An astronomy technique developed in the early 20th century helped researchers measure distances to other galaxies and discover that the universe is expanding. Now, the stars that formed the basis of that century-old technique are part of a survey to map the history of the Milky Way galaxy.
In 1908, Henrietta Swan Leavitt — one of several women who worked as Harvard “computers” in the early 1900s, studying photographic plates of the night sky — discovered a relationship between the changes in the apparent brightness of “variable” stars and those stars’ actual brightness, or absolute magnitude.
Because light dissipates in a predictable way as it travels through space, knowing a star’s absolute magnitude made it possible to measure a star’s distance from Earth. More than 100 years later, astronomers continue to use this discovery, known as the Leavitt’s law, in even more advanced ways. Now, astronomers with the Sloan Digital Sky Survey (SDSS) have included these stars in a survey of stars in the Milky Way that will help reveal the galaxy’s structure and history.
Leavitt’s law came specifically from Leavitt’s studies of Cepheid variable stars, which periodically vary in brightness (or pulse) in cycles that typically last a few days or weeks. The SDSS’ Apache Point Galactic Evolution Experiment (APOGEE) is designed to map the position and composition of cool, old “red giant” stars found throughout the Milky Way galaxy. But the two telescopes now being used for the APOGEE survey can also detect Cepheid variables, which are typically younger than red giants but similar in temperature, according to the statement. Therefore, the APOGEE data allows astronomers to map both young stars and old giant stars in the same way, and, in turn, connect structures from the ancient galaxy to those that formed more recently, according to the statement.
Astronomy undergraduate student Kate Hartman set out to determine whether the variable nature of Cepheids would prevent them from being included in the study.
“It’s been fascinating to work with such historically significant stars,” Hartman, a student at Pomona College, said in the statement. Hartman announced the results of the study Tuesday (Jan. 9) at the American Astronomical Society (AAS) meeting in National Harbor, Maryland.
Hartman studied Cepheid variables over the course of a 10-week summer research project at Carnegie Observatories, during which she worked alongside her research adviser, Rachael Beaton, who is a Hubble and Carnegie-Princeton fellow now based at Princeton University.
Hartman looked at how the APOGEE telescopes measure the chemical composition of Cepheid variables, which can vary greatly in temperature, surface gravity and atmospheric properties during their brightness pulsation cycles. The chemical composition of a star can be measured via a method called spectroscopy, which indicates the presence or absence of chemical elements based on the appearance of specific wavelengths in the light from the star. The results of the study showed that it is possible to collect consistent measurements of the composition of Cepheid variables, regardless of when in their cycle APOGEE observed them, the researchers said in the statement.