Spica, the Ear of Corn in the hand of the Virgin, is immeasurably distant from us, and its companion is invisible even in the most powerful telescopes. It was discovered by means of the spectroscope, for, when the light of the star is drawn out into a long rainbow-coloured ribbon crossed by dark lines, it is found that there are two ribbons, one bright, the other very faint, and that the lines of these two spectra draw apart and then come together again once in every four days, showing that there are two stars close together and revolving round one another in this short time. The joint mass of the pair is two and a half times that of the sun.
Spica is one of the first stars whose invisible companion was discovered in this way, and it is also connected with another discovery, made nearly twenty centuries earlier by Hipparchus in the island of Rhodes. He was patiently plotting the positions of all the visible stars, when he found a slight discrepancy between the places given to Spica by himself and another Greek astronomer, who had observed about a hundred and fifty years earlier. Examining into this led him to discover that all stars change their apparent positions very slowly, completing a cycle of change in nearly 26,000 years, so that 240 centuries more must elapse before any astronomer sees Spica in exactly the same place as where Hipparchus saw her. It remained for modern astronomers to discover that the apparent change is due to a slow nodding motion of the Earth’s axis.
Brightest of all stars in the whole sky is Sirius, the Dog-Star. It was worshipped by the ancient Egyptians, and the day on which it rose just before the sun was counted as the first day of their year. The Arabs, when they learned the astronomy of more ancient nations, were forbidden to adopt their star-worship, hence the saying in the Koran, often quoted by Arab writers: “The Highest saith, He is lord of Sirius.”
Sirius is moving rapidly through space, not uniformly but with an oscillating movement, and Bessel in 1844 “founded the astronomy of the invisible” by showing that these irregularities might be caused by a dark disturbing companion. Eight years later, Mr. Alvan Clark, wishing to test a large lens just made by his firm, turned it on Sirius, and lo! there was the satellite in the position required to explain the vagaries of Sirius. It is not therefore wholly dark, but it shines with so feeble a light that, if it were brought as near to us as our own sun, it would appear only one-hundredth as bright as he is, even though it is a somewhat more massive body. Sirius itself is only about two and a half times as massive as our sun, but immensely more brilliant. It is the typical star of the “Sirian” class, to which belong many of the brightest stars in the heavens, white stars in whose spectra broad hydrogen lines form the most striking feature, indicating a very extensive atmosphere of glowing hydrogen. Bright Sirius and his dim companion revolve round their common centre of gravity in fifty years.
Fomalhaut, the mouth of the Fish, belongs to the Sirian type of stars, and is also very brilliant, giving out fourteen and a half times as much light as our sun. Its distance is 25 light-years.
Canopus, the rudder of the Ship Argo, must be a giant sun, for its distance is altogether beyond reach of measurement and it is steadily receding from us, yet it shines as the brightest star in the sky except Sirius.
In Southern India it is called Agastya, after a Brahmin rishi who led an early Aryan colony to the south, and before whom the Vindhya Mountains prostrated themselves as he passed.
Greek astronomers noticed that this star rose only just above the horizon of Rhodes, but 7½ degrees above it at Alexandria, from which Poseidonius calculated that the circumference of the whole earth, i.e. 360°, must be 240,000 stadia. This is equal to nearly 23,500 miles, a value surprisingly near the correct figure, considering how difficult it must have been to measure the distance over the sea between Rhodes and Alexandria.
Antares, the brightest star in the Scorpion, was so named by the Greeks because it rivals the red planet Mars (Greek Ares) in colour. It is red because a dense atmosphere shuts out most of its blue rays. Like most red stars it is very distant, and its light takes 155 years to reach us. Nevertheless it shines 2000 times as brightly as our sun would do at the same distance, hence it must be of an enormous size. This immense red star is accompanied by a little green satellite, and there is also a very close companion which can never be seen, but is known to exist through the shifting of lines in the spectrum, like that of Spica.
What an amazing variety among these ten stars! Though the eye can only decide that all are much brighter than the average, and that they differ somewhat in colour among themselves, science tells us that they vary enormously in many ways. Half belong to the class of blue Orion stars, others to the white Sirian, yellow solar, red Antarian classes; more than half are known to be double or multiple, and among these we find twins, while others have faint companions differing from themselves in colour. In one case the pair is widely separated, and revolves in a period of half a century, while another pair is so close that the circuit is performed in four days. Again, while one star is the same size as the sun, others are much greater, and their distances from us vary all the way from four light-years to spaces we are powerless to plumb.