Double Stars-Position, Angle, and Distance—Orbits of Double Stars- Binary Systems of Sirius and Procyon - Orbit of Alpha Centauri— System of Capella - Triple and Multiple Systems - Spectroscopic Masses and Densities of the Stars-Diversities among the Stars-Masses and Densities of the Binary Systems- - Gaseous Constitution of the Is the Universe Finite ?-Arrangement of the Stars in Space-Relation of the Milky Way to the Universe - Possible Hypotheses as to the Description of the Milky Way - Lucid Stars belonging to the Milky THE PROGRESSION IN THE NUMBER OF STARS AS THE Ratio of this Increase in Different Regions of the Sky — Higher ratio in Components of the Proper Motion Mean Parallax of the Stars of the Second Magnitude - Motions of the Two Principal Spectral Types of Stars-Kapteyn's Researches-Relation of the Proper Motions to ILLUSTRATIONS PAGE LAW OF CHANGE OF THE MAGNITUDE OF A STAR WITH ITS DISTANCE. 15 PLAN OF THE SPECTRUM 59 65 68 THE MILLS SPECTROGRAPH OF THE LICK OBSERVATORY 86 98 SPECTRUM OF NOVA AURIGÆ. DISTANCE AND POSITION-ANGLE OF A DOUBLE STAR APPARENT ORBIT OF α CENTAURI. RADIAL MOTION OF A BINARY SYSTEM THE GREAT STAR-CLUSTER OF HERCULES THE GREAT STAR-CLUSTER OF CENTAURI THE GREAT NEBULA OF ORION THE GREAT SPIRAL NEBULA M. 51 THE GREAT NEBULA OF ANDROMEDA NEBULOUS MASS IN CYGNUS . 114 120 TWO BINARY SYSTEMS ON THE SAME MODEL POSSIBLE SECTIONS OF THE GALAXY THE STARS WE CHAPTER I REVIEW OF RECENT PROGRESS These are thy glorious works, Parent of good, Thus wondrous fair.-MILTON. E begin our study of the stars by a glance at the structure of the universe. What are familiarly known as the heavenly bodies belong to two classes which are very different as regards their relation to our earth. Those nearest to us form a sort of colony far removed from all the others, called the solar system. The principal bodies of this system are the sun and eight great planets, with their moons, revolving round it. On one of these planets, small when compared with the great bodies of the universe, but large to our every-day conceptions, we dwell. The other planets appear to us as stars. Four of them, Venus, Mars, Jupiter, and Saturn, are distinguished from the fixed stars by their superior brightness and |