The evening times., 1896-04-16

blue end of Tie spectrum, while If it is re-
| ceding they are displaced in the opposite
direction. By careful measurements on
the lines one can therefore find out whether
a star Is moving to ward or frcm us, and how
| fast. Now for the application:
DR.. Vogel of Potsdam in Germany first
succeeded in making the necessary obser-
vations in 1889. He arranged an instru-
ment with which he photographed the spec
trum of the star In all stages of its revo-
luton. and by measuring the photographs
| under powerful microscope he showed
that about seventeen hours before the
minimum the bright star is rece,iing from
us at the tremendous rate OF twenty-six
and half miles a second. while seventeen
hours after the minimum it is rushing
toward us at the same speed. We know,
then, the circumference of the circle on
which the star moves by simply multiplying
the number of seconds in sixty mine hours
by its velocity OF twenty-six and one half
miles second; and all the other dimen-
sions OF the figure follow easily from this.
Here are vogel's results. which, however,
are not to be considered absolutely exact.
but only as near the truth as We can get
at present, and probably very nearly right,
the bright star being designated as A and
the dark body as B:
Distance between centers
or A and B...--...... O23, 900 miles
Diameter or A..............1. .O6O, ,OOG miles
Diameter or B.......-.--.-. $50 900 miles
Velocity OF A. % IL miles per second.
Velocity of B. 90 miles per second.
WHAT is KNOWN.
Ac.or1ino t,t this A 10 ,op.;J.-obl,.
than the sun. a nd B is trifle smaller; but
from the motion OF the bodies it can he
shown that their united mass is only about
two-thirds that of the sun. and their density
only one-fourth as great; so that they are
much lighter than water and cannot be
solid bodies. hut only heavy ciouds-A be-
ing much more brilliant than the sun. while
B. for some reason not yet determined, is
dark.
Algol is the only fixed star in all the beav-
ems whose size we know; as for all the rest
We Can only guess at their real ma gnitude
But this is not all the strange story of our
star. Observation has nladeit plain that the
interval bet ween its minina has been SLOWLY
changing for the past hundred years. first
lengtbening. but later shortening so that
at present it is about six seconds shorter
than il was in Goodricke's time.
Mr.. Chandler Of Cambridge has sho wn that
this effect runs through its changes in about
130 years. and can be explained by suppos-
ing that Aleol and its companion are both
moving together in 4 great orbit al large as
that of the planet Uranus under the attrac-
tion of some invisible body that controls the
movement.
There may be. however. some doubt as to
this explanation. since Tisserand one of the
abiest ofthc French astronomers. sbo wsthat
4 very different one is possible, and, per-
baps. more probable.
Algol will always be memorable in the his
tory Of astronomy as the first variable star
whose changes were thoroughly studied and
explained: as being the first
star whose size has been measured and as
having first made certain the existence OF
stars dark and to us invisible but intimate
" connected with brightly luminous part
ners.
There are about n dozen other variable
stars which undergo changes of the same
sort as Algol's but they are mostly faint,
and it may be long before We shall be able
to deal with them in an equally satisfactory
manner.