The Galactic Heartbeat

Time is a very slow thing when we talk about the universe. Stars can live for many Billions of years, and over human timescales they seem stagnant and unchanging. So it’s no surprise that when we look at distant galaxies, they don’t appear to change at all over the course of centuries. But appearances can be deceiving. Galaxies do change, more quickly than you would imagine.

The monstrous elliptical galaxy M87, located 53 million light-years from Earth is the dominant galaxy at the center of the neighboring Virgo cluster of galaxies. Credit: NASA, ESA, and the Hubble Heritage Team

M87, pictured above, is a monstrous Galaxy of nearly 1 Trillion stars, more than twice as populous as the Milky Way. It looks like a big fuzzy star, and it quite regular in appearance due to it’s lack of gas and dust, all of which has been consumed and converted into stars. It’s stars have been slowly aging without much change, and so the entire galaxy had become older and redder.

But just how old is M87? How can we tell?

changing their brightness significantly over a period of a few hundred days.

“We realized that these stars are so bright and their pulsations so strong, that they are difficult to hide,” said Charlie Conroy, an assistant professor at Harvard University and astronomer at the Harvard-Smithsonian Center for Astrophysics. “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.”

And that’s exactly what they did. Taking photos of the large galaxy M87 over a period of 3 months in 2006, they noticed that individual pixels in the images were changing their brightness subtly. “Amazingly, one in four pixels in the image changes with time,” said Pieter van Dokkum, a professor and chair of the astronomy department at Yale University. “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.”

The amazing thing is that the strength of these pulsations is related to the metallicity of the star. Stars rich in heavy elements have a weaker pulsation than metal-poor stars. Using this knowledge, the astronomers could look at the overall pulsation rates of the Galaxy and determine it’s overall metallicity, and use this to determine its age.

To put it plainly, the astronomers looked at the strength of a galaxy’s heartbeat to determine it’s age. Stronger heartbeats mean younger galaxies, in the same way a strong heartbeat means a younger person.

If we look at the heartbeats of other galaxies, we can understand their ages and put together a historical outline of when galaxies formed in the universe. The astronomers’ work showed that M87 is 10 Billion years old, an age that agrees with estimates using other techniques.

The heartbeats of galaxies will continue on for trillions of years to come, far longer than the current age of the universe. The universe itself may be pretty young.


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