been made using increasingly powerful techniques yielding more and more detail: photometry showed starspot growth and decay and showed cyclic behavior similar to the Sun's; spectroscopy examined the structure of starspot regions by analyzing variations in spectral line splitting due to the Zeeman effect; Doppler imaging showed differential rotation of spots for several stars and distributions different from the Sun's; spectral line analysis measured the temperature range of spots and the stellar surfaces. For example, in 1999, Strassmeier reported the largest cool starspot ever seen rotating the giant K0 star XX Triangulum (HD 12545) with a temperature of , together with a warm spot of . See also Letters on Sunspots Joy's law List of solar cycles Radio propagation Solar cycle Solar rotation Space weather Spörer's law Starspot Wolf number References Further reading External links Sunspot Database based on Terrestrial (GPR/DPD) and Satellite (SOHO/SDO) observations from 1872 to Nowadays with the newest data. () Solar Cycle 24 and VHF Aurora Website (www.solarcycle24.com) Belgium World Data Center for the sunspot index High resolution sunspot image Sunspot images in high-res Impressive collection of sunspot images NOAA Solar Cycle Progression: Current solar cycle. Current conditions: Space weather Lockheed Martin Solar and Astrophysics Lab Sun|trek website An educational resource for teachers and students about the Sun and its effect on the Earth Tools to display the current sunspot number in a browser Propfire – displays current sunspot number in browser status bar HamLinks Toolbar – displays solar flux, A Index and K Index data in a toolbar The Sharpest View of the Sun Daily Sunspot Update and Picture of the Sun (www.spaceweather.com) Animated explanation of Sunspots in the Photosphere (University of South Wales) Sunspot data International Sunspot Numbersunspot maximum and minimum 1610–present; annual numbers 1700–present; monthly numbers 1749–present; daily values 1818–present; and sunspot numbers by north and south hemisphere. The McNish–Lincoln sunspot prediction is also included. American sunspot numbers 1945–present Ancient sunspot data 165 BC to 1684 AD Group Sunspot Numbers (Doug Hoyt re-evaluation) 1610–1995 Solar phenomena Vortices Articles containing video clips