The chemical composition of the Universe is dominated by the hydrogen and helium produced in the Big Bang. The remaining 90 or so chemical elements are produced in stars and constitute only a few percent of the overall mass. Astronomers refer to these elements (all except hydrogen and helium) as metals, even though this includes elements such as carbon and oxygen which are not considered metals in the normal sense.

The abundance of metals with respect to hydrogen is known as the metallicity. While hydrogen and helium are found in high abundance throughout the Universe, the metallicity varies depending on the history of star formation in the region. The chemical composition of the Sun gives us some idea of the chemical composition of the solar neighbourhood:

This indicates that metals constitute only about 2% of the Sun's mass.

The highest metallicities are found in the centres of galaxies. For example, near the centre of the Milky Way, stars with metallicities of up to three times the solar value have been observed. However, there are also stars with only 1/10,000th of the solar value. These stars formed early in the history of the Galaxy, before the interstellar medium (and subsequent generations of stars) became enriched in metals through the actions of other stars.

Although never more than a few percent by mass, the metals content of stars has a significant effect on their stellar evolution, with metal-rich stars being cooler, larger and longer-lived than metal-poor stars of the same mass. Both the length of time spent on the main sequence and the detail of post-main sequence evolution are significantly affected by a star's metallicity.

To fully describe the chemical composition of stars (or galaxies) it is also necessary to define abundance ratios. These relate the relative abundances of metals to each other (e.g. the abundance ratio of magnesium to iron or carbon to oxygen). Astronomers use these abundance ratios to measure how long the object in question has been forming stars.

Both metallicities and abundance ratios are usually expressed in terms of the values for the Sun, and normally on a logarithmic scale.