No achievement of modern science is more remarkable than the
measurement of the velocity with which stars are moving to or from us.
This is effected by means of the spectroscope through a comparison of
the position of the spectral lines produced by the absorption of any
substance in the atmosphere of the star with the corresponding lines
produced by the same substance on the earth. The principle on which the
method depends may be illustrated by the analogous case of sound. It is
a familiar fact that if we stand alongside a railway while a locomotive
is passing us at full speed, and at the same time blowing a whistle,
the pitch of the note which we hear from the whistle is higher as the
engine is approaching than after it passes. The reason is that the
pitch of a sound depends upon the number of sound beats per second.
A B X
* . . . . . . .
* . . . . . . .
Now, we may consider the waves which form light when they strike our
apparatus as beats in the ethereal medium which follow each other
with extraordinary rapidity, millions of millions in a second, moving
forward with a definite velocity of more than 186,000 miles a second.
Each spectral line produced by a chemical element shows that that
element, when incandescent, beats the ether a certain number of times
in a second. These beats are transmitted as waves. Since the velocity
is the same whether the number of beats per second is less or greater,
it follows that, if the body is in motion in the direction in which it
emits the light, the beats will be closer together than if it is at
rest; if moving away they will be further apart. The fundamental fact
on which this result depends is that the velocity of the light-beat
through the ether is independent of the motion of the body causing the
beat. To show the result, let A be a luminous body at rest; let the
seven dots to the right of A be the crests of seven waves or beats,
the first of which, at the end of a certain time, has reached X. The
wave-length will then be one seventh the distance A X. Now, suppose
A in motion toward X with such speed that, when the first beat has
reached X, A has reached the point B. Then the seven beats made by A
while the first beat is traveling from A to X, and A traveling from
A to B, will be crowded into the space B X, so that each wave will
be one seventh shorter than before. In other words, the wave-lengths
of the light emitted by any substance will be less or greater than
their normal length, according to the motion of the substance in
the direction in which its light is transmitted, or in the opposite
direction.
The position of a ray in the spectrum depends solely on the wave-length
of the light. It follows that the rays produced by any substance will
be displaced toward the blue or red end of the spectrum, according
as the body emitting or absorbing the rays is moving towards or from
us. This method of determining the motions of stars to or from us, or
their velocity in the line of sight from us to the star, was first put
into practice by Mr.--now Sir William--Huggins, of London. The method
has since been perfected by photographing the spectrum of a star, or
other heavenly body, side by side with that of a terrestrial substance,
rendered incandescent in the tube of a telescope. The rays of this
substance pass through the same spectroscope as those from the star,
so that, if the wave-lengths of the lines produced by the substance
were the same as those found in the star spectrum, the two lines would
correspond in position. The minute difference found on the photographic
plate is the measure of the velocity of the star in the line of sight.
It will be seen that the conclusion depends on the hypothesis that the
position of any ray produced by a substance is affected by no cause but
the motion of the substance. How and when this hypothesis may fail is
a very important question. It is found, for example, that the position
of a spectral ray may be altered by compressing the gas emitting or
absorbing the ray, and it may be inquired whether the results for
motion in the line of sight may not be vitiated by the absorbing
atmosphere of the star being under heavy pressure, thus displacing the
absorption line.
To this it may be replied that, in any case, the outer layers of the
atmosphere, through which the light must last pass, are not under
pressure. How far inner portions may produce an absorption spectrum we
cannot discuss at present, but it does not seem likely that serious
errors are thus introduced in many cases.
These measures require apparatus and manipulation of extraordinary
delicacy, in order to avoid every possible source of error. The
displacement of the lines produced by motion is in fact so minute that
great skill is required to make it evident, unless in exceptional
cases. The Mills spectrograph of the Lick Observatory in the hands of
Professor Campbell has, notwithstanding these difficulties, yielded
results of extraordinary precision. Quite a number of investigators
at some leading observatories of Europe and America are pursuing the
work of determining these motions. The determinations have almost
necessarily been limited to the brighter stars, because, owing to the
light of the star being spread over so broad a space in the spectrum,
instead of being concentrated on a point, a far longer exposure is
necessary to photograph the spectrum of a star than to photograph
the star itself. The larger the telescope the fainter the star
whose spectrum can be photographed. Vogel, of Potsdam, who has made
the most systematic sets of these measures that have yet appeared,
included few stars fainter than the second magnitude. With the largest
telescopes the spectro of stars down to about the fifth magnitude may
be photographed; beyond this it is extremely difficult to go. The limit
will probably be reached by the spectrograph of the Yerkes Observatory,
which is now being put into operation by Professors Hale and Frost.