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Metallicity refers to the amount of elements in a star other than hydrogen and helium. It depends on the star’s size, age, and fusion of light elements into heavy ones. Astronomers use metallicity and color to classify stars. Population III stars had the least metallicity, but their supernova explosions created heavy elements that gave rise to Population II stars. Metal-poor stars provide insight into the early universe.
Metallicity is a term used in astronomy to refer to the proportion of matter in a star composed of elements other than hydrogen and helium. In astronomical jargon, these elements (lithium, carbon, oxygen) are called metals. The amount of metals in a star depends on its size, age, and most importantly, how much of its light elements it has fused into heavy elements for nuclear fuel. For example, the Sun, a main-sequence star with an age of about 4.57 billion years, has a metallicity of about 1.6% by mass. As the Sun ages, its metallicity will increase until it becomes a red giant star, burn up the rest of its fuel, then sit there for the rest of eternity as a luminous shell called a white dwarf.
Thanks to the magic of spectrometers, astronomers can analyze the chemical composition of distant stars, even some stars in nearby galaxies. Metallicity is one of the primary variables that astronomers have used to classify various stars such as white dwarfs, red giants, main sequence stars, and supergiants. The other variable is color.
Since stars operate on nuclear fusion, the source of their energy comes from the fusion of light atomic nuclei (hydrogen and helium) into heavier nuclei (carbon). The younger a star is, the more light nuclei it has and the lower its metallicity. The stars with the least metallicity of all were hypothetical Population III stars, the first stars to form after matter condensed from the Big Bang. These stars would have had a metallicity of 10-8% by mass.
All modern stars have a significant degree of metallicity, which would be a mystery were it not for the postulate of the earlier existence of Population III stars. As mentioned, they would have been composed almost entirely of light elements. Being very massive and efficient at fusing light nuclei together, these stars would have exploded after less than a million years – a typical life of a star is 10 billion years – producing heavy elements through supernova nucleosynthesis. In the tremendous heat and pressure of a supernova explosion, a percentage of the light elements would have been rapidly crushed together into heavy elements.
These primordial supernovae gave rise to Population II stars, which are the oldest stars observable today. The oldest of these have metallicities on the order of 10-5%, less than 1/10,000 that of the Sun. Some of these stars are about 11 billion years old, not much older than the universe itself, which is estimated to have 13.7 billion years old. Astronomers continue to look for metal-poor stars as a window into the early universe.
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