On the whole, it is characteristic of new stars that the spectrum lines appear double—dark on the violet and bright on the red side. In the spectrum of Nova Aurigæ this peculiarity is, among others, striking in the three hydrogen lines C, F, and H, in the sodium line, in the nebula lines, and also in the magnesium line. In the spectrum of Nova Persei the displacement of the hydrogen lines towards the violet is so great that, according to Doppler’s principle,[13] the hydrogen gas which absorbed the light must have been moving towards us with a velocity of 700 or more kilometres (450 miles) per second. Some calcium lines show a similar displacement, which is less noticeable in the case of the other metals. This would appear to indicate that relatively cold masses of gas are issuing from the stars and streaming with enormous velocities towards the earth. The luminous parts of the stars were either at a stand-still or they were moving away from us. The simplest explanation of these phenomena would be that a star when flashing up by virtue of its high temperature and high pressure shows enhanced (widened) spectral lines, whose violet portion is absorbed by the strongly cooled masses of gas which are moving towards us and are cooled by their own strong expansion. These gases stream, of course, in all directions from the star, but we only become aware of those gases which absorb the light of the stars—that is to say, those situated between the star and the earth, and streaming in our direction.