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The Milky Way hosts more than 400 billion stars, the vast majority of which are main‑sequence stars. In this phase, a star’s core fuses hydrogen into helium, producing the energy that powers its glow. The Sun—our own main‑sequence star—illustrates this chemistry: its bulk consists of hydrogen and helium, with only trace amounts of heavier elements.
Hydrogen is the universe’s most abundant element, accounting for roughly three‑quarters of all baryonic matter. When enormous clouds of gas and dust collapse under gravity, the hydrogen within them fuels the birth of stars. During fusion, protons combine to form helium nuclei, while electrons, positrons, gamma rays and neutrinos are also released. Neutrinos, barely interacting with matter, stream out of the Sun, whereas the other by‑products contribute to the star’s internal heating.
Helium is the second‑most common element and the primary product of hydrogen fusion. In main‑sequence stars like the Sun, helium accumulates in the core, making up about 27 % of the Sun’s mass.
Once core hydrogen is depleted, the fusion chain stalls and the core contracts. Rising temperatures (≈200 million K) ignite helium fusion, where three helium nuclei combine to form a single carbon atom. This marks the onset of the triple‑alpha process.
Further helium fusion can create oxygen by combining four helium nuclei. In more massive stars, successive fusion stages build heavier nuclei—silicon, magnesium, sodium—though these heavier elements represent less than 1 % of a star’s mass. Fusion can produce elements only up to iron; beyond that, stars must undergo catastrophic events such as supernovae to synthesize the heaviest elements.
Thus, the chemical fingerprint of most stars is dominated by hydrogen and helium, with progressively rarer heavy elements formed in advanced fusion stages.
