What are stellar earthquakes that make stars move?

The Gaia mission of the European Space Agency revealed the earthquakes that occur thousands of light years away. Argentine astronomers explain the scope of this space telescope and its discoveries.

In 2013 the European Space Agency (ESA), launched a unique space mission: the Gaia space observatory, a spacecraft designed specifically for astrometry: the science that measures the positions, distances and motions of stars with a precision never seen before.

Scientists say the mission aims to build the largest and most accurate 3D space catalog ever made of the observable universe, with a total of approximately two billion astronomical objects, mainly stars, but also planets, comets, asteroids and quasars, among other fascinating objects in the vastness of space.

This week, ESA released a new update of data collected by Gaia in the last three years, after having observed 10 million stars and space objects, which led to the publication of several scientific papers on some of the discoveries made. This dataset has become the largest catalog to date of binary stars, thousands of solar system objects, such as asteroids and moons of planets, and millions of galaxies and quasars located outside the Milky Way.

One of the most surprising discoveries that Gaia made without intending to and without being the focus of its main mission was the detection of stellar earthquakes or 'Starquakes', which consist of small movements recorded on the surface of a star that change the shape of the stars and could provide new insights into the inner workings of these distant suns.

Marcelo Miller Bertolami, an astronomer from La Plata and senior researcher at Conicet explained to Infobae the work Gaia has been doing since it was launched almost 10 years ago. "Gaia is an ESA space telescope that aims to determine the distance of stars very accurately. It observes the sky, seeing the position of the stars precisely. The Earth is changing its position in space. And so do the stars. Gaia is a somewhat rare telescope. It's not a conventional telescope, like Hubble is. Its main advantage is how its silicon carbide mirrors are polished to observe stars and objects at great distances. They are polished at a very high pressure: 10 nanometers, which are 10 millionths of a millimeter. Almost the separation of atoms," explained Miller Bertolami.

"Their work allows several parallel studies, primarily on the stars and the origin of the Milky Way. By knowing the distance, one can know the intrinsic brightness of the stars. This will also allow us to improve the age of the stars and the history of our galaxy itself. Gaia measures each star many times. In addition to measuring positions, it measures brightnesses. It measures several million stars several times. Each star on average will be measured about 70 times every 2 to 4 weeks, which allows us to observe how they change their brightness," added the astronomer.

"The discovery of stellar earthquakes is quite surprising since the spacecraft was not designed to do that job," said Dr. Conny Aerts, of the Catholic University of Leuven in Belgium, a world astronomy reference, at a press conference of the European Space Agency (ESA) on Monday, June 13, in which Infobae participated. "These vibrations make the stellar gas move up and down. And it changes the brightness of the star as a function of time. Then it makes the stars twinkle in the sky," the expert added.

"You can then determine whether the stars have pulsations. With Gaia's precision, it is possible to determine the period of oscillation of the stars. These are variations in brightness that occur due to the vibration of the star itself. And that vibration tells us about earthquakes or earthquakes. And depending on how they are made, on their changing core, they vibrate in different ways. When a star vibrates, there are variations in the brightness of the star over time. And that is what asteroseismology, a branch of astronomy that is very much in vogue these days, is studying," Miller said.

The expert clarified that although the material with which stars are formed is usually similar, such as hydrogen and helium, over time, the universe "has become dirty", due to the heavier chemical elements that can originate with material from planets, comets, asteroids and other dying stars. "Every star changes its internal chemical composition. Today the sun generated more helium than when it was born because of nuclear generation over the last 5 billion years it has," Miller said.

The stellar earthquakes were discovered in a subset of observations focused on the distribution of variable stars in the Milky Way galaxy, that is, stars whose brightness changes with time. "Twinkling stars offer astronomers a very powerful tool to study their internal physics and chemistry," Aerts said. "It's like earthquakes on Earth. Seismologists love earthquakes if they're not too violent, because they allow us to understand what's going on inside our planet. And asteroseismologists do the same thing, but with the stars."

The data released by ESA generated multiple published scientific studies. One of them is the one in which the Argentine Nicolás Unger, a physics graduate from the University of Buenos Aires with a PhD in astrophysics and exoplanets, participated.

"I study the universe and look for planets outside our solar system. For this, Gaia is a very important mission, since it not only generates the largest star catalog, which today counts more than 1.8 billion stars, but it is also very precise in measuring their position, which is what we call astrometry in astronomy. This allows us to know where the stars are in the sky and the distance between them and us," Unger explained to Infobae.

"These observations are very useful for finding exoplanets, because if one observes that if a star moves in a circular shape in the sky, it means that there is an object orbiting around it. By analyzing the type of movement we can deduce whether it is a planet or another star orbiting around it. Recently, the third Gaia data release was published, with the first exoplanet catalog. I was able to participate in this research," said the specialist in non-Earth neighboring worlds.

"From all the stars that Gaia observes, some are extracted that we think may harbor exoplanets. From those stars we calculate what the orbit of that exoplanet would be. Then, we extract its orbit and pass it through a validation filter because the measurement that Gaia performs today is the first of its kind and needs permanent checks. That's my job. To check and validate the Gaia data. We do that, looking to see if that exoplanet had already been discovered by some other method. For that we use radial velocity, which is measuring the velocity of the star. If one sees the oscillatory motion in the star over time, we can deduce that there is an exoplanet. This is a well-studied technique, with which the first exoplanet was discovered, for example," said Unger, who has been working with Gaia today in Geneva, along with collaborators from Italy and Spain.

In August 2015, Gaia completed its first year of scientific observations, during which it recorded 272 billion positional or astrometric measurements, 54.4 billion photometric data points, and 5.4 billion spectra. On September 14, 2016, ESA released its first Gaia dataset that included positions and G magnitudes of approximately one billion stars based on observations from July 25, 2014 to September 16, 2015.

On April 25, 2018, ESA released a second data set that included the positions of approximately 1.7 billion stars, as well as a measurement of their overall brightness at optical wavelengths. Now, with this third report, astronomers are celebrating and promise to unveil more astonishments captured by Gaia. The June 13 Gaia data release also contains the largest dataset ever compiled of binary star systems in our galaxy, that is, pairs of stars (or stars and black holes) that orbit each other. "This is something that the astronomical community is very excited about because binary stars, for example, are the only way you can measure the mass of stars directly," said Anthony Brown, an astronomer at Leiden University, the Netherlands.

The name Gaia was originally derived as an acronym for Global Astrometric Interferometer for Astrophysics. This reflected the optical interferometry technique that was originally planned for use on the spacecraft. While the method of work evolved during the studies and the acronym is no longer applicable, the name Gaia was retained to give continuity to the project. The total cost of the mission is around $1 billion, including fabrication, launch and ground operations, for a mission that will take 15 years to complete. Although given that its detectors are not degrading as fast as initially expected, the mission could be extended.