From the first stars to us: Our search for our cosmic origins

After about 13 billion years of cosmic evolution, we, humans, are the legacy of a universe initially composed of hydrogen and helium. Our origins are entwined with the remnants of the earliest stars. The first stars were colossal, requiring around 300 times the mass of our sun to ignite nuclear fusion in their cores. Their existence was fleeting, living short lives due to their massive size.

However, when these first stars met their explosive end in supernova events, they scattered heavier elements like carbon and iron into space. These elements became the building blocks for new stars, initiating a cycle. Each stellar generation contained more of these elements, a property astronomers refer to as “metallicity.”

Determining a star’s generation can be complex. The very first stars, born from pristine hydrogen and helium, are the true first-generation stars. Yet, stars come in various sizes, blurring the generational lines. Some massive second-generation stars might have met their fate before some smaller first-generation ones.

The distribution of stars in our galaxy. Credit: NASA, ESA, and A. Feild [STScI]

The composition of stars often includes a mix of material from different generations. Modern stars are categorized based on their metallicity, quantified as the iron-to-helium ratio [Fe/He] on a logarithmic scale. Population I stars, like our sun, have [Fe/He] values of at least -1, indicating they have 10% or more of the sun’s iron ratio. Population II stars have [Fe/He] values below -1. Population III is reserved for genuine first-generation stars.

In our Milky Way galaxy, most stars in the galactic plane are population I stars, younger and richer in metals. Population II stars, older and less metal-rich, are typically found in the galactic halo or ancient globular clusters. Some population II stars in the halo might actually be second-generation stars due to the galaxy’s evolving nature.

A recent study on the arXiv preprint server aimed to distinguish these second-generation stars from others. By examining distant quasars and simulating population III stars, the study identified carbon and magnesium-to-iron ratios, [C/Fe] and [Mg/Fe], as the crucial factors for recognizing second-generation stars in the Milky Way halo. While rare, these stars may indeed be hidden among us.

Identifying second-generation halo stars. Credit: Vanni, et al

Carbon, a vital element, is produced within stars through the CNO cycle, which follows the initial hydrogen fusion process. On the other hand, magnesium results from a more complex fusion involving carbon and helium in three stages. While many first-generation stars met their end in powerful supernovae, some did so with less energy. These lower-energy supernovae expelled elements like carbon and magnesium, but not much iron. Consequently, stars exhibiting a notably high [C/Fe] ratio likely originated from the remnants of a single first-generation star. Conversely, lower [C/Fe] ratios suggest that population II stars formed from a mix of first and second-generation star materials.

In essence, the key lies in identifying halo stars with [C/Fe] values exceeding 2.5. As of now, we have yet to discover such stars, but with the advent of more extensive sky surveys, it’s only a matter of time. While we may still need to search the farthest reaches of galaxies to locate a true first-generation star, we might soon encounter their descendants closer to our cosmic neighborhood.

Source: Universe Today

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