III populiacijos žvaigždės: pirmoji Visatos karta

III population stars: the first generation of the Universe

Massive, metal-free stars whose explosions contributed heavier elements to later star formation

It is believed that Population III stars are the first generation of stars in the Universe. They formed within the first few hundred million years after the Big Bang and played a crucial role in the evolution of cosmic history. Unlike later stars, which contain heavier elements (metals), Population III stars were composed almost exclusively of hydrogen and helium — the products of Big Bang nucleosynthesis, with trace amounts of lithium. In this article, we will discuss why Population III stars are so important, how they differ from modern stars, and how their spectacular explosions had a huge impact on the formation of subsequent stars and galaxies.

1. Cosmic context: the primordial Universe

1.1 Metallicity and star formation

In astronomy, any element heavier than helium is called a “metal.” Immediately after the Big Bang, nucleosynthesis produced mostly hydrogen (~75% by mass), helium (~25%), and trace amounts of lithium and beryllium. Heavier elements (carbon, oxygen, iron, etc.) had not yet formed. Therefore, the first stars — Population III stars — practically had no metals. This near-total absence of metals critically affected how they formed, evolved, and ultimately exploded.

1.2 The epoch of the first stars

It is believed that Population III stars illuminated the dark, neutral Universe shortly after the cosmic “Dark Ages.” They formed in dark matter mini-halos (with masses ~105–106 M⊙) — early gravitational “wells” — and announced the cosmic dawn: the transition from a dark Universe to the emergence of shining stars. Their intense ultraviolet radiation and subsequent supernova explosions initiated the reionization process and enriched the intergalactic medium with chemical elements (IGM).

2. Formation and properties of Population III stars

2.1 Cooling mechanisms in metal-free environments

In later epochs, very important cooling channels for star formation are metal spectral lines (e.g., iron, oxygen, carbon), which help gas clouds cool and fragment. However, in metal-free environments, the main cooling methods were:

  1. Molecular hydrogen (H2): The main coolant in primordial gas clouds, radiating energy through rotational-vibrational transitions.
  2. Atomic hydrogen: Partial cooling occurred through atomic hydrogen electronic transitions, but it was less efficient.

Due to limited cooling capabilities (absence of metals), early gas clouds often did not fragment into large star cluster clumps as easily as in later, metal-rich environments. As a result, the protostar mass here was generally larger.

2.2 Exceptionally large mass

Simulations and theoretical models show that Population III stars could have been very massive compared to current stars. Estimates range from tens to hundreds of solar masses (M⊙), and some models even suggest several thousand M⊙. The main reasons are:

  • Reduced fragmentation: With limited cooling, the gas mass remains larger until one or several protostars form.
  • Inefficient radiative feedback: In the initial stage, a large star could continue accreting material because the metal-free environment feedback (which limits the star's mass) acted differently.

2.3 Lifetime and temperature

Massive stars burn their fuel very quickly:

  • ~100 M⊙ A star lives only a few million years — an extremely short period in cosmic terms.
  • Besides metals that help regulate internal processes, Population III stars likely had extremely high surface temperatures, intensely emitting ultraviolet radiation capable of ionizing surrounding hydrogen and helium.

3. Evolution and death of Population III stars

3.1 Supernovae and element enrichment

One of the most striking features of Population III stars is their impressive “deaths.” Depending on mass, they could end their lives as various types of supernovae:

  1. Pair-instability supernova (PISN): If a star's mass was 140–260 M⊙, extremely high temperatures inside the star cause some gamma photons to convert into electron-positron pairs, triggering a gravitational collapse followed by an explosion that completely disrupts the star (no black hole remains).
  2. Core-collapse supernova: Stars weighing ~10–140 M⊙ could evolve according to a more typical collapse scenario, after which a neutron star or black hole may remain.
  3. Direct collapse: The collapse of extremely massive (>260 M⊙) stars could be so strong that it immediately formed a black hole without causing a large element ejection wave.

Regardless of the mechanism, even the material from a few Population III star supernovae (metals: carbon, oxygen, iron, etc.) enriched the environment. Later gas clouds, containing even a small amount of these heavier elements, could cool the gas much more effectively, thus creating conditions for another generation of stars with some metals (Population II). This chemical evolution later allowed conditions similar to our Sun's to form.

3.2 Black hole formation and early quasars

Some particularly massive Population III stars could have turned into “black hole seeds,” which, rapidly growing (through accretion or mergers), quickly became supermassive black holes powering quasars at high redshifts. A key research question in cosmology is how black holes managed to reach millions or billions of solar masses within the first billion years?

4. Astrophysical impact in the early Universe

4.1 Contribution to reionization

Population III stars intensely emitted ultraviolet (UV) light capable of ionizing neutral hydrogen and helium in the intergalactic medium. Together with early galaxies, they contributed to the Universe's reionization, transforming it from mostly neutral (after the Dark Ages) to mostly ionized within the first billion years. This process radically changed the temperature and ionization state of cosmic gas, influencing subsequent stages of structure formation.

4.2 Chemical enrichment

Metals produced by Population III supernovae had a huge impact:

  • Enhanced cooling: Even a small amount of metals (~10−6 solar metallicity) can significantly improve gas cooling.
  • Next generation stars: Chemically enriched gas fragmented more strongly, allowing the formation of lower-mass, longer-lived stars (called Population II, and later Population I stars).
  • Planet formation: Without metals (especially carbon, oxygen, silicon, iron), it is almost impossible to form Earth-like planets. Thus, Population III stars indirectly pave the way for planetary systems and ultimately life as we know it.

5. Searching for direct evidence

5.1 Challenges in detecting Population III stars

Detecting direct traces of Population III stars is difficult:

  • Short-lived: They lived only a few million years and disappeared billions of years ago.
  • High redshift: Formed at z > 15, so their light is extremely faint and strongly "stretched" into the infrared.
  • Galaxy mergers: Even if some survived theoretically, they are overshadowed by stars of later generations.

5.2 Indirect traces

Rather than directly detecting Population III stars, astronomers seek their traces:

  1. Chemical abundance patterns: Metal-poor stars in the Milky Way halo or dwarf galaxies may show unusual elemental ratios reflecting the influence of Population III supernovae.
  2. High-redshift GRBs: Massive stars can cause gamma-ray bursts (GRBs) upon collapse, which can be detected at cosmic distances.
  3. Supernova signatures: Telescope surveys searching for extremely bright supernovae (e.g., pair-instability SNe) at high redshift may capture Population III explosions.

5.3 The role of JWST and future observatories

With the launch of the James Webb Space Telescope (JWST), astronomers have gained unprecedented sensitivity for observations in the near-infrared, increasing the chances of detecting very distant, extremely faint galaxies that may host Population III star clusters. Future missions, including next-generation ground-based and space telescopes, will further expand these boundaries.

6. Current research and unanswered questions

Although many theoretical models have been developed, fundamental questions remain:

  1. Mass distribution: Did a broad mass spectrum of Population III stars exist, or were they essentially extremely massive?
  2. Initial star formation sites: How and where exactly did the first stars form in dark matter mini-halos, and did this process differ among various halos?
  3. Impact on reionization: How precisely did Population III stars contribute to the Universe's reionization compared to early galaxies and quasars?
  4. Black hole seeds: Did supermassive black holes efficiently form from the direct collapse of especially massive Population III stars, or are other models necessary?

Answers to these questions require combining cosmological simulations, observational campaigns (surveying metal-free halo stars, high-redshift quasars, gamma-ray bursts), and advanced chemical evolution models.

7. Conclusion

Population III stars formed all subsequent cosmic evolution. Born in a Universe without metals, they were likely massive, short-lived, and could have had a lasting impact — ionizing their surroundings, forging the first heavier elements, and forming black holes that became the fuel for early quasars. Although they cannot be directly detected, chemical "signatures" remain in the composition of the oldest stars and the widespread cosmic metal distribution.

Studies of these already extinct star populations are crucial to understanding the early epochs of the Universe, from the cosmic dawn to the origin of galaxies and clusters we see today. With the advancement of future telescopes and deeper observations into high redshifts, scientists hope to more clearly identify the traces of these no longer existing giants — the "first light" in the dark Universe.

Links and further reading

  1. Abel, T., Bryan, G. L., & Norman, M. L. (2002). “The Formation of the First Star in the Universe.” Science, 295, 93–98.
  2. Bromm, V., Coppi, P. S., & Larson, R. B. (2002). “The Formation of the First Stars. I. The Primordial Star-forming Cloud.” The Astrophysical Journal, 564, 23–51.
  3. Heger, A., & Woosley, S. E. (2002). “The Nucleosynthetic Signature of Population III.” The Astrophysical Journal, 567, 532–543.
  4. Chiaki, G., et al. (2019). “Formation of Extremely Metal-poor Stars Triggered by Supernova Shocks in Metal-free Environments.” Monthly Notices of the Royal Astronomical Society, 483, 3938–3955.
  5. Karlsson, T., Bromm, V., & Bland-Hawthorn, J. (2013). “Pregalactic Metal Enrichment: The Chemical Signatures of the First Stars.” Reviews of Modern Physics, 85, 809–848.
  6. Wise, J. H., & Abel, T. (2007). “Resolving the Formation of Protogalaxies. III. Feedback from the First Stars.” The Astrophysical Journal, 671, 1559–1577.
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