Starspotting

Stars have no privacy. Not only does NASA unashamedly collect extensive records of star’s data, they’re not even denying they do. NASA’s surveillance program has the sole intent to stare at stars, and to stare intensely, just for the sake of collecting knowledge.

NASA’s Kepler satellite has been looking for more than three years at a small patch of the Milkyway that hosts an estimated 145,000 stars similar to our own sun. The data that Kepler gathered, and still gathers, is analyzed for transits of planets that temporarily block part of the star’s surface and diminish its emission. The precision by which this detection can meanwhile been done is simply amazing. The Kepler mission so far found more than 2000 planet candidates that are now subject to closer investigation.

Ray Jayawardhana gave a great lecture on exoplanets at Nordita’s recent workshop for science writers, slides are here. The progress in the field in the last decades can’t be called anything but stellar.

To see just how much progress has been made, look at page 13 of Ray's second lecture. You see there a time-series of measurements of the flux from some star observed with Kepler. You clearly see the dips when then planet covers part of the surface, a decrease that isn’t more than a tenth of a percent.

Image: Lisa Esteves.

A decade ago that would have been an amazing observation all by itself. Now look at the (red marked) data taken between the transits. If the planet doesn’t cover part of the star’s surface it will reflect light that is in principle also observable. This reflection should be largest when the planet is just about to vanish behind the star, and then dip. That means there should be a fine-structure in the flux between the transits, at about two orders of magnitude smaller still than the already small transit signal. And in fact, the data and data analysis is meanwhile so good that even the vanishing of the planet behind the star can be measured, as you can see on page 14 of Ray's slides

Image: Lisa Esteves.

I went away from Ray’s lecture thinking I should read his book, but was also left wondering if not the close monitoring of the stars should pick up sunspot activity and what we know about solar cycles of stars other than our own.

So I looked for literature and found two good reviews on starspots. A Living Review by Svetlana Berdyugina and an Astronomy and Astrophysics Review by Klaus Strassmeier. In comparison to exoplanets, starspots seem to be a fairly small research area. For those of you who don’t want to dig through 90+ pages and for my own record, here are the most interesting facts that I learned from my reading

• Prior to the Kepler mission, about 500 spotted stars were known and had been analyzed. (I expect that this number will go up dramatically when the Kepler data is taken into account.)
• That starspots exist was first proposed in 1667 by the French astronomer Ismael Bouillau.
• The first starspots were recorded in the 1940s, but not recognized as such. In the late 60s and early 70s, several research groups independently proposed star spots as an explanation for certain types of light variability of stars that had been observed.
• Some white dwarfs show spectral variations on a timescale of hours and days that are believed to be caused by sunspots. Similar structures are probably present on the surfaces of neutron stars as well.
• Monitoring star spots allows to extract the rotation period of the star. According to presently existing models of stellar evolution, the rotation of stars changes with mass and age. Star spots can thus provide relevant data to find out which of these models is correct and thereby teach us how stars like our own evolve.
• Doppler imaging of the stars emission spectrum allows in principle to reconstruct the latitude of the spot, though in practice reconstruction can be difficult.
• There’s two different types of spots. The spots at low to mid latitude that we know from the sun, and polar spots that cover a pole of the star. The polar spots are thought to be caused by magnetic fields produced by a distinctly different mechanism than the other spots.
• The polar spots can be HUGE. Look at this amazing image that is a reconstruction from Doppler imaging. This polar spot is about 10,000 larger than typical spots on our sun:
  
  

  

  Image: Strassmeier, Astron. Astrophys.,347, 225–234 (1999)
  In: Berdyugina, "Starspots: A Key to the Stellar Dynamo"

  
  
  
• Out of 65 stars whose surface emission was analyzed with the Doppler technique, 36 had polar spots.
• The polar spots can be very long lived and survive up to a decade.
• There seems to exist no strong correlation between temperature and size of the star spots.
• Most spotted stars have a cycle similar to that of the sun with a period in the range 3-21 years. Some stars probably have longer cycles, but existing observations don’t yet allow extracting it. But about a third of the observed stars seem to have no cycle or have one with large variations between the periods.
• The lifetime of a sunspot is probably not determined by the decay of its magnetic field but by the surface shear due to differential rotation.
• Spots on tidally locked binary systems live longer (on the average several months) than spots on single main-sequence stars (on the average several weeks).
• Solar-type stars show the most sunspot activity when in the surface temperature is in the range 4900 - 6400 K.
• Some starspots spin. This has not (yet) been observed on our sun’s spots.

There’s much that astrophysicists can learn about our own sun from other stars like our own. It seems to me this scientific invasion of solar privacy deserves to be called intelligence service :o)