"Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun." "The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun." "Our Sun itself frequently has sunspots, relatively cool dark magnetic depressions that move across its surface. HD 12545, however, exhibits the largest starspots yet observed. Doppler imaging - the use of slight changes in color caused by the rotation of the star - was used to create this false-color image. The vertical bar on the right gives a temperature scale in kelvins. This giant, binary, RS CVn star, also known as XX Trianguli, is visible with binoculars in the constellation of Triangulum. The starspot is thought to be caused by large magnetic fields that inhibit hot matter from flowing to the surface." "A giant starspot was revealed on the K0 giant star XX Triangulum, also known as HD12545, using Doppler imaging on the Kitt Peak National Observatory's 0.9m Coude Feed telescope. This picture shows a series of views around the star". "To observe spots on the surfaces of other 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 inhomogeneities on 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 at the preceding limb of the star causes a blue-shifted asymmetry in each spectral line profile. This asymmetry moves into the line center at the time of meridian passage, and turns into a red-shifted asymmetry after meridian passage. The asymmetry fades away when the spot disappears at the receding limb. The higher the latitude of the spot, the shorter will be its visible path across the projected disk of the star, or the spot may even be circumpolar if the stellar rotation axis is inclined. All this information 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 stellar rotation." "XX Triangulum is an active K0 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 are 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. NSF's Kitt Peak National Observatory is one of the few facilities worldwide that offers this capability with the 0.9-m coude feed telescope." "During the observations, XX Triangulum had its brightest magnitude since the discovery of its light variability in 1985 and showed the largest photometric amplitude so far (0.63 magnitudes in V). The large photometric amplitude was explained when the Doppler-imaging inversion algorithm also recovered a not-quite-as-large equatorial warm spot (350 K above the photospheric temperature) in the hemisphere opposite the dark spot. Strassmeier speculates that the warm spot harbors the same magnetic field as the cool spot but opposite polarity. Surprisingly, the fact that the star was brighter at a time of high spot activity is in agreement with the solar analogy despite the "unsolar" dimension of the gigantic spot." Zeta Andromedae is "a large old star 180 light-years away[.] Zeta Andromedae, a star 15 times the size of the sun, has starspots dotted all over in an apparently random fashion, even at high latitudes (i.e. near the poles). [...] Our sun rotates at a leisurely pace of 2 kilometers per second. Zeta Andromedae is spinning at a rate of 40 kilometers per second and has a smaller binary partner." "A computer reconstruction [on the right] shows the surface of the young star AB Doradus to be mottled by the giant starspots that betray its twisting rotation. The St Andrews-Toulouse team have also mapped the magnetic field patterns on the star's surface. Tenuous hydrogen plasma, heated to 15 million degrees C, is trapped in the magnetic fields that arch over the star's surface. This plasma glows in X-rays, shown here in red." The image of AB Doradus is in the near-infrared. Stars/Nova-likes "Starspot". San Francisco, California: Wikimedia Foundation, Inc. October 5, 2012. Retrieved 2012-11-05. Robert Nemiroff & Jerry Bonnell (November 2, 2003). "Astronomy Picture of the Day". K. Strassmeier (November 2, 2003). "Giant starspot". NOAO. Caty Pilachowski (December 1999). "XX Marks the Spot". NOAO Newsletter. NOAO. Ian O'Neill (6 May 2016). "Spotty Star Reveals Magnetic Weirdness". Space.com. A. Cameron, M. Jardine and K. Wood (2000). "The AB Dor Picture Gallery". Scotland: University of St Andrews.
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