The Discovery Of Helium

To the Romans the god of the Sun was Sol. To the Greeks, he was Helios. The ancient Greeks were more proficient in science and mathematics than the Romans, which is why Greek words are often employed in scientific matters (a term like "heliocentric," for example, or "telescope" from the Greek "tele" meaning "distant").

The element helium gets its name in the same way. This is the second member of the periodic table of elements (the sequence of naturally occurring atoms). It is always found as a monatomic gas— that is, a molecule containing just one atom—because it is inert, meaning that it does not undergo any chemical reactions with other atoms. As a consequence, although it occurs on the Earth its existence had escaped the notice of science until being identified as a major constituent of the Sun—using an eclipse, of course.

The story is quite peculiar. We discussed the so-called Fraunhofer spectrum of the Sun earlier. This consists of the continuum from the lower photosphere, superimposed on which are the many dark lines produced by the cooler atoms in its uppermost layer absorbing at their specific wavelengths. This absorption spectrum can be detected at any time; its intensity swamps any other solar light except in a total solar eclipse. During such an eclipse we see the corona and other structures that are normally drowned by the photosphere. (Similarly, as you look out the window of a railway carriage on a clear day you see the countryside whizzing past. When making the same trip at night, though, or when passing through a tunnel, you see mainly your own reflection and the interior of the carriage from the inside of the glass window. That reflection is always there, but it is not easy to see in broad daylight.)

Turning a spectroscope upon the corona, nineteenth-century astronomers found that the spectrum they recorded was quite unlike the familiar Fraunhofer spectrum. There was no sign of the dark lines, but the exact opposite: all they detected was a series of bright lines. An example of this emission spectrum is shown in Figure 5-7. This is a spectrum of the chromosphere and corona, displaying a series of bright lines produced by the hot gases just above the photosphere. Many of these lines could be identified with known elements, and in particular the red coloration of the chromosphere was recognized to be due to the strong "Ha" line (seen at far right in Figure 5-7). This allowed hydrogen to be identified as the major constituent of the solar atmosphere. (H is the chemical symbol for hydrogen; the Greek letters applied here are simply used by convention to label the specific spectral lines.) But two other distinct lines in Figure 5-7 had an unknown origin.

In 1868 eclipse observations allowed the wavelength of the line immediately to the left of the Ha line (labeled "5875") to be

FIGURE 5-7. A spectrum of the chromosphere photographed by nineteenth-century astronomers, showing some of the features that perplexed them. The spectrum was obtained by dispersing the light received during a total solar eclipse using a large prism. The shape of the chromosphere, around the limb of the Sun, is obvious, as are several prominences. At the top various spectral lines are labeled. Five hydrogen lines are labeled (Ha, p, y, § plus another denoted simply by an H at wavelength 3968 angstroms), two of helium (He), one of potassium (K), and one of titanium (Ti). Many other fainter lines are left unlabeled, but are identifiable.

FIGURE 5-7. A spectrum of the chromosphere photographed by nineteenth-century astronomers, showing some of the features that perplexed them. The spectrum was obtained by dispersing the light received during a total solar eclipse using a large prism. The shape of the chromosphere, around the limb of the Sun, is obvious, as are several prominences. At the top various spectral lines are labeled. Five hydrogen lines are labeled (Ha, p, y, § plus another denoted simply by an H at wavelength 3968 angstroms), two of helium (He), one of potassium (K), and one of titanium (Ti). Many other fainter lines are left unlabeled, but are identifiable.

measured accurately. It was realized that this could not possibly be due to sodium (the yellow of sodium street lamps is due to two very close lines at a shade longer wavelength than this). Two astronomers realized concurrently that this was evidence of a previously unknown element: the aforementioned Jules Janssen and British scientist Sir Norman Lockyer. It was Lockyer, who later made a name for himself with his astronomical theories about megalithic monuments and the Egyptian pyramids, who suggested the name helium for the new element, and hence the symbol "He" used in Figure 5-7. Because it is chemically inert, helium was not identified on Earth until some time later, in 1895.

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