2020 January 11

Could V Sagittae become a bright nova this century?

Jeremy Shears

It’s not often that variable stars make the mainstream news, let alone the popular press! The current fading of Betelgeuse has naturally drawn much attention, with some speculation about when it might go supernova. The story even made the main evening news on the BBC. But now there is more excitement….

Another variable star to gain press coverage recently (at least in the scientific press like New Scientist, but with some spill-over into the broadsheets) is V Sagittae. This followed an announcement from Prof. Bradley Schaefer at Louisiana State University and his colleagues at the 235th meeting of the American Astronomical Society in Honolulu earlier this week. The team had examined images and photometry of V Sge from 1890 to the present which led them to conclude that the binary system has been getting brighter. It’s now 2.5 mag brighter than in 1890. They suggest that the two stars in the binary system could merge spectacularly in a nova in 2083 (plus or minus 11 years).

V Sge is a cataclysmic variable binary star system composed of a main sequence star of about 3.3 solar masses and a white dwarf of about 0.9 solar masses. The orbital period is 0.514 days and the stars eclipse each other each orbit. The stars are gradually spiralling in towards each other and the period is increasing at a rate of -4.73 x 10^-10 days/orbit. The fact that the white dwarf is lighter than its companion is unusual and V Sge is the only super soft X-ray source nonmagnetic cataclysmic variable found so far.

Although not well placed at the moment, the latest visual observation in the BAA Variable Star Section database is by Gary Poyner who puts it at mag 10.8 on January 3. Gary used a 22 cm reflector, but a smaller instrument could also be used.

Jeremy Shears

Director, Variable Star Section

Thumbnail image: the remnant of Nova Persei 1901, the variable star now known as GK Per. Image contains X-ray data from Chandra (blue), optical data from NASA’s Hubble Space Telescope (yellow), and radio data from the National Science Foundation’s Very Large Array (pink). Credit: NASA.