Harvard’s Center for Astrophysics hosts the world’s largest collection of glass photographic plates, and thanks to the efforts of DASCH — which stands for Digital Access to a Sky Century @ Harvard — all of the information they contain will soon be at your fingertips. We caught up with Principal Investigator Dr. Josh Grindlay to find out how the digitization project works and what they hope to accomplish by making all of the data available to the public.
Collected from 1885 to 1992 from Harvard’s observatories and expeditions all over the world, the half-million glass negatives weigh about 300 tons altogether and chronicle the entire night sky through time. A closer look at any given plate might reveal 50,000 to 100,000 individual stars, each recorded as a tiny black dot on the light-sensitive photographic emulsion. The brighter the object, the broader the dot, and by measuring all of the dots on all of the plates spanning a century, the project aims to make a comprehensive brightness history of the universe accessible to anyone with an internet connection and a question to ask.
Columbia University graduate student Maria Charisi had one such question, prompted by the announcement that quasar PG1302 — a distant object that varies in its brightness over time — was really a pair of supermassive black holes orbiting each other. Charisi and her colleagues wanted to know if there might be another component to the variation, a longer-period change in brightness superimposed on the observed 5.2-year variation. Current data, however, only goes back for a couple of decades, making it well-nigh impossible to identify something like a regular 25-year oscillation. That’s where DASCH comes in. By adding an extra century of observations to the available data, Harvard’s digitized plate stacks made it possible to search the past for much longer-term brightness variations.
These types of objects — the ones that look the same for decades and suddenly brighten up for just a few weeks at a time — are among the most exciting for Grindlay, who studies compact binary systems. “A hundred years of data gives you an opportunity to look at the extreme events that nature produces,” he says. “One of the reasons why I
wanted to do this whole project is because time-domain astrophysics on these decadal to century time scales has just never been possible to do.”
DASCH is changing that. Having developed custom equipment and software to clean, scan, and analyze 400 plates every day, the project is nearing the home stretch. About 120,000 plates have been fully digitized already, and all of their brightness information is already available in the electronic database. With 320,000 more plates to go, the full century of data covering the entire sky — about a petabyte in total — will be made available within the next three years.
Looking back on her early career, astronomer Annie Jump Cannon — architect of the modern stellar classification system and one of the Harvard computers responsible for processing the plates in the archives — noted the profound impact of photography on the field of astronomy. “Just at the time when astronomers were peering at starlight,” she wrote in 1926, “chemists were eagerly at work with compounds of bromides and silver, little dreaming that in their mixing bowl lay the means of solving the riddle of the centuries concerning the great inverted bowl above our heads.” Now, by making these same observations digitally available, DASCH is facilitating a similar leap in our access to the skies.
Podcast and post by Meg Rosenburg