In February, 1901, in the constellation Perseus appeared the most brilliant nova of recent years. It was first discovered by Dr. Anderson, an amateur of Glasgow, and at maximum on February 23 it outshone Capella. There were many unusual fluctuations in its waning brightness. Its spectrum closely resembled that of Nova Aurigæ, with calcium, helium, and hydrogen lines. In August, 1901, an enveloping nebula was discovered, and a month later certain wisps of this nebulosity appeared to have moved bodily, at a speed seventy-fold greater than ever previously observed in the stellar universe. According to Sir Norman Lockyer's meteoritic hypothesis, a vast nebulous region was invaded, not by one but by many meteor swarms, under conditions such that the effects of collision varied greatly in intensity. The most violent of these collisions gave birth to Nova Persei itself, and the least violent occurred subsequently in other parts of the disturbed nebula, perhaps immeasurably removed. This explanation would avoid the necessity of supposing actual motion of matter through space at velocities heretofore unobserved and inconceivably high. A recent photograph of Nova Persei, by Ritchey, reveals a nebulous ring of regular structure surrounding the star. The great power of the 60-inch has made it possible to photograph even the spectra of many of the novæ of years ago which are now very faint. After the lapse of years the characteristic lines of the nebular spectrum generally vanish, as if the star had passed out of the nebula--a plunge into which is generally thought to be the cause of the great and sudden outburst of light. Many novæ have recently been found in the spiral nebulæ, especially in the great nebula of Andromeda. CHAPTER LIII THE DOUBLE STARS Examining individual stars of the heavens more in detail, thousands of them are found to be double; not the stars that appear double to the naked eye, as Theta Tauri, Mizar, Epsilon Lyræ, and others; but pairs of stars much closer together, and requiring the power of the telescope to divide or separate them. Only a very few seconds apart they are or, in many cases, only the merest fraction of a second of arc. Some of them, called binaries, are found to be revolving around a common center, sometimes in only a few years, sometimes in stately periods of hundreds of years. Many such binary systems are now known, and the number is constantly increasing. Castor is one, Gamma Virginis another, Sirius also is one of these binaries, and a most interesting one, having a period of revolution of about 52 years.