[F] ‘Astrophysical Journal’, Vol. VII, January, 1898.
It should be remarked that these numbers rest on spectroscopic results,
which need further confirmation. They are, therefore, liable to be
changed by subsequent investigation. What is most remarkable is that we
have here to deal with a case to which we have no analogy in our solar
system, and which we should never have suspected, had it not been for
observations of this star.
The gap between the variable stars of the Algol type and those of
the Beta Lyræ type is, at the present time, being filled by new
discoveries in such a way as to make a sharp distinction of the two
classes difficult. It is characteristic of the Algol type proper that
the partial eclipses are due to the interposition of a dark planet
revolving round the bright star. But suppose that we have two nearly
equal stars, _A_ and _B_, revolving round their common center of
gravity in a plane passing near our system. Then, _A_ will eclipse
_B_, and, half a revolution later, _B_ will eclipse _A_, and so on
in alternation. But, when the stars are equal, we may have no way of
deciding which is being eclipsed, and thus we shall have a star of the
Algol type, so far as the law of variation is concerned, yet, as a
matter of fact, belonging rather to the Beta Lyræ type. If the velocity
in the line of sight could be measured, the question would be settled
at once. But only the brightest stars can, so far, be thus measured, so
that the spectroscope cannot help us in the majority of cases.
The most interesting case of this kind yet brought to light is that
of Tau Cygni. The variability of this star, ordinarily of the fourth
magnitude, was discovered by Chandler in December, 1886. The minima
occurred at intervals of three days. But in the following summer he
found an apparent period of 1 d. 12 h., the alternate minima being
invisible because they occurred during daylight, or when the star was
below the horizon. With this period the times of minima during the
summer of 1888 were predicted.
It was then found that the times of the alternate minima, which,
as we have just said, were the only ones visible during any one
season, did not correspond to the prediction. The period seemed to
have greatly changed. Afterward, it seemed to return to its old
value. After puzzling changes of this sort, the tangle was at length
unraveled by Dunér, of Lund, who showed that the alternate periods were
unequal. The intervals between minima were one day nine hours, one
day fifteen hours, one day nine hours, one day fifteen hours, and so
on, indefinitely. This law once established, the cause of the anomaly
became evident. Two bright stars revolve round their common center of
gravity in a period of nearly three days. Each eclipses the other in
alternation. The orbit is eccentric, and, in consequence, one-half
of it is described in a less time than the other half. If we could
distinguish the two stars by telescopic vision, and note their relative
positions at the four cardinal points of their orbit, we should see the
pair alternately single and double, as shown in the following diagrams:
A B
Position (1), stars at pericenter * *
Interval, 16 hours.
Position (2), A eclipses B *
Interval, 20 hours.
B A
Position (3), stars at apocenter * *
Interval, 20 hours.
Position (4), B eclipses *
Interval, 16 hours.