Will 10 “low-metallicity stars” discovered in the Large Magellanic Cloud change our view of the early universe? | sorae universe portal website

To learn about the characteristics of the first stars that were born in the universe, it is currently believed that they were born from the remains of the first star.Low metal starWe rely on indirect methods to find out “. Low-metallicity stars are extremely rare, accounting for about 1 in 100,000 stars, and most of the stars whose properties have been studied in detail so far belong to the Milky Way.

A research team led by Anirudh Chetty from the University of Chicago discovered the Milky Way's galaxy,Large Magellanic Cloud“They chose 10 low-metallicity stars to analyze. The results,Low-metallicity stars in the Large Magellanic Cloud have different element ratios than most low-metallicity stars in the Milky Way.It turns out that. Differences in element ratios are thought to reflect differences in the environments in which low-metallicity stars formed, and thus provide important clues for exploring the state of the early universe.

■ “Low-metallicity stars'' are evidence of first-generation stars that have not yet been seen

What kind of properties did the first stars (Group III) born in the universe have? It is believed that such stars do not exist near us, as their mass is enormous, equivalent to several hundred times the mass of the Sun, and they reach the end of their lives quickly (less than a million years). It is expected that the James Webb Space Telescope, which recently began operating, will be able to monitor the history of early galaxies in the very distant universe and learn about the properties of the first stars, but it is unclear whether this will be successful. I do not know yet.

Therefore, we now look to the second generation star, which formed from the remains of the first generation star, as evidence of the properties of the first generation star.Low metal star“(The second race) is attracting attention. Unlike the first stars, the low-metallicity stars have a small mass like the Sun, so they are still present in the Milky Way even now, more than 10 billion years after their birth.

A low-metallic star is a term that literally means a star with a lower metal content than new stars such as the Sun, but the metal here is not limited to chemical metallic elements such as iron, but also includes carbon and other metals. It includes chemically non-metallic elements such as oxygen. This has to do with the composition of the elements in the universe.

Immediately after his birth, the universe contained almost only hydrogen and helium.(※)The first star can be considered a mass of pure hydrogen and helium. However, elements heavier than lithium are made in nuclear fusion reactions that occur in the cores of stars. When the first star eventually reached the end of its life and exploded as a supernova, the heavy elements that had been synthesized through nuclear fusion reactions dispersed.

* In fact it is believed that a small amount of minerals higher than lithium were also produced, but the amount is less than 1/10 billion and their presence can be ignored.

The second generation of stars formed from this debris must contain heavier elements that were not present in the first star, but they must also contain less massive elements than later generations of stars. For these reasons, it is customary to collectively refer to heavy elements other than hydrogen and helium as metals, and to refer to stars with very low metal content as low-metallicity stars.

Low-metallicity stars are extremely rare, occurring at a rate of about one in every 100,000 stars, but many have been discovered in the Milky Way, some of which are estimated to be as old as the universe itself. On the other hand, little progress has been made in detailed research on low-metallicity stars outside the Milky Way, as it is impossible to prove that they are low-metallicity stars unless we know the element ratio in detail, and their presence. In itself, it's rare. I wasn't there.

■10 low-metallicity stars discovered in the Large Magellanic Cloud

Chetty and his team conducted a detailed study of low-metallicity stars that do not belong to the Milky Way. The object was a galaxy belonging to the Milky Way.Large Magellanic Cloud“It is. The Large Magellanic Cloud is a galaxy that was recently occupied by the Milky Way (about 2 billion years ago) and has a different origin than the Milky Way. In addition, since it is located close to Earth at a distance of 163 thousand light-years Almost every star can be studied in detail.

However, finding rare stars with low metallicity requires a large amount of stellar data, and it is necessary to analyze them and “find the needle in the haystack.” Chetty et al., a space telescope launched by ESA (European Space Agency),GaiaWe've narrowed down the candidates for low-metallicity stars from observational data for…Magellan telescopeWe have been working to obtain more detailed monitoring data.

Results,A total of 10 low-metallicity stars have been found to have less than 0.3% of the Sun's metal content.LMC-119, the star with the least amount of metal discovered this time around, contains only 13,500 times as much metal (about 0.007%) as the Sun, making it the most metal-rich star in the Milky Way. A metallic star that can be compared to few stars.

■ Will we change common knowledge about the properties of low-metallicity stars?

Moreover, all ten low-metallicity stars discovered this time are…Very low carbonThis is an interesting discovery. Most low-metallicity stars in the Milky Way are relatively high in carbon, but low-metallicity stars in the Large Magellanic Cloud contain only about 40% carbon.

Since the low-metallicity stars in the Milky Way contain a lot of carbon, it was previously thought that being carbon-rich was a general property of low-metallicity stars, and that there was a special process that increased the amount of carbon. I did. But, recently, a cluster of low-metallicity stars with less carbon has been discovered in the Milky Way, suggesting that this idea may be wrong.

It has now been found that low-metallicity stars in the Large Magellanic Cloud also have low carbon content, suggesting that low-metallicity stars in the Large Magellanic Cloud likely formed in a different environment than the majority of low-metallicity stars. In the Milky Way, this is possible. If the properties of the majority of low-metallicity stars in the Milky Way are not general properties of all low-metallicity stars, then the properties of the first stars considered depend on the low-metallicity stars and the environment of early ages. Universe Your perspective on this may change as well. The results of this study will change conventional wisdom regarding stars with low metallicity.

source

• Anirudh Chetty, et al. “Enrichment by the first extragalactic stars in the Large Magellanic Cloud”. (nature) (arXiv)
• Louise Lerner. “Scientists find one of the oldest stars that formed in another galaxy” (University of Chicago)

Written by Riri Aya