η Lyræ. A double-binary star. Each couple revolves, and the couples probably also revolve round each other. (After Chambers.)
Let us turn the telescope for a short time upon a few of the double stars and we shall have a great treat, for one of the most interesting facts about them is that both stars are rarely of the same colour. It seems strange at first to speak of stars as coloured, but they do not by any means all give out the same kind of light. Our sun is yellow, and so are the Pole-star and Pollux; but Sirius, Vega, and Regulus are dazzling white or bluish-white, Arcturus is a yellowish-white, Aldebaran is a bright yellow-red, Betelgeux a deep orange-red, as you may see now in the telescope, for he is full in view; while Antares, a star in the constellation of the Scorpion, which at this time of year cannot be seen till four in the morning, is an intense ruby red.
Plate II.
It appears to be almost a rule with double stars to be of two colours. Look up at Almach (γ Andromedæ), a bright star standing next to Algol the Variable in the sweep of four bright stars behind Cassiopeia (see Fig. 58). Even to the naked eye he appears to flash in a strange way, and in the telescope he appears as two lovely stars, one a deep orange and the other a pale green, while in powerful telescopes the green one splits again into two (Plate II.) Then again, η Cassiopeæ, the sixth star lying between the two large ones in the second V of Cassiopeia, divides into a yellow star and a small rich purple one, and δ Geminorum, a bright star not far from Pollux in the constellation Gemini, is composed of a large green star and a small purple one. Another very famous double star (β Cygni), which rises only a little later in the evening, lies below Vega a little to the left. It is composed of two lovely stars; one an orange yellow and the other blue; while ε Boötis, just visible above the horizon, is composed of a large yellow star and a very small green one.[5]
There are many other stars of two colours even among the few constellations we have picked out to-night, as, for example, the star at the top of the tailboard of Charles's Waggon and the second horse Mizar. Rigel in Orion, and the two outer stars of the belt, α Herculis, which will rise later in the evening, and the beautiful triple star (ζ Cancer) near the Beehive (see Fig. 54), are all composed of two or more stars of different colours.
Why do these suns give out such beautiful coloured light? The telescope cannot tell us, but the spectroscope again reveals the secrets so long hidden from us. By a series of very delicate experiments, Dr. Huggins has shown that the light of all stars is sifted before it comes to us, just as the light of our sun is; and those rays which are least cut off play most strongly on our eyes, and give the colour to the star. The question is a difficult one but I will try to give you some idea of it, that you may form some picture in your mind of what happens.
We learnt in our last lecture (p. 131) that the light from our sun passes through the great atmosphere of vapours surrounding him before it goes out into space, and that many rays are in this way cut off; so that when we spread out his light in a long spectrum there are dark lines or spaces where no light falls.[6] Now in sunlight these dark lines are scattered pretty evenly over the spectrum, so that about as much light is cut off in one part as in another, and no one colour is stronger than the rest.
Dr. Huggins found, however, that in coloured stars the dark spaces are often crowded into particular parts of the long band of colour forming the spectrum; showing that many of those light-rays have been cut off in the atmosphere round the star, and thus their particular colours are dimmed, leaving the other colour or colours more vivid. In red stars, for example, the yellow, blue, and green parts of the spectrum are much lined while the red end is strong and clear. With blue stars it is just the opposite, and the violet end is most free from dark lines. So there are really brilliantly coloured suns shining in the heavens, and in many cases two or more of these revolve round each other.