THE VARIABLE STARS
Spectacular as they are to the layman, novae, or temporary stars, are to
the astronomers simply a class among many thousands of stars which they
call variables, or variable stars. There are a few objects classified 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 class 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 class 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.
Probably they are double stars revolving in very small orbits compared
with their dimensions, their plane of motion passing nearly through the
earth.
But the most interesting of all the variables are those of the Algol
type, their light curves being just the reverse of the Omicron Ceti
type; that is, they are at their maximum brightness most of the time,
and then suffer a partial eclipse for a relatively brief interval. Algol
goes through its variations so frequently that its period is very
accurately known; it is 2d. 20h. 48m. 55.4s. For most of this period
Algol is an easy second magnitude star; then in about four and a half
hours it loses nearly five-sixths of its light, receding to the fourth
magnitude. Here at minimum it remains for fifteen or twenty minutes, and
then in the next three and a half hours it regains its full normal
brilliancy of the second magnitude. During these fluctuations the star's
spectrum undergoes no marked changes. The spectra of all the Algol
variables are of the first or Sirian type.
To explain the variation of the Algol type of variables is easy: a dark,
eclipsing body, somewhat smaller than the primary is supposed to be
traveling round it in an orbit lying nearly edgewise to our line of
sight. The gravitation of this dark companion displaces Algol itself
alternately toward and from the earth, because the two bodies revolve
round their common center of gravity. With the spectroscope this
alternate motion of Algol, now advancing and now receding at the rate of
26 miles per second, has been demonstrated; and the period of this
motion synchronizes exactly with the period of the star's variability.
Russell and Shapley have made extended studies of the eclipsing
binaries, and developed the formulae by which the investigations of their
orbits are conducted. Heretofore, visual binaries and spectroscopic
binaries afforded the only means of deriving data regarding double
systems, but it is now possible to obtain from the orbits of eclipsing
variables fully as much information relating to binary systems in
general and their bearing on stellar evolution. After an orbit has been
determined from the photometric data of the light curve, the addition
of spectroscopic data often permits the calculation of the masses,
dimensions and densities in terms of the sun. Shapley's original
investigation included the orbits of ninety eclipsing variables, and
with the aid of hypothetical parallaxes, he computed the approximate
position of each system in space. The relation to the Milky Way is
interesting, the condensation into the Galactic plane being very marked;
only thirteen of the ninety systems being found at Galactic latitudes
exceeding 30 degrees.
If we can suppose the variable stars covered with vast areas of spots,
perhaps similar to the spots on the sun, and then combine the variation
of these spot areas with rotation of the star on its axis, there is a
possibility of explanation of many of the observed phenomena, especially
where the range of variation is small. But for the Omicron Ceti type, no
better explanation offers than that afforded by Sir Norman Lockyer's
collision theory. First he assumes that these stars are not condensed
bodies, but still in the condition of meteoric swarms, and the
revolution of lesser swarms around larger aggregations, in elliptic
orbits of greater or less eccentricity, must produce vast multitudes of
collisions; and these collisions, taking place at pretty regular
periods, produce the variable maximum light by raising hosts of meteoric
particles to a state of incandescence simultaneously.
The catalogues of variable stars now contain many thousands of these
objects. They are often designated by the letters R, S, T, and so on,
followed by the genitive form of the name of the constellation wherein
they are found. Most of the recently found variables have a range of
less than one magnitude. They are so distributed as to be most numerous
in a zone inclined about 18 degrees to the celestial equator, and split
in two near where the cleft in the Galaxy is located. Nearly all the
temporary stars are in this duplex region. Bailey of Harvard a quarter
century ago began the investigation of variables in close star clusters,
where they are very abundant, with marked changes of magnitude within
only a few hours.
Many amateur astronomers afford very great assistance to the
professional investigator of variable stars by their cooperation in
observing these interesting bodies, in particular the American
Association of Observers of Variable Stars, organized and directed by
William Tyler Olcott.
For a high degree of accuracy in determining stellar magnitudes the
photo-electric cell is unsurpassed. Stebbins of Urbana has been very
successful in its application and he discovered the secondary minimum of
Algol with the selenium cell. His most recent work was done with a
potassium cell with walls of fused quartz, perfected after many trial
attempts. The stars he has recently investigated are Lambda Tauri,
and Pi Five Orionis. Combining results with those reached by the
spectroscope, the masses of the two component stars of the former are
2.5 and 1.0 that of the sun, and the radii are 4.8 and 3.6 times the
sun's.
Russell of Princeton thinks it probable that similar causes are at work
in all these variables. In the case of the typical Novae there is
evidence that when the outburst takes place a shell of incandescent gas
is actually ejected by the star at a very high velocity. What may be the
forces that cause such an explosion can only be guessed. Repeated
outbursts have not, in the case of T Pyxidis, destroyed the star,
because it has gone through this process three times in the past thirty
years. Russell inclines to regard it as a standard process occurring
somewhere in the stellar universe probably as often as once a year.
Novae, then, cannot be due to collisions between two stars, for even if
we suppose the stars to be a thousand millions in number, no two should
collide except at average intervals of many million years. The idea is
gaining ground that the stars are vast storehouses of energy which they
are gradually transforming into heat and radiating into space. "Under
ordinary circumstances, it is probable that the rate of generation of
heat is automatically regulated to balance the loss by radiation. But it
is quite conceivable that some sudden disturbance in the substance of
the star, near the surface, might cause an abrupt liberation of a great
amount of energy, sufficient to heat the surface excessively, and drive
the hot material off into infinite space, in much the form of a shell of
gas, as seems to have been observed in the case of Nova Aquilae.... With
the rapid advance of our knowledge of the properties of the stars on one
hand, and of the very nuclei of atoms on the other, we may, perhaps
before many years have passed, find ourselves nearer a solution of the
problem."
The Cepheid variables increase very rapidly in brightness from their
least light to their maximum, and then fade out much more slowly, with
certain irregularities or roughnesses of their light-curves when
declining. Their spectral lines also shift in period with their
variations of light. In the case of these variables, whose regular
fluctuation of light cannot be due to eclipse, and is as a rule embraced
within a few days, there is a fluctuation in color also between maximum
and minimum, as if there were a periodic change in the star's physical
condition. Eddington and Shapley advocate the theory of a mechanical
pulsation of the star as most plausible. Knowledge of the internal
conditions of the stars make it possible to predict the period of
pulsation within narrow limits; and for Delta Cephei this theoretical
period is between four and ten days. Its observed period is five and
one-third days, and corresponding agreement is found in all the Cepheids
so far tested.
Shapley of Mount Wilson finds that the Cepheid variables with periods
exceeding a day in length all lie close to the Galactic lane. So greatly
have the studies of these objects progressed that, as before remarked,
when we know the star's period, we can get its absolute magnitude, and
from this the star's distance. On all sides of the sun, the distances of
the Cepheids range up to 4,000 parsecs. So they indicate the existence
of a Galactic system far greater in extent than any previously dealt
with.