American Association of Variable Star Observers
An international nonprofit organization that coordinates, analyzes, publishes, and preserves variable-star observations from amateur and professional astronomers.
Last updated August 25, 2026
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Overview
The American Association of Variable Star Observers, commonly known as the AAVSO, is an international nonprofit organization devoted to the observation and study of variable stars. Founded in 1911 by William Tyler Olcott, it developed around the idea that carefully conducted observations by amateur astronomers could provide a continuous and scientifically valuable record of stellar behavior. Professional astronomers generally cannot monitor every known variable star on a regular basis, so the AAVSO coordinates a distributed observing network that complements the work of professional observatories and research institutions. The organization receives observations from amateur and professional observers in the United States and many other countries. These observations record changes in stellar brightness and are used to construct light curves showing how a star varies over time. The resulting records support research into stellar evolution, novae, supernovae, pulsating stars, eclipsing binaries, and other transient or periodic astronomical phenomena. The AAVSO also responds to observing requests and alerts from professional astronomers when particular stars require intensified monitoring. A central institutional resource is the AAVSO International Database, which archives submitted observations and makes them available to researchers, educators, and other users. The database passed 20 million observations in 2011, the organization's centennial year, and contained more than 35 million observations by 2019. The organization has reported receiving close to one million observations per year from an estimated community of roughly 2,000 observers. Its data are regularly cited in scientific literature. The AAVSO also maintains the International Variable Star Index, or VSX, a catalog of variable stars that listed more than 10 million entries by November 2023. In addition to data collection and archiving, the AAVSO publishes scientific and technical material, issues observing alerts, organizes workshops, and supports citizen-science education. Its educational work has included the Hands-On Astrophysics curriculum, later developed as Variable Star Astronomy with support from the U.S. National Science Foundation. The organization has also promoted professional-amateur collaboration by involving amateur observers in research projects and by publishing papers with amateur co-authors. The AAVSO's institutional history includes periods of close association with the Harvard College Observatory. After its incorporation in 1918, it moved informally to Harvard, which served as its official headquarters from 1931 to 1953. The organization subsequently occupied several locations in the Cambridge area before purchasing the Clinton B. Ford Astronomical Data and Research Center in 1985. In 2007 it acquired and moved into premises previously occupied by Sky & Telescope magazine. Its work remains centered on citizen science, long-term astronomical data stewardship, and cooperation between professional and amateur observers. The minor planet 8900 AAVSO was named in recognition of the organization.
History
The AAVSO was founded in 1911 by William Tyler Olcott to organize observations of variable stars. Its founding reflected a distinctive feature of astronomy: systematic observations could be made by trained amateurs using small telescopes, visual estimates, photographic methods, and later electronic detectors, while professional observatories often lacked the personnel and telescope time to follow large numbers of stars continuously. The organization was incorporated in 1918. After incorporation, its activities became closely associated with the Harvard College Observatory. Harvard served as the official AAVSO headquarters from 1931 through 1953, although the organization subsequently operated from several locations in the Cambridge area. Its early headquarters had been at Olcott's residence in Norwich, Connecticut, illustrating the association's origins in an observer-led network rather than a large institutional observatory. A major part of the AAVSO's historical role has been the preservation and standardization of long observational time series. Individual reports are checked, organized, and made useful for analysis of stellar light variation. The resulting light curves can reveal regular pulsations, eclipses, outbursts, novae, and other changes. By maintaining a long-running archive, the AAVSO has helped ensure that observations made by successive generations remain available to professional astronomers and other researchers. The organization acquired its first dedicated building in 1985, the Clinton B. Ford Astronomical Data and Research Center. In 2007, it purchased and moved into premises recently vacated by Sky & Telescope magazine. These facilities supported the association's data-management, publication, education, and administrative functions. Education and public participation became increasingly prominent in the late twentieth and early twenty-first centuries. During the 1990s, the AAVSO developed Hands-On Astrophysics, an educational program later known as Variable Star Astronomy. The curriculum was supported by the National Science Foundation and was designed to connect classroom learning with real astronomical observations. The association also conducts workshops and training activities for citizen scientists and encourages collaboration in which amateur observers contribute to research papers. The Citizen Sky project was one of the AAVSO's notable professional-amateur initiatives. Supported by a three-year, $800,000 National Science Foundation grant announced in 2009, the project coordinated observations of the unusual eclipsing binary Epsilon Aurigae during its 2009–2011 eclipse. It demonstrated how a distributed observer network could address a focused research question while also involving participants in scientific interpretation and communication. The scale of the AAVSO's archive expanded substantially over its first century. The organization received its 20 millionth variable-star observation in 2011. By 2019, the AAVSO International Database held more than 35 million observations, and the association reported receiving nearly one million observations each year from approximately 2,000 professional and amateur observers. About half of its members and observers are outside the United States, giving the organization an international operating profile. The AAVSO also maintains the International Variable Star Index, a catalog intended to organize information about variable stars. By November 2023, the index was reported to contain more than 10 million cataloged entries. Alongside its databases, the association publishes the Journal of the American Association of Variable Star Observers, issues AAVSO Alert Notices, and historically published the AAVSO Circular from 1970 to 2000. The organization continues to serve as a bridge between citizen observers, professional astronomers, educators, and the wider public. Minor Planet 8900 AAVSO commemorates its contribution to astronomy.
- 2023
Variable Star Index exceeds ten million entries
[1]The International Variable Star Index was reported to catalog more than 10 million variable-star entries by November 2023.
- 2019
Database exceeds 35 million observations
[1]The AAVSO International Database was reported to contain more than 35 million observations.
- 2011
20 millionth observation received
[1]The AAVSO database reached 20 million variable-star observations during the association's centennial year.
- 2009
Citizen Sky receives NSF support
[1]The National Science Foundation supported the Citizen Sky professional-amateur project with a three-year grant for observations of Epsilon Aurigae.
- 2007
Move to former Sky & Telescope premises
[1]The AAVSO purchased and moved into premises recently vacated by Sky & Telescope magazine.
- 1990
Hands-On Astrophysics developed
[1]During the 1990s, the AAVSO developed the Hands-On Astrophysics educational curriculum, later known as Variable Star Astronomy.
- 1985
Dedicated data and research center acquired
[1]The organization purchased the Clinton B. Ford Astronomical Data and Research Center.
- 1970
AAVSO Circular begins publication
[1]The AAVSO Circular began publication as an observing and information publication and continued through 2000.
- 1931
Harvard becomes official headquarters
[1]Harvard College Observatory served as the association's official headquarters from 1931 until 1953.
- 1918
Organization incorporated
[1]The AAVSO was formally incorporated, establishing the institutional structure through which it could manage observations and publications.
Products and positioning
A global nonprofit hub for long-term variable-star monitoring, citizen science, and professional-amateur astronomical collaboration.
Astronomical data archive
AAVSO International Database
The AAVSO International Database stores observations submitted by the association's global observer community. Its long time series are used to create and analyze variable-star light curves, support professional observing programs, preserve historical records, and provide research material for astronomers and educators. The archive contained more than 35 million observations by 2019.
Astronomical catalog
International Variable Star Index
The International Variable Star Index, commonly called VSX, is a catalog maintained by the AAVSO that organizes information about variable stars. It provides a searchable reference for identifying objects and their variability characteristics. More than 10 million entries were reported in the index by November 2023.
Astronomical alerts
AAVSO Alert Notices
AAVSO Alert Notices communicate observing opportunities and requests, including situations in which professional astronomers need coordinated monitoring of variable or transient objects. The notices help mobilize the association's distributed observer network.
Scientific publication
Journal of the American Association of Variable Star Observers
The Journal of the American Association of Variable Star Observers publishes research and technical contributions concerning variable-star observation and analysis. It reflects the AAVSO's role in connecting professional researchers with amateur observers and citizen scientists.
Educational curriculum
Variable Star Astronomy
Variable Star Astronomy is the later form of the Hands-On Astrophysics curriculum developed by the AAVSO during the 1990s. It uses variable-star observation to support astronomy education and practical citizen-science learning, with support from the National Science Foundation.
Marketing campaigns
- 2009Citizen Sky
International
Citizen Sky was a professional-amateur collaboration organized around observations of the 2009–2011 eclipse of Epsilon Aurigae. The project used the AAVSO network to coordinate observations, involve citizen scientists in research, and improve public understanding of the target system.
Outcome. The project created a structured observing effort around the eclipse and expanded the AAVSO's model for organized citizen-science research.
Brand decisions
2009
Launch the Citizen Sky observing collaborationStrategyBackground
The eclipse of Epsilon Aurigae created an opportunity for coordinated long-duration observations involving both professional and amateur astronomers.
Action taken
The AAVSO operated Citizen Sky with support from a three-year National Science Foundation grant.
Aftermath
The project strengthened the association's role in organized citizen science and professional-amateur research.
National Science Foundation grant. $800,000 (Three-year grant announced in 2009)
2007
Acquire premises formerly occupied by Sky & TelescopeStrategyBackground
The association needed an institutional base for its data, research, publication, education, and administrative activities.
Action taken
The AAVSO purchased and moved into recently vacated Sky & Telescope premises.
Aftermath
The move provided the organization with a continuing physical base after earlier relocations around Cambridge.
Recent events
- 2022
Brian Kloppenborg becomes executive director[1]
Leadership change
Brian Kloppenborg was identified as the AAVSO executive director on September 16, 2022, following an interim period led by Kathy Spirer.
- 2019
AAVSO International Database exceeds 35 million observations[1]
Other
The organization's principal observation archive was reported to contain more than 35 million variable-star observations.
- 2011
AAVSO reaches its 20 millionth variable-star observation[1]
Other
The organization recorded its 20 millionth observation in the year marking its centennial, highlighting the scale and historical value of its observational archive.
- 2009
National Science Foundation funds Citizen Sky project[1]
Campaign
The AAVSO received a three-year National Science Foundation grant to operate Citizen Sky, a professional-amateur observing project focused on the eclipse of Epsilon Aurigae during 2009–2011.
- 2007
AAVSO moves into former Sky & Telescope premises[1]
Other
The organization purchased and moved into premises that had recently been vacated by Sky & Telescope magazine.
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