ful telescope. The sun has a diameter of 860,000 miles and, as its distance from the earth is only 93,000,000 miles, an extremely small fraction of the distance of the other stars, it is possible to observe and to study in detail its extraordinary phenomena, which are incomparably more violent than anything observed on the earth. When we speak of the sun we speak collectively of a great number of phenomena, some of which extend for millions of miles from the sun's visible disk. Chief of these is the corona, a vast filmy atmosphere so rare that it offers little or no resistance to the passage of a comet, as it sweeps around the sun under the action of gravitation and returns into the space from which it came. The polar streamers of the corona (Fig. 9) suggest the

action of magnetic forces and offer material for long continued study of this, the most mysterious of all the solar appendages. At the base of the corona, rising out of a sea of flame which completely encircles the sun, are the prominences, some of which occasionally attain a height of nearly 400,000 miles. Like the corona, the prominences are hidden by the brilliant illumination of our own atmosphere, and are visible to the naked eye only when the direct light of the sun's disk is cut off by

the interposition of the moon at a total eclipse. But methods have been devised by which they can be observed or photographed on any clear day through the agency of a modified form of spectroscope. The prominences are constantly changing in form, sometimes slowly, as in the case of this group (Fig. 10), a photograph of which, taken at the eclipse of May 28, 1900, by the Astronomer Royal of England in Spain, is shown for comparison with the photograph taken about two hours carlier by the Yerkes Observatory party in North Carolina. Here the

a, BY

CLOUD-LIKE PROMINENCES PHOTOGRAPHED AT THE ECLIPSE OF MAY 28, 1900. YERKES OBSERVATORY PARTY AT WADESBORO, N. C. b, BY ASTRONOMER ROYAL OF ENGLAND AT OVAR, PORTUGAL, TWO HOURS LATER. (THE BRIGHT CROSS ON THE RIGHT OF THIS PICTURE IS DUE TO A DEFECT IN THE ORIGINAL PHOTOGRAPH.)

change in the form of the mass of gas which constitutes the prominence, is comparatively small, but that violent forces are sometimes at work may be illustrated by photographs of an eruptive prominence taken at the Kenwood Observatory in 1895 (Fig. 11). At the moment when the first photograph was made the prominence had attained a height of 160,000 miles and was rising rapidly. Eighteen minutes later another picture was taken; during the interval the prominence had been going upward at the rate of six thousand miles. a minute, and when the exposure was made it had reached an elevation of 280,000 miles. When looked for a few minutes later it had completely disappeared.

The constitution of the chromosphere, the sea of flame some 10,000 miles deep from which the prominences arise, increases in complexity as the surface of the solar disk is approached. In its upper part only the vapor of calcium and the light gases, hydrogen and helium, are found. But in proceeding downward the vapors of magnesium, sodium, iron, chromium, and last of all, carbon, are successively encountered. At this part of the solar atmosphere the dark lines of the solar spectrum take their rise through the effect of absorption.

Tme does not permit a detailed description of the phenomena of

the sun's disk. When photographed with an instrument which excludes from the sensitive plate all light except that which is characteristic of the vapor of calcium, its surface is found to be dotted over with extensive luminous regions. Associated with these are the sun-spots, the minute study of which has revealed some strikingly beautiful phenomena, which have been most successfully drawn by Langley. The surface of the sun in the regions devoid of spots is shown by the photographs of Janssen to consist of brilliant granules separated by darker spaces. Much might be said of the peculiar law of rotation of the sun, which causes a point near the equator to complete an axial rotation in much less time than a point nearer the poles. Much might also be said of the periodicity of sun-spots, which at times are

ERUPTIVE PROMINENCE PHOTOGRAPHED IN FULL SUNLIGHT AT THE KENWOOD OBSERVATORY, CHICAGO, MARCH 25, 1895. a, AT 10H. 40м. (HEIGHT, 162.000 MILES). b, AT 10H. 58M HEIGHT, 281,000 MILES). (FIGS. 10 AND 11 ARE REPRODUCED ON THE SAME SCALE.)

very numerous and again, as at present, are absent from the sun's disk for weeks together. But enough has already been told to indicate some of the chief characteristics of this central star of the solar system, which has thousands of counterparts among other stars of the same spectral class.

We are now approaching the last chapters in the life history of a star. After the solar stage has passed the color changes from yellow to orange, and subsequently to red, as the temperature falls. The spectral lines of hydrogen become fainter and fainter and finally disappear completely. The lines of the metallic elements, on the contrary, become nore and more complex and the changes in their relative intensities are those which are characteristic of lower temperatures. But curiously enough,

there are two well-defined classes of these older stars, which until recently were not known to have any points in common except their red color. These are the stars of Secchi's third and fourth types. In general appearance their spectra are wholly unlike, particularly on account of the absence from third class spectra of the broad dark bands due to the absorption of carbon vapor, the most characteristic feature of the fourth type. But in spite of this apparent dissimilarity, photographs recently taken with the 40-inch Yerkes telescope show that in certain regions of the spectrum stars of the two types are practically identical and are thus probably more closely related than formerly appeared to be the

SPECTRA OF FOUR RED STARS (HALE AND ELLERMAN). SHOWING HOW THE DARK BAND DUE TO CARBON INCREASES IN INTENSITY AS THE STAR COOLS.

case. The measurements and reductions of a long series of photographs of fourth type spectra now in progress at the Yerkes Observatory should soon permit us to form an opinion of the nature of these interesting

stars.

In both the third and fourth types it is easy to trace the successive stages of development. In stars of the fourth type the signs of increasing age are particularly striking. The carbon vapor which produces the broad dark bands becomes denser and denser, until it is not difficult to imagine that through the further increase of such absorption the light of the star might be completely extinguished (Fig. 12).

The phenomena of the red stars indicate that this final stage is close at hand, and curiously enough, in further testimony of the remarkable power of the spectroscope, the total extinction of a star's light does not always prove sufficient to place that star beyond the reach of this instrument. It is true that a spectroscope cannot reveal the chemical composition of a solid body which is devoid of intrinsic light, but such a body may form a system with another object which is still lumin

LUNAR CRATER THEOPHILUS AND SURROUNDING REGION. PHOTOGRAPHED WITH THE FORTY-INCH YERKES TELESCOPE (RITCHEY).

ous, and its gravitational power may cause the luminous body to move in an orbit. As we have already seen, the spectroscope is capable of revealing the motions of such a body. From a knowledge of these motions and the time in which the revolution is effected it is possible to determine the mass and dimensions of the system, and in some special cases like that of Algol, the diameter and density of the invisible component of the pair.