Star of the Month
KIC 8462852
AKA: Tabby’s Star; Boyajian's Star; TYC 3162-665-1
Position: 20 hrs 06 min 15.5 sec +44 degrees 27 min 25 sec
Due south at 23.29 (BST) on 15 August 2021
KIC 8462852 (the star with the blue cross)
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
I usually make the star of the month relatively easy to see, but sometimes a star is sufficiently interesting in its own right I include it despite it being very faint. Tabby’s star is certainly faint as it is only magnitude 12. This star was first discovered in 1890 as part of the Harvard University Observatory’s famous photographic survey of the sky, and while it duly appeared in several recent catalogues it remained largely overlooked for almost 120 years until it became of the stars observed by the Kepler space observatory between 2009 and 2013, thus gaining the designation KIC 8462852. It became part of the Kepler mission purely by chance by virtue of being in that area of the sky between Deneb and Delta Cygni scanned by Kepler. It was a group of citizen scientists that first noticed that it seemed to have marked light fluctuations. The star was then studied by a team lead by the Armenian-American astronomer Tabetha S. Boyajian – hence its popular name Tabby’s star or more formally Boyajian’s star – who published a paper about these light variations in 2015; the paper posed the question “where’s the flux?” and hence it is known as the WTF paper. Tabby’s star dims by as much as 22% in an irregular manner and there was no obvious reason for this behaviour. It is an F type main sequence star which means it should not display the erratic behaviour of red giants or young stars which often usually variable. Interest in Tabby’s star grew enormously when it was suggested that the light fluctuations could be explained by a Dyson sphere, an artificial energy gathering device previously postulated by the British-born theoretical physicist Freeman Dyson. However this has now been ruled out. The real reason for this variability remains elusive. It could be a cloud of disintegrating comets, a flock of asteroids or debris from the destruction of an exoplanet. More recently two less exotic explanations have been suggested. The dimming could the result of large exomoons being stripped from their planets and taking up new orbits round the star or it could just be due to the photosphere of the planet cooling as a consequence of reduced convection. Perhaps further observation will eventually solve this mystery.
In the meantime, you can observe Tabby’s star yourself if you have a large enough telescope (or use imaging). A six inch refractor, an eight inch Schmidt-Cassegrain or ten inch reflector should be large enough in Havering; in a darker area, a smaller telescope may be successful. It lies roughly half-way between Deneb and Delta Cygni but it has no bright stars close to it, so you will need to use goto. It sits just outside a triangle of stars of similar magnitude as shown in the above image. It is also just to the north of NGC 6866, but Tabby’s star is 1,470 light years away whereas the rather unremarkable open cluster is almost 4,000 light years distant. Tabby’s star has a dim red dwarf companion and hence is a binary star system.