Janssen was examining sunlight with a telescope and spectroscope on August 18,
1868, when he noticed an unknown bright yellow spectral line signature—it was the
first evidence of helium.
with a telescope and spectroscope on August 18, 1868,
when he noticed an unknown bright yellow spectral line
signature—it was the first evidence of helium. It appeared at
a wavelength of 587.49 nm in the spectrum of the chromosphere of the sun. Janssen is also credited with demonstrating
the gaseous nature of the red solar prominences that were
observed during solar eclipses.
Janssen had studied mathematics and physics, but he was
mainly involved in scientific expeditions to Asia, Africa,
Europe, and the Pacific Islands from 1860 to 1905. On these
trips, he studied eclipses, and, that year, he had discovered a
technique to observe solar prominences that occurred in the
absence of an eclipse.
Tests were soon carried out at the College of Chemistry
in London to reproduce the line that Janssen had observed.
Although its existence was confirmed, no one knew its source.
Janssen postulated that it was an element that had not yet
been discovered on our planet—a conjecture that attracted
much ridicule since no elements had been detected on the sun
that were not known to exist on earth.
Meanwhile, on October 20 of that same year, another
scientist made the same independent discovery. In a remarkable leap of intuition, the British astronomer Joseph Norman
Lockyer (1836-1920) combined an astronomical telescope,
which he built himself, with a prism spectroscope. Using this
setup, Lockyer discovered the same yellow line that Janssen
had observed. He named it the D3 Fraunhofer line because
it was near the known D1 and D2 lines of sodium. He also
concluded that its origin was an unknown chemical element
that had not yet been found on earth.
Lockyer collaborated with the English chemist Edward
Frankland, who had previously investigated how the spectrum
of elements related to their physical state, on the identification
of helium. They named the element helium, after the Greek
word for the sun (helios).
(Lockyer is also well-known for establishing the journal
Nature in 1896. One of his primary objectives in doing so was
to facilitate the communication of ideas between disparate
scientific disciplines; this goal seems so relevant today. For
the next 50 years, he remained the journal’s editor until just
before his death in 1920.)
Some 25 years after Janssen and Lockyer discovered helium
on the sun, a Scottish chemist found the first evidence of it on
our home planet. In 1895, William Ramsay discovered helium
in uranium minerals. In another example of independent
discovery, two Swedish chemists—Per Teodor Cleve and Nils
Abraham Langlet—also discovered helium in a mineral called
cleveite at around the same time.
In 1835, the French positivist philosopher Auguste Comte
wrote that people would never understand the chemical
constitution of the stars. Thankfully, through spectroscopy,
science has proven Comte wrong. In 1904, after Pierre Janssen
and J. Norman Lockyer independently discovered helium on
the sun, Lockyer wrote at the beginning of his book Studies In
Spectrum Analysis: “The work of the true man [woman] of Science is a perpetual striving after a better and closer knowledge
of the planet on which his lot is cast, and of the universe in
the vastness of which that planet is lost.” t
Barry R. Masters ( firstname.lastname@example.org), OSA Fellow, SPIE Fellow, is
with the department of biological engineering, Massachusetts
Institute of Technology, Cambridge, Mass., U.S.A. Member
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