Star of the Month
Z Andromedae
AKA: HD 221650; SAO 53146
Position: 23 hrs 33 min 40.0 sec +48 degrees 49 min 06 sec
Due south at 00.54 (BST) on 16 September 2021
Z Andromedae (shown with a cross)
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
In July, we met a cataclysmic variable called SS Cygni. This month we have another cataclysmic variable in our sights, namely Z Andromedae, which is a distant star, being 6,400 light years away. Like SS Cygni, Z Andromedae is a prototype star, in this case for the symbiotic binary variables. Since it was discovered by Scottish-American astronomer Williamina Paton Fleming at the Harvard University Observatory in 1901, five years after SS Cygni was discovered at the same observatory. She noticed the star’s peculiar visible spectrum and it was studied more closely by her colleague Annie Jump Cannon. It was proposed as a prototype star by Paul Merrill at Mount Wilson observatory in 1941. The binary system consists of a distended red giant (of two solar masses) whose radiation pressure often exceeds the weak gravity at its surface. The resulting stellar wind blows onto the surface of a companion white dwarf (of 0.75 solar masses). When enough matter has accumulated onto the white dwarf it flares up and brightens. As the matter starts to pile up, an accretion disc is created round the white dwarf and when there is too much material blowing in from the red giant, the white dwarf even produces jets. As with SS Cygni, it is obvious that the process is going to be fairly regular but not a precise cycle. The two stars co-exist quietly for much of the time, but the white dwarf flares up with a period of about 759 days (25 months).
Z Andromedae is usually about magnitude 11.3 (i.e. invisible to most of us) but flares up to about magnitude 7.7 (exactly the same as SS Cygni) for a day or so. It last flared up on 19 December 2019 when it brightened from 10.3 to 7.9 and back to 10.3 within a day. There were several similar outbursts in November 2019. Ever since 19 December 2019, it has been between 10.3 and 11. This suggests that another outburst could occur soon. To find Z Andromedae, it is almost half way between the Andromeda Galaxy and M39 in Cygnus, and to the south of Caph in Cassiopeia and to the north of the Blue Snowball planetary nebula. More precisely find Kappa Andromedae (mag. 4.2), draw a line to nearby Lambda Andromedae (mag. 3.8) to its south-west and continue on that line just a bit further in the same distance. Z Andromedae lies just east of the bright but unremarkable open cluster NGC 7686. It sits on the edge of a triangle of 9th magnitude stars (as in the above image) which can be used for comparison purposes (Z And is going to be significantly dimmer or obviously brighter than these three stars).