October 2019 Sknotes: Almach as a triple star once more

Almach as a triple star once more

The beautiful double star Almach, or γ Andromedae, is one of the great telescopic sights of autumn skies and one of the finest colour-contrasted doubles in the heavens. Until 2003 a good telescope of at least 10 inches (0.25m.) aperture would have shown this beauty as a triple at high power under the best seeing, the blue component of the obvious double itself being a famous binary OΣ 38, revolving in a highly elongated orbit every 62 years. For most observers that ceased to be the case around mid-2003, however, as OΣ 38’s highly eccentric orbital motion took the apparent separation of its component stars below 0.4 seconds of arc, where they have been ever since. The system passed through periastron in mid-2014, at which point, at a separation of somewhere about 0.021 arcsec., γ2 Andromedae was well beyond the resolution-limit even of the Hubble Space Telescope.

In truth, the exact separation of the stars at their closest remains completely unknown, as there appear to have been no published measures of the binary since early 2010, despite this being its first periastron which could have been observed with a technology – speckle interferometry on 4-metre class telescopes – powerful enough to resolve it. It will now be nearly another 60 years before this deeply unfortunate omission can be rectified. Meanwhile, however, the two stars of OΣ 38 are now swinging apart again and just, infinitesimally just, rising above the uttermost limits of double-star detection in amateur-size instruments. This has been closely followed with the 12½-inch reflector at Hanwell, working at x825: in 2011 and 2015 γ2 And. was absolutely ‘round as a button’ to the most intense scrutiny in very good seeing – not a ghost of a hint of its being a double star – but over 12 nights determined examination in this last evening apparition of 2018/19 it was repeatedly seen just beginning to tease apart into a very slightly, but quite unmistakeably, oval ‘olive’ shape. This is still well short of actual resolution, which will not be possible in this aperture for at least another four or five years but the writer urges readers with any experience of, or appetite for, high-power observation of very close, sub-arcsecond double stars to have a go at following this. It is not an easy one.

In a detailed paper due to appear* in the Journal of the British Astronomical Association, the writer has revised the orbital elements of OΣ 38 to optimize their fit to all the best published measures on the relevant periastron-arc, from which this ephemeris computes:

t: 2018.86 2019.16 2019.8 2020.8 2021.8
ρ: 0.20 0.21 0.23 0.26 0.29 (separation, arcsec.)



2018.86-2019.16 being the epoch spanned by last season’s Hanwell observations.  This is a fine challenge presented by a gorgeous system – do have a go!

                             Christopher Taylor, Hanwell Community Observatory, 22.x.19.


* Probably in mid-2020.