Helonium: The Universe’s First Molecule and How It Changed Everything

A Cosmic Detective Story

Imagine being able to witness the very first chemical reaction in the history of the universe. Not just early chemistry, but the absolute beginning of all molecular bonds—the moment when atoms first joined together to create something new. That moment happened roughly 13.8 billion years ago, just 100,000 years after the Big Bang, and it involved a molecule so special that scientists searched for it in space for nearly 50 years before finally finding it.

This molecule is called helonium, and its story is one of the most remarkable detective tales in modern science. Despite being created in a laboratory as early as 1925, helonium remained hidden in the cosmos until 2019, when astronomers finally detected it in space. The discovery confirmed one of the most important predictions about how chemistry began in our universe.

What Exactly Is Helonium?

Let’s start with the basics. Helonium, also known by its scientific name “helium hydride ion,” has the chemical formula HeH+. If you’re not a chemistry person, don’t worry—we’ll break this down in simple terms.

Think of helonium as an unlikely friendship between two very different characters. On one side, you have helium—the second most abundant element in the universe and a member of the noble gas family. Noble gases are called “noble” because they’re standoffish and rarely form bonds with other elements, much like aristocrats who keep to themselves. Helium is so unreactive that it doesn’t even form molecules with itself under normal conditions.

On the other side, you have hydrogen—specifically, a hydrogen ion, which is really just a lonely proton looking for electrons to bond with. A proton is the positively charged core of a hydrogen atom that has lost its electron.

When these two unlikely partners come together, they form helonium: one helium atom bonded to one hydrogen ion, with a positive charge. The plus sign in HeH+ indicates that this molecule is missing an electron, making it an ion—a charged particle.

Why Is Helonium So Special?

Helonium holds a title that no other molecule can claim: it is believed to be the very first molecule ever formed in the universe. To understand why this matters, we need to travel back in time to the moments after the Big Bang.

When the universe exploded into existence 13.8 billion years ago, it was unimaginably hot—we’re talking millions of degrees. At these temperatures, matter couldn’t exist as we know it. There were no atoms, no molecules, nothing solid or stable—just a soup of elementary particles zipping around at incredible speeds.

As the universe expanded, it cooled down. After just a few seconds, temperatures dropped enough for the simplest atomic nuclei to form: hydrogen (one proton) and helium (two protons and two neutrons). This process is called Big Bang nucleosynthesis, and it created the raw ingredients that would eventually build everything in the universe.

But these nuclei were still too hot to hold onto electrons. That wouldn’t happen until about 380,000 years after the Big Bang, during a period called the “epoch of recombination.” This is when the universe finally cooled enough for electrons to stick to nuclei, forming complete atoms.

Here’s where the story gets interesting. Helium and hydrogen didn’t all form atoms at the same time. Because helium holds onto electrons more tightly than hydrogen (it has a higher ionization potential), helium atoms formed first, while most hydrogen was still just loose protons floating around.

During this transitional period—between 120,000 and 380,000 years after the Big Bang—the universe contained neutral helium atoms alongside ionized hydrogen (protons). When these two bumped into each other, they formed helonium through a process called radiative association. Helonium became the universe’s first molecule, marking the beginning of chemistry itself.

The Birth of Cosmic Chemistry

To appreciate why helonium matters, we need to understand what came next. The formation of helonium wasn’t just a curiosity—it was a crucial stepping stone that led to all the chemistry we see in the universe today.

Once helonium formed, it immediately became part of a chemical chain reaction. Helonium is incredibly reactive (we’ll get to why in a moment), and when it encountered hydrogen atoms, it would break apart and help form hydrogen molecules (H2). Here’s the simplified version of that reaction:

HeH+ encounters a hydrogen atom → They react → This produces H2+ (a hydrogen molecular ion) and releases helium back out → The H2+ then reacts with another hydrogen atom → This creates H2 (molecular hydrogen) plus a lone proton

Molecular hydrogen became the most abundant molecule in the universe—and still is today. About 75% of all normal matter in the universe is hydrogen, and most of it exists as H2 molecules rather than individual atoms.

But here’s the truly important part: hydrogen molecules played an essential role in the formation of the first stars. To understand how, we need to talk about star birth.

How Helonium Helped Create the First Stars

After the Big Bang, the universe went through what astronomers call the “cosmic dark ages.” Atoms had formed, but there were no stars yet—no light sources except the fading glow of the Big Bang itself. The universe was transparent but dark, filled with massive clouds of hydrogen and helium gas.

For stars to form from these gas clouds, something had to happen: the gas needed to cool down and compress. When gas cools, it contracts under gravity, and if it contracts enough, the pressure and temperature at the core can trigger nuclear fusion—the process that powers stars.

The problem is that cooling isn’t simple. For gas to cool, it needs to shed energy, which it does by having its atoms and molecules collide with each other. These collisions excite the particles, which then release energy as light (photons). But at temperatures below about 10,000 degrees Celsius, simple hydrogen atoms aren’t very good at this cooling process anymore.

This is where molecules become crucial. Molecules can rotate and vibrate, not just bounce around like atoms. These additional motions give molecules extra ways to release energy. It’s like the difference between clapping your hands (one way to make noise) versus playing a whole drum set (many ways to make noise and release energy).

Helonium turned out to be especially good at this cooling process. Even though it’s fragile and easily destroyed, helonium has what’s called a strong dipole moment—basically, one end of the molecule is slightly more positive, and the other is slightly more negative. This property made it remarkably efficient at radiating away energy, even at relatively low temperatures.

The hydrogen molecules (H2) that formed from helonium’s destruction were also excellent coolants. Together, helonium and molecular hydrogen helped the primordial gas clouds cool just enough to collapse and form the first stars—which emerged several hundred million years after the Big Bang.

Without helonium, the chemistry of the early universe would have unfolded very differently, and the first stars might have taken much longer to form—if they formed at all in the same way.

The Properties of Helonium: A Chemical Extremist

Now let’s talk about what makes helonium so unusual from a chemistry perspective.

First, helonium holds a rather violent distinction: it is the strongest acid known to science. An acid, in chemical terms, is something that really wants to donate a proton (a hydrogen ion) to other substances. Helonium is so desperate to give away its hydrogen that it will protonate—essentially attack with a proton—absolutely anything it touches, except other positively charged ions.

How strong is it? Scientists measure acid strength using something called pKa. Water has a pKa of about 15.7. Hydrochloric acid (the acid in your stomach) has a pKa of about -6. Even fluoroantimonic acid, previously considered one of the strongest acids, has a pKa around -28. Helonium? Its pKa is approximately -63. That’s extraordinarily acidic.

This extreme reactivity means helonium cannot be stored in any container. It would immediately react with the walls of any vessel you tried to keep it in. On Earth, even in the best vacuum chambers, helonium would react with any stray molecules it encountered. That’s why, even though scientists first created it in 1925, helonium can only be studied “in situ”—meaning right where and when it’s created, before it has a chance to react with anything.

In space, however, conditions are very different. The density of matter in interstellar space is extraordinarily low—far lower than any vacuum we can create on Earth. With so few particles around, the average distance between them is huge, and helonium can exist for much longer periods because it rarely bumps into anything it can react with.

Second, helonium has some interesting structural properties. The bond between the helium and hydrogen is surprisingly short—only about 0.772 ångströms (that’s 0.0000000772 millimeters). Despite helium’s reputation for not forming bonds, this bond is real and measurable.

In the helonium molecule, the single electron they share spends most of its time (about 80%) near the helium nucleus rather than the hydrogen. This unequal sharing creates that strong dipole moment we mentioned earlier—the molecule is electrically lopsided.

The Great Search: Why Was Helonium So Hard to Find?

If helonium was supposed to be the first molecule in the universe, you’d think it would be everywhere, right? Well, it’s not quite that simple.

Scientists predicted helonium’s existence in interstellar space as far back as the 1970s. They knew it should be out there, forming in places where conditions mimic those of the early universe. The most promising hunting grounds were planetary nebulae—the beautiful, glowing shells of gas that dying stars puff out into space.

The problem was detection. To spot a molecule in space, astronomers look for its spectroscopic signature—essentially, the specific frequencies of light that the molecule absorbs or emits when it rotates or vibrates. Every molecule has its own unique fingerprint.

For helonium, one of the most important spectral lines occurs at a wavelength of 149.14 micrometers, which is in the far-infrared part of the spectrum. Here’s where things got tricky: this wavelength is absorbed by Earth’s atmosphere. Ground-based telescopes couldn’t see it. You’d need to observe from space or from high altitude.

Making matters worse, helonium’s signature line coincides almost exactly with a spectral line from another molecule called methylidyne (CH). It’s like trying to hear one specific voice in a crowded room when someone else is talking at almost the same pitch. Separating these two signals required extremely precise instruments.

For decades, astronomers tried and failed to find helonium in space. Some began to wonder if the theoretical models were wrong. Maybe helonium wasn’t as common as predicted. Maybe there was something about chemistry in space that scientists didn’t understand.

The 2019 Breakthrough: Finally Finding Helonium

The breakthrough came in 2019, thanks to a flying telescope called SOFIA (Stratospheric Observatory for Infrared Astronomy). SOFIA isn’t your typical observatory. It’s a Boeing 747 airplane that’s been modified to carry a 2.7-meter telescope. By flying at altitudes of 38,000 to 45,000 feet—above 99% of Earth’s water vapor—SOFIA can observe infrared wavelengths that ground-based telescopes can’t see.

SOFIA carried an instrument called GREAT (German REceiver for Astronomy at Terahertz Frequencies), which had been specifically upgraded to search for helonium. The target was NGC 7027, a planetary nebula about 2,900 light-years from Earth in the constellation Cygnus.

NGC 7027 was an ideal hunting ground. This nebula represents a Sun-like star at the end of its life, puffing off its outer layers into space. The conditions in its glowing gases—hot, ionized regions next to cooler neutral gases—were thought to be perfect for helonium formation, somewhat similar to conditions in the early universe.

On April 17, 2019, the international team of astronomers announced their discovery in the journal Nature. They had found it: the unmistakable spectroscopic signature of helonium in NGC 7027. After decades of searching, the universe’s first molecule had finally been detected in its natural habitat.

“The chemistry of the universe began with this ion,” the researchers wrote. “The unambiguous detection reported here brings a decades-long search to a happy ending at last.”

The discovery was more than just checking a box. It confirmed that our understanding of chemistry in extreme environments was correct. The theoretical models that predicted helonium’s presence and behavior were validated. It showed that the same physical laws that work in laboratories on Earth also apply to the exotic conditions of planetary nebulae and, by extension, the early universe.

The 2025 Discovery: Helonium’s Reactions Are Faster Than Expected

The story didn’t end in 2019. In fact, some of the most exciting discoveries about helonium came in 2025, when scientists made new measurements that challenged previous assumptions.

Researchers at the Max Planck Institute for Nuclear Physics in Heidelberg used a remarkable instrument called the Cryogenic Storage Ring (CSR) to study helonium under conditions that mimic the early universe. This ring is 35 meters in diameter and can cool ions down to just a few degrees above absolute zero (about -267 degrees Celsius).

They wanted to understand one specific reaction: what happens when helonium encounters a hydrogen atom. This reaction is crucial because it’s the main way helonium gets destroyed in space, and it’s the pathway that leads to molecular hydrogen formation.

Previous theoretical calculations suggested this reaction would be too slow to matter much in the cold temperatures of the early universe (a few thousand degrees, which is cold by cosmic standards). But when the Heidelberg team actually measured the reaction, they found something surprising: it was much faster than expected—essentially instantaneous, with no energy barrier to overcome.

This discovery has major implications. It means that helonium was destroyed more quickly in the early universe than scientists thought, which in turn means it helped produce molecular hydrogen faster than expected. This faster chemistry would have affected how quickly the first stars formed.

“This is a fundamental breakthrough in our understanding of chemical evolution during the first phases of the universe,” commented one scientist not involved in the study. The findings require astrophysicists to update their models of the early universe and reconsider the timeline of how chemistry evolved in those crucial first few hundred million years.

What Helonium Teaches Us About the Universe

The story of helonium is really the story of how we’ve come to understand cosmic chemistry. It demonstrates several profound principles about our universe.

Everything starts somewhere. The universe’s complexity—from molecules to cells to planets to galaxies—all had to begin somewhere. Helonium represents that first step, the moment when individual atoms became something more than the sum of their parts. It’s humbling to think that every molecule in your body—the DNA, the proteins, the water—traces its ancestry back to those first simple bonds formed in the infant universe.

Nature is relentlessly creative. Even in the harshest conditions imaginable—temperatures that would vaporize any material we know, densities far lower than any vacuum we can create—nature found a way to form molecules. The bond between helium and hydrogen seems impossible: noble gases aren’t supposed to bond, and yet they did. It shows that given enough time and space, chemistry will find a way.

The simple enables the complex. Helonium itself is fragile and short-lived, especially in cosmic terms. But its brief existence made molecular hydrogen possible, and molecular hydrogen made star formation possible, and stars made everything else possible—heavier elements, planets, life. The simplest molecule enabled everything that came after.

Science requires patience and persistence. The search for helonium took nearly 50 years from prediction to detection. Scientists had to develop new instruments, fly telescopes on airplanes, and overcome countless technical challenges. The story reminds us that some answers take time, but they’re worth the wait.

The Bigger Picture: From Helonium to Us

When you look up at the night sky, you’re seeing the products of nearly 14 billion years of chemistry—chemistry that began with helonium.

The first stars that formed thanks to helonium’s cooling effect were massive and short-lived. When they exploded as supernovae, they scattered heavier elements—carbon, oxygen, nitrogen, iron—into space. These elements became the building blocks for new generations of stars and planets.

Our Sun is a third-generation star, made from material that was processed through previous stars. The Earth beneath your feet, the air you breathe, the water you drink—it’s all made of atoms that were forged in stellar furnaces and scattered through space.

And it all traces back to that first molecular bond, formed in the darkness between the Big Bang and the first stars: one helium atom, one hydrogen ion, joining together to create helonium.

In a very real sense, we are the descendants of that first molecule. The chemistry it initiated continued to build complexity: from molecular hydrogen to water, from water to organic compounds, from organic compounds to amino acids, from amino acids to proteins, from proteins to life.

Why Scientists Still Study Helonium

You might wonder why scientists continue to study helonium when we’ve already confirmed its existence in space. The answer is that each new discovery raises new questions.

The 2025 findings about helonium’s reaction rates mean that models of the early universe need to be revised. How much does this change our understanding of when the first stars formed? How does it affect our predictions about what the James Webb Space Telescope might see when it looks at the most distant, earliest galaxies?

Scientists are also interested in finding helonium in other locations. NGC 7027 was just the first confirmed detection. Researchers want to know how common helonium is in other types of nebulae and whether it might be detectable in the even more distant, primitive galaxies that formed in the first billion years after the Big Bang.

There are also practical questions about chemistry in extreme environments. Understanding how molecules form and react at temperatures of thousands of degrees, in densities far lower than any laboratory vacuum, helps us predict chemical reactions in other exotic locations—from fusion reactors to the edges of black holes.

Conclusion: A Molecule That Changed Everything

Helonium is more than just a molecule—it’s a bridge between the simplicity of the early universe and the complexity we see today. It’s a reminder that great things can have humble beginnings, that even the most unlikely partnerships can create something new, and that the universe is continually building complexity from simplicity.

The next time you hear about the Big Bang or look at a photograph of a distant nebula, remember helonium. Remember that in the darkness after the Big Bang, before stars lit up the cosmos, chemistry was already beginning. Two atoms joined together, and in that joining, they set in motion a chain of events that would eventually lead to stars, planets, and—on at least one small rocky world orbiting an ordinary star—to creatures capable of understanding where they came from.

Helonium is the universe’s first molecule, but more importantly, it’s proof that from the simplest beginnings, given enough time, anything is possible. That’s a profound and beautiful thought—one that makes this tiny, highly reactive, impossible-to-store molecule one of the most significant discoveries in all of astronomy and chemistry.

The search for helonium took almost a century from first creation to first detection in space. Its study continues to reveal new secrets about how our universe evolved from a hot, simple state to the rich, complex cosmos we inhabit today. And that makes helonium not just the first molecule, but one of the most important molecules ever discovered.

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Key Takeaways:

• Helonium (HeH+) is believed to be the first molecule formed in the universe, about 100,000 years after the Big Bang
• It’s an unlikely bond between noble helium and reactive hydrogen, forming the strongest acid known to science
• Helonium helped create the first hydrogen molecules, which in turn helped the first stars form
• Scientists searched for helonium in space for nearly 50 years before finally detecting it in 2019 using SOFIA, a flying telescope
• New 2025 discoveries show helonium reacts faster than previously thought, changing our understanding of early universe chemistry
• The story of helonium shows how complexity builds from simplicity and how patient scientific investigation can answer profound questions about our cosmic origins