Current Science at NOAO

XX Marks the Spot

Klaus Strassmeier, University of Vienna

An image of the K0 giant star XX Triangulum and its super starspot (the dark area in the upper hemisphere). An image of the solar disk is shown to scale as a comparison.

December, 1999  |  The largest starspot ever seen was discovered recently on the Coude Feed telescope. Astronomer Klaus Strassmeier from the University of Vienna used a technique similar to a CAT scan to map the surface of the star XX Triangulum, revealing a starspot about 8 times larger than the diameter of the Sun - 60 times larger than the largest sunspot group every observed on the Sun.

To observe spots on the surfaces of stars, astronomers need to "resolve" the stellar disk. This cannot be done directly with the largest telescopes even planned, but Doppler imaging can be used to obtain a map of a star's surface. The principle is similar to medical tomography, but instead of a scanner rotating around a fixed object, a rotating star is observed with a fixed telescope. A cool starspot rotating into view on one side of the star causes a change in the profile of absorption lines in the spectrum of the star. These changes move from the short-wavelength to the long-wavelength side of the absorption line profile as the spot rotates across the face of the star and disappears around the other side. The map of the star is hidden in the variation of the spectral line profiles and is reconstructed by mathematical inversion to create a true picture of the stellar surface. For a successful application, the telescope needs to "see" the entire stellar surface during at least one full stellar rotation.

XX Triangulum is a cool, giant, binary star, approximately 10 times larger and twice as massive as the Sun. Its rotation period is 24 days, so that 24 consecutive (clear) nights of telescope time with an excellent high-resolution optical spectrograph were needed to obtain a good Doppler image. Because starspots vary on the same (short) time scales as Sunspots do (they are stable for about one stellar rotation), all the observations must be made on one rotation cycle. The Coude Feed telescope at Kitt Peak is one of the few facilities worldwide that offers this capability.

For more information see:
   NOAO Newsletter

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