A revival of RS Ophiuchi means that the normally invisible white dwarf star can now be seen unaided.

Observant astronomers have spotted a new star in the constellation Snake Bearer. It concerns RS Ophiuchi: a white dwarf that is currently undergoing a rather rare revival and is therefore (temporarily) visible to the naked eye.

Recurrent nova
RS Ophiuchi is a so-called recurrent nova. The white dwarf star is part of a binary star system that also harbors a red giant. Material from that red giant flows away to the white dwarf. This is where the material (mainly hydrogen) accumulates. If enough hydrogen is collected, the pressure and temperature in it can rise to such an extent that hydrogen nuclei fuse into helium. We know the same process from the hydrogen bomb and it ends in the same way: with an explosion. And the brightness of the white dwarf increases considerably. In RS Ophiuchi’s case, this is a repetitive process; after the explosion, material flows from the red giant to the white dwarf again and work is done towards another explosion.

The last time RS Ophiuchi increased significantly in brightness due to an explosion was in 2006. And now it’s hit again. Astronomers report a magnitude of 4.8, making the star visible to the naked eye.

It’s a relatively rare revival; On average, RS Ophiuchi’s brightness increases every 15 years (see also the light curve below). However, the increase in brightness is not the same every time. RS Ophiuchi, for example, is now brighter than it was in 1985. The length of the period between explosions also varies; sometimes there is only nine years between them. Sometimes 21 too.

Image: PopePompus (via Wikimedia Commons).

The increase in brightness is temporary. So the star will soon disappear again from the part of the firmament visible to the naked eye. A good reason to try again in the coming days to spot the star.

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