AAVSO observers help solve a mystery about an important variable star

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.

Dr. Matthew Templeton, Science Director of the AAVSO, commented on the role of the AAVSO's worldwide community of observers:

"The amateur astronomers who participated in the SS Cygni observing campaign really enabled a great observational result by the radio astronomers.  Having the amateur community participate in the optical monitoring aspect of the work freed up time and resources of the professional community that were put to more effective use making radio observations only, instead of both the optical and radio.  It also improved the efficiency of the professional-led observations, since they could utilize observing time at a major facility like the VLBA or EVN when they could obtain the most useful results, instead of having to either monitor SS Cygni continuously or observe it at times when conditions weren't ideal.

"The AAVSO community often rallies when projects like this come up, and it is very committed to doing good science.  It's gratifying to see the work of the community lead to something that produces exciting new results like this.  We're very proud to have played a part in fundamental research at this level."

The results of this study by Miller-Jones et al. will be published in the May 24, 2013 issue of Science Magazine.

About the AAVSO:

The American Association of Variable Star Observers (AAVSO) is a 501(c)(3) non-profit research and educational organization of amateur and professional astronomers who are interested in stars that change in brightness, known as variable stars.  The AAVSO was founded in 1911 to coordinate variable star observations made largely by amateur astronomers for Harvard College Observatory.  A century and more later, we continue as an independent, private research organization headquartered in Cambridge, Massachusetts dedicated to the observation and study of variable stars.  With members and observers in 52 countries, and an archive of over 23 million variable star observations, the AAVSO is the world's largest association of variable star observers.

Dr. Matthew Templeton
American Association of Variable Star Observers (AAVSO)
49 Bay State Road
Cambridge, MA 02138
email: matthewt@aavso.org