Nebulium
Encyclopedia
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
found in a nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
by William Huggins
William Huggins
Sir William Huggins, OM, KCB, FRS was an English amateur astronomer best known for his pioneering work in astronomical spectroscopy.-Biography:...
in 1864. The strong green emission
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
lines of the Cat's Eye Nebula
Cat's Eye Nebula
The Cat's Eye Nebula is a planetary nebula in the constellation of Draco. Structurally, it is one of the most complex nebulae known, with high-resolution Hubble Space Telescope observations revealing remarkable structures such as knots, jets, bubbles and sinewy arc-like features...
, discovered using spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
, led to the postulation that an as yet unknown element was responsible for this emission. In 1927, Ira Sprague Bowen
Ira Sprague Bowen
Ira Sprague Bowen was an American astronomer. In 1927 he discovered that nebulium was not really a chemical element but instead doubly ionized oxygen.-Life and work:...
showed that the lines are emitted by doubly ionized oxygen
Doubly ionized oxygen
Doubly ionized oxygen is a forbidden line of the ion O2+. It is significant in that it emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres and secondarily at 495.9 nm. Concentrated levels of [O III] are found in diffuse and planetary nebulae...
(O2+), and no new element was necessary to explain them.
History
William Hyde WollastonWilliam Hyde Wollaston
William Hyde Wollaston FRS was an English chemist and physicist who is famous for discovering two chemical elements and for developing a way to process platinum ore.-Biography:...
in 1802 and Joseph von Fraunhofer
Joseph von Fraunhofer
Joseph von Fraunhofer was a German optician. He is known for the discovery of the dark absorption lines known as Fraunhofer lines in the Sun's spectrum, and for making excellent optical glass and achromatic telescope objectives.-Biography:Fraunhofer was born in Straubing, Bavaria...
in 1814 described the dark lines within the solar
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
spectrum. Later, Gustav Kirchhoff
Gustav Kirchhoff
Gustav Robert Kirchhoff was a German physicist who contributed to the fundamental understanding of electrical circuits, spectroscopy, and the emission of black-body radiation by heated objects...
explained the lines by atomic absorption
Atomic absorption spectroscopy
Atomic absorption spectroscopy is a spectroanalytical procedure for the qualitative and quantitative determination of chemical elements employing the absorption of optical radiation by free atoms in the gaseous state. In analytical chemistry the technique is used for determining the concentration...
or emission, which allowed the lines to be used for the identification of chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s.
In the early days of telescopic astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, the word nebula
Nebula
A nebula is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases...
was used to describe any fuzzy patch of light that did not look like a star. Many of these, such as the Andromeda Nebula
Andromeda nebula
Andromeda nebula may refer to:* Andromeda Galaxy, a spiral galaxy in the Andromeda constellation* Andromeda , a 1957 science fiction novel by Ivan Efremov* The Andromeda Nebula, a 1967 Russian film based on the above novel...
, had spectra
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
that looked like stellar spectra, and these turned out to be galaxies. Others, such as the Cat's Eye Nebula
Cat's Eye Nebula
The Cat's Eye Nebula is a planetary nebula in the constellation of Draco. Structurally, it is one of the most complex nebulae known, with high-resolution Hubble Space Telescope observations revealing remarkable structures such as knots, jets, bubbles and sinewy arc-like features...
, had very different spectra. When William Huggins
William Huggins
Sir William Huggins, OM, KCB, FRS was an English amateur astronomer best known for his pioneering work in astronomical spectroscopy.-Biography:...
looked at the Cat's Eye, he found no continuous spectrum like that seen in the Sun, but just a few strong emission lines. The two green lines at 495.9 nm and 500.7 nm were the strongest. These lines did not correspond to any known elements on Earth. The fact that helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
had been identified by the emission lines in the Sun in 1868, and had then also been found on Earth in 1895, encouraged astronomers to suggest that the lines were due to a new element. The name nebulium (occasionally nebulum or nephelium) was first mentioned by the wife of Huggins in a short communication in 1898, although it is stated that Huggins occasionally used the term before.
The development of the periodic table by Dimitri Mendeleev and the determination of the atomic numbers by Henry Moseley
Henry Moseley
Henry Gwyn Jeffreys Moseley was an English physicist. Moseley's outstanding contribution to the science of physics was the justification from physical laws of the previous empirical and chemical concept of the atomic number. This stemmed from his development of Moseley's law in X-ray spectra...
in 1913 left nearly no room for a new element. John William Nicholson
John William Nicholson
John William Nicholson, FRS was an English mathematician.Based on the results of astronomical spectroscopy of nebula he proposed in 1911 the existence of several yet undiscovered elements. Coronium with an atomic weight of 0.51282, nebulium wit a weight of 1.6281 and protofluorine with a weight of...
added one more elements protofluorine and created an atomic theory. In this theory all known elements consist out of the four protoelements. In 1914 French astronomers where able to determine the atomic weight
Atomic weight
Atomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
of nebulium. With a measured value of 2.74 for the lines near 372 nm and a slightly lower value for the 500.7 nm line indicating two elements responsible for the spectrum.
Ira Sprague Bowen
Ira Sprague Bowen
Ira Sprague Bowen was an American astronomer. In 1927 he discovered that nebulium was not really a chemical element but instead doubly ionized oxygen.-Life and work:...
was working on UV spectroscopy and on the calculation of spectra of the light elements of the periodic table when he became aware of the green lines discovered by Huggins. With this knowledge he was able to suggest that the green lines might be forbidden transition
Forbidden mechanism
In physics, a forbidden mechanism or forbidden line is a spectral line emitted by atoms undergoing nominally "forbidden" energy transitions not normally allowed by the selection rules of quantum mechanics. In formal physics, this means that the process cannot proceed via the most efficient route...
s. They were shown as due to doubly ionized oxygen
Doubly ionized oxygen
Doubly ionized oxygen is a forbidden line of the ion O2+. It is significant in that it emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres and secondarily at 495.9 nm. Concentrated levels of [O III] are found in diffuse and planetary nebulae...
at extremely low density, rather than the hypothetical nebulium. As Henry Norris Russell
Henry Norris Russell
Henry Norris Russell was an American astronomer who, along with Ejnar Hertzsprung, developed the Hertzsprung–Russell diagram . In 1923, working with Frederick Saunders, he developed Russell–Saunders coupling which is also known as LS coupling.-Biography:Russell was born in 1877 in Oyster Bay, New...
put it, "Nebulium has vanished into thin air." Nebulae are typically extremely rarefied
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, much less dense than the hardest vacuums produced on Earth. In these conditions, lines can form which are suppressed at normal densities. These lines are known as forbidden lines, and are the strongest lines in most nebular spectra.