+---------------+----------+----------+------------+
| Name. | Maximum | Minimum | Period. |
| |Magnitude.|Magnitude.| |
+---------------+----------+----------+------------+
| | | |Days. Hours.|
|RS Sagittarii | 5.9 | 6.3 | 2 9 |
|R Canis majoris| 5.8 | 6.4 | 1 2 |
|δ Librae | 4.8 | 6.2 | 2 7 |
|R Arae | 6.8 | 7.9 | 4 9 |
+---------------+----------+----------+------------+
Altogether nearly a hundred Algol stars are now known, and seventy-four
of these lie in or near the Milky Way.
Unlike the mysterious Mira stars, the variation of Algol stars has been
explained. The sudden drop in the light is a partial eclipse, caused
by a dark or partly dark companion which for a time hides the bright
star from us. When a source of light is coming towards us, the lines in
its spectrum are shifted towards the violet, when going away they are
shifted towards the red, exactly as the whistle of an engine becomes
more shrill when approaching us, and falls to a lower pitch when
going away. In this way it has been discovered that an Algol star is
revolving round an invisible companion, for it alternately approaches
and recedes, and these movements correspond with its light-changes.
It is, in fact, a spectroscopic binary which happens to have an orbit
whose plane lies just in our line of sight, so that at every revolution
one star passes behind the other.
The speed of the star in its orbit can be accurately determined (by
the amount of shift in the spectrum lines) in miles per second, even
when we do not know its distance from us; hence the size of its orbit
can be calculated, since we know the period in which it is completed;
and, further, the size of the orbit gives us the mass of the stars,
for their movements depend upon the attraction they exercise over one
another, and this is proportional to their mass; and so we are able to
picture the system, although the eclipsing star is never seen and the
distance from us may never be known. Here is indeed a triumph of modern
astronomy.
Very curious are the systems thus discovered. Algol stars are
extraordinarily light for their size, their density being always less,
and sometimes immensely less, than that of water, and the companions
are usually extraordinarily close together. In some pairs they seem to
be actually touching. Nearly all are Sirian stars; a few are of Orion
and solar types.
Sometimes the companion star gives light also, instead of being dark,
and then we have a different type of variation. There are two eclipses
in one revolution, each star passing alternately behind the other, but
neither is a very dark eclipse, only a lessening of light. β Lyrae was
the first star of this kind to be discovered; its southern counterpart
is U Scuti, which varies from magnitude 9.1 to 9.6, and runs through
its two maxima and two minima in less than twenty-three hours! It is a
Sirian star.
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