The times., 1899-10-01
THE THREE NORTH STARS.
The Wonderful Results of Spectro-
scopIc nBSerVnfion.


companion that owners of small telescopes
have often used il to test the power of their
eyes and instruments. News has lately
come from Lick Observatory az Mt. Hamii-
ton, Cal. of the discovery that PolaFis con-
sists OF three stars two OF which revolve
around common centre Of gravity in
about four days At the same time these
two revolve around third star in much
the same way as the earth and moon re-
yolve around the sun. In this case the test
Is so severe that no telescope can see the
individual stars. We can only infer their
existence by noticing peculiar phenom-
enon in the spectrum of Polaris.
The stars. and. in fact all luminous
bodies. emit series of waves which are
transmitted by the ether to US. Those
waves that fall upon the retiua Of our
eyes produce sensation that we call
light In the case OF many millions OF
stars the effect OF these waves upon our
4 eyes IS tnappreciable. Without the aid OF
some instrument such stars are Invisible.
The lens OF a telescope condenses a large
area CT waves into a small compass and
! this increases the effect upon the eyes OF
+ the observers. Consequently telescopes
+ with large lenses can gather IN SO. much
I light as to make exceedingly faint stars
I visible TO the observer.
But light has another characteristics be-
sides intensity-that is. color When a GER
Z
second We call the resulting sensation red
When a larger number enter each second
we call the sensation violet etc II the
\ light gathered by some large telescope
passes through a prisp. or. better. several
prisms, the differend colors are arranged
in the order of theii wave length. Suer
/ an arrangement of cfiors, has been named
/ the spectrum.
The spectrum of the sun and of most OF
the stars is crossed 6Na number Of fine
dark lines These lines are really gaps
caused by the absence of rays OF light OF
certain wave lengths. The light before
| 1t can reach our telescope. has to pass
| through an extensive solar Atmosphere
which absorbs certain parts. The gaps
I in the spectrum caused by atmospheric
! absorption of the sun and stars are called
lines.
When a luminous body is at rest the
waves of light are emitted in all directions
alike. But when it is moving rapidly the
waves in front are crowded while those
in the rear are correspondingIy Iengthen-
/ ed. OF course, at the sides the waves con-
! tinue to be emitted al their normal wave
length. The same phenomenon is exhib-
tied in sound. IL an express train whistles
as 1t passes the pitch of the whistle is
/ pereeptibly lower when the train is re-
eeding than when it is approaching There
fore, when the star is approaching the
earth with sufficiently large velocity all
the lines are displaced a trile toward the
violet that being the color with the short
est wave length. Conversely. when the
star is Teeeding from the earth the lines
! are displaced toward the red end of the
I spectrum. This displacement is so ex-
ceedingly slight that measurements of its
amount have had little value until recent
ly. The first successful investigation On
the motion in the line of sight was upon
the rotation of the sun by Vogel in 1871
Since then the spectTa have been photo
graphed and measured. In this way dis
placements corresponding 10 a velocity
considerably less than a mile per second
can be detected.
If two stars are so close together that
they appear in the telescope as a single OBJ
iect, the spectra will be superimpCsed. So
long as these two stars move away from
I US, or toward us at the same rate the lines
in this compound spectrum Will continue
| to maintain the same relative positions
But if the two components OF double star
move in opposite directions in such a way
that one Is approaching while the ether is
reeeding from the earth, then the lines of
the two superimposed spectra will be dis
placed in opposite directions. Sometimes
only one Of the cOmponents OF binary
star is bright enough to be seen As this
I star revolves around the centre of gravity
! between il and its dark companion it is
| sometimes approaching and sometimes re-
! ceding from the earth The lines of its
spectrum are therefore displaced first tC-
ward the violet and then toward the red
end of the spectrum. Hence when star
| is found whose motion in the line OF a
sight is variable. it is iustifable TO con-
clude that there is close to 1t a fainter
companion The period of revolution Of
such bodies about their centre OF gravity
is, OF course, obtained directly from the
observations
In 1887 a photograph taken at the Har-
ward College Observatory OF the spectrum
of Zeta Ursae Majoris (the middle star in
the handle of the Dipperl was found TO con-
tain several double lines. An examination
of the photographs showed that the lines
are sometimes double and sometimes
4 single Evidently We have here a double
star whose eOmponents are so close TO-
gether that no telescope can show them as
double stars. They are revolving around
common centre OF gravity in such plane
that when one is approaching the other
is receding from the earth. Twice during
every revolution they are nearly in a line
with the earth. They are then neither ap-
preaching nor receding. the line OF their
spectra are in juxtaposition and appear
single.
Beta Persci. named Algol fu nisAes an
example of a binary star where on y one
of the components is visible. Nogel. at
Potsdam in 1888. found f-Om the cisplace-
ment CT the lines in the spsct- um that Al
col is moving in a small orbit at the ia
| of tyenty-six miles per second To az
count for this ii is necessary to assume
that there is dark body, and that Algol
and its companions are moving around
common centre OF gravity In this in
stance the existence Of the dark cOmpan-
ion is still further demonstrated by the
! variation in AIgol's light GoodTicke in
1872 discovered that Algol was a variable
star. Ii is usually constant in brightness,
hut at times it will decrease in light for
few hours and then in a few 10 rs more
regain its normal brightness. T.. ese mini
ma in light occur with great regularity,
the period being little less than seventy
hours. Such phenomenon naturally sug-
gests a partial eclipse caused b. some DARK
body periodically passing betwe- Us and
the star.
By means Of the enormous iight-sather-
ing power Of the Lick telescope (thirty-six
inches aperture) Prof. Keeier has been able
to measure the velocity in the line OF sight
of the Great Orion Nebula. He finds that
it is receding from the earth at the rate
Of about eleven miles per second While
at the Allegheny Observatory Prof. Keeler
used this same spsctToscopie method TO
prove that Saturn's rings are neither solid
nor liquid. They coasist of small bodies
that revolve around Saturn
SpeCtroscOpic determinations Of motion II
| the line OF sight is one OF the most prom
ising fields Of investigation in astronomy.
Many problems have been solved that fer