At the end of 2019, I wrote about the speculation around Betelgeuse and its recent behaviour (see: Space Sunday: a look at Betelgeuse). However, when it comes to a star going nova or supernova, there is a candidate out there that will more than likely do so before the end of this century.
V Sagittae, a variable star in the constellation Sagitta that is actually a binary system some 1100 light years away. In particular, it is in a class of stars called cataclysmic variables (CVs). These comprise a large, ordinary star orbiting a much smaller white dwarf at a distance where the white dwarf (referred to as the primary) distorts the larger (called the donor), drawing off mass to form an accretion disk around itself.
Usually, material at the inner edge of this disk heats as a result of both increasing density and proximity to the primary, causing dwarf nova outbursts, resulting in the pair to suddenly brighten. In some cases the material drawn into the primary can trigger a nova explosion or even a Type Ia supernova explosion, which would completely destroy the white dwarf.
However, the V Sagittae is amongst the most extreme group of CV stars, with the donor star almost 4 times as massive as its companion, and an extremely hot star. The material being drawn off of it is correspondingly hot (around 12,000ºK), and on contact with the white dwarf is either being accreted, building up the white dwarf’s mass and heat, or violently accelerated away in an extreme solar wind. This wind irradiates the inner hemisphere of the donor, further heating it, fuelling a circle of activity that is seeing mass transferred between the two at an ever-faster rate.
Checks back through archival images show the binary has been steadily brightening for around a century – and the rate of brightening has been accelerating. The net result of this is that astronomers believe V Sagittae is now in the closing decades of this life: over the next several decades, it will continue to brighten and the mass transfer accelerate further. Around the 2080s this will reach a point where the donor will spiral into the primary, triggering a catastrophic nova – one so powerful it could, with the assistance of the extreme solar wind, border on a supernova event.
Astronomers studying the system are so confident of their findings, they are prepared to to put a year on when the collision and subsequent explosion will occur: 2083 ±16 years.
When it happens, the normally faint V Sagittae will be between Venus and Sirius in brightness – not as bright as any supernova from Betelgeuse by any means, but it will be bright enough for even casual observers of the night sky to notice it – although it will be somewhat short-lived as a “new star”; after a month or so, it will dim down once more. The explosion itself will totally destroy whatever was left of the donor star at the time it occurred, with the primary likely converted into a red giant with a core of degenerate electron matter, surrounded by a hydrogen burning shell in turn layer surrounded by a vast halo of mostly hydrogen.
Happy Anniversary Yutu-2 and Chang’e 4
China greeted the New Year with some impressive lunar milestones. January 3rd, 2020 the Chang’e 4 mission to the lunar far side achieved its first anniversary of surface operations, while its Yutu-2 mini-rover completed its 13th lunar day of science operations.
Yutu-2 in particular has proven impressive. Designed to have a primary mission of 90 days, it has survived a full year of operations, which include the little rover having to put itself to sleep for 14 out of the 28 days of a lunar day in order to survive the cold lunar night, and it is still going strong. In that time, it has travelled 357.7m, the longest distance travelled by any vehicle on the lunar surface.
In during so, the rover has revealed a great deal about the composition of lunar soil – regolith – within the South Pole Aitken-Basin, including about materials that are believed to have originated from deep inside the lunar mantle. This has helped scientists understand more about the composition, formation, and evolution of the Earth-Moon system. It’s anticipated that Yutu-2 will continue these explorations, helping to better understand the Moon’s composition and locate accessible resources that might be used in establishing and operating a permanent lunar base.
Chang’e 4 is part of a broader Chinese Lunar Exploration Programme, which includes the Chang’e 5 sample return mission scheduled for launch towards the end of 2020. It will be followed by a second sample return mission in 2024, (Chang’e 6), while Chang’e 7 will continue and extend the work of Chang’e 4, and Chang’e 8, scheduled for a 2027 launch, will test technologies and lay the groundwork for a crewed mission.