Composition of stars
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Hydrogen and Helium: The Primary Elements in Star Composition
Most stars are made almost entirely of hydrogen, with helium as the next most abundant element. At the time of their formation, at least 99% of a star’s mass is hydrogen, with only a tiny fraction made up of heavier elements, often called “metals” in astronomy 23. As stars age, some of this hydrogen is converted into helium through nuclear fusion, but in stars like the Sun, the overall composition remains dominated by hydrogen throughout most of their lifetimes 23.
Heavy Elements and Metallicity in Stars
The amount of elements heavier than helium—collectively called “metals”—varies between different types of stars and their ages. Main sequence stars and hot supergiants (Population I stars) have a chemical composition similar to the interstellar medium, which is mostly hydrogen and helium with small amounts of metals 19. In contrast, older Population II stars, such as those in the galactic halo, have much lower metal content, sometimes with heavy elements reduced by factors of 10 to 500 compared to the Sun 19. This difference reflects the gradual enrichment of the galaxy with heavy elements produced by earlier generations of stars.
Elemental Abundances and Stellar Spectroscopy
The chemical composition of stars is determined by analyzing their spectra. By comparing observed spectral lines with models, astronomers can measure the abundances of elements like carbon, oxygen, magnesium, silicon, iron, and others 15710. Recent studies using high-precision spectroscopy have shown that Sun-like stars in our neighborhood have very similar abundance patterns, especially for key elements like carbon, oxygen, magnesium, and silicon, with variations typically less than 10% . This suggests that exoplanets around these stars may also have similar compositions.
Changes in Composition During Stellar Evolution
As stars evolve, their internal nuclear reactions gradually change their composition. Hydrogen is fused into helium, and in more massive stars, heavier elements are created through further nuclear reactions 89. However, for most of a star’s life, especially in stars like the Sun, the surface composition remains largely unchanged, with hydrogen and helium still dominating 238.
Diversity and Homogeneity Among Stars
While the overall pattern is that stars are mostly hydrogen and helium, there are subtle differences in the abundances of heavier elements. Some young stars show peculiar chemical patterns, possibly due to processes like planet engulfment or differences in their formation environments 510. However, for Sun-like stars of similar age and metallicity, the chemical composition is remarkably homogeneous .
Conclusion
In summary, stars are primarily composed of hydrogen and helium, with small but important amounts of heavier elements. The exact mix of elements varies with a star’s age, type, and evolutionary history, but the basic pattern—hydrogen and helium dominance—remains consistent across most stars. Advances in spectroscopy and stellar modeling continue to refine our understanding of these compositions and their implications for the evolution of stars and planetary systems 123578910.
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