On the morning of August 18, 1868, a French astronomer named Pierre Janssen stood within the coastal city of Guntur, in Andhra Pradesh on the southeastern coast of India, and watched the Moon slide throughout the face of the Solar. He had travelled from France for a window of totality lasting a number of minutes. In his arms was a spectroscope — a brass instrument the dimensions of a small telescope that cut up incoming mild by means of a prism right into a rainbow of slender traces. When the sky darkened and the photo voltaic corona flared out across the black disk of the Moon, Janssen aimed his prism at an enormous purple loop of gasoline rising off the Solar’s edge. Within the ensuing spectrum, close to the yellow band the place sodium all the time sits, a second vibrant yellow line burned at a wavelength of roughly 587 nanometres. It matched nothing on Earth.
He had simply discovered helium. The component wouldn’t be pulled out of a rock in a terrestrial laboratory for one more twenty-seven years.
A prism, a prominence, and a line that didn’t match
Janssen’s instrument labored on a precept solely 9 years outdated on the time. In 1859, the German physicist Gustav Kirchhoff had proven that each chemical component, when heated to incandescence, emits mild at a set set of wavelengths — a barcode signature distinctive to that component. Level a spectroscope at a sodium lamp and also you see a particular yellow doublet at 589 nanometres. Level it at hydrogen and also you see a purple line, a blue-green line, a violet line, all the time in the identical locations. Kirchhoff’s perception meant that starlight, in precept, carried the chemistry of the star that produced it. Astronomers now not wanted a pattern. They wanted a prism.
The 1868 eclipse was the primary large check of that concept in a working photo voltaic remark, as Science Friday recounts in its historical past of the component. Janssen’s spectroscope caught the flaming prominences that erupt off the Solar’s floor — arches of plasma taller than the Earth is broad. When he decomposed their mild, the anticipated hydrogen traces have been there. The surprising yellow line, near sodium’s however distinctly off, was not.
He was so struck by the brightness of that line that he suspected it could possibly be seen with out an eclipse in any respect — that the Moon’s shadow had merely made it simpler to identify. The subsequent morning, in strange Indian daylight, he pointed the spectroscope on the Solar’s edge once more. The road was nonetheless there.
Two males, 5,000 miles aside, chasing the identical yellow glow
Whereas Janssen was establishing in Guntur, a British astronomer named Joseph Norman Lockyer was doing the identical work from London. Lockyer had not travelled to the eclipse. He had spent months designing a spectroscope delicate sufficient to pick photo voltaic prominences in broad daylight — no Moon required. Later in 1868, Lockyer independently caught the identical anomalous yellow line, at primarily the identical wavelength.
He in contrast the place of the road in opposition to each component then identified and concluded, as Large Science describes in its historical past of eclipse discoveries, that the supply needed to be a component current within the Solar however not but recognized on Earth. He named it helium, from helios, the Greek phrase for the Solar.
The coincidence turned one of many nice small dramas of nineteenth-century science. Each males’s letters arrived on the French Academy of Sciences across the similar time, and the Academy credited each. A commemorative medal was later struck bearing each their profiles.
Why a chemist wouldn’t contact it for 27 years
To fashionable ears, discovering a component within the Solar sounds just like the ending of the story. In 1868 it was nearer to a hearsay. Most chemists dismissed the yellow line for the higher a part of three many years. A component that existed solely within the Solar, inferred from a single spectral function 150 million kilometres away, sat awkwardly with a self-discipline constructed on beakers, precipitates, and atomic weights measured on a steadiness. You can not weigh helium. You can not put it in a bottle. You can solely take a look at a vibrant line in a prism.
The deadlock broke in 1895. The Scottish chemist William Ramsay was finding out a uranium-bearing mineral referred to as cleveite. When he handled it with acid, a gasoline hissed out. Ramsay ran the gasoline by means of his personal spectroscope, anticipating to search out nitrogen. He discovered the yellow line at 587 nanometres — the identical signature Janssen had caught throughout the Indian eclipse. He despatched a sealed pattern to Lockyer for affirmation. The spectroscopic evaluation confirmed helium’s presence.
Twenty-seven years after the component had been named for the Solar, somebody lastly held a jarful of it in a London laboratory. Earth.com’s account of the invention notes that humanity had recognized helium in starlight earlier than anybody on the planet had ever measured a single gram of it straight.
Why the Solar made helium straightforward and Earth made it laborious
The delay was not a failure of chemistry. It was a reality about geology. Helium is the second-lightest component within the universe, and on a planet with an environment as skinny as Earth’s, any helium that reaches the floor merely drifts upward and escapes into house. The Solar, in contrast, is sort of 1 / 4 helium by mass. Its core constantly fuses hydrogen into helium. The component is the Solar’s dominant ash.
What terrestrial helium exists on Earth right this moment is buried, not floating. It’s produced by the gradual radioactive decay of uranium and thorium deep within the crust, over a whole lot of tens of millions of years, and will get trapped inside the identical geological formations that maintain pure gasoline. Ramsay’s cleveite labored as a result of uranium had been quietly producing helium inside it for the reason that rock shaped. Earlier than spectroscopy pointed chemists at uranium ores, there was no apparent purpose to search for a gasoline that vents itself into the sky.
The 587-nanometre line is now catalogued because the helium D3 line, and it stays one of the crucial dependable spectroscopic fingerprints in astrophysics. It exhibits up in stellar atmospheres, in planetary nebulae, and within the particles of exploding stars. A 2026 infrared survey detected helium in a Kind Ic supernova — a category of stellar explosion lengthy outlined by its obvious lack of helium — and compelled a partial rewrite of how astronomers classify dying large stars. The identical barcode Janssen caught in a six-minute window over the Bay of Bengal continues to be the instrument doing the figuring out.
Eclipses as a chemistry lab
Janssen’s expedition belongs to a really specific scientific custom: the eclipse as a pure laboratory. For a couple of minutes at a time, the Moon does one thing no man-made instrument might do reliably within the nineteenth century — it obliterates the disk of the Solar and lets the faint ambiance round it present. Each nice eclipse of the period carried a analysis query with it. In 1868, the query was chemical. In 1919, Arthur Eddington used the full eclipse over the island of Príncipe to measure the deflection of starlight by the Solar’s gravity, confirming Einstein’s common idea of relativity to throughout the tolerance of his photographic plates.
Janssen himself remained a compulsive eclipse chaser. When a complete eclipse crossed Algeria in 1870, Paris was beneath siege by the Prussian military. Based on The Dialog in its historical past of eclipse expeditions, he escaped town by hot-air balloon to achieve the eclipse path, solely to have clouds damage the remark as soon as he arrived.
A few of the similar instincts that despatched Janssen to Guntur drove astronomical observations for generations. The 1868 eclipse expedition exemplified a second when a quick window of entry to the sky produced data that would not have been assembled from the bottom.
The Guntur coast, then and now
Guntur shouldn’t be a spot most fashionable histories of physics point out. It sits on the Andhra Pradesh coast of southeastern India, a district city of shrimp farms and cotton fields. In August 1868, it occurred to lie on a slender strip of floor the place the Moon’s shadow would contact the Earth for a couple of minutes on a Tuesday morning. Janssen had chosen it after months of correspondence with the British Indian survey authorities, who supplied the observing website and the logistical assist. The Instances of India has famous the city’s function because the geographic anchor of the invention.
India in 1868 was a busy vacation spot for European astronomers. The subcontinent’s monsoon-belt latitudes put it within the path of a number of nineteenth-century eclipses, and Britain’s imperial infrastructure — railways, telegraph traces, cantonment cities — made expeditions there logistically far simpler than in distant components of Africa or the Pacific. Janssen’s Guntur camp was one in all not less than a dozen scientific events strung alongside the eclipse observe from the Arabian Sea to the Malay Peninsula. Most have been taking a look at prominences. Most noticed the yellow line. Solely Janssen and, later, Lockyer labored out that it belonged to one thing new.
The a hundred and fiftieth anniversary of the invention was marked in India in 2018, with commemorations at observatories that hint their institutional lineage again to the colonial-era survey groups that hosted Janssen. His unique spectroscope survives within the assortment of the Paris Observatory.
A finite reward from an infinite supply
The unusual coda to the story is that helium — the component recognized in probably the most plentiful object within the photo voltaic system, the second most plentiful component within the universe — is working low on Earth. International demand has climbed roughly 10 % per yr over the past decade, and costs have greater than tripled. Helium cools the superconducting magnets inside each MRI machine in each hospital, chills the electromagnets at CERN’s Giant Hadron Collider, and shields specialty metals throughout welding. When Qatar, the world’s second-largest producer, was blockaded in the summertime of 2017, MRI departments and physics labs on three continents felt the provision pinch inside weeks.
The paradox is geological. Helium retains being produced contained in the Earth by uranium and thorium decay, however the course of is so gradual — on the order of a whole lot of tens of millions of years to build up business portions — that from a human perspective the provision is mounted. Most of what escapes rock formations rises straight by means of the ambiance and out into house, as a result of helium atoms are mild sufficient to achieve escape velocity on their very own thermal movement. The Solar holds onto its helium as a result of the Solar’s gravity is 28 instances Earth’s. Earth can not.
Which suggests the state of affairs Janssen would recognise, if he got here again tomorrow, is roughly this: the yellow line he caught in his prism on a coastal morning in 1868 nonetheless marks the identical component, nonetheless burns in the identical photo voltaic prominences, nonetheless sits at 587 nanometres. The Solar has not run out. The get together balloons and the MRI coolant loops are those on the clock. The component named for a star turned out, on this planet, to be a fossil.
