RS Ophiuchi, artist impression. Credit: David A. Hardy.
RS Ophiuchi is a recurrent nova that lies several thousand light-years away in the constellation Ophiuchus. Typical of all stars of this type, RS Ophiuchi is close binary star system consisting of a white dwarf in orbit with a red giant. The two stars are so close together that the intense gravitational field of the white dwarf continuously pulls hydrogen-rich gas from the outer layers of the red giant. About every 20 years, so much gas builds up that a runaway thermonuclear explosion occurs on the white dwarf's surface. In less than a day, its energy output increases to over 100,000 times that of the Sun, and a quantity of gas equivalent to the mass of the Earth is ejected into space at speeds of several thousand km/s.
Explosions such as this on short timescales of decades can only be explained if the white dwarf is near the maximum mass it could have without having collapsed to become an even denser object – a neutron star during a supernova explosion.
What is also very unusual about RS Oph is that the red giant is losing enormous amounts of gas in a wind that envelops the whole system. As a result, the explosion on the white dwarf occurs effectively inside its companion's atmosphere and the ejected gas then slams into it at very high speed.
The last nova outburst of RS Oph was observed to occur on the night of 12 February 2006. From its normal apparent brightness of 12.5, RS Oph suddenly grew in brightness to become visible to the naked eye. Although this was the latest in a series of such outbursts of this star that have been spotted over the last hundred years or so, it was the first one since 1985 and gave scientists an opportunity to study it with new, more powerful, telescopes on the ground and in space.
|9.6-15.5 (quiet), < 5 (burst)
|M2III / white dwarf
|1,950–5,000 light-years (600–1,600 pc)
|RA 17h 50m 13.2s, Dec +06° 42' 28"