How were the oldest stellar relics in the Milky Way discovered by Chinese scientists?

13.8 billion years ago, when the universe was first born, the Big Bang produced a large amount of hydrogen, helium and extremely small amounts of lithium. In such an environment, the first generation of stars in our universe were born and lit up the entire universe for the first time.

In 2005, Science magazine published a list of the 125 most challenging scientific problems on its 125th anniversary. The first generation of stars was one of them. To this day, the nature of the first generation of stars is still an important scientific problem that needs to be solved, and is even one of the key scientific goals of the James Webb Space Telescope.

What did the first stars look like? Legend has it that they were probably very large and heavy, but how big could they be? Fifty suns? Or a hundred suns? These questions have puzzled astronomers for many years.

However, not long ago, on June 7, 2023, the international academic journal Nature published online an important discovery led by Chinese astronomers. They found for the first time the remains of the first generation of supermassive stars in the Milky Way, and confirmed that the remains originated from a pair-instability supernova (PISN). In other words, the mass of this first-generation star is as high as 260 times that of the sun! It is a veritable supermassive star.

One of the key players who helped Chinese astronomers make this important discovery is the LAMOST telescope, also known as the Guo Shoujing Telescope, which was independently developed by my country. LAMOST has broken through the technical bottleneck of having a large aperture and a large field of view. It is currently the world's largest telescope with a large aperture and a large field of view. It has also completed the world's first survey project to publish a spectrum of more than 10 million stars in the Milky Way.

Focusing on the three core scientific goals of the Milky Way structure and evolution, multi-band celestial body cross-verification, and galactic physics, LAMOST has successfully completed the first phase of the five-year spectral survey, and the second phase of the survey is about to come to a perfect end. In the latest 10th data release, the amount of spectral data obtained by LAMOST has reached 2.9 times the total number of spectra released by other survey telescopes in the world! Using such a rich and massive amount of observational data, astronomers at home and abroad have made many interesting and important scientific discoveries in the fields of Milky Way research, stellar physics, and special celestial body searches. For example: revealing the growth process of the Milky Way in its childhood, discovering the most massive stellar black hole, tracking intergalactic immigrant stars in the Milky Way, rewriting the stellar initial mass function, etc., are all important breakthroughs with international influence.

Schematic diagram of the starry sky, picture from Tuchong.com

Let's go back to the first generation of stars. Most of these progenitor stars are huge, bright, but extremely short-lived. Coupled with the limitations of existing observation capabilities, we cannot directly see the first generation of stars today. So how was this discovery made? The first generation of stars synthesized and produced some metal elements heavier than lithium during their growth, and at the end of their short lives, they ejected the various metal elements they produced through supernova explosions and buried them in the interstellar dust around them.

The second generation of stars was born in these dusts. Their surface atmospheres perfectly retain the properties of the first generation of stars, and their lifespans are long enough to survive and be seen by us today. In other words, they are equivalent to cosmic fossils. By excavating these fossil stars and analyzing their properties and composition, we can reverse the environment during their formation and recreate the appearance of the first generation of stars. Since these fossil stars are very rare, the massive data provided by LAMOST also provides a crucial "selection" database for this successful discovery.

This time, we not only successfully confirmed the most massive first-generation star to date, a "fat guy" weighing more than 260 times the sun, but also confirmed the existence of a special supernova for the first time. Usually, first-generation stars with less than 100 times the mass of the sun end their lives in the form of core-collapse supernova explosions; and first-generation stars with masses between 140 and 260 times the mass of the sun, the positron-electron pairs produced in their cores will weaken the radiation pressure inside the star and cause the star to collapse to form a pair-instability supernova. Before this, the pair-instability supernova was like a legend, only heard of in theory. And this time, our Chinese astronomers finally confirmed its existence.

In the future, with the assistance of China-made astronomical observation instruments such as LAMOST and the China Space Station Project Survey Telescope, we will construct larger cosmic fossil samples, re-understand the distribution pattern of the mass of the first generation of stars, reveal the origin of elements, and reproduce the birth and growth images of stars and galaxies in the early universe.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Li Haining

Reviewer: Han Wenbiao, researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.