Helonium is the informal name for HeH⁺, the helium hydride ion. It is a tiny charged molecule made of one helium atom and one proton. Scientists think it was the first molecule to form after the Big Bang.
This guide explains helonium in plain language, using peer-reviewed papers and research institute reports. You will learn what HeH⁺ is, how it was made in a lab in 1925, and why astronomers needed until 2019 to spot it in space. You will also see what a 2025 experiment changed.
The story matters beyond one ion. HeH⁺ connects the first atoms, the first chemical bond, molecular hydrogen, and the birth of the first stars. Claims are checked against sources, and open questions are marked as open.
Direct Answer: What Is Helonium?
Helonium is another name for HeH⁺, the helium hydride ion. It is a positively charged molecule made of one helium atom bonded to one proton, which is the nucleus of a hydrogen atom.
Scientists think it was the first molecule to form in the early universe. It was created in a lab in 1925 and first detected in space in 2019. You may also see it called the helium hydride cation or hydridohelium(1+). All three names describe the same particle.
Quick Facts
| Feature | Detail |
| Common names | Helonium, helium hydride ion, hydridohelium(1+) |
| Formula | HeH⁺ |
| Type | Molecular ion (cation) |
| Made of | One helium atom and one proton |
| Electrons | Two, both from helium |
| Lab discovery | 1925, Hogness and Lunn |
| First space detection | 2019, planetary nebula NGC 7027 |
| Detection line | About 149 micrometres (2.01 terahertz) |
| Key 2025 result | Reaction with hydrogen stays fast at very low temperatures |
| Acid strength | Widely described as the strongest known acid |
| Everyday use | None; it is a research tool |
Is HeH⁺ a Chemical Element?
No. An element is defined by the number of protons in its nucleus. Helium has two protons and hydrogen has one. HeH⁺ contains both, so it is a molecular ion, not an element.
The name causes confusion. The ending “-ium” looks like sodium or helium, but it also appears in ions such as hydronium and ammonium. Helonium follows the ion pattern.
HeH⁺ will never appear on the periodic table. It has no place there, just as water or carbon dioxide do not.
What Is HeH⁺ Made Of?
HeH⁺ has two nuclei and two electrons:
- One helium nucleus, usually helium-4 with two protons and two neutrons
- One hydrogen nucleus, which is a single proton
- Two electrons, both originally from helium
- A net charge of plus one, because three protons are balanced by only two electrons
The two electrons hold the ion together by binding the proton to the helium nucleus. This gives HeH⁺ the same electron count as molecular hydrogen (H₂). That is why quantum chemists often use it as a test case for calculations.
Helium is a noble gas that normally refuses to bond. HeH⁺ is an exception because the positive charge changes the rules. The ion also has an uneven charge distribution, called a dipole moment. Researchers note that this comparatively large dipole moment makes it a potential coolant during the epoch of first star formation.
Why Is HeH⁺ Linked to the First Chemistry in the Universe?
Chemistry needs neutral or nearly neutral pieces that can bond. The early universe did not offer them at first.
Here is the sequence:
- Within seconds of the Big Bang, temperatures dropped enough for hydrogen and helium to form, but both stayed fully ionized.
- Free electrons and bare nuclei filled the cosmos as a hot plasma. Any molecule would have been torn apart.
- Helium captured electrons first and became neutral, while hydrogen was still present as bare protons.
- Neutral helium then met free protons and formed HeH⁺, the universe’s first molecular bond.
- Neutral hydrogen atoms only appeared about 380,000 years after the Big Bang, during recombination.
The formation reaction is called radiative association. A helium atom and a proton stick together and release a particle of light to carry away extra energy. This is rare on Earth but important in space.
Was HeH⁺ the First Molecule?
It is the leading candidate, but the claim rests on models, not direct observation. No telescope can see molecules from that era.
Current models predict HeH⁺ to be the very first molecule, followed rapidly by He₂⁺, once helium atoms start to recombine. These models use lab-measured reaction rates and well-tested physics. That gives the claim strong support.
What is confirmed is narrower. HeH⁺ exists, it forms by known reactions, and it has been detected in a nebula. The “first molecule” label describes a prediction that fits all current evidence.
Researchers usually word it carefully. You will see “thought to be” or “probably” in scientific papers, and that caution is worth keeping.
How Did HeH⁺ Help Lead to Molecular Hydrogen?
Molecular hydrogen (H₂) is the most common molecule in the universe. HeH⁺ gave the early cosmos one of its first routes to making it.
The process has two steps:
- HeH⁺ collides with a neutral hydrogen atom. This produces a neutral helium atom and an H₂⁺ ion.
- The H₂⁺ ion then reacts with another hydrogen atom to form neutral H₂ and a proton.
This matters for star formation. Gas clouds must lose heat before they can collapse and ignite. The growing abundance of such molecules gave the gas a dominant way to cool and form stars.
The timeline is long. After recombination came the cosmic dark ages, with no stars at all. It took several hundred million years for the first stars to appear.
When Was HeH⁺ First Found in a Laboratory?
HeH⁺ was first identified in a lab in 1925. Physicists T. R. Hogness and E. G. Lunn reported it in Physical Review. Their paper is the standard citation for the earliest laboratory work on the ion.
They studied hydrogen ionized by electron impact and used positive ray analysis, an early form of mass spectrometry. It sorted charged particles by mass.
This early work set an important precedent. Astronomers later knew that HeH⁺ was stable enough to exist and could be measured. Later lab work pinned down its spectral lines, which told observers exactly where to look in space.
Why Was HeH⁺ So Hard to Find in Space?
Several problems stacked up. Each one alone would have made the search difficult.
- It is very reactive. HeH⁺ is destroyed quickly by collisions, so it never builds up in large amounts.
- Earth’s atmosphere blocks its signal. Its rotational ground state sits near 149 micrometres, and ozone and water in the air block this light. Telescopes had to fly above the blocking layers.
- Neighboring lines crowd the spectrum. Signals from the far more common carbon-hydrogen molecule appear at 149.09 and 149.39 micrometres, right beside the HeH⁺ line.
- Older instruments lacked resolution. A previous space observatory could not separate the HeH⁺ line from the neighboring signal, so it could only set an upper limit.
Searches began decades earlier. In the late 1970s astronomers thought the nebula NGC 7027 might suit HeH⁺, but early observations were inconclusive. The long non-detection even led some scientists to question their chemical models.
Where Was HeH⁺ Found in Space?
HeH⁺ was found in the planetary nebula NGC 7027. It lies about 3,000 light-years away in the constellation Cygnus. The result appeared in Nature in 2019, led by Rolf Güsten.
The team used the GREAT spectrometer aboard SOFIA, a telescope built into a modified jet. The plane flew at 13 to 14 kilometres, above most of the atmosphere that blocks this radiation. SOFIA has since retired, but its data remain a landmark.
A second confirmation followed. Astronomers later detected HeH⁺ through vibrational infrared emission lines in the same nebula. Two separate kinds of evidence made the detection much stronger.
One caution: this is not primordial HeH⁺. In planetary nebulae, HeH⁺ mainly forms from He⁺ meeting neutral hydrogen, which differs from the early universe route. The detection proves the ion exists in space, not that it was seen from the Big Bang era.
Why Was NGC 7027 a Good Place to Look?
NGC 7027 offers a rare mix of conditions that favor HeH⁺:
- A hot central star. Ultraviolet radiation and heat from the aging star create conditions suitable for HeH⁺ to form.
- A young, dense, compact shell. Dense gas raises the chance of collisions that build the ion.
- A sharp boundary between zones. Hubble imaging shows a clear transition between the ionized region and the cool envelope. HeH⁺ forms where helium ions meet neutral hydrogen.
- Bright signals overall. The ion was detected at the inner edge of the hot gas cloud, where strong radiation flows.
Together these features made NGC 7027 the best-studied candidate for decades.
How Do Scientists Identify HeH⁺ in Space?
Every molecule emits light at fixed frequencies, like a fingerprint. Spectroscopy reads those fingerprints.
The process runs in five steps:
- Lab experiments measure the exact frequencies of HeH⁺ lines.
- A telescope records the spectrum from a target such as a nebula.
- Astronomers look for an emission line at the predicted frequency. For HeH⁺ this is the fundamental rotational transition at 2.010 terahertz, or 149.1 micrometres.
- They rule out nearby lines from other molecules, especially carbon-hydrogen.
- They look for a second line set, such as vibrational lines, and compare the strength with chemical models.
Models like the Cloudy photoionization code test whether a nebula can produce the observed amount. A detection is only trusted when the frequency, strength, and chemistry all agree.
What Did Scientists Learn About HeH⁺ in 2025?
A team at the Max Planck Institute for Nuclear Physics in Heidelberg reported the key 2025 result. The paper by Grussie and colleagues appeared in Astronomy & Astrophysics on 24 July 2025.
They used the Cryogenic Storage Ring, a device built to mimic space conditions. Its inner chambers sit near 6 kelvin, and stored ions cool to their lowest rotational states. Ions were held at minus 267 °C for up to 60 seconds before meeting a beam of deuterium atoms.
Deuterium is heavy hydrogen. The team studied HeH⁺ plus deuterium instead of ordinary hydrogen. Both reactions follow the same potential energy surface, so the theory applies to both.
The findings surprised researchers:
- The reaction rate stayed nearly constant as temperature fell, contradicting earlier predictions.
- The reaction is barrierless and fast at low collision energies.
- Older calculations underestimated the low-temperature rate because of an artifact in a widely used potential energy surface.
Why Does the 2025 Result Matter?
Chemical models of the early universe depend on reaction rates. If a key rate is wrong, predictions about the first stars shift too.
The paper title itself points to the consequence. It suggests a lower abundance of the first molecules at very high redshifts. In simple terms, HeH⁺ was probably destroyed faster than models assumed. The authors call for a reassessment of helium chemistry in the early universe.
The picture is not one-directional. A 2019 Science study found that electron recombination rates for HeH⁺ were far below values used in earlier models, which implied higher abundance. One result pushes abundance up and another pushes it down. Modelers must now combine both.
Some limits apply:
- The lab used deuterium as a stand-in for hydrogen.
- Lab conditions approximate, but do not fully copy, the early universe.
- No one has observed primordial HeH⁺ directly.
What Changed in 2026?
As of 24 September 2026, I found no new confirmed detection of HeH⁺ in another object. I also found no new headline experiment that overturns the 2025 result. The year mostly brought follow-up work and renewed coverage.
- Coverage of the 2025 study continued. Articles in mid-2026 repeated the Heidelberg findings. One June 2026 report described the same recreated reaction and its possible effect on first-star models. Do not mistake these for new discoveries.
- Theory supports the fast reaction. A late 2025 theory paper found rates near 2 × 10⁻⁹ cubic centimetres per second at 10 kelvin, matching the classical Langevin value and exceeding the only earlier lab measurement.
- New lab tools appeared. A January 2026 preprint described an ion trap instrument for measuring radiative association rates, the process behind HeH⁺ formation. Preprints are not yet peer reviewed.
The safe summary is simple. The 2025 result still stands, and the science is moving toward updated models.
Is HeH⁺ a Strong Acid?
Yes, in a specific chemical sense. Acid strength here means how eagerly an ion gives up its proton. HeH⁺ is widely described as the strongest known acid.
The reason is helium’s very weak grip on protons. Helium has the lowest proton affinity of any atom, at about 178 kilojoules per mole. Nearly any other atom or molecule takes the proton from HeH⁺ instantly.
This has two practical results. HeH⁺ cannot exist in water or any normal solution. It also cannot be stored in a bottle, because it would react with the container wall.
Do not confuse this with everyday danger. HeH⁺ only exists in tiny amounts in special gas conditions. Astronomers describe it as a light, highly reactive molecule with strong acid character.
Can Scientists Make HeH⁺ on Earth?
Yes, but only as a short-lived ion inside specialized equipment. Common methods include:
- Passing electric discharges through mixtures of helium and hydrogen
- Using electron impact in ion sources and mass spectrometers
- Holding the ions in traps or cryogenic storage rings for study
- Colliding the stored ions with beams of atoms to measure reaction rates
None of these makes a stock of HeH⁺ that you could hold. Researchers make it, measure it, and let it react away. That is enough for spectroscopy and reaction studies.
Does HeH⁺ Have Everyday Uses?
No. HeH⁺ is not used in products, medicine, or industry. It exists only in labs and in space.
Its value is scientific:
- Testing theory. It is a simple two-electron system, ideal for checking quantum chemistry methods.
- Reading nebulae. Its emission helps astronomers study helium and hydrogen conditions in planetary nebulae.
- Improving cosmic models. Accurate rates help predict how the first stars formed.
- Future searches. Researchers have suggested that redshifting could stretch the HeH⁺ line about tenfold, making distant sources visible to large ground telescopes.
Helium vs HeH⁺
| Feature | Helium (He) | HeH⁺ (helonium) |
| Category | Chemical element | Molecular ion |
| Charge | Neutral | Plus one |
| Bonding | Almost never bonds | Bonded helium and proton |
| Stability | Very stable | Reactive, short-lived |
| Where found | Air, gas wells, stars | Labs, nebulae, early universe models |
| Everyday role | Balloons, cooling, welding | None |
Key Timeline
| Year | Milestone |
| 1925 | Hogness and Lunn report HeH⁺ in a lab |
| Late 1970s | Astronomers suggest NGC 7027 as a good place to search |
| 1980s to 2010s | Several searches fail or stay inconclusive |
| 2019 | SOFIA and GREAT detect HeH⁺ in NGC 7027 |
| 2019 | Science study finds lower electron recombination rates |
| 2020 | Vibrational emission confirms HeH⁺ in NGC 7027 |
| 2025 | Cryogenic ring experiment shows a fast, barrierless reaction |
| 2026 | Follow-up theory, new lab tools, and renewed coverage |
Common Claims Checked
| Claim | Verdict | Reason |
| Helonium is a chemical element | False | It is a molecular ion |
| HeH⁺ is proven to be the first molecule | Not fully proven | The claim rests on models |
| HeH⁺ was found in the early universe | False | It was found in a nebula in 2019 |
| HeH⁺ is stable like helium gas | False | It reacts almost instantly |
| The 2025 study rewrote all early universe chemistry | Overstated | It revised one key rate |
| A new 2026 discovery overturned it | Not verified | 2026 items mostly follow up 2025 |
Why Does This Ion Still Matter?
HeH⁺ sits at the start of cosmic chemistry. Without a path to molecules, gas could not cool easily and stars would form later or differently.
It also shows how science works. A prediction stood for decades before a detection confirmed it. Then lab work showed that a widely used model contained an error. Each step improved the picture.
Open questions remain. Researchers still need a full early universe model that combines the 2019 electron recombination results with the 2025 hydrogen reaction results. Any confirmed sighting of HeH⁺ at high redshift would be a major advance.
FAQ’s
What is helonium?
Helonium is the informal name for HeH⁺, the helium hydride ion. It is a positively charged molecule made of one helium atom and one proton.
Is HeH⁺ the first molecule in the universe?
It is the leading candidate, based on well-tested models of the early universe. It has not been observed from that era, so scientists say “thought to be.”
Where was HeH⁺ first detected in space?
It was first detected in planetary nebula NGC 7027, about 3,000 light-years away in Cygnus. The 2019 detection used the GREAT instrument aboard SOFIA.
Is HeH⁺ the strongest acid?
It is widely described as the strongest known acid because helium holds protons so weakly. It cannot survive in solution and only exists in special gas conditions.
Does helium form chemical bonds?
Helium almost never bonds as a neutral atom. HeH⁺ is a rare exception, where a positive charge lets helium share electrons with a proton.
Why was HeH⁺ hard to detect in space?
The ion is very reactive and short-lived, and Earth’s atmosphere blocks its signal. Nearby carbon-hydrogen lines also overlap with it in the spectrum.
What did the 2025 HeH⁺ experiment show?
A Heidelberg team found that the HeH⁺ reaction with hydrogen isotopes stays fast at very low temperatures. Models of early universe helium chemistry now need updating.
Final Thoughts
Helonium, or HeH⁺, is small but historically important. It is a helium atom joined to a proton, and it is thought to mark the start of chemistry in the universe. It was made in a lab in 1925 and first seen in space in 2019.
The 2025 experiment gave the story a modern update. The reaction that destroys HeH⁺ stays fast in the cold, which changes how scientists model the first molecules and the first stars. As of September 2026, that finding still stands.
Key takeaways:
- HeH⁺ is a molecular ion, not an element.
- The “first molecule” claim is a strong prediction, not a direct observation.
- The 2019 detection proved that HeH⁺ exists in space, but not in primordial gas.
- The 2025 result revised one key reaction rate, not the whole picture.
- HeH⁺ has no everyday use, but it is a valuable research tool.
