Part 22 (1/2)
E) Radial Velocity Km. per sec.
F) Spectral Type G) Luminosity (Sun=1) H) Star Stream
==================================================================== Star's Name | A | B | C | D | E | F | G | H ---------------+-----+------+------+-----+-----+------+-------+----- Groombridge 34 | 8.2 | 0.28 | 2.85 | 48 | .. | Ma | 0.010 | I Eta Ca.s.siop | 3.6 | 0.20 | 1.25 | 30 | +10 | F8 | 1.4 | I Tau Ceti | 3.6 | 0.33 | 1.93 | 28 | -16 | K | 0.50 | II Epsilon Erid | 3.3 | 0.31 | 1.00 | 15 | +16 | K | 0.79 | II CZ 5h 243 | 8.3 | 0.32 | 8.70 | 129 |+242 | G-K | 0.007 | II Sirius |-1.6 | 0.38 | 1.32 | 16 | -7 | A |48.0 | II Procyon | 0.5 | 0.32 | 1.25 | 19 | -3 | F5 | 9.7 | I ?
Lal. 21185 | 7.6 | 0.40 | 4.77 | 57 | .. | Ma | 0.009 | II Lal. 21258 | 8.9 | 0.20 | 4.46 | 106 | .. | Ma | 0.011 | I OA (N) 11677 | 9.2 | 0.20 | 3.03 | 72 | .. | .. | 0.008 | I Alpha Centauri | 0.3 | 0.76 | 3.66 | 23 | -22 | G,K5 |{2.0 | I | | | | | | |{0.6 | OA (N) 17415 | 9.3 | 0.27 | 1.31 | 23 | .. | F | 0.004 | II Pos. Med. 2164 | 8.8 | 0.29 | 2.28 | 37 | .. | K | 0.006 | I Sigma Draco | 4.8 | 0.20 | 1.84 | 43 | +25 | K | 0.5 | II Alpha Aquilae | 0.9 | 0.24 | 0.65 | 13 | -33 | A5 |12.3 | I 61 Cygni | 5.6 | 0.31 | 5.25 | 80 | -39 | K5 | 0.10 | I Epsilon Indi | 4.7 | 0.28 | 4.67 | 79 | -62 | K5 | 0.25 | I Kruger 60 | 9.2 | 0.26 | 0.92 | 17 | .. | .. | 0.005 | II Lacaille 9352 | 7.4 | 0.29 | 7.02 | 115 | +12 | Ma | 0.019 | I --------------------------------------------------------------------
These stars are distant less than five pa.r.s.ecs (about 16 light-years) from the sun, so they make up the closest fringe of the stellar universe immediately surrounding our system. The large number of binary systems is quite remarkable. Why some stars are single and others double is not yet known. By the spectroscopic method the proportion is not so large; Campbell finding that about one quarter of 1,600 stars examined are spectroscopic binaries, and Frost two-fifths to a half. The exceptional number of large velocities is very remarkable; the average transverse motion of the nineteen stars is fifty kilometers per second, whereas thirty is about what would have been expected.
As to star streams to which these nearest stars belong, eleven are in Stream I and eight in Stream II, in close accord with the ratio 3:2 given by the 6,000 stars of Boss's catalogue. ”We are not able,” says Eddington, ”to detect any significant difference between the luminosities, spectra, or speeds of the stars const.i.tuting the two streams. The thorough interpenetration of the two star streams is well ill.u.s.trated, since we find even in this small volume of s.p.a.ce that members of both streams are mingled together in just about the average proportion.”
[Ill.u.s.tration: THE RING NEBULA IN _Lyra_. This is the best example of the annular and elliptic nebulae, which are not very abundant.
(_Photo, Mt. Wilson Solar Observatory._)]
[Ill.u.s.tration: THE DUMB-BELL NEBULA OF _Vulpecula_. To take the photograph required an exposure of five hours. (_Photo, Mt.
Wilson Solar Observatory._)]
CHAPTER L
ACTUAL DIMENSIONS OF THE STARS
We have seen that the distances of the stars from the solar system are immense beyond conception, and millions upon millions of them are probably forever beyond our power of ascertaining by direct measurement what their distance really is. After we had found the sun's distance and measured the angle filled by his disk, it was easy to calculate his actual size. This direct method, however, fails when we try to apply it to the stars, because their distances are so vast that no star's disk fills an angle of any appreciable size; and even if we try to get a disk with the highest magnifying powers of a great telescope our efforts end only in failure. There is, indeed, no instrumentally appreciable angle to measure.
How then shall we ascertain the actual dimensions of the vast spheres which we know the stars actually are, as they exist in the remotest regions of s.p.a.ce? Clearly by indirect methods only, and it must be said that astronomers have as yet no general method that yields very satisfactory results for stellar dimensions. The actual magnitude of the variable system of Algol, Beta Persei, is among the best known of all the stars, because the spectroscope measures the rate of approach and recession of Algol when its invisible satellite is in opposite parts of the orbit; the law of gravitation gives the ma.s.s of the star and the size of its...o...b..t, and so the length of the eclipse gives the actual size of the dark, eclipsing body. This figures out to be practically the same size as that of our sun, while Algol's own diameter is rather larger, exceeding a million miles.
If we try to estimate sizes of stars by their brightness merely, we are soon astray. Differences of brightness are due to difference of dimensions, of course, or of light-giving area; but differences of distance also affect the brightness, inversely as the squares of the distances, while differences of temperature and const.i.tution affect, in very marked degree, the intrinsic brilliance of the light-emitting surface of the star. There are big stars and little stars, stars relatively near to us and stars exceedingly remote, and stars highly incandescent as well as others feebly glowing.
We have already shown how the angular diameters subtended by many of the stars have been estimated, through the relation of surface brightness and spectral type. Antares and Betelgeuse appear to be the most inviting for investigation, because their estimated angular diameters are about one-twentieth of a second of arc. This is the way in which their direct measurement is being attempted.
As early as 1890, Michelson of Chicago suggested the application of interference methods to the accurate measurement of very small angles, such as the diameters of the minor planets, and the satellites of Jupiter and Saturn, as well as the arc distance between the components of double stars. Two portions of the object gla.s.s are used, as far apart as possible on the same diameter, and the interference fringes produced at the focus of the objective are then the subject of observation. These fringes form a series of equidistant interference bands, and are most distinct when the light comes from a source subtending an infinitesimal angle. If the object presents an appreciable angle, the visibility is less and may even become zero.
Michelson tested this method on the satellites of Jupiter at the Lick Observatory in 1891, and showed its accuracy and practicability.
Nevertheless, the method has not been taken up by astronomers, until very recently at the Mount Wilson Observatory, where Anderson has applied it to the measurement of close double stars. It is found that, contrary to general expectation, the method gives excellent results, even if the ”seeing” is not the best--2 on a scale of 10, for instance.
To simplify the manipulation of the interferometer, a small plate with two apertures in it is placed in the converging beam of light coming from the telescope objective or mirror. The interference fringes formed in the focal plane are then viewed with an eyepiece of very high power, many thousand diameters. The resolving power of the interferometer is found to be somewhat more than double that of a telescope of the same aperture. By applying the interferometer method to Capella, arc distances of much less than one-twentieth of a second of arc were measured. More recently the method has been applied to the great star Betelgeuse in Orion, whose angular diameter was found to be 0”.46, corresponding to an actual diameter of 260,000,000 miles, if the star's parallax is as small as it appears to be.
CHAPTER LI
THE VARIABLE STARS
Spectacular as they are to the layman, novae, or temporary stars, are to the astronomers simply a cla.s.s among many thousands of stars which they call variables, or variable stars. There are a few objects cla.s.sified as irregular variables, one of which is very remarkable. We refer to Eta Argus, an erratic variable in the southern constellation Argo and surrounded by a well-known nebula. There is a pretty complete record of this star. Halley in 1677 when observing at Saint Helena recorded Eta Argus as of the fourth magnitude. During the 18th century, it fluctuated between the fourth magnitude and the second. Early in the 19th it rapidly waxed in brightness, fluctuating between the first and second magnitudes from 1822 to 1836. But two years later its light tripled, rivaling all the fixed stars except Canopus and Sirius. In 1843 it was even brighter for a few months, but since then it has declined fairly steadily, reaching a minimum at magnitude seven and a half in 1886, with a slight increase in brightness more recently. A period of half a century has been suggested, but it is very doubtful if Eta Argus has any regular period of variation.
Another very interesting cla.s.s of variables is known as the Omicron Ceti type. Nearly all the time they are very faint, but quite suddenly they brighten through several magnitudes, and then fade away, more or less slowly, to their normal condition of faintness. But the extraordinary thing is that most of these variables go through their fluctuations in regular periods: from six months to two years in length. The type star, Omicron Ceti, or Mira, is the oldest known variable, having been discovered by Fabricius in 1596. Most of the time it is a relatively faint star of the 12th magnitude; but once in rather less than a year its brightness runs up to the fourth, third and sometimes even the second magnitude, where it remains for a week or ten days, and afterward it recedes more slowly to its usual faintness, the entire rise and decline in brightness usually requiring about 100 days. The spectrum of Omicron Ceti contains many very bright lines, and a large proportion of the variable stars are of this type.
Another cla.s.s of variables is designated as the Beta Lyrae type. Their periods are quite regular, but there are two or more maxima and minima of light in each period, as if the variation were caused by superposed relations in some way. Their spectra show a complexity of helium and hydrogen bands. No wholly satisfactory explanation has yet been offered.