May 2017 - Double Star of the Month
24 CBe (12 35 07.76 +18 22 37.4) sits just on the northern edge of the Coma Cluster of galaxies and about 10 degrees to the south of the coarse open cluster of stars in Coma.
It is a fine pair for the small aperture and well worth looking for. The different spectral types of K2III and A9Vm promise a substantial colour contrast. Smyth notes
A, 5.5 orange colour, B 7 - emerald tint,- the colours very brilliant. Hartung, from Australia finds deep yellow and white, whereas Sissy Haas records citrus orange and fainter royal blue.
The WDS gives the visual mags as 5.1 and 6.3. Hipparcos gives similar proper motions but differing parallaxes - 7.24 mas for A and 19.29 for B but the latter comes with an error of +/-14.58 mas possibly due to the fact that this star is known to be a spectroscopic binary (SB). The relative positions are very useful for calibration purposes. At present the PA and separation are close to 270 degs and 20".
Starting at beta Crucis and moving eastwards by about 3 degrees will bring you to a pair of barely naked-eye stars separated by about a degree in right ascension.
The first of these is the fine pair R 213 (6.6, 7.0, 22 degs, 0".7) whilst the second is the multiple star I 424 (13 12 17.63 -59 55 13.9). To the user with 15-cm or more, here is an unequal pair of stars of mags 4.6 and 8.4, separated by 2". This has opened slightly and moved 20 degrees in increasing PA since being found by R.T.A. Innes.
Two fainter and wider companions are also catalogued. C is mag 12 at 258 degs, 46" whilst D, a much greater test, is mag. 14.9 at 24 degs and 53".
Whilst in the neighbourhood, about 7' north-east is the wide pair COO 152 (mags 6.2, 9.4, 146 degs, 25").
If you are able to bring 50-cm aperture to bear then there are two further tests. I 424 A is the close binary SEE 170 - currently at 0".17 and with a period of 27.4 years; the mags are 5.3 and 6.2. One of these stars is also a SB - actually an eclipsing beta Lyrae-type system.
The A star of COO 152 was resolved by Hipparcos. The components of this system are 6.4, 9.7 and the separation is 0".4. One of these stars is a SB of 4.23 day period and the 25" component is also physical meaning that COO 152 is a quadruple system.
Bob Argyle - Double Star Section Director