4 ly away. Neptune is about 4¼ light-hours from the Sun.

Newtonian and beginning astrocam on the left, more recent 'deep sky' setup on the right.

When I started getting deeper into the hobby, buying a shorter focal length refractor telescope (Altair Astro 80EDR, focal length 440 mm with 0.8 reducing lens) and an astrocam with a far larger chip (Altair Astro 533c) – both bought second-hand from people I knew – imaging binary stars became a useful ‘filler’ between long runs on larger and fainter objects like galaxies and nebulae. The images below are all taken with this new setup, which has a square field-of-view of 1.4°. There are binary star systems pretty much everywhere one looks …

Even Polaris is a binary system

Next stop, star clusters.

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P.s. there are several earlier posts which will give you the 'backstory' to this current short series. They'll be pretty obvious if you peruse the list of posts on my blog. The first one I wrote on this post-retirement hobby of mine was uploaded a little over five years ago.