By: Paramjot Kaur
ABSTRACT
Metal-poor stars, typically found in the early universe, offer a unique window into the origins of stellar formation and the evolution of galaxies. These stars, often formed in environments with very low levels of heavy elements, provide valuable insights into the processes that shaped the first generations of stars. Understanding their lifecycle dynamics and evolutionary pathways is crucial for unraveling the history of our universe. This research examines metal-poor stars' formation, characteristics, and evolutionary paths, focusing on their formation in the early universe, the key traits that distinguish them from their metal-rich counterparts, and how they evolve. Additionally, the ultimate fate of these stars remains a topic of great interest: Do they end as black holes, neutron stars, or perhaps through different processes altogether?
INTRODUCTION
The concept of time, as shaped by the stars, has been scientifically demonstrated to follow a narrative outlined by Freeman and Bland-Hawthorn (Frebel & Norris, 2015). Over recent centuries, stellar observation has been a key tool for investigating the nature of the Milky Way, Andromeda, and numerous other galaxies. Metal-poor stars offer insights into the early chemical composition of the universe, shedding light on stellar formation processes. Later in this research, we may explore the concept of metallicity. Stellar Metallicity refers to the direct quantification of metals in a galaxy, which is ultimately prevalent in the stars. Ultraviolet light can be used to examine stellar photospheric absorption lines, which act as indicators of metallicity. (Bouwens, R. J., Illingworth, G. D., Oesch, P. A., et al, 2015). Metallicity is the primary factor influencing the color distribution of the Horizontal Branch, as it impacts the stars' light-blocking ability (envelope opacity) and is closely linked to stellar mass. Additionally, metallicity and temperature are negatively correlated.
LITERATURE REVIEW
I. FORMATION OF METAL-POOR STARS
Stars and galaxies don't reveal the full picture. Yet they bring the story to its peak. Big Bang Nucleosynthesis (BBN) refers to the early process in the Universe’s formation, during which the first light elements were created, namely quarks and electrons. (The Early Universe, 2024). Soon after, quarks combined to form neutrons and protons, which, within minutes, fused to create nuclei. This process resulted in the formation of the most abundant atoms in our universe: helium and hydrogen. However, in the early stages of their development, stars experienced more intense physical processes due to the high gas fraction, which indicates the star formation potential. In most galaxies, gas is depleted through various processes, such as being used up in star formation or being expelled during supernova explosions. (Forming Stars in the Early Universe | Center for Astrophysics | Harvard & Smithsonian, 2024).
The formation of metal-poor stars is intricately linked to the conditions of the early universe. As the universe evolved, hydrogen and helium, primordial elements, became abundant in the cooled gas clouds after their collapse. These stars, which lacked heavier metals, came to be known as metal-poor stars. What occurs during a collapse is largely influenced by the temperature and the evolution of the cloud. In this way, a giant molecular cloud can give rise to a group of stars, with their mass distribution shaped by the fragmentation process. This process is influenced by the cloud's physical and chemical properties, including factors like ambient pressure, magnetic fields, rotation, composition, dust content, and stellar feedback. We can apply the virial theorem to understand the formation of metal-poor stars and gas clouds, as it explains how these clouds collapse to form stars.
2E = −Ω
E denotes the total kinetic energy of the system,
Ω represents the gravitational potential energy of the system.
The balance between these factors determines the cloud's stability or collapse. In metal-poor stars, cooling mechanisms are less effective because of the scarcity of heavy metals, which are crucial for energy dissipation. This results in a different fragmentation process.
In 1944, Walter Baade classified stars within the Milky Way into distinct groups, known as stellar populations (Wikipedia Contributors, 2024). Baade established two main divisions,
Population I and Population II, along with a hypothetical Population III. According to population analysis, stars with extremely low metal content are classified as Population III, while stars with low metal content belong to Population II, and those with high metal content are placed in Population I.
Population I stars are typically young, have the highest metal content, and are primarily located in the spiral arms of the Milky Way galaxy. An example of a Population I star is the Sun. Population II stars, or metal-poor stars, contain fewer elements heavier than helium and formed in an earlier era of the universe. Intermediate Population II stars are common in the Milky Way’s bulge, while those in the galactic halo are older and more metal-deficient. Population III stars are a theoretical class of extremely massive, luminous, and hot stars, characterized by the absence of "metals" except possibly for traces of material from nearby early Population III supernovae. (The Editors of Encyclopaedia Britannica, 1998)
II. STELLAR FORMATION
The first phase of stellar energy production, hydrogen burning, involves transforming hydrogen into helium through the CNO cycle. The CNO cycle is a stellar nucleosynthesis process in which hydrogen is fused into helium through a six-stage reaction sequence within stars. (CNO Cycle | COSMOS, n.d.). The CNO cycle, or carbon-nitrogen-oxygen cycle, is a fusion process in stars more massive than the Sun, where carbon, nitrogen, and oxygen act as catalysts to convert hydrogen into helium. This cycle is essential for energy production in massive stars and contributes to stellar nucleosynthesis by aiding the formation of heavier elements throughout a star's life. (Cno Cycle - (Principles of Physics IV) - Vocab, Definition, Explanations | Fiveable, n.d.). A recent study investigating the impact of metallicity on envelope inflation in massive stars examined the relationship between metallicity levels and stellar envelope expansion. The findings revealed that stars with higher metallicity begin to exhibit inflation at comparatively lower masses, around 29 solar masses. In contrast, stars with lower metallicity, such as Population III, require significantly greater masses, approximately 150 solar masses, to undergo similar inflation.
III. THE RED GIANT PHASE
As previously mentioned, the majority of stars in the universe are main sequence stars, which generate energy by converting hydrogen into helium through nuclear fusion in their cores. The red giant branch (RGB) exhibits a metallicity dependence due to its influence on stellar opacity. Since metals contribute free electrons that form H⁻, higher metallicity increases opacity, causing an optical depth of τ ≈ 2/3 to be reached at lower densities. As a result, metal-rich stars of the
same mass have larger radii and lower effective temperatures than metal-poor stars. This also shifts the RGB of metal-rich stars to slightly lower temperatures.
● The blue curve represents metal-rich stars, which have lower effective temperatures and larger radii at a given mass.
● The red curve represents metal-poor stars, with higher effective temperatures and relatively smaller radii at the same mass.
IV, END STAGES
Stars evolve over millions to billions of years, and their end stages are influenced by factors such as mass, composition (metallicity), and the internal processes at play.
For low-mass stars (up to ~8 solar masses), the end stages begin with hydrogen burnout, leading to a red giant phase. As the core contracts and helium fuses, the star sheds its outer layers in a planetary nebula, leaving behind a white dwarf that gradually cools into a black dwarf. Metal-poor stars tend to have fewer heavy elements, which affects their cooling rates, potentially resulting in more massive white dwarfs and a higher interior temperature. For intermediate-mass
stars (~8 to 12 solar masses), the evolution includes the red supergiant phase, where multiple stages of fusion occur before the core collapses after the fusion of iron, leading to a Type II supernova. Depending on the mass of the remaining core, a neutron star or black hole may form. Metal-poor intermediate-mass stars have weaker stellar winds, allowing them to retain more mass and be more likely to form neutron stars rather than black holes. High-mass stars (greater than ~12 solar masses) undergo rapid fusion, evolving into blue supergiants before their core collapses in a Type II supernova. If the core is massive enough, it forms a black hole. Metal-poor high-mass stars have lower opacity in their outer layers, resulting in reduced mass loss and an increased chance of black hole formation. Very metal-poor (Population III) stars, which formed from primordial hydrogen and helium, likely had massive sizes and intense fusion processes, contributing to the first heavy elements in the universe. These stars end their lives in supernovae, possibly leaving black holes or neutron stars behind. Extremely metal-poor stars (Population II) have longer lifetimes, as they are more efficient in fusion, and many still exist today as the oldest stars in the galactic halo or globular clusters. These stars’ remnants often form white dwarfs, neutron stars, or black holes after supernovae.
Finally, very massive and rapidly rotating stars experience rotational mixing in their cores, leading to quicker evolution and the production of different elements, even in metal-poor environments.
V. METALLICITY
Metallicity refers to the abundance of elements heavier than hydrogen and helium in a star. To quantify metallicity, we typically compare it to solar metallicity, expressing the ratio on a logarithmic scale. This allows us to define the metallicity of a star in terms of how much it deviates from the Sun's metal content. (Metallicity of Stars, n.d.)
If a star has a metallicity of [Fe/H] = 0, it indicates that its iron abundance is identical to the Sun's. For [Fe/H] = -1, the star's iron abundance is one-tenth of the Sun's. This scale helps quantify the relative abundance of elements, particularly iron, in stars compared to our Sun.
VI. CONCLUSION
The study of metal-poor stars provides essential insights into the early universe and the processes that shaped the first generations of stars. These stars, primarily formed in environments with low levels of heavy elements, offer a unique perspective on stellar formation, evolution, and the cosmic history of metallicity. From their formation in primordial gas clouds to their eventual fates as white dwarfs, neutron stars, or black holes, metal-poor stars follow distinct evolutionary pathways influenced by their low metallicity, which affects their cooling mechanisms, mass loss, and fusion processes.
By examining the formation, characteristics, and end stages of these stars, we gain a deeper understanding of the conditions that prevailed in the early universe and the role of stellar populations in the development of galaxies. The relationship between metallicity and stellar evolution highlights key differences between metal-rich and metal-poor stars, affecting their size, temperature, and lifespan. Metal-poor stars, such as those in Population II and the hypothetical Population III, hold clues to the formation of the first heavy elements and the mechanisms that contributed to the chemical enrichment of the universe.
Further research into these stars, particularly about their birth, evolution, and death, will continue to shed light on the history of our galaxy and the broader cosmos. As we refine our understanding of stellar metallicity and its effects on stellar behavior, we move closer to unraveling the complex processes that have governed the formation of the universe and the development of stars throughout cosmic time.
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