A team of astronomers has measured the ‘heartbeats’ of stars within Messier 87, a giant elliptical galaxy located some 53 million light-years from us in the constellation Virgo, and used that data to determine the galaxy’s age.
Messier 87 is the dominant galaxy at the center of the neighboring Virgo cluster of galaxies. Image credit: NASA / ESA / Hubble Heritage Team.
It is the first time astronomers have measured the effect that pulsating red stars have on the light of their parent galaxy.
“We tend to think of galaxies as steady beacons in the sky, but they are actually ‘shimmering’ due to all the giant, pulsating stars in them,” explained team member Prof. Pieter van Dokkum, an astronomer with Yale University and co-author of a paper published today in the journal Nature.
Later in life, stars like our Sun undergo significant changes. They become very bright and swell up to huge size, swallowing any planets within a radius equivalent to Earth’s distance from the Sun.
Near the end of their lifetime, these stars begin to pulsate, increasing and decreasing their brightness every few hundred days.
Until now, no one had considered the effects of these stars on the light coming from more distant galaxies. In distant galaxies the light of each pulsating star is mixed with the light of many more stars that do not vary in brightness.
“We realized that these stars are so bright and their pulsations so strong, that they are difficult to hide. 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,” said co-author Dr Charlie Conroy of the Harvard-Smithsonian Center for Astrophysics.
The astronomers examined images of Messier 87 taken with the NASA/ESA Hubble Space Telescope over the course of three months in 2006. They quickly found what they were looking for.
“Amazingly, one in four pixels in the image changes with time,” Prof. van Dokkum said.
Analysis of the Hubble images showed that the average pixel varies on a timescale of approximately 270 days. The regular up and down changes in brightness are reminiscent of a heartbeat.
“It’s as if we’re taking the pulse of the galaxy,” Dr Conroy said.
The discovery offers a new way of measuring the age of a galaxy, because the strength and speed of a galaxy’s heartbeat varies depending on its age.
The scientists find that Messier 87 is approximately 10 billion years old, a number that agrees with previous estimates using different techniques.
The next step, according to the team, is to take the pulse of other galaxies.
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Charlie Conroy et al. Ubiquitous time variability of integrated stellar populations. Nature, published online November 16, 2015; doi: 10.1038/nature15731
