Fig. 30. Solar prominences, photographed with the spectroheliograph without an eclipse (Ellerman).
In these luminous gaseous clouds, which sometimes rise to elevations exceeding half the sun's diameter, the new gas helium was discovered by Lockyer in 1868. Helium was not found on the earth until 1896. Since then it has been shown to be a prominent constituent of nebulæ and hot stars.
The chief importance of helium lies in the clue it has afforded to the constitution of matter and the transmutation of the elements. Radium and other radioactive substances, such as uranium, spontaneously emit negatively charged particles of extremely small mass (electrons), and also positively charged particles of much greater mass, known as alpha particles. Rutherford and Geiger actually succeeded in counting the number of alpha particles emitted per second by a known mass of radium, and showed that these were charged helium atoms.
To discuss more at length the extraordinary characteristics of helium, which plays so large a part in celestial affairs, would take us too far afield. Let us therefore pass to another case in which a fundamental discovery, this time in physics, was first foreshadowed by astronomical observation.
SUN-SPOTS AS MAGNETS
No archæologist, whether Young or Champollion deciphering the Rosetta Stone, or Rawlinson copying the cuneiform inscription on the cliff of Behistun, was ever faced by a more fascinating problem than that which confronts the solar physicist engaged in the interpretation of the hieroglyphic lines of sun-spot spectra. The colossal whirling storms that constitute sun-spots, so vast that the earth would make but a moment's scant mouthful for them, differ materially from the general light of the sun when examined with the spectroscope. Observing them visually many years ago, the late Professor Young, of Princeton, found among their complex features a number of double lines which he naturally attributed, in harmony with the physical knowledge of the time, to the effect of "reversal" by superposed layers of vapors of different density and temperature. What he actually saw, however, as was proved at the Mount Wilson Observatory in 1908, was the effect of a powerful magnetic field on radiation, now known as the Zeeman effect.
Fig. 31. The 150-foot tower telescope of the Mount Wilson Observatory.
An image of the sun about 16 inches in diameter is formed in the laboratory at the base of the tower. Below this, in a well extending 80 feet into the earth, is the powerful spectroscope with which the magnetic fields in sun-spots and the general magnetic field of the sun are studied.