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.