How ESA's Gaia telescope and stellar music help astronomers measure the universe

One of the scientists who use Gaia's data is Prof. Richard Anderson from the Swiss Federal Institute of Technology in Lausanne (EPFL). He leads a research group that studies how the Universe is expanding. He says Gaia is a very powerful tool for measuring distances to stars. "Gaia improved the accuracy of measuring parallaxes by 10,000 times compared to the previous ESA mission, Hipparcos," he says.

Parallax is a method of measuring distance by looking at how an object appears to move when you change your point of view. For example, if you hold your finger in front of your nose and close one eye, then switch to the other, you will see your finger move slightly. The same thing happens with stars but on a much larger scale. Gaia measures the parallax angles of stars by observing them from different positions in its orbit around the Sun. The smaller the angle, the farther the star.

But measuring parallax takes work, even for Gaia. There are some errors and uncertainties that need to be corrected. That's why scientists from EPFL and the University of Bologna in Italy have developed another way of measuring distances to stars, using asteroseismology.

Asteroseismology

Asteroseismology is the study of how stars vibrate and make sounds. Yes, stars make sounds, but we can't hear them because they are in space. Instead, we can see them as tiny changes in the star's brightness. These changes form a frequency spectrum like a musical instrument's notes.

Saniya Khan, a scientist in Anderson's group and the lead author of a new study published in Astronomy & Astrophysicsexplains how asteroseismology works. "The frequency spectrum tells us how big a star is, just like the sound of a musical instrument tells us how big it is. For example, a violin sounds lower than a cello because it is bigger. By knowing the size of a star, we can calculate how far it is, and get the asteroseismic parallax," she says.

In their study, the scientists used asteroseismology to measure the distances of over 12,000 red giant stars, some as far as 15,000 light-years away. They compared their results with the ones from Gaia and found that they agreed very well. This means both methods are reliable and accurate and can map the Universe.

But how do they get the size of a star from the frequency spectrum? The scientists use a simple fact: the speed of sound waves depends on the temperature and density of the star's interior. "By analyzing the frequency spectrum of stellar oscillations, we can estimate the size of a star, much like you can identify the size of a musical instrument by the kind of sound it makes – think of the difference in pitch between a violin and a cello," says Andrea Miglio, a full professor at the University of Bologna's Department of Physics and Astronomy and the study's third author.

Once they know the size of a star, they can use other information to calculate its distance. They measure the star is brightness and compare it to how bright it looks from Earth. They also measure the star's temperature and chemical composition using spectroscopy. They use these data to run sophisticated analyses and get the asteroseismic parallax. Then, they check if it matches the Gaia parallax and correct any errors.

"Asteroseismology is the only way we can check Gaia's parallax accuracy across the full sky – that is, for both low- and high-intensity stars," says Anderson. And the future of this field is bright, as Khan outlines:

"Upcoming space missions like TESS and PLATO intended to detect and survey exoplanets will employ asteroseismology and deliver the required datasets across increasingly large regions of the sky. Methods similar to ours will therefore play a crucial role in improving Gaia's parallax measurements, which will help us pinpoint our place in the Universe and benefit a plethora of subfields of astronomy and astrophysics."