+----------------------+----------+----------+-------+
| Name. | Maximum | Minimum |Period.|
| |Magnitude.|Magnitude.| |
+----------------------+----------+----------+-------+
| | | | Days. |
|W Sagittarii | 4.3 | 5.1 | 7½ |
|κ Pavonis | 4.3 | 5.2 | 9 |
|_l_ Carinae | 3.6 | 5.0 | 35½ |
|S Crucis | 6.5 | 7.6 | 4½ |
|V Centaurii | 6.4 | 7.8 | 5½ |
|S Trianguli australi | 6.4 | 7.4 | 6½ |
|S Normae | 6.6 | 7.6 | 9¾ |
+----------------------+----------+----------+-------+
The three first on the list have been discovered to be spectroscopic
binaries, the motions varying with the light, and as this is found to
be the case with all those whose motions are known, there can be little
doubt that all variables of this class are binaries. Their orbits,
however, are somewhat larger than those of the Algol variables, and are
tilted towards us so that neither star can be seen to pass in front
of the other. For the gradual and continual change of light cannot
possibly be caused by an eclipse. Yet it seems clear that it is in some
way connected with the revolution of two stars about one another.
Various suggestions have been made, such as that the revolving star
is unequally bright over its surface, and shows us now its brighter
and now its duller face; that the two pull one another out of shape
when they approach most nearly; that they are permanently elliptical,
and turn to us first the broad and then the narrow face. Any or all
of these may play some part in the variations of the light. But the
most hopeful theory takes into account a very strange fact lately
established, viz. that the variable is at its brightest not when
approaching its fellow but when coming directly toward us, and at its
faintest when receding directly from us.
This theory is that a dark and bright star are involved in a kind
of thin nebulous cloud, and that the bright revolving star has an
enveloping atmosphere. As it moves through the cloud this atmosphere
is continually brushed back from its advancing face, and therefore we
shall see it when coming straight towards us through a less thickness
of atmosphere, and it will look brighter than when it is retreating
from us.
There are grave difficulties in accepting this view, but it receives
some support from the case of β Scorpii, in which, as we saw, a double
star is suspected, for quite a different reason, of being surrounded by
a thin cloud. And the atmosphere supposed to be surrounding the bright
star may resemble the “smoky veil” which we know envelops our own sun,
and causes a considerable absorption of light, for these variables are
solar stars. The resistance of the cloud to the motion of the revolving
star ought in time to shorten its period, and some variables have been
found to be shortening their periods.
Short-period variables are found chiefly in the Milky Way.