Page:Popular Science Monthly Volume 46.djvu/764

744 components are of magnitudes four and a half and ninth, distance 57″, p. 110°. Burnham discovered that the ninth-magnitude star consists of two of the tenth less than 2″ apart, p. 24°.

No astronomer who happens to be engaged in this part of the sky unless his attention is entirely absorbed by something of special interest, ever fails at least to glance at β Libræ, which is famous as the only naked-eye star having a decided green color. The hue is pale, but manifest.

The star δ, is a remarkable variable, belonging to what is called the Algol type. Its period, according to Chandler, is 3 days, 7 hours, 51 minutes, 22·8 seconds. The time occupied by the actual changes is about twelve hours. At maximum the star is of magnitude five and at minimum of magnitude 6·2.

We may now conveniently turn northward from Virgo in order to explore Boötes, one of the most interesting of the constellations (map No. 11). Its leading star α, Arcturus, is the brightest in the northern hemisphere. Its precedence over its rivals Vega and Capella has been settled by the Harvard photometry. You notice that the color of Arcturus, when it has not risen far above the horizon, is a yellowish red, but when the star is near mid-heaven the color fades to yellow. The hue is possibly variable, for it is recorded that in 1852 Arcturus appeared to have nearly lost its color. If it should eventually turn white, the fact would have an important bearing upon the question whether Sirius was once a red or flame-colored star.

But let us sit here in the starlight, for the night is balmy, and talk about Arcturus, which is perhaps actually the greatest sun within the range of terrestrial vision. Its parallax is so minute that the consideration of the tremendous size of this star is a thing that the imagination can not placidly approach. Calculations, based on its assumed distance, which show that it outshines the sun several thousand times may be no exaggeration of the truth! It is easy to make such a calculation. Dr. Elkin's parallax for Arcturus is 0⋅018″. That is to say, the displacement of Arcturus due to the change in the observer's point of view when he looks at the star first from one side and then from the other side of the earth's orbit, 186,000,000 miles across, amounts to only eighteen one-thousands of a second of arc. We can appreciate how small that is when we know that it is about equal to the apparent distance between the heads of two pins placed an inch apart and viewed from a distance of a hundred and eighty miles!

Assuming this estimate of the parallax of Arcturus, let us see how it will enable us to calculate the probable size or