About every 15 years, enough material from the red giant builds up on the surface of the white dwarf to produce a thermonuclear explosion.

Williamina Fleming discovered a nova-like spectrum in the Henry Draper Memorial photographs and announced it as a potential nova in 1904.

This diagnosis was affirmed by Edward Charles Pickering in 1905, after which Annie Jump Cannon determined that RS Ophiuchi had likely reached maximum in 1898.

In January 1985, Warren Morrison of Peterborough, Canada discovered RS Oph to again be in outburst, reaching a maximum brightness of magnitude 5.4.

It was notably observed with the VLTI by Olivier Chesneau, who discovered an elongated fireball as early as 5.5 days after the explosion (see the figure below).

[9] On 8 August 2021, the Brazilian amateur astronomer Alexandre Amorim, from Florianópolis, Brazil detected a new outburst of RS Oph at 21:55 UT and sent a notification to AAVSO.

[10] The Fermi Gamma Ray Space Telescope corroborated optical observations made by Amorim and Geary of a new outburst associated with RS Oph, with an estimated visual magnitude of 5.0.