Helium: a discovery you can't take 'lightly'

The earliest evidence of the existence of the chemical element helium came during the observations of the total solar eclipse, in Guntur, in India, on August 18, 1868. On that day, French astronomer Jules Janssen observed in the spectrum of the chromospheres of the Sun a bright yellow line with a wavelength of 587.5 nanometres. Atomic spectroscopy was still in its infancy then and the line was at first attributed to the D line of sodium which is very close to the above wavelength. The characteristic wavelengths of the spectra of various elements were already well established and many were seen as absorption lines in the solar spectrum. However, in October of the same year, Sir Norman Lockyer observed a yellow line in the solar spectrum, which he dubbed D3 as it was near the well-known D1 and D2 Fraunhofer lines of the sodium. It was the same line seen by Janssen and did not correspond to any other known element at that time.

To account for this unknown line in the solar spectrum, Lockyer boldly proposed the existence of a new element on the Sun, unknown on Earth. Lockyer and Sir Edward Frankland named the new element helium, after the Greek word ‘helios’ for the Sun. Lockyer was widely ridiculed for his boldness but terrestrial confirmation for the existence of the new element came in 1881, when Luigi Palmieri detected helium on Earth for the first time through its D3 spectral line while analysing material that had sublimated during an eruption of Mount Vesuvius.

Complete confirmation came on March 26, 1895, when Sir William Ramsay isolated helium on Earth by treating mineral cleveite (a form of uraninite) with acids. He was looking for argon, but after separating nitrogen and oxygen from gas liberated by the acid, he saw a bright yellow line that matched the D3 line observed on the Sun. The samples were identified as helium by Lockyer and William Crookes. It was independently isolated by Swedish chemists, in the same year, who collected enough of the gas to accurately determine its atomic weight. Thus helium was at last confirmed to exist on earth (as a new element), 30 years after its first discovery on the Sun. In 1907, Rutherford showed that alpha particles which are a perennial product of the decay of heavy radioactive elements like uranium are indeed helium nuclei. That is how helium accumulates in the rocks on Earth and is present in natural gas. Ionised helium lines are also seen in the spectra of hot O-stars. Helium emission lines are also seen in the recombination line series of planetary nebulae.

Second lightest, abundant

Helium is the second lightest element after hydrogen, or twice as heavy as hydrogen. Helium is also the second most abundant chemical element after hydrogen, which constitutes three-fourths or 75% of the chemical composition of the universe. Helium constitutes about 24%, while all other elements make up the remaining 1% to 2% of the chemical composition of the universe. It is surprising that although humans were aware of and indeed making use of several chemical elements like iron, copper, gold, silver, etc early in history, the discovery of helium happened only 150 years ago and that of hydrogen by Henry Cavendish about 100 years earlier, in 1766.

But it’s perhaps not surprising after all as the Earth’s atmosphere has hardly any hydrogen or helium. Being light atoms they would have much higher velocities (at a given temperature) than heavier atoms like those of oxygen or nitrogen, implying that at any given time a substantial fraction would have velocities greater than the escape velocity of the Earth. The Earth would have lost all the hydrogen and helium in its atmosphere long back. Objects like the moon or Mars which are much lighter than the Earth would have lost even the heavier gases from their atmosphere many billion years ago. However, the giant planets like Jupiter or Saturn, which are much more massive and thereby have much higher escape velocities retain substantial amounts of helium in their atmosphere.

Very recently, in May 2018, helium was detected for the first time in the atmosphere of an exoplanet. This is WASP 107b who’s atmosphere is being stripped by it’s host star. The planet’s atmosphere is sweltering hot at 500 degrees Celsius and extends to over a 1,000 km. This has enabled an excited state of helium to be identified. The planet has the size of Jupiter but is bloated up so that its average density is much less than water.

Helium on earth has been put to several uses. It liquefies at four degrees Kelvin and is used in cryogenics to cool complex systems to very low temperatures. Helium becomes a superfluid at two degrees Kelvin. The Large Hadron Collider uses large amounts of liquid helium in its vacuum chambers and huge superconductor magnets. Helium is also used in balloons.

Hydrogen was earlier used in airships, even to cross the Atlantic, like in the Zeppelins, 80 years ago. But the explosion of the Hindenburg in 1938 at the end of its voyage put an end to this mode of air travel. Helium being inert is not flammable and is currently used in airships to transport freight. It is also used in diving equipment, industrial processes like welding and, of course, in large-scale cooling. It is feared that the supply of helium may run out in the next few decades, although new sources are being discovered.

(The writer is with Indian Institute of Astrophysics, Bengaluru)