Metallicity
Metallicity is a measure of the abundance of chemical elements heavier than helium in a star or an astrophysical medium. In astronomy, by a historical convention that may come as a surprise, all elements heavier than helium— including carbon, oxygen, and iron—are called metals. The metallicity ofa star is therefore its content of these elements, generally expressed relative to the abundance of iron in the Sun.
Why metallicity varies among stars
The first elements to appear in the universe after the Big Bang were almost exclusively hydrogen and helium. Heavier elements were gradually synthesized by successive generations of stars— through stellar nucleosynthesis and during supernova explosions. Old stars, formed early in the galaxy’s history, are therefore generally metal-poor. Younger stars, such as the Sun, benefit from an interstellar medium enriched by several generations of predecessors.
Metallicity as an Indicator of Age
Metallicity is an indirect tracer of a star’s age. Galactic halo stars, which are very old, have metallicities ranging from 1/1000th to 1/100th that of the Sun. Young stars in the galactic disk have metallicities comparable to or higher than that of the Sun. By measuring the metallicity of a globular cluster, astronomers can determine its age and reconstruct the chemical history of the galaxy.
Importance in Astronomy
Metallicity profoundly influences planet formation. Metal-rich stars have more material available to form rocky planets. Statistical studies of exoplanets show that high-metallicity stars are more likely to host giant planets. The Sun, with a metallicity slightly above the galactic average, was able to form a rich planetary system.
Concrete example
Stars known as Population III—the very first stars in the universe— are thought to have zero or near-zero metallicity, composed almost exclusively of hydrogen and primordial helium. None have yet been directly observed, but the James Webb Space Telescope is searching for traces of them in the earliest galaxies.
Frequently Asked Questions
Why do astronomers call carbon and oxygen “metals”?
It is a historical convention in astrophysics. Early spectroscopists classified stars according to their spectra into two broad categories: hydrogen/helium on one hand, and everything else on the other. This “everything else” was called “metals”—a misnomer—and the convention has persisted despite its chemical imprecision.
Is a high metal content always conducive to life?
Not necessarily. A sufficient metal content is necessary to form rocky planets. But too high a metallicity can favor the formation of gas giants, which disrupt the orbits of rocky planets. There appears to be an optimal range of metallicity for potentially habitable planetary systems.
How is a star’s metallicity measured?
Through spectroscopy. Analysis of a star’s light spectrum reveals the absorption lines characteristic of each element. By comparing the intensity of these lines to theoretical models, astronomers can measure the relative abundances of different elements with great precision.