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A rare nova visible to the eye. See it while you can!

Nova visible to the eye: Outline of Ophiuchus with nova RS Ophiuchi labeled.
View at EarthSky Community Photos. | Filipp Romanov in Yuzhno-Morskoy, Russia, captured this photo of RS Ophiuchi – a rare nova visible to the eye – on August 10, 2021. He wrote: “Recurrent nova RS Oph in outburst. I observed it with the unaided eye and estimated it at +4.6 mag. I took photos from my small hometown on August 9, 2021, from 15:19 to 15:22 UT. I had been waiting for several hours, and the sky cleared for a few minutes.” Thank you, Filipp!

A rare nova visible to the eye

On August 8, 2021, the star RS Ophiuchi burst into view in Earth’s sky as what astronomers call a nova. It’s a rare nova visible to the eye alone. It’s nowhere near as bright as the brightest stars. But, in a dark sky, you can see it shining in the constellation Ophiuchus the Serpent Bearer. Earlier this week, the nova was said to have brightened to magnitude 4.3. If true, that would be huge amount of brightness increase for this star. RS Oph normally shines at around 12th magnitude. It’s usually much too dim to see with the eye or even most backyard telescopes. But a sudden thermonuclear explosion on this star’s surface has increased its brightness several hundredfold.

This isn’t the first time RS Oph has gone nova. RS Oph is part of a recurrent nova star system. That is, the system has regular explosions. RS Oph is part of a 2-star system. One is a white dwarf, a highly compact, evolved star. The other is a red giant. The explosions happen when the white dwarf accretes or pulls too much mass from its red giant companion. The excess mass triggers a thermonuclear explosion in a hydrogen layer on the white dwarf’s surface. You might see that this can happen periodically, as the two stars orbit each other, and one keeps pulling from the other. The system’s most recent nova was in 2006, but known explosions of the star go back to 1898.

The explosions of RS Oph are spaced out approximately every 15 years. That means we can expect a future nova from RS Oph too. A system like RS Oph – whose outbursts recur on the order of decades – are rare. So the current outburst of this star is a special treat.

How to find RS Ophiuchi

The nova brightened this week, but now may be beginning to fade. Over the past day today (August 10, 2021), we didn’t see any brightness estimates from the American Association of Variable Star Observers any brighter than 5th magnitude. That’s still within the limit of seeing with the eye alone, but just barely. See it while you can.

The constellation Ophiuchus is currently in the south after sunset. It lies above the better-known constellation Sagittarius and its familiar Teapot asterism, which is closer to the horizon. If you look above the spout of the Teapot and cross the cloudy swath of the Milky Way, you’ll come to the region of sky where RS Oph lies.

The brightest star in Ophiuchus is Rasalhague, located near the top of the constellation, and the 2nd-brightest is Sabik, located near the bottom. RS Oph lies about midway between the two stars but farther to the left (east). You can use the photo above as a star chart to get you to the right area, then use the photo below to narrow down the field. You can also find a star chart at SpaceWeather.com.

Star field with brightest point labeled RS Oph.
View at EarthSky Community Photos. | Peter Forister in Charlottesville, Virginia, captured this photo of RS Ophiuchi on August 9, 2021. He wrote: “I captured a very rare astronomical event – a recurrent nova outburst! This nova is visible to the unaided eye currently in the constellation Ophiuchus, but will not last long as the brightness diminishes over the next few days. This is an unpredictable/sudden thermonuclear explosion on the small star RS Oph which last occurred in 2006. I got lucky with clear skies the night after the outburst was observed.” Thank you, Peter!

Observer reports of RS Oph

Observers took to Twitter to share their reports of RS Oph.

Bottom line: RS Ophiuchi is a stellar system that undergoes recurrent novas, and on August 8, 2021, a nova appeared to observers with the unaided eye. Is it still visible? The only way to know is to look.

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