A few seconds after the Big Bang, the first elements appeared in the newborn Universe – ionized forms of hydrogen and helium. These particles combined to form helium hydride, the first molecule in history. Several hundred million years passed before the first stars appeared, and scientists have long puzzled over the exact nature of the chemical processes that led to their formation.
To try to understand the origin of stars, scientists at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, recreated helium hydride in the laboratory. They found that it probably played a much larger role in the birth of stars than they previously thought, helping primary gas clouds to lose enough heat to collapse into stars.
In the study, the researchers recreated the collision between helium hydride and deuterium in an experiment they believe is the first of its kind, according to a press release. The results of the study, published on July 24 in the journal Astronomy & Astrophysics, show that the reaction rate remains constant as the temperature decreases, which contradicts the results of previous studies.
“Previous theories predicted a significant decrease in the probability of reaction at low temperatures, but we could not confirm this either in the experiment or in the new theoretical calculations of our colleagues,” Holger Kreckel, a researcher at the Max Planck Institute and the study’s lead author, said in a statement.
“Thus, the reactions of [helium hydride] with neutral hydrogen and deuterium appear to have had a much greater impact on chemical processes in the early Universe than previously thought,” he added.
Two reactions of helium hydride form molecular hydrogen and probably contributed to the formation of stars in the early Universe. In the first reaction reproduced in the study, deuterium, an isotope of hydrogen that contains a neutron in addition to a proton, collides with helium hydride to form hydrogen deuteride, a form of molecular hydrogen composed of a hydrogen atom and a deuterium atom. The other reaction occurs when helium hydride collides with a neutral hydrogen atom to form neutral molecular hydrogen. Both forms of molecular hydrogen act as coolants, helping nebulae to lose heat, condense, and eventually collapse into stars.
To conduct the experiment, the researchers used the Max Planck cryogenic storage ring. This low-temperature reaction chamber allows scientists to study molecular and atomic reactions in space-like conditions. The team kept helium hydride ions inside the chamber for a minute at about -450 degrees Fahrenheit (-267 degrees Celsius) and then superimposed a beam of neutral deuterium atoms on them. To observe how the collision speed varies with the collision energy, which is directly related to temperature, they adjusted the relative speed of the two particle beams.
Previously, scientists believed that the speed of reactions slows down with decreasing temperature, but the results of this experiment show the opposite. The researchers found that the rate remained almost unchanged despite the temperature drop. This unexpected result suggests that helium hydride remains chemically active even in cold conditions. The scientists argue in their article that this discovery should prompt a reassessment of the chemistry of helium in the early Universe.
