The American Association of Variable Star Observers is one of the most active and most venerable amateur science organizations in the United States--or anywhere else, for that matter. Since 1911 (If memory serves) they have been carefully collection millions of observations on variable stars such that if you are an astronomer doing work on variables, you pretty much need to work with AAVSO. This piece from their web site is a tribute to their outstanding tradition of careful work and tireless education.
A group of astronomers led by Dr. James Miller-Jones of Curtin University and the International Centre for Radio Astronomy Research have published a revised distance to the famed variable star SS Cygni, solving a major mystery surrounding this star and its behavior. Their work was made possible due to the dedication of dozens of observers worldwide who monitor SS Cygni and report their observations to the American Association of Variable Star Observers (AAVSO).
Miller-Jones and collaborators sought to resolve a discrepancy between the distance to SS Cygni observed with the Hubble Space Telescope (HST) and theoretical models of accreting binary stars like SS Cygni. SS Cygni is a type of variable known as a "dwarf nova", consisting of a normal star and a white dwarf in close orbit around one another. The gravitational pull of the white dwarf pulls matter from the normal star, creating a disk of accreted material around the white dwarf. At irregular intervals, this accretion disk gets hot and bright, going into outburst. These outbursts give the "dwarf nova" class of variables its name.
The mystery around SS Cygni was that HST measurements suggested it was farther away than expected, and therefore brighter than expected. HST was used to measure the "parallax" of SS Cygni -- its apparent side-to-side shift in position relative to more distant stars that occurs as the Earth orbits our own Sun. Parallaxes can be used to determine the distance of an object from the Sun. HST measured the parallax of SS Cygni to be over 450 light years. If SS Cygni was as bright as the HST parallax implied, theoretical models predicted the star should be permanently in outburst, rather than undergoing episodic outbursts as observed. Thus the HST distance measurement cast doubt on current models of why dwarf novae and other similar stars behave the way they do.
Miller-Jones and collaborators used information about SS Cygni's optical behavior relayed to them by the AAVSO to trigger radio observations with the Very Long Baseline Array (VLBI) and the European Very Long Baseline Interferometry Network (EVN). SS Cygni is bright in radio waves only within hours of the start of its optical outbursts, and the radio observers needed prompt notification of these events from amateur variable star observers to obtain useful data. Fortunately, SS Cygni has long been a favorite target for variable star observers, and the worldwide community of variable star observers participated in this observing campaign with enthusiasm. Due to the amateur community's extensive coverage and prompt reporting of SS Cygni's behavior over several months, Miller-Jones was able schedule radio observations of SS Cygni and a number of background radio-bright galaxies. Doing so allowed them to re-measure SS Cygni's parallax more precisely than HST could, and yielded a much shorter distance -- just 372 light years -- indicating SS Cygni's brightness is lower than HST implied. This new information resolves the mystery and brings SS Cygni's behavior into agreement with that predicted by the theoretical models.