Using spectroscopy, helium was first discovered in the Sun. Spectral lines labeled coronium and nebulium did not fit into the periodic table of the elements. Discovery of Helium in the Sun. Spectroscopy, Coronium, Nebulium, and the Periodic Table.
In 1869 Dmitri Mendeleev discovered the patterns that led to the modern periodic table of the elements. With the periodic table, Mendeleev and other chemists could predict where an element was missing and chemical properties of the missing element. The periodic table also told chemists when all the elements had been discovered.
In 1859 Gustav Kirchoff built an improved spectroscope and began the science of spectroscopy. Astronomers soon mounted spectroscopes on telescopes to study the chemical compositions of celestial objects.
Discovery of Helium
During the total solar eclipse in 1868, a French scientist, P.J.C. Janssen, observed the spectrum of the Sun’s chromosphere from India. Janssen observed a bright yellow spectral emission line that he initially thought was a well known line from the element sodium. Unlike most of the chromosphere, this line was bright enough to observe outside of solar eclipses, so Janssen continued to study the line. About the same time, Norman Lockyear, a British scientist, independently observed this line. Detailed study showed that this line did not quite match the sodium line, which is actually two very closely spaced lines.
Janssen and Lockyear had discovered a new element, named helium from the Greek word for Sun, helios. Sir William Ramsay finally isolated helium in a laboratory in 1895. He heated a uranium-containing mineral, Norwegian cleveite, and found it gave off helium gas.
Mendeleev did not predict the existence of helium and other noble gasses with his periodic table because he originally based it on atomic weight rather than atomic number. However, the discovery of helium filled an important gap in the modern periodic table, atomic number 2.
Coronium and Nebulium
In 1869, several astronomers independently discovered a bright green emission line from the Sun’s corona. This line did not match any spectral lines known at the time for any elements on Earth. This possible new element was called coronium. From its location in the Sun’s outermost atmosphere, the corona, scientists inferred that coronium must be lighter than hydrogen. However, Mendeleev’s periodic table left no room for elements lighter than hydrogen, which has an atomic number of 1.
In 1864 Sir William Huggins studied the spectrum of the Orion nebula and found a bright green emission line that did not match any spectral lines known on Earth. Huggins observed this line in about a third of the nebulae he studied. He proposed the name nebulium for this possible new element. The perfected periodic table left no place for nebulium.
In 1927, I.S. Bowen finally solved the mystery of coronium and nebulium. Under high temperature near vacuum conditions, such as found in nebulae and in the solar corona, ordinary elements emit spectral lines that they do not emit in conditions found in laboratories on Earth. To produce these lines, atoms must remain in highly ionized states (missing multiple electrons) for relatively long times. These states do not occur on Earth without special effort, so these spectral lines are called forbidden lines.
The spectral lines of coronium and nebulium turned out to be forbidden lines of ordinary elements. Nebulium was oxygen that was missing two electrons, doubly ionized. Coronium was highly ionized iron with 13 electrons missing. Further study and understanding revealed many other forbidden lines of highly ionized common elements in the corona and nebulae.
It took about six decades to solve the mystery, but the periodic table of the elements gave scientists a unifying principle to distinguish between new elements, like helium, and common elements in unusual states, like coronium and nebulium.