When the red giant star devours the earth

It sounds like the script of an apocalyptic sci-fi film, but it's considered a plausible scenario by scientists: In about three and a half billion years, our sun will begin to cool on the surface and in the process expand into a red giant star that may contain all four devours inner planets - including Earth.

This terrifying vision is not new, but astronomers have now determined the time and course of this catastrophe more precisely than ever before, as reported by the European Space Agency Esa. To do this, the researchers used recently published data from the Gaia satellite, which has been observing billions of stars with several telescopes since 2014.

"Only when we understand our Sun - and there is still a lot we don't know about it - can we expect to understand all the other stars in our Milky Way," explains Orlagh Creevey from the Observatory Côte d'Azur in France the extensive data analysis that she and her colleagues have carried out.

In order to gain new insights into our sun, the researchers have to look at a large number of similar stars. Because we only see our sun at the present time of its development, 4.57 billion years after its formation. Astronomers can only predict the evolution of the sun by observing many sun-like stars of different ages.

The Gaia data offer this possibility, because in addition to the movement, they also record the size, temperature, mass and chemical composition of the stars. And a detailed analysis of this data enables astronomers to determine the type of star - including its resemblance to the Sun - and its age.

In a first step, Creevey and her colleagues filtered out the stars with the most accurate values ​​from the gigantic amount of data. "We wanted to have the cleanest possible data set with high-precision measurements at our disposal," emphasizes the researcher. In a second step, the team filtered out all those stars from this selection that resemble our sun in terms of their mass and chemical composition.

The bottom line is that although all of these stars are similar to the sun, they are very different in age. Because as long as the stars burn hydrogen to helium in their interior and thus generate their radiation, their mass and composition only change slightly. The situation is different, however, with the size and temperature of the stars: both are slowly increasing.

Since the stars are similar to the sun but are of different ages, Creevey and her colleagues were able to determine how these stars – and thus our sun – evolved over time. The result: When the sun is eight billion years old, its surface will reach a maximum temperature - which is only about 20 degrees higher than today's value of 6045 degrees Celsius.

Then it begins to cool and expand into a red giant. As a giant that keeps expanding, it could swallow up the four inner planets Mercury, Venus, Earth and Mars. Only when there is no longer any nuclear fusion inside the sun - at an age of ten to eleven billion years - does the red giant collapse into a white dwarf star just about the size of the earth, which slowly cools down over billions of years.

As if that weren't enough, Creevey and her colleagues went one step further: From their data, they picked out all those stars that also resemble the sun in terms of temperature and size - and are therefore about the same age as our central star. They found a total of 5883 "solar analogues" in this way - and this list is extremely valuable for astronomers all over the world.

Because precise observations of these stars can answer the question of how "normal" our sun is. Or whether the emergence of life on Earth depended on the fact that the sun has special properties that distinguish it from other stars.

"Aha! Ten minutes of everyday knowledge" is WELT's knowledge podcast. Every Tuesday and Thursday we answer everyday questions from the field of science. Subscribe to the podcast on Spotify, Apple Podcasts, Deezer, Amazon Music, among others, or directly via RSS feed.