August 9, 2021: Every 20 years or so, a thermonuclear explosion occurs on the surface of RS Oph, a white dwarf in the constellation Ophiuchus. Yesterday, it happened again. On Aug. 8th, the brightness of the tiny star increased 600-fold, from magnitude +12 to +5. Keith Geary of Ireland was the first to notice. Hours later, Italian astronomer Ernesto Guido and colleagues photographed the outburst using a remote-controlled telescope in Australia:

This is called a “recurrent nova,” and it is rare. In the whole Milky Way galaxy, only 7 star systems are known to produce such explosions.

RS Oph is actually a binary star–a very lopsided one. On one side is a white dwarf, on the other is a red giant. There’s very little distance between the two, so the gravity of the white dwarf is able to pull gaseous material off the larger star down onto itself. Every couple of decades, enough matter accumulates to trigger an explosion. The last time this happened was back in 2006.

At 5th magnitude, the current outburst is visible to the unaided eye, albeit just barely. Binoculars or a telescope will allow you to see it with ease. Look south after sunset. Ophiuchus hangs high in the sky just above the better known constellations Scorpius and Sagittarius. Sky maps: simple, detailed, really detailed.

Update: Variable star observer Filipp Romanov of Yuzhno-Morskoy, Russia, has just seen RS Oph and estimates that its magnitude has increased further to +4.6.