Trillions of degrees. That was the temperature a fraction of a second after the Big Bang. So darn hot it took 380,000 years for it to cool down enough for individual protons and neutrons to come together to form the first atoms. Cool is a relative term. The temperature at the time simmered around 6,700° F (3,700° C), not exactly a spring afternoon. About three-quarters of those early atoms were hydrogen, the simplest element, and most of the remainder helium.
Hydrogen is the simplest atom with a single positively-charged proton for a nucleus orbited by a negatively-charged electron. Helium has two protons and two neutral particles called neutrons in its core orbited by two electrons. Atoms join together to form molecules. When lots of atoms join in a variety of ways, complex molecules result.
Our bodies are made of a mix of simple molecules like water (H2O) and complicated ones like hemoglobin, the molecule that transports oxygen to our blood cells and carries away carbon dioxide “waste” to our lungs. Hemoglobin looks like this: C2952H4664N812O832S8Fe4. That’s right — 2,952 carbons atoms, 4,664 hydrogens, 812 nitrogens, 838 oxygens, 8 sulfurs and four irons. A big molecule for a big job.
To get the ball rolling toward complex matter and life, individual atoms have to link up. Since the early universe was mostly hydrogen and helium you won’t be surprised to learn that hydrogen and helium came together to make the first molecules. It wasn’t easy. More like an arranged marriage than freedom of choice. Helium doesn’t like combining with anything. It’s holds on so tightly to its electrons, you have to apply a tremendous amount of energy to pry one away so another atom can hook in. But in the extreme heat of the early universe, it happened and helium hydride or HeH+ was born. Because of its fragility however, it didn’t last long — one of the reasons it’s been nearly impossible to find.
Long before the molecule was detected in space, it was concocted in a lab in 1925. Now for the first time, after decades of searching, scientists have discovered it in space for the first time using NASA’s Stratospheric Observatory for Infrared Astronomy or SOFIA. The observatory is a specially outfitted aircraft that flies above much of Earth’s atmosphere to make sensitive observations in infrared light. Air blocks much of the infrared, a type of light that we sense as heat.
SOFIA found modern helium hydride in the planetary nebula called NGC 7027 located 3,000 light-years away in the Northern Cross. As a sunlike star ages into a white dwarf, it expels its atmosphere as a beautiful, flower-form cloud called a planetary nebula. The discovery is important for the same reason the recent photograph of the black hole in the galaxy M87 is — we know that helium hydride can exist in space. Theory predicted it would and now we have proof it does. That’s a big deal.
“This molecule was lurking out there, but we needed the right instruments making observations in the right position — and SOFIA was able to do that perfectly,” said Harold Yorke, director of the SOFIA Science Center, in California’s Silicon Valley.
Helium hydride turns out to be a crucial molecule. As the early universe continued to expand and cool, hydrogen atoms interacted with helium hydride to make molecular hydrogen (H2) — two hydrogens bonded together. This molecule is primarily responsible for the formation of the first generation of stars. Hydrogen molecules helped to cool the clouds of collapsing gases, so that gravity could draw the material into stars. As those early suns aged, they forged these simpler substances into more complex elements in their cores including carbon, the backbone of the hemoglobin molecule. For all you know, some of those first-generation carbon atoms might be swimming around in your very own blood.
Astronomers suspected that even if the original helium hydride might forever elude detection, the planetary nebula NGC 7027 would be a good place to look for current-day material. Ultraviolet light radiating from the exceedingly hot dwarf star (342,000° F / 190,000° C) star strips electrons from hydrogen and helium around the star, creating the right conditions for helium hydride to form. In 2016, scientists boarded SOFIA and flew to 45,000 feet (13.7 km), high above the interfering layers of Earth’s atmosphere. A recent upgrade on one of its instruments enabled them to tune into the frequency of helium hydride similar to how you’d tune in an FM radio station. Bingo! They picked up the signal loud and clear.
With that, astronomers have that much more confident they’re on the right track when it comes to figuring out the chemistry of the early universe.