An inconspicuous star in the constellation Sagitta, the Arrow, may become one of the most luminous objects in our sky later this century.
An inconspicuous star in the constellation Sagitta, the Arrow, may become one of the most luminous objects in our sky later this century. According to Bradley Schaefer (Louisiana State University), the 11th-magnitude variable star V Sagittae will outshine Sirius and maybe even Venus, around the year 2083 — despite its distance of some 7,500 light-years.
“It’s going to be an awesome sight,” Schaefer said at a press conference at a meeting of the American Astronomical Society in Honolulu. The celestial fireworks aren’t expected until 2083 or so, though, so only the younger readers of Sky & Telescope may have a change to witness the event.
V Sagittae is a cataclysmic variable, a system in which a main sequence star is losing mass to its white dwarf companion. Cataclysmic variables pepper the sky, but V Sagittae is special because the main sequence star is so massive: 3.3 times the Sun’s mass, which makes it 3.9 times the mass of the white dwarf. As a result, the star is losing its outer layers at a prodigious rate of some 6 Earth masses per year.
According to Schaefer and his colleagues, the situation is quickly running out of hand.
Old photographic plates at the Harvard College Observatory, combined with archival data from the American Association of Variable Star Observers (AAVSO), reveal that V Sagittae has been gradually brightening, gaining 2.5 magnitudes (a factor of 10 in brightness) since its discovery in 1904. According to the team, which has submitted the results to Monthly Notices of the Royal Astronomical Society, this indicates a ten-fold increase in the mass falling onto the white dwarf.
As the larger star loses mass to the white dwarf, the binary pair’s orbit should tighten. Indeed, a careful comparison of older and more recent observations show that the two companions eclipse each other from Earth’s perspective, and their orbital period is decreasing. Right now, the stars orbit each other every 12.34 hours, but this value is decreasing by about 0.015 seconds per year.
According to Schaefer and his colleagues, the mass loss is currently doubling every 89 years, an exponential increase that will end catastrophically in the second half of this century. Sometime between 2067 and 2099, the main-sequence star will be ripped apart, dumping about one final solar mass worth of gas onto the white dwarf over just a few days.
The result, which Schaefer describes as a “merge burst,” would maintain a luminosity between that of a regular nova and a supernova explosion for weeks on end. “What remains,” he says, “is a red giant star with a hydrogen-burning mantle and a white dwarf core.”
“It’s an interesting suggestion, that may even be true,” comments astrophysicist Gijs Nelemans (Radboud University, the Netherlands). However, he cautions that the team may be putting too much confidence in rather simplistic theories about the future development of the mass transfer. “I don’t believe that their prediction can be as precise as they suggest,” Nelemans says, “but it would of course be great if the current trend will indeed continue.”
Schaefer expects that future observations will lead to more precise predictions of V Sagittae’s showtime. But he is quite sure that “people the world over […] will see a wondrous guest star shining as the brightest in the sky for a month or so.”