Double Star of the Month:
Sigma Cassiopeiae
AKA: STF 3049
Position: 23 hr 59 min 01 sec +55 degrees 45 min 18 sec
Due south at 01:21 (BST) on 16 September 2022
Sigma Cassiopeiae
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission

If Achird (Eta Cassiopeiae), the double star of the month for October 2020, is the easy double in Cassiopeia, Sigma Cassiopeiae is the difficult one. The two stars have quite a large magnitude difference being magnitude 5.0 and 7.2, but the real killer is the tight separation of 3.1 arcseconds. You will need a telescope with an aperture of at least four inches, five is better, and you will have to crank up the magnification to at least 200x. This also means you will require steady seeing or the highly magnified image will just skittle around in the field of view. So it is tough, especially if you have houses around you, but it is worth the effort. The main star is usually seen as yellow or yellowish white, but the colour of the secondary is less certain with reported colours ranging from blue to tawny or ash-white. I see them as yellow and blue. These colours fly in the face of the actual spectral type of the stars. The primary is a B1 subgiant which means it is in the process of leaving the main sequence and its companion is a B3 dwarf star which is still firmly on the main sequence. Hence they are both large hot blue-white stars and the main star is 12 times heavier than our sun. They are a binary system. The distance was previously said to 4,200 or even 5,000 light years away, but the latest results from Gaia put them much closer at 1,384 light years for the main star. Gaia also puts the secondary at 194 light years from the main star. Sigma Cassiopeiae was seen to be a double by William Herschel on 31 August 1780. He described the colours as white slightly inclining to red and dusky. To find Sigma Cassiopeiae, look to the right (west) of Caph (Beta Cassiopeiae) and you will see a very narrow isosceles triangle (as noted by Herschel), with 4.5 magnitude Rho Cassiopeiae at the apex and two dimmer stars at the base. The left-hand star is Sigma Cassiopeiae (mag. 5.0) and the right-hand star is V1022 Cassopeiae (mag.5.6) which is an eclipsing binary. In the middle of this triangle, as it happens, is the star cluster NGC 7789. 
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