Helium

• 1. Introduction

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  Helium hid inside sunlight before anyone found it on Earth. On the 18th of August 1868, during a total solar eclipse seen from Guntur, India, the French astronomer Jules Janssen recorded a bright yellow line at a wavelength of 587.49 nanometers in the spectrum of the Sun's chromosphere. At first the line was mistaken for sodium. Months later, the English astronomer Norman Lockyer studied a yellow line in the solar spectrum and reached a stranger conclusion. He decided it belonged to an element present in the Sun but unknown on our planet. He named it helium, from the Greek word for the Sun. The questions that follow are these. How did a substance discovered in starlight turn out to be the second most abundant element in the universe? Why does it refuse to freeze even at the coldest temperature physics allows? And why is something so common in the cosmos so scarce, and so hard to keep, here at home?

• 2. A Line In The Sun

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  Norman Lockyer called the new yellow line D3, because it sat near the known D1 and D2 Fraunhofer lines of sodium. The name he chose for the element ended in -ium, an ending normally reserved for metals. As an astronomer, Lockyer was probably unaware of that chemical convention. It is sometimes said that the English chemist Edward Frankland helped with the naming, but this seems unlikely, since Frankland doubted the new element existed at all. Janssen is often credited jointly with Lockyer for detecting helium, though only Lockyer proposed that the line came from something new. The first detection on Earth came in 1881, when the Italian physicist Luigi Palmieri spotted the D3 line while analyzing material sublimated during an eruption of Mount Vesuvius. The element would not be isolated in a laboratory for another fourteen years.

• 3. Pulled From The Rocks

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  On the 26th of March 1895, the Scottish chemist Sir William Ramsay isolated helium by treating the mineral cleveite, a variety of uraninite, with mineral acids. Ramsay had been hunting for argon. After separating nitrogen and oxygen from the gas liberated by sulfuric acid, he noticed a bright yellow line matching the D3 line seen in the Sun. Lockyer and the British physicist William Crookes confirmed the samples were helium. The same year in Uppsala, Sweden, the chemists Per Teodor Cleve and Abraham Langlet isolated it independently from cleveite, gathering enough gas to measure its atomic weight accurately. Before he knew his gas matched Lockyer's, Ramsay had intended to call it krypton, a name he later gave to a heavier noble gas. The American geochemist William Francis Hillebrand had actually freed helium from uraninite before Ramsay, but attributed its unusual spectral lines to nitrogen. His letter of congratulations to Ramsay stands as a quiet study in near-discovery. In 1907, Ernest Rutherford and Thomas Royds proved that alpha particles are helium nuclei, by letting the particles pass through a thin glass wall and studying the spectrum of the gas that gathered inside.

• 4. The Element That Will Not Freeze

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  In 1908, the Dutch physicist Heike Kamerlingh Onnes cooled helium below 5 K and turned it into a liquid for the first time. He tried to make it solid by lowering the temperature further, and failed, because helium does not solidify at atmospheric pressure. Helium remains liquid down to absolute zero at ordinary pressure. This is a direct consequence of quantum mechanics. The zero point energy of the system is simply too high to allow freezing. To make it solid requires pressures above about 25 atmospheres, and a temperature of 1 to 1.5 K, roughly minus 272 degrees Celsius. Onnes's student Willem Hendrik Keesom finally solidified one cubic centimeter of helium in 1926 by applying extra external pressure. The solid is sharply defined and crystalline, yet astonishingly compressible. Pressure in a laboratory can shrink its volume by more than 30 percent. With a bulk modulus of about 27 megapascals, solid helium is roughly 100 times more compressible than water.

• 5. The Strange Liquid

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  Helium I, the normal liquid phase, has a density only one-fourth of what classical physics predicts. Below its boiling point of 4.22 K and above the lambda point of 2.1768 K, helium-4 sits as a colorless liquid. Its index of refraction of 1.026 makes its surface so hard to see that floats of Styrofoam are used to mark where it lies. Below the lambda point it stops boiling, and instead of contracting when cooled, it expands. In 1938, the Russian physicist Pyotr Leonidovich Kapitsa found that near absolute zero helium-4 has almost no viscosity, a state called superfluidity. This phase, helium II, behaves like nothing else. Flowing through capillaries as thin as 10 to 100 nanometers, it shows no measurable viscosity at all. When heat is introduced, it travels as waves at 20 meters per second at 1.8 K, a phenomenon known as second sound. Its thermal conductivity is a million times that of helium I and several hundred times that of copper. Helium II also creeps. It climbs the walls of an unsealed vessel against gravity in a film about 30 nanometers thick, a Rollin film named for Bernard V. Rollin, until it reaches a warmer region and evaporates. That escape habit makes the superfluid extremely difficult to confine. In 1972, the American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson observed superfluidity in helium-3, but only at temperatures far closer to absolute zero.

• 6. Born In The First Minutes

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  About 24 percent of the mass of the universe's ordinary matter is helium-4, nearly all the ordinary matter that is not hydrogen. The vast majority formed by Big Bang nucleosynthesis, one to three minutes after the Big Bang. As the soup of free protons and neutrons, created in roughly a 6:1 ratio, cooled enough for nuclear binding, almost all the first nuclei to form were helium-4. The tight binding of helium-4 consumed nearly all the free neutrons within minutes, before they could beta-decay. That left few neutrons to build heavier atoms like lithium, beryllium, or boron. Helium-4's binding energy per nucleon is stronger than in any of those, so once it formed, no energetic push remained to make elements 3, 4, and 5. Fusing helium into carbon would need three helium nuclei to strike almost simultaneously, the triple-alpha process, because the intermediate beryllium-8 is so short-lived. The early universe cooled past that point before significant carbon could form. The result was a ratio close to 3 parts hydrogen to 1 part helium-4 by mass, much as we see today. Every heavier element, including those needed for rocky planets and for life, was made afterward, in stars hot enough to fuse helium itself.

• 7. The Trouble With Keeping It

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  In the Earth's atmosphere, helium is present at only 5.2 parts per million by volume. Most terrestrial helium comes from the radioactive decay of heavy elements like thorium and uranium, whose alpha particles are helium-4 nuclei. An estimated 3000 metric tons are generated each year throughout the lithosphere. This radiogenic helium gets trapped with natural gas, in concentrations as high as 7 percent by volume, and is extracted commercially by fractional distillation, a low-temperature separation process. Helium is a non-renewable resource. Once released into the atmosphere, it promptly escapes into space, and its supply is thought to be diminishing rapidly. The American story of helium extraction began with a curiosity. After a 1903 oil drilling operation in Dexter, Kansas produced a gas geyser that would not burn, the state geologist Erasmus Haworth collected samples and took them to the University of Kansas at Lawrence. There the chemists Hamilton Cady and David McFarland found the gas was 72 percent nitrogen, 15 percent methane, 1 percent hydrogen, and 12 percent an unidentified gas. Of the sample, 1.84 percent proved to be helium, hidden in quantity beneath the American Great Plains. The United States set up the National Helium Reserve in 1925 at Amarillo, Texas. The Helium Act of 1925 banned the export of the scarce gas, on which the country held a production monopoly. That ban forced German Zeppelins to use hydrogen, which would gain infamy in the Hindenburg disaster.

• 8. What Helium Does

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  Cryogenics is the largest single use of helium, about 32 percent of total production in 2014, much of it cooling the superconducting magnets in MRI scanners and NMR spectrometers. The Large Hadron Collider at CERN uses 96 metric tons of liquid helium to hold its temperature at 1.9 K. Helium serves as a protective gas for growing silicon and germanium crystals and as a shielding gas in arc welding, chosen over cheaper argon for materials like aluminium and copper. Because it diffuses through solids three times faster than air, it works as a tracer for leak detection, with sensitivity as fine as a leak rate of 10 to the minus 9 millibar-liters per second. Deep divers breathe mixtures like trimix and heliox, since helium has no narcotic properties and its low molecular weight eases breathing under pressure. The Saturn V rocket of the Apollo program needed about 370,000 cubic meters of helium to launch. Inhaling helium can be dangerous, because it is a simple asphyxiant that displaces the oxygen needed to breathe. Helium was approved for medical use in the United States in April 2020, for humans and animals. As scarcity loomed, the Nobel laureate physicist Robert Coleman Richardson argued in 2010 that the price of helium would need to be multiplied by 20 to stop the gas from being wasted.