For the first time, scientists were able to calculate how the slow pulsation of older stars affect the amount of light we get from distant galaxies. Scientists think that looking at these "stellar heartbeats" could represent a new method to estimate the age of a galaxy. 

For the study, astronomers from Harvard and Yale used a unique series of images of a galaxy called M87, located 53 million light-years from us. The images were taken by Hubble over the course of three months and show that almost one-quarter of the pixels fluctuate in brightness. The team established that these periodical changes in brightness, or pulsations, occur over 270 days.  

Each pixel in the images contains between 10,000 and 10 million stars, and the observed variation in light was found to be between 0.1 to 1 percent. This difference comes from the presence of long-period variable (LPV) stars, such as red giants and even older stars. These objects undergo regular pulsations, within hundreds of days, as the outer shell expands and contracts, producing the observed changes in brightness.

These stars and their pulsations have been studied in detail in the Milky Way; observations beyond our galaxy are more difficult because it’s tricky to isolate the light of single stars among millions of others. This project circumvents the problem by looking at a large number of stars and isolating the contribution from the LVPs. 

“We realized that these stars are so bright and their pulsations so strong, that they are difficult to hide,” Professor Charlie Conroy, who led the research, said in a statement. “We decided to see if the pulsations of these stars could be detected even if we couldn’t separate their light from the sea of unchanging stars that are their neighbors.”

Using their approach one can calculate the age of the galaxies, as the younger a galaxy is, the more significant the contribution of the LPV stars will be. Stars in the early universe are on average bigger than stars born nowadays, so they age more quickly. They estimated M87 is about 10 billion years old, which is consistent with other estimation methods. 

“Our models suggest that the pulsations will be stronger in younger galaxies, and that’s something we’d love to test,” said Jieun Choi, a graduate student at Harvard and a co-author of the study. 

The research was published in Nature.