We have discovered the oldest star to date, and its "father" is a "fat guy"

When talking about stars, the first thing that comes to mind is the sun, but the sun is a young star, only about 4 billion years old. Are there stars older than the sun? What does the oldest and earliest star in the universe look like?

The latest research tells us that there were "fat guys" among the first generation of stars in the universe.

On June 7, 2023, the international academic journal Nature published online an important result of an international team led by Zhao Gang, a researcher at the Astronomical Observatory of the Chinese Academy of Sciences. The research team was the first to discover chemical evidence of the existence of pair-instability supernova (PISN) formed by the collapse of the first generation of supermassive stars after evolution in the Milky Way halo stars. The result confirmed that this supernova originated from a first-generation supermassive star with a mass of up to 260 times that of the sun , refreshing people's understanding of the mass distribution of the first generation of stars.

This conclusion is actually drawn from the "fossils" of the first generation of stars. Let us travel through billions of years and start the journey of "stellar archaeology" together.

Although the first generation of stars has basically disappeared, you and I have its mark.

Astronomers are used to calling the first batch of stars born in the universe the first generation of stars, also known as Population III stars. Cosmological theory holds that the Big Bang created the universe 13.8 billion years ago. For a period of time after the birth of the universe, everything was dark. About 100-200 million years after the Big Bang, the oldest first generation of stars were born, emitting the first ray of light that illuminated the universe, and also opening the prelude to the evolution of chemical elements in the universe.

After the Big Bang, only hydrogen, helium and a very small amount of lithium were produced. It is generally believed that the first generation of stars were born in the earliest gas clouds in the universe that contained almost no metal elements ( in astronomy, elements other than hydrogen and helium are collectively called metal elements ). It is impossible to effectively cool down by radiation here (the first excitation energy level of hydrogen and helium atoms is higher than the excitation energy level of metal elements). Therefore, the metal content in the atmosphere of these oldest first-generation stars is extremely low, with only hydrogen, helium and a very small amount of lithium.

Schematic diagram of the first generation of stars formed in the primordial gas cloud (Image credit: NASA)

Theory holds that the first generation of stars were extremely massive[1][2], ranging from tens to hundreds of times the mass of the sun, and had a lifespan of only a few million years (because the fatter the star, the shorter its lifespan). Therefore, as early as more than 13 billion years ago, most of the first generation of stars ended their lives in the form of violent supernova explosions.

Supernova explosion of the first generation of stars (Image source: National Astronomical Observatory)

In this process, nuclear fusion reactions produce new metal elements that are ejected into the surrounding environment. These metals help the gas cloud to cool by radiation, allowing the gas cloud to give birth to a second generation of stars with smaller mass and longer lifespan. Generation after generation of stars "follows one another", and the metal content of each new generation of stars is slightly higher than that of its ancestors, and the types and quantities of chemical elements in the universe continue to increase. It can be said that many of the elements that make up our bodies were originally produced by the first generation of stars.

However, the lifespan of the first generation of stars is so short that it is extremely difficult to directly observe them. To this day, astronomers have not actually observed the first generation of stars, and "how big they are" (termed as mass distribution) has always been a hot topic for astronomers.

Second-generation stars: I am a "living fossil" of the first-generation stars

As the saying goes, "When a whale dies, all things come to life." Although the massive first-generation stars have disappeared, when they ended in supernova explosions, the released metal elements were inherited and preserved by the second-generation stars.

The remains of the first generation of stars gave birth to the second generation of stars (Image source: National Astronomical Observatory)

Some of these second-generation stars have very low masses, even lower than the mass of the Sun, and therefore have relatively long life spans. They have survived to this day and have been directly observed by astronomers, and are the oldest stars that we can directly observe. These second-generation stars have very low metal content, for example, their iron content is even less than one percent of the iron content of the Sun, and astronomers usually call them metal-poor stars.

For a long time, astronomers have been committed to studying and tracing the properties of the first generation of stars by searching for the second generation of stars with extremely low metal content. These metal-poor stars carrying the chemical "genes" of the first generation of stars are like "living fossils" recording the oldest history of the universe. Therefore, finding them is of great significance for understanding the story of the early universe after the Big Bang.

By analyzing the chemical composition (types and contents of elements contained) of these existing low-mass second-generation stars, astronomers can use supernova theoretical models to infer the nature of the first-generation stars from which the material that makes up the star originated. This analysis process is figuratively called "stellar archaeology" by astronomers.

Moreover, if special second-generation stars are found, there may be unexpected gains!

A "living fossil" carrying special evidence reveals the secrets of two "infant" universes

In this study, the researchers not only confirmed the most massive first-generation stars to date, but also confirmed the existence of a special type of supernova for the first time.

First, let me tell you about the process of the "death" of the first generation of stars:

Usually, first-generation stars with masses less than 100 times that of the sun end their lives in the form of core-collapse supernova explosions; while first-generation stars with masses between 140 and 260 times that of the sun, the positron-electron pairs produced in their cores weaken the radiation pressure inside the star and cause the star to collapse to form a pair-instability supernova (PISN).

Compared with core-collapse supernovae, theory predicts that the products of pair-instability supernovae have extremely special chemical element compositions [4]. The most important feature is that the content of elements with odd atomic numbers is much smaller than the content of adjacent elements with even atomic numbers, also known as the "odd-even effect", such as sodium to magnesium and cobalt to nickel. Therefore, the second generation of stars formed by the evolution of pair-instability supernova products will also show a rare chemical abundance pattern, which provides clues for searching for the chemical remains of the first generation of supermassive stars.

However, all of the above studies are theoretical , and the sample size of metal-poor stars is limited . People have never found observational evidence for the first generation of stars with a mass greater than 100 times that of the sun [3] and their "death" to form unstable supernovae.

In this study, the research team launched the world's largest metal-poor star search project based on my country's large spectroscopic survey telescope LAMOST[5], and obtained a sample of tens of thousands of metal-poor stars, which is equivalent to obtaining a large number of "living fossils" for studying stellar archaeology and early cosmic chemical evolution.

The research team then used Japan's Subaru Telescope to accurately determine the chemical element composition of a large number of metal-poor stars, and was the first to discover a star with an extremely special content of chemical elements . It is the star with the lowest sodium content among all known stars, and the chemical abundance of this star also shows a strong "odd-even effect".

The chemical element composition of stars can be known through stellar spectra (Image source: National Astronomical Observatory)

The chemical genetic characteristics of this star indicate that it cannot be explained by the core collapse supernova theoretical model, but it is highly consistent with the theoretical calculation results of PISN with a mass of 260 times that of the sun. Therefore, astronomers believe that this is a metal-poor star that retains the chemical remains of PISN, providing clear observational evidence for the first generation of supermassive stars and their evolutionary formation of PISN, which have been hidden for a long time.

It can be said that the "parent" of this star is a "big fat guy" with 260 times the mass of the sun. Since the lifespan of its "parent" is very short, it can also be said to be the oldest star observed so far .

Comparison of the chemical abundance of the peculiar star LAMOST J1010+2358 with the supernova model. The red dots represent the element abundance of LAMOST J1010+2358, and the black solid lines represent the core-collapse supernova with a progenitor mass of 10 times the mass of the sun (a); the core-collapse supernova with a mass of 85 times the mass of the sun (b); and the pair-instability supernova with a mass of 260 times the mass of the sun (c).

Lighting up the path of “stellar archaeology”

The discovery of this special star allowed astronomers to find observational evidence for the existence of unstable supernovae of the first generation of supermassive stars and their evolutionary products for the first time. It also confirmed from observations that the mass of the first generation of stars can reach hundreds of times the mass of the sun, revealing that unstable supernovae contributed a large amount of metal elements in the process of chemical enrichment in the early universe. This is of great significance for studying the distribution law of the initial mass of the first generation of stars, and will have a profound impact on the research on the origin of elements, star formation in the early universe, and chemical evolution of galaxies.

When, where and how the elements that make up the world and our bodies were formed, and how they drove the birth of life, the answers to these questions may be hidden in the history of stellar evolution.

In the future, we expect astronomers to use LAMOST and the China Space Station Engineering Survey Telescope to discover more stars with special chemical element content, so that humans can capture more chemical relics left by the first generation of stars, travel through the cosmic time tunnel to ancient times, and recognize and understand the "original appearance" of the highest generation of first-generation stars and the early universe.

Artist's impression of the first generation of supermassive stars evolving into pair-instability supernovae, which eject element-rich material into the interstellar medium, aiding the formation of the next generation of stars.

References:

[1]Susa, H., Hasegawa, K. & Tominaga, N. The Mass Spectrum of the First Stars, Astrophys. J. 792, 32 (2014)

[2]Abel, T., Bryan, GL & Norman, ML The Formation and Fragmentation of Primordial Molecular Clouds. Astrophys. J. 540, 39–44 (2000)

[3]Ishigaki, MN, Tominaga, N., Kobayashi, C. & Nomoto, K. The Initial Mass Function of the First Stars Inferred from Extremely Metal-poor Stars. Astrophys. J. 857, 46 (2018)

[4]Heger, A. & Woosley, SE The Nucleosynthetic Signature of Population III. Astrophys. J. 567, 532–543 (2002)

[5] Zhao, G., Zhao, Y.-H., Chu, Y.-Q., Jing, Y.-P. & Deng, L.-C. LAMOST spectral survey — An overview. Research in Astron. and Astrophys. 12, 723–734 (2012)

Author: Xing Qianfan and Li Shuang

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