Metallicity

In astronomy, metallicity is the amount of elements in an object that are heavier than hydrogen and helium. Most of the matter we can see in the universe is either hydrogen or helium. But astronomers use the word metals to talk about all the other elements, even ones that are not usually called metals in chemistry.

The globular cluster M80. Stars in globular clusters are mainly older metal-poor members of population II.

Heavier elements are made inside stars over time. As stars change and explode, they spread these heavier elements into space. This enriches the material around them, helping new stars form with more of these heavier elements. Older stars have fewer of these heavier elements because they formed when the universe had less of them.

Scientists often measure a star’s metallicity using a value called [Fe/H]. This shows how much iron a star has compared to hydrogen, using our Sun as a reference point. Iron is used because it’s easy to measure and changes in a way that helps us understand how stars evolve.

Metals in early spectroscopy

In 1802, William Hyde Wollaston saw dark lines in the sun's light. Later, in 1814, Joseph von Fraunhofer studied these lines closely and measured their wavelengths. These lines are now called Fraunhofer lines.

Then, Gustav Kirchhoff and Robert Bunsen found that some of these lines matched the light patterns of heated elements. They realized the dark lines came from chemical elements in the sun's atmosphere absorbing light. They saw strong lines from metals like sodium, potassium, and iron. Early studies of the sun's makeup found only hydrogen and metals in its light. Today, in astronomy, any elements besides hydrogen and helium are called metallic.

Origin of metallic elements

The heavier elements in the universe, called metals by scientists, are made inside stars as they change over time. When stars end their lives, they spread these metals into space. This enriches the material around them and helps create new stars. Because of this, older stars usually have fewer metals than younger stars, which formed later in a universe that had more metals.

Stellar populations

Population I star Rigel with reflection nebula IC 2118

Astronomers noticed that different types of stars have different amounts of elements heavier than hydrogen and helium. In 1944, an astronomer named Walter Baade suggested that there are two main groups of stars. These groups are called population I stars, which have more of these heavier elements, and population II stars, which have fewer. Later, in 1978, a third group was theorized called population III stars. These are thought to be the very first stars that formed in the universe, and they would have very few heavier elements.

Common methods of calculation

Astronomers use different ways to figure out how many heavier elements are in stars and other space objects. They look at how much of the object is made of gas compared to heavier elements, or they compare the numbers of atoms of different elements to what we see in the Sun.

One way they do this is by looking at the mass of hydrogen and helium in a star. Most of a star’s mass is usually just hydrogen and helium, with the rest being all the other elements, which astronomers call “metals.” By measuring how much of the star is hydrogen, helium, and the rest, they can find out how many heavier elements are there.

Another way is to look at how much iron is in a star compared to the Sun. Iron is easier to measure, and it helps tell us about the history of the star. By comparing the amount of iron to hydrogen, astronomers can figure out if a star has more or less of these heavier elements than our Sun does.

Description	Solar value
Hydrogen mass fraction	X ⊙ = 0.7438 ± 0.0054 {\displaystyle \ X_{\odot }=0.7438\pm 0.0054\ }
Helium mass fraction	Y ⊙ = 0.2423 ± 0.0054 {\displaystyle \ Y_{\odot }=0.2423\pm 0.0054\ }
Metal mass fraction	Z ⊙ = 0.0139 ± 0.0006 {\displaystyle \ Z_{\odot }=0.0139\pm 0.0006}
Metal-to-hydrogen ratio	Z ⊙ / X ⊙ = 0.0187 ± 0.0009 {\displaystyle \ Z_{\odot }/\ X_{\odot }=0.0187\pm 0.0009\ }

Metallicities in various astrophysical objects

Stars

Stars with fewer heavy elements are a little warmer than stars with more heavy elements of the same size and age. Some stars, called Population II stars, have about one-thousandth to one-tenth of the heavy elements found in our Sun. These stars look cooler overall because the big ones have already ended their lives.

For stars heavier than 40 times our Sun, having more or fewer heavy elements changes how the star ends its life. Stars with fewer heavy elements might collapse directly into a black hole, while stars with more heavy elements might explode and leave behind a neutron star.

Relationship between stellar metallicity and planets

A star’s amount of heavy elements helps decide if it might have large planets. Scientists have found that stars with more heavy elements are more likely to have big planets, such as gas giants like Jupiter and Saturn. The heavy elements help form these planets in the disk of material around a young star. The Sun, which has eight planets and nine dwarf planets, is used as a reference point for comparing other stars.

H II regions

Big, hot young stars in H II regions shine with a special kind of light that can help scientists measure how many heavy elements are in these areas. These stars send out energy that knocks electrons loose from hydrogen atoms. When these electrons settle back down, they give off light at specific colors. By studying these colors, scientists can figure out how many heavy elements are present. Oxygen is one of the most common elements used for these measurements.

Metallicity calibrations at high redshift

New telescopes like the James Webb Space Telescope can now study very distant galaxies that existed when the universe was very young. These galaxies are so far away that their light has taken billions of years to reach us. Looking at these galaxies helps scientists understand how the amount of heavy elements has changed over time.