History of LPV Program
At the beginning of the 20th century, variable stars in general were a mystery. Many had been discovered, but the mechanism of their light variability and their precise nature was unknown.
In order to obtain regular observations of these stars over a period of months or years, amateur astronomers were enlisted by Harvard College Observatory Director Edward Pickering in 1911 to observe a series of 372 LPVs. His original list and the names of some of these observers was published as Harvard College Observatory Circular 166 in that same year.
Many of that original group of observers went on to become the nucleus of the AAVSO. That group of 372 variable stars went on, too, forming the bulk of the AAVSO's LPV observing program for nearly a century. These LPVs have periods in some cases approaching two years; many patient years of gathering data are necessary to properly describe the behavior of these stars. This list of 372 original program stars can be found in our file section (the file is titled "AAVSO LPV Program Stars_3")
Over the years, the list of LPVs observed by AAVSO observers grew in a sometimes haphazard fashion. In 2009, Mike Simonsen and Kate Hutton worked through this problem and determined which LPVs out of the thousands of stars in the AAVSO program should be our core, or "Legacy" stars. These would be the stars included in future Bulletins, and those stars whose long term light curves we expect to remain scientifically useful for decades to come. Their work is summarised in this paper.
The list used information on how many observations of each star are in the AAVSO International Database, and how many references to each star were found in the scientific literature. The assumptions being:
- If we don't have a lot of observations on the star, how can it be a 'legacy' variable.
- we should be sure to include stars that astronomers have in the past been, and are currently interested in.
The criteria Mike and Kate adopted was that the stars must have at least 15,000 observations in the AID over more than 50 years and that there must be at least 100 references to the star in the scientific literature.
Furthermore the list contained stars with a smaller number of observations but which are too bright to be followed by automated surveys and/or they are located in fields too crowded for the surveys. The Legacy star list numbered approximately 100 stars and therefore is a smaller subset of the original Pickering 372 star program list described above.
In early 2017, Andrew Pearce revisited the Legacy list determined by Mike and Kate. Using the same 2009 criteria, a further 21 stars were added to the Legacy list.
The current list of AAVSO Legacy Stars can be found in our file section (the file is titled "AAVSO Legacy LPVs 2017 Update").
Development of the Southern Program
The vast majority of LPV's in the legacy list are north of the celestial equator which is understandable given most observers have been located in the northern hemisphere. However to ensure there is sufficient coverage south of the celestial equator, a separate legacy list has been developed for LPV's south of declination 20S (southern stars in the existing legacy list will remain there, so this new list just contains additional stars). The selection criteria as first proposed by Simonsen and Hutton remains, however it is necessary to relax the criteria somewhat. All stars on this southern list have been under observations since before 1950, have over 5,000 observations in the AID and have been referenced in over 50 scientific publications over the years. This resulted in a list of approximately 30 stars and again is a subset of the original Pickering program list.
The current list of AAVSO Legacy Stars South can be found in our file section (the file is titled "AAVSO Legacy South"). It should also be noted that the Pickering LPV Program star list , LPV Legacy star list and LPV Legacy South list can all be searched in VSX under "Campaign or Program".
Why should you observe LPV Legacy Program stars?
We collect data on variable stars in order to understand their physical behavior. The light curves of these Legacy stars are valuable for several reasons. First, these are primarily long-period variables, and having long light curves improves our ability to understand exactly how they vary. When studying periodic behavior in any object, you should make observations over many, many repetitions of the cycle shown by the object. Since many LPVs take a year or more to complete a cycle, it takes a long, long time to collect a lot of cycles!
Second, nearly all of the stars classified as Long-Period Variables are "old" stars, meaning that they've exhausted most of the nuclear fuel in their cores, and are evolving toward the ends of their lives as stars. When stars are burning hydrogen in their cores, changes may take millions of years to be measurable, but in later stages of a star's life, these changes may take as little as a few decades. So by observing stars over a long period of time, you can not only track the variability that makes them so prominent, you may also be able to see evolution in that behavior caused by evolution in the star. Two good examples of stars undergoing internal changes of some kind in recent years are the Mira variable T Ursae Minoris and the semiregular variable RU Vulpeculae.
Finally, the reason why we've chosen these particular stars is that we already have a lot of data for them. You might think that if we have a lot of data we don't need to observe them anymore, but the opposite is true. If they're already well-observed, then continued observations have a better chance of yielding useful physical information. The logic is as simple as this: if you have a hundred years of data, two hundred years would be even better! More eloquently, a century of data may allow you to investigate changes that occur on timescales of several decades. The more data we collect, the longer are the timescales we can study. If (big if) we could collect data for several centuries—or even a millenium—astronomers of the future will be better able to understand changes that occur. But they can only do that with observational data – hopefully your observational data!
The data you take will certainly be useful to researchers studying these stars today. They may also be useful to researchers in the future. It is our plan—not just our hope—that our data will long outlive us, and our work will contribute to future astronomical studies that we can only guess at now.