Double stars - Mizar and Alcor (1st May 2024)

Having got my telescope set up correctly on this evening (see previous post) unfortunately the weather wasn't going to play ball and it was pretty hazy. So I decided to observe something where the weather didn't matter too much and these were double stars. The SynScan handset of the HEQ-5 has a number of double stars preprogrammed into it and so I began to take a tour of them and to photograph them as I went. The first was Mizar and Alcor. This is an extremely wide but very well-known pairing but the interesting part is that Mizar is itself a double which I had only realized recently. Here is my photograph:-

This was a 1s exposure at ISO 1600 on a Nikon D90 at the prime focus of a 140mm Maksutov Cassegrain (an Orion OMC-140). The picture can be better seen by 'clicking' on it. The image is being viewed at full scale (in other words it has not been binned up). Mizar A and B are marked and are magnitude 2.23 and 3.88 respectively. Off to the left is Alcor which is magnitude 3.99. The other star, Sidus Ludoviciana is magnitude 7.58.

To get an idea of the image scale here we can determine the angular separation of Mizar A and Alcor using their coordinates. Mizar A is at RA 13h 23m 55.543s Dec. +54 deg. 55' 31.30" and Alcor is RA 13h 25m 13.538s Dec. +54 deg. 59' 16.65". Using the tool here, the separation is found to be 0.1968 degrees. Their separation in pixels is 1209.25 which gives 0.5859 arcseconds per pixel. 

It should be noted that this image here has been cropped and is 1600x1066 pixels. The full image is 4288x2844 pixels and this corresponds to 0.698x0.463 degrees. To get the right orientation with North upwards and West to the right I used this article for reference (the article is worth a read in itself) where there is a diagram showing the relative positions of the stars relative to North and South. I then rotated my image so that the stars were orientated correctly.

For a double star, a couple of measurements are given. The first is their separation of their components in arcseconds and the second is their position angle. The latter gives the position of the fainter star relative to the brighter and is an angular measure from the north anticlockwise through the east. I measured the separation of Mizar A and B to be 25.08 pixels which corresponds to 14.7 arcseconds. I must be to within about a pixel so + or - 0.59 arcsecond. The measured value is 14.4 arcseconds which is fortuitously close. The measured postion angle is 153 degrees and so by my image I don't think I am too far off in my orientation.

As a footnote, if you read Bob King's article highlighted above, you will discover that all three stars Alcor, Mizar A and Mizar B are doubles in themselves making this a sextuple system and all bound together gravitationally (in the case of Mizar and Alcor, just!).

All text and images © Duncan Hale-Sutton 2024