Helium
I
just read with interest on 25th August an article in THE DECCAN
HERALD by Shri C Sivaram of the Indian Institute
of Astrophysics, Bengaluru. It is about the gas Helium. It is stated
that Helium is the second lightest element after hydrogen, or twice as heavy as
hydrogen. Hydrogen gas constitutes three-fourths or 75% of the chemical
composition of the universe and Helium is the second most abundant chemical
element after hydrogen constituting 24% of the chemical composition of the
universe. All other elements make up the remaining 1% to 2% of the
chemical composition of the universe. He says that 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.
Helium
was discovered in the gaseous atmosphere surrounding the Sun by the French
astronomer Pierre Janssen, who detected a bright yellow line in the spectrum of
the solar chromosphere during an eclipse in 1868; this line was initially
assumed to represent the element sodium. That same year the English astronomer
Joseph Norman Lockyer observed a yellow line in the solar spectrum that did not
correspond to the known D1 and D2 lines of sodium, and so he named it the D3
line. Lockyer concluded that the D3 line was caused by an element in the Sun
that was unknown on Earth; he and the chemist Edward Frankland used the Greek
word for sun, hēlios, in naming the element. The British chemist Sir William
Ramsay discovered the existence of helium on Earth in 1895. Ramsay obtained a
sample of the uranium-bearing mineral cleveite, and, upon investigating the gas
produced by heating the sample, he found that a unique bright yellow line in
its spectrum matched that of the D3 line observed in the spectrum of the Sun;
the new element of helium was thus conclusively identified. In 1903 Ramsay and
Frederick Soddy further determined that helium is a product of the spontaneous
disintegration of radioactive substances.
Shri
Sivaram says that “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”.
Commenting
about its usage, he says “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”.
