6 Acknowledgments
This work has been supported by grant ATM-0447159 to Lowell Observatory from the National Science
Foundation. Writing this review was greatly facilitated by the resources available from NASA’s Astrophysics
Data System and the SIMBAD Astronomical Database, operated at CDS, Strasbourg, France. The author
thanks Drs. Mark Giampapa and Tom Ayres for their time and thoughtfulness in reading a lengthy
manuscript and returning excellent reviews with a number of useful suggestions that improved the final
paper.
