The recurrent nova RS Ophiuchi just went into outburst — its first burst in 15 years — and it’s bright enough to see with the naked eye.

RS Oph animation
The recurrent nova RS Ophiuchi dramatically brightened from magnitude 11.2 to 4.8 over August 8-9, 2021. The outburst image was taken on August 9.42 UT and paired with an older photo from the Palomar Observatory Sky Survey (POSS) from July 1989 when the star was at minimum. Ernesto Guido, Marco Rocchetto & Adriano Valvasori / telescope.live

A star that normally slumbers around 12th magnitude suddenly "woke up" seven magnitudes brighter this past weekend. Now you can see it with the naked eye!

Irish amateur Keith Geary was the first to report the surprise outburst of RS Ophiuchi, one of the few known recurrent novae. He captured photos of it glaring at magnitude 5.0 with his DSLR camera at 22:20 UT on August 8th and confirmed his observation in binoculars. Fellow AAVSO member Alexandre Amorim of Brazil also spotted the eruption 25 minutes earlier.

RS Ophiuchi wide-view map
RS Ophiuchi is located in eastern Ophiuchus near the Serpens border and well-placed for observing at nightfall.
Stellarium

Since then, the star has brightened up to magnitude 4.5 (August 9.7 UT), making it a relatively easy naked-eye object even from outer suburban areas and a cinch to see in binoculars.

RS Oph light curve
The light curve of RS Ophiuchi during its last outburst in 2006 reveals the rapid rise, relatively fast decline and a slower drop-off back to minimum light over a period of about six weeks. Courtesy of the AAVSO

Typically, RS Oph rises steeply and rapidly to maximum and then quickly declines, dropping about two magnitudes over a week's time, before leveling off and fading more gradually. The outburst will make for exciting observing in binoculars and small telescopes in the coming days. Try to catch the star at every opportunity you can because the next blast won't likely recur for another 15 or 20 years!

Use this zoomed-in map to pinpoint the recurrent nova. Both Mu (μ) and Nu (ν) Ophiuchi make helpful guide stars. Numbers are magnitudes with the decimals omitted and correspond with the stars on the more detailed AAVSO map below.
Stellarium

RS Ophiuchi last blew its top in February 2006 and before that in January 1985. The process that makes a recurrent nova is essentially the same as in the classical variety: A white dwarf and a donor star orbit in a close binary system. As the donor star evolves — a red giant in the case of RS Ophiuchi — it overflows its Roche Lobe and the white dwarf snatches the spilling gas (mostly hydrogen), which forms a spinning accretion disk around the compact star.

Nova illustration
In this artist view, a white dwarf (left) robs gas from its companion star. The material forms an accretion disk around the dwarf before spiraling down to the surface where it's compressed and ignites in a thermonuclear blast.
ESO / M. Kornmesser

Material in the disk funnels to the dwarf's surface, where it's compacted and heated until the base layer reaches about 10 million degrees Celsius. It then ignites in a thermonuclear explosion which blows the envelope of material into space at high speed and makes one heck of a bright blast. According to recent observations reported in the Astronomer's Telegram, RS Ophiuchi is expelling material at around 2,600 kilometers per second.

Recurrent novae repeat the nova process approximately every 10 to 100 years, while nova outbursts are thought to repeat on much longer timescales from around 1,000 to 100,000 years. You can't wait around for a nova to go nova again, but for the approximately 10 recurrent novae (yes, they're rare!), if you live well you can see a repeat or two in a lifetime.

RS Ophiuchi AAVSO chart
This chart from the American Association of Variable Star Observers (AAVSO) includes magnitudes (with decimals omitted) so you can compare RS Oph against stars of known brightness to watch its light vary.
Courtesy of the AAVSO

I love it when amateurs discover things. I asked Geary if he would share his discovery account, which you'll find below. I think you'll wholeheartedly agree with his conclusion.

"I have been following this star and many other variables since the year 2003. I regularly submit my observations to the AAVSO," said Geary.

Keith Geary portrait
Irish amateur astronomer Keith Geary was the first to report the current RS Ophiuchi outburst.
Courtesy of Keith Geary

"I had been on a family holiday visiting my parents in Waterford, Ireland. Up to last night I had eight nights of cloudy weather, however I persevered and made my way to a familiar observing spot called Dunbratton County awaiting nightfall.  I began my visual and DSLR nova patrol using my APM 20×100 binoculars and Canon 6D at 21:30 UT, until I took a regular shot of the RS Ophiuchi region at 22:20 UT with my Canon 200-mm f/2.8 lens.

I examined the image and immediately thought that I had made an error. I thought I was off target. To my amazement after re-examining my image I could see RS Ophiuchi shining at magnitude 5.0. I was astounded as it was my first ever success!  I immediately emailed CBAT and the AAVSO to raise the alert.

Persistence pays off! I would encourage anyone reading this article to take up variable star astronomy, who knows what the heavens will show us all next!"

Image of RS Oph August 8, 2021
Brazilian amateur Willian Souza snapped this photo of RS Oph (arrowed) at 23:35 UT August 8, 2021 with his Samsung mobile phone using the ground for a "tripod." He estimated its magnitude at 4.9 with the naked eye. Willian Souza

Comments


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jfleborgne

August 10, 2021 at 6:02 am

Hi
It is interesting to know when was the first observation of the outburst of RS Oph. My colleague Michel Dumont (GEOS) phoned me on August 8 at about 21h30 UT. He had visually seen it at mag 5 from Jura mountains in France. I made my first image of RS Oph at 22h08 UT from Marseilles (France). It was indeed in outburst. instrument: 5cm telephoto lens, focal length 135mm+CCD camera.
J.F. Le Borgne
GEOS
Astronome émérite, IRAP, Observatoire Midi-Pyrénées, Toulouse
Chercheur associé, Laboratoire d'Astrophysique de Marseille

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Bob King

August 10, 2021 at 10:54 am

Dear J.F.,
Thank you for the information about your early imaging of RS Oph as well as the Michel's 21:30 UT observation. Those are certainly among the earliest reports of the outburst. I'm also interested in the first observation.

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Alex Amorim

August 11, 2021 at 1:00 pm

Dear friends,

It seems that our friend Keith Geary observed RS Oph at 22:20 UT and quickly reported to CBAT, following his this report to VSNET list at 22:35 UT:
http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26131

I observed RS Oph at magnitude 5.0 at 21:55 UT and reported to the AAVSO database and to the AAVSO Forum at 22:22 UT.
https://www.aavso.org/rs-ophiuchi

So, I back to my observation session. After clouds in sky, I back to computer e send an e-mail to Dr. Green who wrote in CBET 5013 including my name as an independent detection:
http://cbat.eps.harvard.edu/iau/cbet/005000/CBET005013.txt

and quoted the link to my photo:
http://www.geocities.ws/costeira1/img/20210808_2159ut.jpg

Alex!

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Bob King

August 11, 2021 at 1:34 pm

Dear Alex,

Thank you for this additional information, and congratulations on your independent discovery!!

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Valentin Grigore

August 18, 2021 at 7:13 pm

Hi friends,

I made photos in the nova area capturing Perseid meteors starting at 19:46 UT on August 8. The last capture in this area was made at 21:24 UT. RS Oph seems to have same magnitude (estimated at +5) during this period.
Place: Runcu Stone, Runcu, Romania.
Do you know another photo capture of RS Oph made early?
At 21:23 UT, I captured a Perseid meteor that, in its final part, indicated exactly the direction where the nova was! 🙂
You can see this pictures here:
https://www.facebook.com/photo?fbid=10227164770087757&set=a.1431551511448

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Bob King

August 19, 2021 at 12:56 am

Beautiful image, Valentin! Your image was taken more than two hours before Keith Geary's. The earliest images I'm aware of were taken by two Chinese observers starting at 13:47 UT, August 8.

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Valentin Grigore

August 19, 2021 at 3:02 am

Thank you, Bob!
It is interesting to find when nova eruption started. To know if the outburst was directly at magnitude of +5 or fainter.

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Dylan Rude

September 12, 2021 at 11:01 am

I believe my friend Mr. Janzen and I witnessed this event with our naked eyes. It's a long story, to keep it short... we were outside in a birch Wilderness Steam Bath, as Mors Kochanski or Dragan Uzelac might call it... Our wives went to bed and we thought we would do one more steam session in the complete darkness of the night. We sat and talked in the steam bath, some moments later Mr. Janzen said "do you hear something " and he stuck his head out the door, I followed him. Mr. Janzen exclaimed "Whoa" as he turned his eyes up to the sky, I followed him and looked up... or was it down? A matter of perspective I suppose. The stars revealed themselves to us as we lay there in awe. In the moments that followed we saw a bright flash in the sky, a circular expression of light. I said "what was that?". Mr. Janzen replied "I think it was a star exploding". I didn't know what this might look like but I had to agree with Mr. Janzen.

With in the following days I saw an Instagram post by the Royal Astronomical Society of Canada... sure enough a star "exploded" the night we were out star gazing. This very particular series of events leads me to believe that we witnessed this event. A very profound experience to say the least...

Dylan J.L. Rude

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Bob King

September 12, 2021 at 11:33 am

Dear Dylan,
It sounds like you witnessed an amazing sight, but unfortunately it would not have been the explosion of this star. When RS Ophiuchi underwent the initial nova outburst it was only magnitude 5.0, much too faint to notice unless you knew exactly where to look. In which case you would have seen a barely visible star. A few days later it rose to 4.5, which is still faint to the unaided eye. Also, stellar explosions, whether they be novae or supernovae, don't pop on and off like a flashbulb but linger many days, weeks and even months. Frequently, they continue to brighten during that time before fading.

So what did you see? Two likely possibilities come to mind: a bright glint from a passing satellite (I've seen many of these myself over the years) or a head-on meteor. Depending on its location it could also have been a reflection from a satellite in geostationary orbit Thanks for sharing your observation!

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Anthony Barreiro

August 10, 2021 at 2:25 pm

Thanks very much for the heads up! Yesterday evening I saw the nova with 10x42 image-stabilized binoculars, through breaks in fast-moving low clouds. With Ophiuchus culminating at dark, and the nova in a neat little triangle with nu and mu Ophiuchi, it was an easy catch, even with the clouds. Very cool to think that we're witnessing the afterglow of a sudden huge thermonuclear explosion about 5000 light years away.

Congratulations to Mr. Geary, and to everyone else who witnessed the initial outburst and knew what they were seeing.

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L.A.B.

August 10, 2021 at 3:28 pm

Came to this site to confirm what I saw last night and I felt honored to witness that Nova star. I was amazed and I did wish. hehehe

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Bob King

August 10, 2021 at 3:45 pm

L.A.B.,

May your wish come true 🙂

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Bin

August 10, 2021 at 9:36 pm

On the night of August 8, my friend and I were observing in the High Mountain(Shaoxing, Zhejiang, China),We took images in the first 3 hours of the nova outburst earlier than Geary ,at 20210808 13:47(UT) ,we find the nova outburst,it brightness to 9.1 mag; 13:53(UT), it's 8.4 mag; 14:10(UT), it's brightened to 7.0 mag.
the image URL:
http://nova.astrometry.net/image/11658797
http://nova.astrometry.net/image/11658847
http://nova.astrometry.net/image/11658900

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Bob King

August 10, 2021 at 11:49 pm

Dear Bin,

We are grateful for your photos and observations here at Sky & Telescope. Thank you! Check your email for a message from me about your images.

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SNH

August 10, 2021 at 10:41 pm

Extremely quick and informative post there, Bob. I saw it naked-eye (my estimate was +4.8) last evening thanks to you! I checked Gaia briefly and it seems that RS Oph might be around 8,500 light-years away.
But, at the risk of being a nitpicking weasel, I have to point out that on your AAVSO chart, you have the area around Feb 1/2 labeled as "Jan. 29, 2006". Just letting you know.

Scott

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Bob King

August 11, 2021 at 12:06 am

Hi Scott,

Thanks for your kind words and for pointing out the mislabeled light curve date. Now corrected.

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Anthony Barreiro

August 11, 2021 at 3:05 pm

I'm trying to figure out where this nova is in the galaxy. In the wikipedia article on RS Ophiuchi, the main article says it's "approximately 5000 light years away," while the sidebar lists a distance of 3900 - 6500 light years. Now Scott says Gaia says it might be around 8500 light years away. Any of the wikipedia distances would put it in the next spiral arm in from us, the Sagittarius arm. At 8500 light years, it looks like it would be above the galactic plane over the Scutum Centaurus arm. If anybody has any insight on this, I would appreciate it.

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Andrew James

August 11, 2021 at 7:27 pm

Don't trust Wikipedia for astronomical information or presented results because contributors cherry-pick sources often without comprehending the accuracy nor the significance of the method employed. The 2008reference source is here[1], and the discussion will answer your questions. It's clear the ref does really specify any of the quoted values except their 1.4^+0.6-0.2 kpc., which is based on peak luminosity. This is equal to 4570 light years and ranging between 6570 to 3910 ly. The 5000 likely comes from the average of the range or 6570+3910/2=5240 ly averaged to 5000. The paper says: "Given this,we recommend that estimates should always be reported with an associated uncertainty interval." Wikipedia ignores that bit.

GAIA gives parallax as 0.4419 mas = 2.263 kpc or 8194 light years.

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goodricke1

August 13, 2021 at 5:17 pm

2.263 kpc is 7381 LY.

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Anthony Barreiro

August 13, 2021 at 10:24 pm

Thanks Goodricke. At 7400 light years it looks like the nova would be on the inside edge of the Sagittarius arm. I like thinking of it there.

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Andrew James

August 15, 2021 at 6:33 pm

Sorry. Conversion error by me. Accidentally multiplied by 3.6216 instead of 3.2616. Also I always think in terms of parsecs and avoid light years for this exact reason. BTW. 0.4419±0.0527 mas is 2.2630±0.2738 kpc. Thanks.

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Andrew James

August 11, 2021 at 8:11 pm

Wikipedia should be using 1.4 kpc or 4570 or rounded 4600 ly. If using GAIA the distance is better said as 8200 light years.

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Anthony Barreiro

August 11, 2021 at 11:47 pm

Thank you Andrew. I'll imagine the nova at 8200 light years, floating above the Scutum Centaurus arm of our galaxy.

I take everything I read on wikipedia with a grain of salt. But there is one nice thing about wikipedia: you could edit the article and improve it. It's an endlessly iterative encyclopedia.

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David Benn

August 14, 2021 at 5:16 am

Congrats and nice work Keith and Alexandre! Recurrent novae are so cool, as are novae in general, especially bright ones. I made a few visual observations of RS Oph and wrote a couple of blog posts about it in the last week, in an attempt to encourage interest: https://strangequark.me/category/astronomy/

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