When the spectroscope is directed towards the corona, it is found that a great deal of the corona' light yields the ordinary Fraunhofer spectrum and is therefore reflected sunlight. But there is also light which must originate in the corona itself, for it gives a spectrum of which part is continuous and part is corn posed of bright lines which have not yet been matched terrestrially. The name coronium has been given to a hypothetical element which has been held to radiate the lines, but it is now generally believed that coronium is a familiar element in disguise. Elements have a habit of changing their spectra when certain changes are made in the conditions of their radiation.
Spectra and Physical Conditions.—This change of spectrum with change of physical conditions is one of the most important phenomena of spectroscopy. We shall at this stage consider three of its aspects without inquiring into their theoretical foundations. They are, respectively, the effect on a line spectrum of tem perature, a magnetic field, and motion of the source of light.
When a luminous compound body is gradually raised in tern perature, changes of spectrum of two kinds take place successively.
First, the spectrum of the compound gives place to the lines of the constituent elements as the compound becomes dissociated by heat ; secondly, the earliest elementary lines to appear slowly fade out, while new lines come into view and are gradually strengthened. Spectrum lines may therefore be roughly classified into "high temperature" lines and "low temperature" lines. These observations have an important application when the spectra of the photosphere and a sunspot are compared. The spectrum of the spot contains evidence of compounds of which only the separated elements contribute to the photospheric spectrum, and, furthermore, the spot spectrum shows a strengthening of "low temperature" and weakening of "high temperature" lines when compared with the spectrum of the photosphere. The obvious conclusion is that the vapours over a sunspot have a lower tem perature than the rest of the solar atmosphere.
A source of light placed in a magnetic field (i.e., a region such as the neighbourhood of the poles of a magnet, in which a freely suspended compass needle is constrained to lie in a definite direc tion) has each of its spectrum lines split into a number of com ponent lines, lying side by side. Unless the field is a very strong one, the components are too close together to be seen as distinct individuals and the lines merely appear to be broadened. By
means of special appliances, however, the existence of the separate components can be detected. In 1908 George Ellery Hale, using such appliances, found that the lines in the spectrum of a sunspot were of just the composite character that would result from the existence of a magnetic field in the spot in a direction perpendicular to the sun's surface.
Further observation showed that all sunspots are magnets, some presenting a north, and others a south pole to the surrounding space. In each pair of spots, the leader and the follower in the journey round the sun's axis had opposite polarities. Indeed, many apparently single spots were found by this magnetic criterion to have invisible companions. Moreover, there was a reversal of polarity in the southern solar hemisphere as compared with the northern ; thus, if the leader of a pair were a N-pole .in the northern hemisphere, it would be a S-pole in the southern hemi sphere, and vice versa and, with few exceptions, this order would characterise all spots from one minimum to the next of a sunspot sphere, and vice versa, and, with few exceptions, this order would be reversed, so that the approximate period of a complete set of sunspot phenomena turned out to be, not II but 22 years.
More refined investigations of the same kind have shown that the sun as a whole is a magnet, just as the earth is. Its field, however, is much weaker than that of a spot, being only 5c) gausses at its strongest observable region (the base of the revers ing layer) as compared with about 4,000 gausses in large spots. (The maximum value of the earth's field is roughly 0.5 gauss.) The third of the effects mentioned above—that of motion of the source of light, or Doppler as it is called—has had many and various applications. When the distance between an observer and a source of light is decreasing the wave-length of every line emitted is lessened by an amount proportional to the wave-length itself and to the rate of approach. When the distance is increasing the wave-length is augmented in a similar manner. This shows itself by a displacement of the spectrum lines compared with their normal positions, and the amount and direction of the displace ment indicate the velocity of approach or recession.