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From primordial deuterium to stars: How to decipher the rarity story of deuterium in the universe?
In the grand context of the universe, deuterium, an isotope of hydrogen, plays an important role. Deuterium fusion, also known as deuterium burning, is a nuclear fusion reaction that mainly occurs in stars and some substellar objects. During this process, deuterons combine with protons to form helium-3 nuclei. This process is the second stage of the proton-proton chain reaction and can also originate from raw deuterium. Understanding deuterium and its role in star formation not only helps us explore the origin of the universe, but also provides rich research directions for astrophysics.
Deuterium is an easily fused nucleus, especially in the center of the accreting protostar. When the temperature exceeds 1,000,000 K, deuterium burning starts.
For newly formed protostars, the fusion efficiency of deuterium depends on the surrounding environmental conditions. When the temperature at the center increases, the nuclear fusion of deuterium generates large amounts of energy, which drives convection processes that transport heated gas toward the star's surface. Without deuterium for fusion, the protostar would not be able to obtain enough mass and would collapse prematurely, causing the hydrogen fusion process to hinder further accretion of matter.
Once deuterium fusion proceeds, it will act like a temperature control device inside the star, temporarily preventing the core temperature from rising enough to promote hydrogen fusion, which also provides the necessary time for the star to accumulate further mass. As the energy transport mechanism changes from convection to radiation, the speed of energy transport will slow down, and the core temperature and hydrogen fusion will enter a stable stage.
As deuterium is gradually depleted within a star, its original supply declines over time, eventually exhausting itself after millions of years.
In addition to stars, deuterium burning can also occur in substellar objects. These substellar objects are called brown dwarfs and have masses ranging from 13 to 80 Jupiter masses. Brown dwarfs are capable of burning deuterium without the energy required to burn conventional hydrogen, so their existence in the range leads experts to believe that the process of forming star-like objects is still possible at certain masses.
Brown dwarfs may shine for up to 100 million years before their deuterium supply is exhausted.
For these brown dwarfs, the onset of deuterium burning is called a deuterium flash, a phenomenon that not only helps stabilize their existence but also advances researchers' understanding of star and planet formation. Especially in observations of low-mass stars, although studies have shown the possibility of deuterium burning, no changes related to this have been observed so far.
On planets, research shows that deuterium fusion can also occur, especially above solid cores of about 13 Jupiter masses. However, despite growing understanding of gas planets, how exactly this process occurs and its consequences remain a mystery. How to produce nuclear fusion in such an extreme environment is a major challenge in current scientific exploration.
Although the fusion reaction of deuterium with other nuclei is not as common as the reaction of deuterium with protons, it also exhibits different nuclear physics properties, including the production of helium-3, tritium and rare helium-4.
Due to the rarity of deuterium in the universe, its supply is generally limited, which makes its role in the evolution of stars and the formation of matter even more compelling. In future research, how to fully understand the overall impact of deuterium in the universe and the role it plays in the evolution of different celestial bodies will be an important topic in the scientific community. Are there undiscovered universal laws underlying these processes?
