Cepheid + Light Echo = Accurate Distances

Posted By Pamela on Feb 11, 2008 | 15 comments

phot-05c-08-preview.jpgI am so so frustrated that I can’t get the full journal article associated with this press release. I’m going to have to do some emailing tomorrow to see if someone can get it to me.

Here is what has me excited. In a new paper in Astronomy and Astrophysics (which my Uni doesn’t get) with Pierre Kervella as lead author,  the distance to a Cepheid variable has finally been accurately measured in a method so simple I can’t believe it wasn’t done before. The binocular-bright Cepheid RS Pup is embedded in a nebula. As it’s light varies, it causes the dust and gas to also vary in brightness. By measuring how long after the star varies in brightness the blob of gas and dust varies in brightness, it is possible to tell how far apart the star and blob are located (sort of like measuring the distance between two cities based on how long it takes to drive between them going 100 km/hr). The next step is to measure the angle on the sky between the two. This gives us one angle and one side on a triangle. Everything else is than calculatable – including the distance from us to them.

Beautiful. Clean. Simple. I wish I knew why no one did this before. (Hopefully that’s addressed in the paper.)

The other thing the press release doesn’t do is tell me if these new results significantly changed our understanding of our place in space. Cepheid variable stars are one of the standard  candles used to measure the distances to other galaxies and to calibrate the supernovae distance scale. If it turns out that we misplaced the Cepheids it will rescale things a bit. We shouldn’t be off by more than a few percent (we have some not totally accurate ways to measure distances today with bad parallax measurements), but still… It will be interesting to know how close we got by averaging a whole bunch of imperfect measurements.

Once I get my hands on the paper, I’ll let you know.


  1. Please give more detail, because I don’t think it gives us all the info we need to calculate the distances to the Cepheids. Knowing the length of a side and the opposite angle to that side tells you very little about the length of either of the unknown legs. It can give you a maximum length, but that’s about it. Perhaps I’m missing something.

  2. Looks like A&A knows they have a hot one and is using the press release as a tease to get subscriptions. It might work, too, but 2,669 Euros/yr. is a fairly large chunk of change…
    Too uh, “rich” for my blood.

  3. This only works because we assume the separation on the sky is such that from us to dust and from us to star is the same, making it an equilateral isosceles triangle. How well they were able to get that alignment will probably (but I still need to read the paper) be the dominant source of error.

  4. Do you mean isosceles triangle? Cool math at work here.

  5. Hi Beth – Yes I did. that’s what I get for posting before drinking coffee. Good catch!

  6. Pamela, if I didn’t catch it, my daughter would have when I described it to her. 9th grade math whizzes keep you on your toes. There’s a geometry puzzle here.

    The diagram makes it look like the nebula is a reasonable distance from the star, but other pictures show that the star is more embedded in the nebula that sort of surrounds it. So the distance to the dust blobs (as the ESO press release puts it) based on their separation in the sky seems hard to measure. A dust blob could be closer to us or further away from us rather than at the same distance as the star. It seems that would affect the calculations.

    Not that I’m going to do so here, but can I just use HTML-format links to add a link in this blog?

  7. Hi Beth,

    I really love it when students catch my mistakes – it shows they’re paying attention. It’s nice to know your daughter there keeping us mathematically honest.

    The star is embedded in the nebulae. I need the paper before I can address your concerns about the star and blob not being equally distant. Sorry 🙁

  8. My university doesn’t get that journal either. I’ll wait. This seems to be an intriguing approach to the distance calculations.

    I also appreciate it when my students point out my mistakes as long as they’re nice about it.

  9. This result is featured here as the 12 Feb 2008 Astronomy Picture of the Day. There’s the pretty picture and links to various terms.

  10. Light echoes are cool things, and not quite what they intuitively appear. (Not catching this is the only time I can point to of Fritz Zwicky being wrong about something). If you track the light from a given time of peak brightness, we will see that reflected from dust which lies along an ellipsoid with the star at one focus and us at the other (it is usually fine to approximate this as a paraboloid). As time goes on, the ellipsoid expands (the sum of focal distances grows at the speed of light). What we see is where this intersects reflecting material. If we measure both the projected separation of the illuminated material from the star, and how fast the light echo moves transversely, that tells us how far from the star the dust is. This has been used to make a 3D map of interstellar material in from of Supernova 1987A, for example. There is a crude diagram of this at http://www.astr.ua.edu/keel/galaxies/lightecho.gif

  11. NGC3314, doesn’t that mapping depend on knowing how far away the source is?

    Could you use Doppler shifts to work it out, on the assumption that the nebulosity has been ejected by the star? Reflections from material at exactly the same distance as the star should then (I think) have half the shift, relative to the overall motion of the system, of reflections from material (almost) directly behind it.

    Also, does the existence of the nebulosity mean RS Pup is atypical for Cepheids, and can’t be trusted for calibrating the scale?

  12. More often than not it’s the simple ideas that get over looked.I’m looking forward to reading the full journal article.

  13. The full paper is now available via a link from the Sky & Telescope web site’s (SkyTonight.com) own article on the subject. I have not had time to read it yet, but as NGC3314 explained, the geometry is not quite trivial.

  14. David, thanks for the link.

    Shorter version: they calculated the distance for a number of blobs in the nebula, on the assumption that they are in the plane of the sky with the star. They averaged the result, and then cross-checked by various clever means such as throwing away the highest and lowest two estimates.

    Also, the nebulosity is associated with the star, lending confidence that it’s roughly symmetrical so the averaging should work.

  15. Yes ngc3314, that makes great sense because only in an ellipsoid does light travel the same distance from the source to us. [I read it in a cool book called “The Sky at Einstein’s feet”. (plug) :)]

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