Sunday, August 29, 2010

Spots in Southern California Part 2

Pop quiz: what is the source of the strongest signal in Kepler's long-term observations of stars?  The answer:  magnetic spots.  Here's an example of the raw data from Kepler (click to enlarge):
The planetary transits are the little collections of dots well below the main curve, while the lower-frequency signal around a relative brightness of 1 are long-lived star spots rotating in and out of view.  Using star spots on rotating stars, Kepler data can measure things like spot life-times, average spot coverage, and the temperatures of the spots.  With long time series and many spots, one can measure the rotation rate of the star and even the differential rotation profile of the stellar surface.

Additionally a technique called Zeeman Doppler Imaging (ZDI) can be used to map the magnetic fields on rapidly rotating stars.  This technique uses the Doppler shift of spectral lines from rotation that also show Zeeman splitting caused by magnetic fields to map the magnetic field on surface of stars.  Zeeman splitting only gives line of sight magnetic field, however in many cases spectro-polorimiters can provide full vector magnetic field.  For example, here's the magnetic field of a star slightly larger than the sun:
So while it's hard to image other stars directly, it's much easier to image their magnetic fields.


  1. Once again, very interesting Nick. I think Kepler is going to go down as a very important mission.

    I see it's habitable planet count is at 7. I think by the time Kepler is done those who are "skeptical" that life exits outside of earth will be viewed as too extreme in their skepticism. (They already are viewed that way in my book given everything we have found coupled to the vastness of the universe with a helping of the principle of mediocrity.)

    I mean, their like the modern day version of those who believe earth is the one and true special place in the universe for no other reason than to worship the god of skepticism.

  2. THANK YOU for posting! Your blog is a Must Read!

    Common Cents

  3. Nick, how do you get that type of spatial resolution for the magnetic fields? It's incredible!

  4. Andrew,

    It's all in the rotation. The star itself cannot be physically resolved, but if the star is rotating much faster than its surface features are changing, then the integrated light from the entire star will change as magnetic fields move on and off the visible portion of the star. If one can get good spectropolarimetric measurements densely sampled in time, one can then invert the data for a best-fit magnetic field topology, essentially getting somewhere around 3 to 10 spherical harmonic modes, which is what you see in the images. It's a great technique, however it's very tough to do since it requires multiple long exposures with a spectropolarimeter, generally with continuous or nearly continuous time coverage for something like a week. As of right now, there are something like 5 solar-type stars where ZDI has been successful.


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