The Perseids, a rare eruption of nova RS Ophiuchi and a challenging dawn comet round out an amazing week of skywatching.

It couldn’t have happened at a better time. While we’re all gearing up for the peak of the Perseid meteors this New Moon week on August 12th, two more astronomical events have given us a reason to step outside on warm August nights: the eruption of recurrent nova RS Ophiuchi, and the brief appearance of comet C/2021 O1 Nishimura.


Naked eye nova RS Ophiuchi, along with the outline of the constellation Ophiuchus. Credit: Filipp Romanov

RS Ophiuchi Erupts

First up, is this past weekend’s eruption of RS Ophiuchi. This variable star is a member of a rare class of stars known as recurrent novae, which erupt in a spectacular fashion. T Pyxidis and U Scorpii are members of this same rare club of variable stars. Only 10 recurrent novae have been identified in our galaxy to date: they’re that rare. What you’re seeing is a white dwarf and red giant star in a tight orbital embrace, with the white dwarf siphoning off material from the red dwarf star until in compresses and ignites briefly.


A wide field finder for RS Ophiuchi, looking south at 9PM local from 35 degrees north latitude. Credit: Stellarium.

RS Ophiuchi erupted six times in the 20th century, and most recently flared up in 2006. Though it averages an eruption every 20 years or so, its irregular period has seen gaps as brief as nine years. It has topped out at +4 magnitude in the past, and the American Association of Variable Star Observers AAVSO currently lists it at +4.5 magnitude ‘with a bullet’.


A narrow field binocular finder view for nova RS Ophiuchi, along with a five degree wide Telrad finder. Credit: Stellarium.

Already, the Fermi Gamma Ray Space Telescope has backed up visual observations of RS Ophiuchi, suggesting that an outburst is indeed underway. The American Association of Variable Star Observers now has Alert Notice No. 752 posted for RS Ophiuchi.


A radio/VLTI map of the 2006 eruption of RS Ophiuchi. Credit: ESO/VLTI/VLBA Creative Commons 4.0 license.
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