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Elemental Abundances

Most of the atoms in the universe are either hydrogen or helium, formed within the first few minutes after the Big Bang. The other elements are mostly made by nuclear fusion in stars, especially fusion during supernova explosions. Other elements are born in the collisions of neutron stars or extreme environments around black holes. By measuring the amount of each type of atom in and around galaxies, astronomers can trace the history of the stars, nebulas, and other objects in the cosmos.

The Origins of Matter

Nearly all the hydrogen and helium in existence was formed in the first three minutes or so after the Big Bang, when the entire universe was extremely hot and dense. However, even those conditions weren’t right for keeping nuclear fusion going, so very few heavier elements were produced. Most of the rest of the elements, known to astronomers as “metals”, were created by nuclear fusion in stars.

Though we only have indirect evidence for them so far, the first stars in the universe were very massive and made entirely of hydrogen and helium. The supernova explosions of these early stars produced and spread the first metals. Younger generations of stars formed partly from these heavier elements, and continued the cycle. As a result, the number of metal atoms has increased with every generation of stars, although hydrogen and helium still make up about 99% of all atoms in the universe today.

 

Stellar Metallicity and Planets

The fraction of heavier elements is called “metallicity”, and it provides one way for astronomers to measure when a particular star or nebula formed. A low metallicity star must have formed a long time ago, while a higher metallicity star is of more recent vintage. The first stars would have had effectively zero metallicity; many astronomers are looking for traces of these ancestors of all later generations.

Stellar metallicity is also related to whether a star has planets. Higher metallicity stars like the Sun are more likely to have planets than lower metallicity stars, based on data from NASA’s Kepler telescope and other observatories. Using that information, astronomers took the observed metallicity of stars throughout the Milky Way and estimated the total number of potential planets.

 

Galactic Metallicity

Stars tend to form in clusters, and many remain in those clusters for their entire life cycle. As a result, the metallicities of stars reveal information about a cluster’s age and environment where it formed. For instance, in the Milky Way, old low metallicity star clusters are found in the halo, the region of the galaxy surrounding the bright inner regions where the Sun lives.

This is important because similar low metallicity clusters are common in small galaxies. According to the theory of galaxy formation, big galaxies like the Milky Way form out of smaller galaxies, even eating them whole. The various metallicities of star clusters give a partial history of our galaxy’s cannibalism, providing some details of how it formed and grew.

The same principles apply to other galaxies. Individual stars are usually too faint to see within distant galaxies, but astronomers can measure the collective metallicity of many stars together. High metallicity galaxies tend to be brighter than their low metallicity cousins, and many galaxies have higher metallicities in their inner regions than out toward the edges. As with the Milky Way, this is a reflection of the individual history of the galaxy, measured in part by where the various chemical elements live.