Stars are luminous spheres of plasma that generate light and heat through processes within their cores. Understanding their primary composition provides insights into their behavior, life cycles, and the structure of the cosmos. The elements that make up stars dictate how they form, shine, and contribute to the interstellar medium.
The Fundamental Elements: Hydrogen and Helium
Stars are primarily composed of hydrogen and helium. These elements were the main constituents of the universe after the Big Bang, a period known as Big Bang nucleosynthesis. Consequently, the gas clouds from which stars form are rich in hydrogen and helium. For instance, stars in the Milky Way galaxy are typically about 71% hydrogen and 27% helium by mass.
Hydrogen and helium fuel nuclear fusion, the process that powers stars for most of their lives. During this main sequence phase, hydrogen atoms in a star’s core fuse to create helium, releasing energy. This energy prevents the star from collapsing under its own gravity, maintaining a stable state for billions of years. Our Sun, for example, is a main-sequence star that has been fusing hydrogen into helium for approximately 4.6 billion years.
How We Know: Peering into Starlight
Astronomers determine the elemental composition of stars by analyzing the light they emit, a technique known as spectroscopy. This involves collecting starlight and separating it into individual wavelengths, much like a prism splits sunlight. This process creates a spectrum, which contains distinct dark or bright lines. These lines are specific to each element and act as unique “fingerprints.”
When light from a star passes through its cooler outer atmosphere, certain elements absorb specific wavelengths, creating dark absorption lines. By identifying these patterns, scientists determine which elements are present and their relative abundance. Cecilia Payne-Gaposchkin was among the first to demonstrate that stars are primarily composed of hydrogen and helium based on these spectral analyses.
The Origin of Heavier Elements: Cosmic Alchemy
While hydrogen and helium constitute the bulk of a star, trace amounts of heavier elements are also present. These elements, which astronomers collectively refer to as “metals” (meaning any element heavier than helium), are formed through processes within stars themselves. This creation of new elements is known as stellar nucleosynthesis.
Inside stars more massive than our Sun, after hydrogen fusion ceases in the core, helium begins to fuse into heavier elements like carbon and oxygen. This process continues in successive stages, forming elements up to iron in the cores of very massive stars. The most massive stars end their lives in spectacular supernova explosions. These events create and disperse the heaviest elements, such as gold and silver, into the cosmos, enriching the interstellar medium from which new stars and planetary systems can form.