Lots of little dots with no apparent pattern: where laymen can just see milky-gray photos dotted with what look like random crumbs, that’s enough to make astronomers’ hearts race. We are talking about historic photographic plates showing negatives of the night sky. In collaboration with the Leibniz Institute for Astrophysics in Potsdam and the universities of Hamburg and Tartu (Estonia), FAU researchers digitized the images and published them online. After a total of 10 years, the project has now been completed, thanks to financial support from the German Research Foundation (DFG).
Even though the oldest image is “only” 129 years old, a blink of an eye from the time scales usually associated with astronomy, they have great historical value and are a treasure trove for academic purposes. Such images are the only way for astronomers today to track the motion or change in intensity of stars over decades. They can be used to answer new research questions and take a closer, more objective look at millions of stars.
Since 2012, the research team has been working to digitize images from the archives of their partner institutes dating from 1893 to 1998 into the APPLAUSE database – which stands for Archives of Photographic Plates for Astronomical USE – and save them in a catalog with details about the images such as date, section of sky and location where the images were taken. Additionally, the research consortium has developed software that uses artificial intelligence to remove errors on plates caused by scratches or dust and to calibrate images, allowing for the first time to compare them to each other. for scientific purposes. Researchers around the world now have access to 4.5 billion measurements of celestial light sources for their research.
More than 94,000 photographic plates digitized
A significant part of the total of 94,090 plates is represented by the 40,000 photographic plates of Dr. Karl Remeis Observatory Bamberg, Astronomical Institute of FAU. These include photographs taken by Franconian researchers between 1963 and 1976 at observatories in the southern hemisphere. These unique images show the southern sky and are the only ones of their kind available anywhere in the world, as no other astronomical project has documented this part of the sky during this period. Since the last images were published four years ago, the photographic plates taken in Bamberg between 1912 and 1968 showing the northern sky are now added to the project. These 17,600 images are the largest addition to what is now the final data update.
But that’s not all: the project attracted the attention of other observatories at an academic conference in Bamberg. The Thüringer Landessternwarte Tautenburg, for example. It gave the research team access to the archives of the Karl Schwarzschild Observatory, the former observatory of the GDR Academy of Sciences for the years 1960 to 1998. Researchers at the astronomical observatory of the Vatican State at Castel Gandolfo have also expressed interest in having their archives incorporated into the database and made available to the global scientific community.
New insights from old images
But what knowledge can we glean from historical photographic plates, and does it have any relevance today? Surveys of the northern and southern skies conducted over the past century by the Bamberg Observatory aimed to study stars of varying intensity. The physical properties of some of the objects, in other words what gases they are composed of, are still unclear. The star “HD49798” is a particularly interesting example. Its erratic fluctuations in intensity were recorded on Bamberg photographic plates in the 1960s and early 1970s, but scientists were only able to analyze them last year. They show that the star steadily increased in intensity between 1964 and 1965, but then began to shine less brightly until 1974. There were also rapid changes in the light it emitted in the space of only a few days. In 1999, satellite readings revealed that the star was emitting X-rays. Today, scientists suspect that those rays are being emitted by an invisible, more compact accompanying object, possibly a neutron star. Until now, scientists had not been able to track long-term intensity variations because measurements spanning such a long period of time, namely ten years, were not available. Historical data from photographic plates is therefore a valuable source of astronomical information that researchers will now analyze in the years to come. This particular pair of stars is still the only such constellation to have been discovered anywhere in the universe.
Access to APPLAUSE published data: https://www.plate-archive.org/
Detailed report on the interim results: https://www.fau.de/2018/10/news/wissenschaft/neues-web-archiv-mit-fotoplatten-der-astronomischen-sammlung-der-fau-online/