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Gaia reveals the past and future of the sun

Gaia reveals the past and future of the sun

An artistic impression of the Gaia satellite.

Photo: ESA

We all wish we could see the future. Thanks to the latest data from the European Space Agency’s Gaia stellar mission, astronomers can now do just that for the Sun. By accurately identifying stars of similar mass and composition, they can see how our Sun will develop in the future. This work goes beyond just a little astrophysical clairvoyance. The third edition of the Gaia Master Data (DR3) was announced on June 13, 2022. One of the main products of this release was the database containing the intrinsic properties of hundreds of millions of stars. These parameters include how hot it is, its size, and the masses it contains.

Gaia measures the apparent brightness of a star, as seen from Earth, and the color of the star with exceptional accuracy. Converting these fundamental observator properties into intrinsic properties of the star is a daunting task. Orlag Crevi, La Côte d’Azur Observatory, France, and Gaia Coordination Unit 8 staff are responsible for extracting such astrophysical parameters from Gaia observations. The team builds on the pioneering work of astronomers at the Harvard College Observatory in Massachusetts in the late 19th and early 20th centuries.

At the time, astronomers focused on classifying the appearance of “spectral lines”. These are the dark streaks that appear in the rainbow of colors that arise when a star’s light is split by a prism. Annie Jump Cannon devised a spectral classification arrangement that ranks stars according to the strength of these spectral lines. It was later discovered that this arrangement was directly related to the temperature of the stars. Antonia Morey created a separate classification based on the width of certain spectral lines. It was later discovered that it was related to the star’s luminosity and age.

By connecting these two properties, every star in the universe can be drawn into a single diagram. This scheme, known as the Hertzsprung-Russell (HR) scheme, became one of the cornerstones of astrophysics. Developed independently in 1911 by Ignar Hertzsprung and in 1913 by Henry Norris Russell, the HR diagram plots a star’s intrinsic luminosity versus its effective surface temperature. It shows how stars evolve during their long life cycles. While a star’s mass changes relatively little during its lifetime, a star’s temperature and size varies greatly as it ages. These changes are determined by the type of nuclear fusion reactions taking place in the star at that time.

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At about 4.57 billion years, our Sun is currently in the middle of its comfortable life, melting hydrogen into helium and is generally stable; even stable. This will not always be the case. If the Sun’s core runs out of hydrogen and changes occur in the fusion process, we would expect the Sun to grow into a red giant star, with a lower surface temperature. Exactly how this happens depends on the star’s mass and chemical composition. This is where DR3 comes in. Orlag and his colleagues rummaged through the data for the most accurate stellar observations the spacecraft could provide. “We wanted to get a really clean sample of stars with very accurate measurements,” Orla says.

They focused their efforts on stars with surface temperatures between 3,000 K and 10,000 K, because they are the longest-lived stars in the Milky Way, and thus can reveal the history of the Milky Way. It is also a promising candidate for finding exoplanets because it is very similar to the Sun, which has a surface temperature of 6000 K. Then Urla and his colleagues filtered the sample so that only stars of the same mass and chemical composition as the Sun would appear in the image. Due to their difference in age, the chosen stars eventually drew a line on the HR chart representing the evolution of our Sun from its past to its future. This explained how our star will change its temperature and luminosity with age.

The evolution of our star, the Sun – Photo: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

It is clear from this work that our Sun will reach a maximum temperature when it is about 8 billion years old, then cool down and increase in size, becoming a red giant star when it is about 10-11 billion years old. After this stage, the Sun will reach the end of its life when it finally becomes a faint white dwarf. Finding Sun-like stars is essential to understanding how we fit into the broader universe. “If we don’t understand our sun – and there are a lot of things we don’t know – how can we expect to understand all the other stars that make up our beautiful galaxy,” Orla says.

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Paradoxically, the Sun is our closest and most studied star, but this proximity forces us to study it with telescopes and instruments very different from the ones we use to look at the rest of the stars. That’s because the sun is much brighter than other stars. By identifying stars that are similar to the Sun, but this time of the same age, we can bridge this perception gap. To identify these “solar isotopes” in the Gaia data, Orlag and colleagues searched for stars with temperatures, surface gravity, compositions, masses, and radii that all resemble the current Sun. They found 5,863 stars matching their criteria.

Now that Gaia has created his list of goals, others can begin to seriously examine them. Some of the questions they want answered are: Do all of their solar counterparts have a similar planetary system to ours? Do all the isotopes of the sun rotate at the same speed as the sun?

With the release of Data 3, the high-resolution Gaia instrumentation has made it possible to determine the stellar parameters of a greater number of stars with greater accuracy than ever before. This accuracy will feed into many other studies. For example, a more accurate knowledge of stars can help in the study of galaxies, the light of which is a fusion of billions of individual stars.

“The Gaia mission had an impact throughout astrophysics,” Orla says.

source: ESA