Title: Exploring the Long-term and Extreme Variability of Stars
Description: Stellar variations over decades are poorly explored. With the unique 100 yr coverage of DASCH (Digital Acess to a Sky Century@Harvard), for the first time, we are able to study the variable sky over long timescales in a systematic way. I have discovered exciting new types of long-term variables, which do not match any of the common classes. The most interesting one to date is a peculiar 10-yr nova. With P=119 days, it is interestingly located in the gap region between classic novae and symbiotic novae. The most probable explanation of the outburst is Hydrogen shell-burning on the WD without significant mass loss, which suggests a promising new channel for Type Ia Supernovae (SNe). We have found peculiar long-term K giant variables with 1-2 mag slow variations over decades. Most of them show strong Ca H&ampK emission, and 5 of them are covered by Kepler and show (in 4 stars) star spots and flaring activities. This suggests that the mysterious long-term variations may be related to strong star spots and magnetic activities which may be a new mechanism for dust formation or ejection. We discovered a 5-yr dust accretion event in KU Cyg, which is probably linked with its accretion disk. I conclude my thesis with the DASCH-Kepler variable catalog. Compared with the ASAS variable catalog, I recovered most of large amplitude variable stars (>0.5 mag).