The first stars formed where gas gathered
The birth of stars and galaxies changed both the appearance of the universe and the material it contained. Stars formed from gas that was initially almost entirely hydrogen and helium, and new elements were made inside them. Some of the material they released later mixed into the ingredients of other stars and planets. The rocks of Earth and the substances in our bodies carry traces of this long history.
Light existed in the universe before the first stars formed. The light left over from the very hot early universe is called the cosmic microwave background. There were no stars producing light yet. The first stars became a new source of light in the universe.
Gas was not spread evenly throughout the universe. Where matter was more densely packed, gravity drew in nearby gas. As the gas collecting at the center grew hotter, conditions for forming stars developed. Unlike the clouds that form stars today, the material of the first stars contained almost no heavy elements or dust. Researchers compare observations of the distant universe with calculations based on physical laws to investigate how massive the first stars were and how they formed.
Many stars formed at different times within galaxies
A galaxy in the night sky is a structure far larger than a single star. It contains huge numbers of stars, along with gas and dust, all held together by gravity. Dark matter, whose presence is inferred from its gravitational effects rather than directly seen through light, also plays a part in that structure. Clouds where stars form lie in many places within galaxies.
Early galaxies changed as they took in gas from their surroundings or merged with other galaxies. While new stars formed from the arriving gas, earlier stars continued their own lives. Stars were not all born together or passing through the same stage at once. This is why young and old stars can coexist in the same galaxy.
Astronomers study distant galaxies to investigate the past. Light takes time to reach us, so light that has traveled for a long time carries a view of a galaxy from long ago. By comparing galaxies at different distances and analyzing starlight and the composition of gas, researchers can learn how galaxies changed.
Light from early galaxies changed the surrounding gas
Hot stars emit ultraviolet light as well as the light our eyes can see. When a hydrogen atom absorbs light with enough energy, an electron around its nucleus can be knocked free. This removal of an electron from an atom is called ionization.
In the early universe, electrons were separate from atomic nuclei. As the universe cooled, they joined together to form atoms. Light from stars and galaxies knocked electrons out of those atoms again. The period when this change spread widely around galaxies is called the era of reionization. The way light traveled through the gas changed as well.
One study using the James Webb Space Telescope examined galaxies as they were when the universe was about 900 million years old. Researchers measured how much light from a bright object farther away passed through the intervening gas, then compared this with galaxy locations found by Webb. Finding regions around galaxies where light passed through more easily supports the explanation that light from those galaxies ionized the surrounding gas.
Reionization does not turn hydrogen into another element. The type of nucleus stays the same while an electron is removed. This differs from the nuclear reactions in the next section, in which the nuclei themselves change and new elements form.
New elements form inside stars and in violent events
Carbon and oxygen were not always as abundant as they are today. In the hot interiors of stars, atomic nuclei combine in a process called nuclear fusion. As stars evolve, reactions involving helium nuclei can produce carbon and oxygen. Which elements a star makes, and how much, depend on its mass—the amount of matter it contains—and its stage of evolution.
Fusion is not the only way to make elements. A nucleus can absorb particles called neutrons and may later become another element through reactions that change particles within the nucleus. This neutron capture also occurs inside aging stars and contributes to the production of some elements heavier than iron.
Heavy elements can also form in material thrown out when two neutron stars merge. A neutron star is a small, dense object that can remain at the end of a massive star's life. A study reanalyzing light from a neutron-star merger observed in 2017 identified signs of an element called strontium. This is observational evidence that such mergers are one place where heavy elements form.
Material released by stars mixes into surrounding gas
Even when an element forms inside a star, that material does not immediately enter the next star. It must first leave the star and mix with surrounding gas. When an aging star sheds its outer layers or a supernova explosion throws out material, the elements within it spread too. Stellar death explores what different stars leave behind.
The Hubble Space Telescope has observed gas released by Sun-like stars surrounding the hot star at the center. Astronomers analyze the light from these objects to investigate which elements the released gas contains. Nuclear reactions that create elements and flows of matter that carry them into space are separate processes.
The released material contains both substances the star had from the beginning and substances it newly produced. If some of this material mixes into a cold gas cloud and gathers again, it can become the ingredients of later stars and planetary systems. Some spreads far away or remains in stellar remnants, so this is not a closed cycle in which every bit of material returns.
The Solar System's material carries traces of earlier stars
The cloud that formed our Solar System also contained substances left by stars that lived before the Sun. The Sun, planets, and smaller objects formed from gas and dust enriched in elements such as carbon and oxygen. Some ingredients, such as hydrogen, came from the early universe, so a single supernova cannot have made all the material in Earth or our bodies.
Some meteorites preserve tiny dust grains that formed around other stars before the Sun was born. Researchers examine the types and proportions of atoms within the grains to trace the kinds of stars their material may have come from. Physical samples, as well as light from distant objects, offer clues to the history before the Sun.
Today's Milky Way contains young stars, old stars, star-forming clouds, and stellar remnants together. In this article, the era of stars and galaxies means the history of the universe as these changes unfold. Its scope differs from the main-sequence stage, when one star shines through hydrogen fusion. To look more closely at how a new star forms in a cloud, continue to Stellar birth.
Sources
- NASA Science — Elements and background radiation in the early universe
- NASA Webb — Research on the first stars and early galaxies
- NASA Science — The contents and evolution of galaxies
- NASA Science — Gas inflow and galaxy mergers
- NASA Webb — Observing reionization around early galaxies
- NASA Webb — Stellar evolution and the production of carbon and oxygen
- Liu et al. — Neutron capture in aging stars and presolar dust
- ESO — Observations identifying strontium in a neutron-star merger
- NASA Hubble — Gas released by stars and the reuse of elements