A classification of variable stars, based on the period of variation
and the law of change, was proposed by Pickering. It does not, however,
seem that a hard and fast line can yet be drawn between different types
and classes of these bodies, one type running into another, as we have
found in the case of the Algol and Beta Lyræ types. Yet the discovery
of the cause of the variation in these types makes it likely that a
division into two great classes, dependent on the cause of variation,
is possible. We should then have:
(1) Stars, or systems, constituting to vision a single star, of which
the apparent variability arises from the rotation of the system as a
whole, or from the revolution of its components around each other.
(2) Stars of which the changes arise from other and as yet unknown
causes.
The main feature of the stars of the first class is that we are under
no necessity of supposing any actual change in the amount of light
which they emit. Their apparent variations are purely the effect of
perspective, arising from the various aspects which they present to us
during their revolution round each other. If we could change our point
of view so that the plane of the orbit of Algol’s planet no longer
passed near our system, Algol would no longer be a variable star.
Under the same circumstances the apparent variations in a star of the
Beta Lyræ type would cease to be noticeable, if they did not disappear
entirely.
The stars of this class are also distinguished by the uniformity and
regularity with which they go through their cycle of change.
The stars of the other class, which we may call the Omicron Ceti type,
are different not only in respect to the length of the period, but
in the character of the variation. There are certain general laws of
variation and irregularities of brightness which stars of this class go
through. Starting from the time of the minimum, the increase of light
is at first very slow. It grows more and more rapid as the maximum is
approached, in which time there may be as great an increase in two or
three days as there formerly was in a month. The diminution of light
is generally slower than the increase. The magnitude at corresponding
times in different periods may be very different. Thus, as we have
already remarked, Omicron Ceti is ten times as bright at some maxima as
it is at others. The periods also, so far as they have been made out,
vary more widely than those of stars of the other type.
The idea has sometimes been entertained that these variations of
light are due to a revolution of the star on its axis. A very little
consideration will, however, show that this explanation cannot be
valid. However bright a star might be on one side, or however dark
on the other, any one region of its surface would be visible to us
half the time and a change of brightness from different degrees of
brilliancy on different sides would be gradual and regular.
It is not impossible that the variability may be in some way connected
with the action of a body revolving round the star. This seems to be
the case with Eta Aquilæ. The radial motion of this object shows the
existence of a dark body revolving round it in the same period as that
of the star’s variation.
From what has been said, it will be seen that, although a sharp line
cannot be drawn, there seems to be some distinction between the stars
of short and long periods. The number of stars which have been known to
belong to the first class is quite small, only about fifteen, all told.
On the other hand, there are still left some stars having a period less
than ten days, which are otherwise not distinguishable from the Omicron
Ceti type. It seems quite likely that the variations in the periods of
these stars are, in some way, connected with the revolution of bright
or dark bodies round them.
They also vary more widely than those of stars of the other two types.
This might easily happen in the case of stars really variable through
a cycle of changes going on in consequence of the action of interior
causes.
The periodic stars of short period, which have not been recognized as
of the Algol or Beta Lyræ type, form an interesting subject of study.
Although the separation between them and the stars of long period is
not sharp, it seems likely to have some element of reality in it. But
no conclusions on the subject can be reached until the light-curves of
a large number of them are carefully drawn; and this requires an amount
of patient and accurate observation which cannot be carried out for
years to come.