Amazing RS Puppis, a star with a groovy echo

Hubble photo of the variable star RS Puppis and its surrounding cloud of gas and dust. RS Pup is a Cepheid variable that brightens and dims as it expands and contracts. Click to learn more about Cepheid variables. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration

Have you ever seen an echo? Maybe if you’re a bat, which can form a mental picture of its environment using echolocation. Well, then, allow me to introduce you to the star RS Puppis in the constellation Puppis the Ship’s Stern. Unlike the steady sun, RS Pup pulsates like a beating heart, becoming brighter and fainter by a factor of five about every 40 days. Instabilities in the star’s core brought on by switching from hydrogen to helium fuel cause regular expansions and contractions – actual changes in the star’s size – that we see as cyclic changes in its brightness.


Time-lapse video using observations from the Hubble to show the variable star RS Puppis. What appears to be an expanding gas cloud are really echoes of light reflecting from a gas cloud around the star.

Lucky for us bat types, the star is swaddled in nebulosity, a swirly cloud of gas and dust. As RS Pup brightens, a pulse of light travels from near to far across the expanse of the nebula, reflecting off gas and dust to create a light echo. Watching the video might make you think you’re seeing an expanding gas cloud. That’s an illusion. Instead, light moves across the nebula like someone pointing a flashlight in a cave, the beam touching nearby walls first and then reaching farther back into the deeper recesses. Every 5-6 weeks another pulse echoes across the murky gas clouds.

Artist’s impression shows the location of RS Pup in the Milky Way Galaxy. RS Pup is about 6,500 light years away, with an uncertainty of 90 light-years. It’s 10 times more massive than the sun and 15,000 times brighter.  Credit: ESO / Hubble

We hear an echo when sound bounces off objects in the environment and reflects back to our ears; echoes around RS Puppis happen the same way except with light instead of sound.

Interestingly, as the reflected light bounces off the nebula’s nooks and crannies, it takes a variety of different paths through the gas and arrives at Earth after light that travels straight from star to telescope. This helps reinforce the ‘expanding gas cloud’ illusion.


Time-lapse video of the light echo around V838 Mon photographed multiple times between 2002 and 2006 by the Hubble Space Telescope.

An even more amazing light echo blossomed into view when the star V838 Monocerotis underwent a tremendous outburst in early 2002, suddenly becoming more than 600,000 times brighter than the sun. Over the next few years the eruption illuminated whorls and eddies of interstellar dust around the star, producing the most spectacular light echo ever recorded. Even modest-sized telescopes showed the light echo as a small, comet-like haze.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

12 thoughts on “Amazing RS Puppis, a star with a groovy echo

  1. Bob, did you ever see that “comet-like haze” from V838 Mono back in ’02? I recall looking for it with my 12.5-inch but not finding it. And, for that matter, have you ever seen the glow from Nova Persei 1901?
    Me neither.
    I have seen the glow from Nova Tauri 1054, though!

    • Richard,
      I’ll never forget V838 Mon. I had been following its outburst using an AAVSO chart. One night I looked and exactly centered on the star I saw a small nebula. My first thought was “could it be a comet?”. I really got excited since I had not seen any observations posted of the nebula. It was very easy to see in the first few nights but rather quickly faded as did the star. V838 Mon was a signature experience for me as an amateur astronomer – never forget it. As for Nova Persei 1901, I’ve tried but not seen it with my 15-inch. Ah, but Nova Tauri 1054. That one just keeps on giving.

      • Sounds like the visible nebula was pretty brief, then. Either it was below the threshold for a 12-inch (you have a 15-inch, right?) or I was too late looking at it. I don’t think I was aware of the echo until I got my print copy of Sky & Tel.
        I guess my signature event as an amateur astronomer was stepping outside one August evening in 1975 to see how the stars were doing and becoming the 42nd reported co-discoverer, on IAUC 2826 http://www.cbat.eps.harvard.edu/iauc/02800/02826.html
        of Nova Cygni. A true thrill!
        But the dark lunar eclipse of December 1963 had more effect on my life, as I’ve done – and still do – research into that phenomenon (dark lunar eclipses, volcanoes, climate, etc.).
        I just read the AAVSO article about V838 Mon, http://www.aavso.org/files/vsots/1202.pdf which has a lot more about the visuals than the Wikipedia article.
        I can see your excitement!

        • Richard,
          Wow – one of the first to see the nova. You must have thought for a minute you’d discovered the star yourself. What a great experience that must have been. Yes, I do use a 15-inch. I wrote lots of notes on what V838 looked like as I was following it intensely (won’t go digging for those right now), but I do remember the nebula / light echo as strikingly obvious even at low power early on.

          • The first discoveries of Nova Cygni 1975 were from Japan, where the longitude was right for the nova becoming bright naked-eye (3rd magnitude) just as twilight deepened. On the IAU circular you can follow the discoveries as twilight swept westward around the earth – Japan, India, Russia, Europe, USA. By the time it got to Colorado the nova was 12+ hours old, and the kind folks at the IAU cut the “discoveries” off at 24 hours.
            Now, finding a 2nd magnitude object is kind of like discovering the moon, and doesn’t take a whole lot of skill except for immediately recognizing that the Northern Cross looked drastically skewed. After thinking it was one of those bright high-flying balloons (Pageos), I watched for a few minutes, then put binoculars on it, and then – after seeing no motion – called it in.
            A friend of mine was observing atop Mt. Evans (14256 feet) and kept thinking there was something odd about Cygnus, but with 40 percent less oxygen up there, didn’t quite piece it together.
            Say, it would be great to see an article about your observations of ol’ V838 Mon. It has the panache of being one of those distant and exotic Hubble kind of things that was actually seen by a mortal.

          • Richard,
            Are you still writing? You should – you’re good at it. Thanks for sharing more on your discovery of V1500 Cyg. I did briefly mention V838 Mon in a blog a few years back, but you’re right, it would be fun to revisit it with the full story.

          • Hey Bob, thanks for the compliment. You know I’ve written a bunch of weather books, and was working on another a dozen years ago when my teaching load was ratcheted up to full-time plus. So I’d get home and opt to have a beer and play with the cat rather than engage in more thoughtful pursuits.
            But I’m retired now, and – like you suggest – need to pick up those things left back by the side of the road.
            How about yourself? You write excellent stuff for this blog – your style, clarity, and selection of subjects are all superb – and for the newspaper. Are you considering something more enduring, like pictographs or a book?

          • Richard,
            What a thought – putting it all in pictographs. Funny you ask about a book. I’ve written a little book with lots of photos and submitted it to a Duluth publisher about 6 months ago. They just contacted me to say they were going to pitch it at a meeting recently, so we’ll see. Maybe they’ll bite.

        • Richard,
          Ahem. I believe that would be IAU Notched Bone Missive #1. Thanks for the link! I paid a visit to Chaco a while back and made the pilgrimage to the famous crescent and star pictograph. It’s located on a rock underhang with a big view of the eastern sky. Almost spiritual experience to be there.

          • Chaco is one of the more remote National Parks (outside of Alaska), and after a 25-mile drive on a road disguised as a creek bed, it’s a three-mile or so walk down a sandy arroyo to the 1054 petrograph (or “pictograph”, which may be more correct). But it is very well worth it. You and I, and most of your raders I’m sure, know the wonder of finding something odd, new, or just plain beautiful in the sky, and the urge to record it. These folks did just that a thousand years ago. That pictograph is their drawing of their “V838″, made with wet ochre instead of a #2 pencil.
            Next, I’d like to visit the Wyoming “medicine wheels”, which are equally old, remote, and neat. But a lot colder!

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