On April 17, 2019 (March 13, 2019 in the lunar calendar), helium hydrogen ions were discovered. If someone asked: What was the product of the first chemical reaction that occurred in the universe? I believe that even people who have studied chemistry for many years will be confused in an instant: Isn't this question a bit difficult? Didn't the chemistry teacher teach it? It doesn't matter if you can't answer it. In fact, this is not bad for chemistry teachers. For a long time, even scientists had no idea where this product was hidden in the universe. But after all, with the name of "first", this product is particularly important. Therefore, astronomers have spent decades tirelessly searching for it in the vast universe. Until recently, they finally got their wish! On April 17, 2019, a research team published a paper in Nature announcing that they had detected helium hydrogen ions (HeH) in the planetary nebula NGC7027. This is the first time that this ion has been detected in the modern universe, and it is the product of the first chemical reaction in the universe. Schematic diagram of the planetary nebulae NGC7027 and HeH, where the red sphere represents helium and the blue sphere represents hydrogen. Source: Reference [2] How was HeH born? Before explaining what scientists did, let's take a moment to go back to the early days of the universe to see what the first chemical reaction in the universe actually happened. About 380,000 years after the Big Bang, the universe, which experienced a high temperature and dense state, began to cool down with expansion. At this time, the temperature had dropped below 4000 Kelvin (K). At this time, ions of the light elements produced during the Big Bang nucleosynthesis period (at that time there was only hydrogen, helium, and very little lithium) began to recombine. Helium ions (He2 and He) first combined with free electrons to form the earliest neutral atoms in the universe. The recombination of hydrogen also occurs subsequently. Although chemistry textbooks tell us that helium is an inert gas, chemical reactions are difficult to occur under conditions of normal temperature and pressure on the earth today. However, in the high temperature and dense environment of the early stage of the universe, the situation was completely different. Neutral helium atoms would combine with protons to form helium hydrogen ions-this occurred in the universe, and the products included the earliest chemical bonds in the universe. The process of chemical reaction, where h¦Í is the energy of a photon. Photo source: Reference [1] Structural diagram of HeH, with protons on the left, helium atoms on the right, and chemical bonds in the middle. Photo source: This is how the dawn of WikimediaCommons Chemistry has come. Helium hydrogen ions then interact with hydrogen atoms to form hydrogen molecules. Hydrogen molecules were the basis for the formation of the first stars. Stars are like "element factories", making all the elements that make up the universe today. The wonderful combination of these elements then created life, and we have the opportunity to discuss this issue here. Witness the germination of life. Photo source: PixabayHeH may be there, but we can't see that although helium hydrogen ions played a very important role in the evolution of the early universe, astronomers have never detected its existence in interstellar space. Therefore, this crucial first step in the chemical birth process cannot be proved. The reason for this is that the space telescope they use cannot distinguish the signal of helium and hydrogen ions from the mixed signals emitted by other molecules. As the paper's corresponding author, Rolf Güsten of the Max Planck Institute for Radio Astronomy in Germany, said: "For decades, the lack of evidence for the existence of such ions in interstellar space has put astronomy in a dilemma." [2]As early as 1925, chemists had synthesized this ion in the laboratory. But it wasn't until the late 1970s that astronomers began to discuss the possibility of such ions in celestial environments and cited planetary nebulae as candidate areas for finding them. Planetary nebulae are made up of material ejected outward by stars similar to the sun at the end of their evolution. The nebula continues to expand, and the gas and dust eventually ejected are dispersed in interstellar space, leaving only the hot star in the center. Astronomers believe that planetary nebulae, whose physical conditions are similar to those of the early universe, are likely to produce such ions and be dense enough to be detected. Planetary nebula NGC7027. Photo source: WikimediaCommons In fact, astronomers targeted the planetary nebula NGC7027, where HeH was discovered from the beginning. The nebula is 3000 light-years away from Earth near the constellation Cygnus. However, observations at that time did not yield definite results, and although subsequent studies suggested the existence of HeH, they still did not achieve direct detection. The answer is that the "observatory on an airplane" started in 2016, and astronomers have tried to detect with the help of the Stratospheric Observatory for Infrared Astronomy (SOFIA). This is a space-based observatory jointly sponsored by the National Aeronautics and Space Administration (NASA) and the German Aerospace Center (DLR). It actually carries observation equipment on a Boeing 747SP wide-body passenger aircraft. You are right, this is the stratospheric infrared observatory in flight. Image source: WikimediaCommonsSOFIA was put into use in 2010. Flying at an altitude of more than 14000 meters above the ground can avoid many interference from the earth's atmosphere. And one of SOFIA's obvious advantages over space telescopes is that after each flight, astronomers can adjust the detection plan and install the latest equipment in a timely manner. Harold York, director of the SOFIA Science Center, said of SOFIA's role: "Helium and hydrogen ions are hidden there, but we need the right equipment to observe in the right place to find it, and SOFIA can do this perfectly." [2]The equipment that helped astronomers discover helium and hydrogen ions this time is the German Receiver at Terahertz Frequencies (GREAT) installed in a recent equipment upgrade. This instrument is a bit like the radio we use to listen to radio, and astronomers tune it to the emission frequency of the ion they want to find, just as we tune the radio to the channel we want to hear. When SOFIA conducts observations at night, scientists on the plane can read data from the device in real time. In May 2016, SOFIA conducted observations on three flights. The research team led by Guusten analyzed the data obtained from the observations, and the signal of helium hydrogen ions was finally discovered. The first discovery of helium and hydrogen ions excited Guusten and many astronomers. A long search finally came to a perfect conclusion. Based on the Big Bang theory, we have developed an understanding of the chemistry of the early universe and inferred how it evolved over tens of billions of years into the complex chemistry of today's universe. The emergence of the first chemical reaction is undoubtedly a key part of this. The discovery of the helium and hydrogen ion dispels our previous concerns, allows us to continue to trust existing theories, and guides us to discover the deeper secrets of the universe. We are all scattered stars. Now we know exactly where this story should start.