is this huge sun of ours, moving with its planet family through the
regions of the Milky Way, where are to be found not only moving
clusters and groups of stars, speeding along their way in obedience
to the laws of motion of the system to which they belong, but also
strangely formed nebulæ covering vast stretches of space, whirling
and seething internally and shining with mysterious light, and still
other stretches of dark obscuring matter shutting off the rays of
suns beyond.
The extent and form of this enormous system of stars and nebulæ and
the laws that govern the motions of its individual members are among
the problems that the astronomers of today are attempting to solve.
On both sides of these regions of the Milky Way, wherein lies our
own solar system, lie other vast systems, such as the globular star
clusters, composed of thousands, possibly hundreds of thousands, of
suns; the Magellanic clouds, which resemble detached portions of the
Milky Way, and, probably, the much discussed spiral nebulæ, possible
"island universes" similar to our own.
We have come far in the past three hundred years from the conception
of an immovable earth at the center of the universe to this
awe-inspiring conception of the universe that we have today, which is
based upon modern astronomical discoveries.
Whatever may be discovered in the future in regard to the form and
extent of the universe the idea of a fixed and immovable center
either within the solar system or among the stars beyond has gone
from the minds of men at last.
[Illustration: SPIRAL NEBULA IN ANDROMEDA VIEWED EDGEWISE
Taken with 60-inch Reflector of the Mt. Wilson Observatory]
Not more than a generation ago a survival of the old idea of a
fixed center was seen in the belief that Alcyone, in the Pleiades
was a "central sun" about which all the stars revolved. It is now
well known that the Pleiades form a moving star cluster. Alcyone is
therefore drifting slowly onward through the universe and the idea
of a fixed and immovable center to which man may anchor his ideas is
drifting away also. There are, it is true, local centers of systems,
such, for instance, as the sun occupies in the solar system or some
group of stars may occupy in the stellar system to which our sun
belongs, yet _as a whole_ these systems move on and their centers
with them. There is no evidence today that any absolutely immovable
point exists in the heavens.
No celestial object has been found to be without the attribute of
_motion_, not only motion _onward_ through the universe, but also
_rotational_ motion about an axis of the body. The planets rotate on
their axes as well as revolve about the sun, and the sun also turns
on its axis as it moves onward through space. This rotational motion
is also found in the nebulæ and star clusters as well as in the stars
and planets. No object in the heavens is known to be without it. Even
the slowly drifting Orion nebula possesses a rapid internal velocity
of rotation. There is no such thing as a body absolutely at rest in
the universe.
TABLE
Showing the number and relative size, velocity and distribution of the various types
of celestial objects.
================+====================+=====================+===========+=====================
| | |Velocities |
Object | Number | Diameter | miles | Distribution
| | | per sec. |
----------------+--------------------+---------------------+-----------+---------------------
1. Solar System | | | |
| | | |
a. Planets |Eight |3,000 to 88,000 mi. |3 to 35 |Revolving in nearly
| | | miles | circular orbits
| | | per sec. | about the sun.
b. Sun | |864,000 mi. |12-1/2 mi. |Travelling through
| | | | galactic systems of
2. Stars | | | | stars (Milky Way).
| | | |
a. Helium | | |8 mi. |
(bluish) | | | |
b. Hydrogen |Approx. | Dwarfs |14 mi. |All types of stars are
(white) | 2,000,000,000 (Two | 500,000 to | | more or less crowded
c. Solar | thousand million) | 1,000,000 mi. |18-19 mi. | toward plane of Milky
(yellow) | | | | Way in lens shaped
d. Type M |Including all types | Giants |21 mi. | formation. (Milky
(red) | | 10,000,000 to | | Way possibly a spiral
| | 400,000,000 mi. | | nebula.)
3. Nebulæ | | | |
| | | |
a. Diffuse or |Numerous |Very extensive, many |Very low |In or close to Milky
Gaseous | | light years. | | Way.
| | | |
b. Spiral |Approx. 700,000 |Size and distance |Average |Far external to Milky
| (seven hundred | doubtful but | 480 mi. | Way and numerous
| thousand) | very great. | | near its poles.
| | | |
c. Planetary |One hundred and |Several times that |Average |In or close to Milky
| fifty (150) | of the solar system | 48 mi. | Way.
| | on the average. | |
| | | |
4. Globular Star|About one hundred |22,000-220,000 |Very high |External to Milky Way
Clusters | known | light-years. | | and spherically
| | | | distributed about it.
| | | |
5. Magellanic |Two (Greater and |Thousands of |Very high |Far beyond Milky Way.
Clouds | Lesser) | light-years. | |
================+====================+=====================+===========+=====================
XXVIII
THE EVOLUTION OF THE STARS--FROM RED GIANTS TO RED DWARFS
The most casual of star-gazers is aware that the stars differ one
from another in color and in brightness. There are red stars, yellow
stars, white stars and bluish-white stars. There are the brilliant
stars of first magnitude such as Vega, Capella and Antares, and
there are, on the other hand, stars so faint that they can barely be
glimpsed with the most powerful telescopes.
In general the most brilliant stars are the nearest and the faintest
stars are the most distant, but there are many exceptions to the
rule, since there are stars that appear faint even when comparatively
near because they are small and shine with a feeble light. Such a
star is the faint, sixth-magnitude star, 61 Cygni, one of the nearest
of all the stars. Again, there are stars in far-distant clusters
visible only in powerful telescopes that in actual brightness exceed
our own sun several thousand times and in volume several million
times. A star the size of the sun would be invisible in the most
powerful telescope in existence if it were at the distance of many
stars in the Milky Way or globular star clusters.
Stars differ in color because they differ in temperature. We are
all aware of the fact that a piece of iron when heated first glows
a deep red, then appears yellowish in color and finally attains to
white heat. It is the same among the stars. The red stars are the
coolest of all the stars and the bluish-white stars are the hottest
of all the stars, while intermediate between them in temperature come
the yellow and the white stars.
Now as the biologist and the geologist see in this world of ours
signs of evolution, or gradual development and change from the simple
to the more complex forms, and of growth and decay, so the astronomer
sees among the stars signs of a continuous, progressive development
from one type of star to another. Stars share in the general
evolution that is the law of the universe, and are born, reach the
height of their development, decline to old age and die.
Within the past few years important astronomical discoveries have
been made that show the true order of this evolution of the stars. It
was believed not so long ago that the blue-white helium stars--the
type B stars the astronomers called them, or the Orion stars,
since there are so many stars of this type in the constellation of
Orion--were not only the hottest but also the youngest of the stars
and that they represented the first stage in the development of a
star from a primitive gaseous nebula such as the Great Orion Nebula.
It is now known that these brilliant, hot helium stars represent the
peak of development of the most massive of all the stars and not the
first stages in the development.
A star, it is now known, comes into existence as a giant, reddish
star of enormous size and of a density only about one-thousandth that
of the earth's atmosphere at sea-level. It is inconceivably tenuous
or rare, and its temperature is comparatively low, about 3,000°
Centigrade or less. It is not evolved from the luminous, gaseous
nebulæ because red stars are never found associated with the gaseous
nebulæ, as are the blue-white stars. The origin of these red giant
stars is uncertain, but it is possible that they may be gradually
evolved in some manner from the dark clouds of obscuring matter or
dark nebulæ that exists so abundantly in the heavens.
In the next stage of its development the deep-red giant star
increases in temperature as it contracts under the action of
gravitation and its color gradually changes from red to yellow. Its
density increases slightly and its volume decreases. It is now a
yellow giant star. As the evolution progresses in the course of ages
the star continues to contract, its temperature increases greatly as
does also its density and it continues to decrease in volume. It is
now a brilliant white star, a hydrogen star, so called because its
spectrum is chiefly characterized by the lines of hydrogen.
As the star contracts under the gravitation of its parts and
increases in temperature and density there comes more and more into
play an important factor that has a great effect upon its future
development. This is light-pressure or radiation pressure which acts
in opposition to gravity and exerts a strong outward pressure upon
matter within the depths of the star, tending to push it outward from
the center where the temperature is greatest and the light is most
intense. It is a most interesting fact that if the mass of a star,
that is the quantity of matter that it contains, exceeds a certain
value the pressure of light or radiation within it overbalances the
gravitational attraction that draws matter towards its center and
the star disintegrates or ceases to exist as a star. This accounts
for the fact that the stars differ very little among themselves in
the quantity of matter that they contain, that is, in their masses,
though they may differ enormously in size. Stars that exceed a
certain mass will become unstable and this may account for the
association of luminous nebulæ with the hottest of all stars, the
nebulæ possibly being puffed off from the surfaces of these stars
under the action of radiation pressure.
After a star has reached the height of its development as a
bluish-white helium star with a temperature of something like 10,000°
Centigrade and a density about one-tenth that of the sun, it begins
to lose heat and to cool gradually though it continues to contract
and increase in density.
It is now on the descending scale of evolution and is to be counted
among the dwarfs instead of the giants. A brilliant blue-white helium
or Orion star is about one hundred times more luminous than the sun,
and its diameter is about ten times that of the sun.
Our own sun, we find, is on the descending scale of stellar
evolution. It is a yellow dwarf star of temperature about 6,000°
Centigrade and density one and one-fourth that of water, which is
probably about as great a density as is attained by any star since
even the non-luminous planets Jupiter and Saturn have lower densities
than the sun.
The last stage in the development of a star is represented by the
dwarf red star of high density and low temperature. The diameter of
the dwarf red star probably averages about five hundred thousand
miles and its temperature is 3,000° Centigrade or less. After this
we have the extinct stars, similar possibly to our planet Jupiter,
though considerably larger, with a dense gaseous atmosphere and a
certain degree of internal heat.
We have traced the evolution of a star from a red giant to a red
dwarf through the intermediate stages from yellow giant to a giant
helium star with increasing temperature and thence to yellow dwarf
and red dwarf as the temperature decreases. Only the most massive
stars pass through this entire chain of evolution. Stars of small
mass never attain to the splendor of brilliant blue-white helium
stars, but begin to decrease in temperature and brightness before
this stage is reached.
The time required for the evolution of a star from red giant to red
dwarf is not known, but it must be very great. The age of the earth,
which is probably equal to that of the solar system, is estimated as
something like one thousand million years. It is probable that the
average life of a star far exceeds this limit.
XXIX
DOUBLE AND MULTIPLE STARS
The plan of the solar system which consists of a central sun
encircled by satellites that are far inferior to their luminary
in size, and that move about it in orbits that are almost perfect
circles, is not the only, nor possibly, even the most general one in
the universe.
Sweeping the heavens with powerful telescopes one is astonished
to find that myriads of stars can be separated into two or more
physically connected suns that are often, moreover, of exquisitely
tinted and contrasting shades. Green and red, orange and blue, white
and golden or white and blue pairs exist in profusion, and strange to
say there are well-authenticated instances of color changes taking
place temporarily within the same system. A pair of white stars has
been known to change within a few decades, first to golden yellow and
bluish green and then to orange and green. The famous pair catalogued
as "95 Herculis" was noted to change from green and red to a palish
yellow and back to the original strongly contrasting hues within
the course of a single year, while at another time they appeared to
be a perfectly white pair. At the present time both of these stars
are decidedly yellowish in color. Such changes in hue are probably
due to temporary disturbances in the atmospheres of the stars,
possibly of an electrical nature or to sudden or unusual outbursts of
activity, concerning the origin of which we are as much in doubt as
we are of the cause of the sun-spot cycle and periodic variation in
the intensity of radiation of our own sun. Temporary changes in the
color of the components of double star systems sometimes take place
when the two stars approach their "periastron" or point of nearest
approach. Owing to the great eccentricity of the orbits of double
stars, such stars are anywhere from twice to nineteen times as near
to each other at periastron as they are at "apastron," or point of
greatest departure. Such great changes in the relative distances
of two physically connected suns would produce marked changes in
the intensity of the tides raised upon each of them by their mutual
gravitational attraction and unusual outbursts of gases or electrical
excitement in the atmospheres of the stars might cause very
noticeable changes in the color of these stars as they drew nearer to
each other, which would subside as they receded toward apastron.
In addition to "visual" double or multiple stars, there exists a very
extensive class of stars known as "spectroscopic binaries," in which
the two components are so close to each other that even the most
powerful telescopes cannot divide them. It is only from the shifting
of the lines of their overlapping spectra, caused by their alternate
motion toward and from the earth as they revolve about their common
center of gravity, that their duplex nature is revealed to us.
In some instances one member of the system is so faint that its
spectrum is not visible and its presence is disclosed only by the
shifting of the lines of the bright star.
According to Doppler's Law, when a star is approaching the earth the
lines of its spectrum shift toward the blue end of the spectrum,
and when the star is receding from the earth the lines are shifted
toward the red end of the spectrum. The amount of this shift can
be very accurately measured, and gives the relative velocities of
the stars in their orbits directly in miles per second. Knowing in
addition, by observation, the period of mutual revolution of the
stars, it is possible to find the dimensions of these spectroscopic
binary systems compared to our own solar system, and also the masses
of the stars compared to the mass of our own sun. If the spectrum of
the fainter star is not visible, only the velocity of the brighter
star with respect to the center of gravity of the system can be
found and the mass found for the system comes out too small. In such
cases we can obtain only a lower limit for the mass of the system.
Then, too, it must be remembered that these systems of stars lie at
all angles with reference to our line of sight, and so we rarely
see the orbits in their true form. The measured velocities are as a
result smaller than the true velocities, and on the average amount to
only sixty per cent. of the true orbital velocities. The calculated
masses of spectroscopic binary stars are, therefore, in general only
about sixty per cent. of the true masses. It has been found from
calculating the masses of a number of binary systems, that the
combined masses of the stars in these systems do not differ very
greatly among themselves, nor as compared to our own sun, though in
light-giving power these stars may differ hundreds, thousands, even
millions of times. For instance, there are stars that give only one
ten-thousandth part of the light of our own sun, and other stars that
give ten thousand times as much light as the sun. Moreover, there
are many instances of physically connected stars differing thousands
of times in luminosity, though in mass, or quantity of matter found
in the stars, they differ only two or three times. Why this is so
remains one of the great mysteries of the heavens, and makes it
extremely difficult to give any satisfactory theory of the origin
of double-star systems. It has never been explained satisfactorily
why of two suns physically connected and, therefore, presumably
originating at the same time, one should be radiating with the
greatest intensity, while the other is practically an extinct sun,
in spite of the fact that the quantity of matter in the two bodies
differs but slightly.
In a few systems the plane in which the stars revolve passes so
nearly through the earth that the two stars temporarily eclipse one
another during each revolution. Such systems are called _eclipsing
binaries_. To such a system belongs the famous _Algol_. Its light
waxes and wanes periodically with the greatest punctuality in a
period of 2^d 20^h 48.9^m, owing to its temporary eclipse by a very
large but extremely faint attendant sun. The diameter of the faint
star is slightly greater than the diameter of the bright star
which is about one million miles in extent. The distance between
the _centers_ of the stars is only about three million miles, which
brings their surfaces within two million miles of each other. The
masses of the two stars are in the ratio of two to one, the brighter
and more massive star being about half as heavy as our own sun,
though its density is only about two-tenths that of the sun. The
density of the fainter star is still less, being only about half
that of the brighter star. Very low density of both components and
extreme faintness of one member compared to the other, appears to be
a very general characteristic of closely associated eclipsing and
spectroscopic binary stars. Among the extremely hot and brilliant
helium and hydrogen stars, spectroscopic binaries exist in great
numbers. In fact, among these types there appear to be as many binary
and multiple systems as there are systems of isolated suns. Sometimes
these close binary stars are egg-shaped or oval and revolve rapidly
almost in contact about their common center of gravity. Inhabitants
of satellites of such a system would see in their heavens the, to us,
strange and startling phenomenon of _two_ suns, each equal to our
own or even greater in size, whirling rapidly about each other and
separated by a space comparable in extent to their own diameters.
Eclipses in such a system would be of daily occurrence, and, if
one star were dark, would produce for the satellite world the same
effect of alternate day and night that results from axial rotation
of a satellite. The two hemispheres of the faint companion sun
would be very unequally illuminated owing to the fact that the side
turned toward the brilliant sun would always reflect its neighbor's
brightness in addition to shining with its own comparatively feeble
inherent light, while the opposite hemisphere would shine only by its
own dim light, and would, therefore, be in comparative darkness.
The spectroscopic binaries generally revolve closely and rapidly
about their common center of gravity; there are to be found, on the
other hand, among the wider visual doubles, many systems wherein the
components are separated by distances comparable to the distances of
the outer planets, Saturn, Uranus and Neptune, from the sun. It is
evident that the individual stars of such binary systems could not
possibly be encircled by any such extensive system of satellites as
attends our own sun, though satellites such as our own planet Earth,
or the inferior planets Mercury and Venus, might conceivably encircle
the individual components of such binary systems at distances not
greater than that of the earth from the sun. No planet could safely
exist at a much greater distance from one of these suns without being
subject to most dangerous perturbations and disruptive tidal forces
arising from the vicinity of the second sun. Granted that planets
might encircle one of these suns at a distance approximating that of
Venus or our own planet from the sun, the inhabitants of such worlds
would behold the strange phenomenon of _two_ suns in the heavens,
not almost in contact as in spectroscopic binary systems, but at
one time comparatively near and again in opposite portions of the
heavens as is the case with the sun and moon in our own heavens. As
the planet advanced in its orbit about the ruling sun, the secondary
sun would be visible at first by day and again by night. If the two
suns were of contrasting hues, as, for instance, green and red, there
might appear in the nearby heavens at a distance of one hundred
million miles or so a magnificent sun of deep reddish hue, equal to
or surpassing our own in splendor, while in a far distant part of
the sky, at a distance as great as that which separates us from the
planet Saturn, might appear a rival sun of greenish hue, smaller and
fainter, but nevertheless, hot and extremely brilliant and capable of
exerting through its great gravitational attraction a most disturbing
effect upon the motion of the planet of its neighbor. At times the
rays of the two suns, red and green, would combine to produce a day
characterized by terrific heat and intense illumination. Again the
green orb would rise in the east as the red sun set in the west and
night would be turned into a weird, dimly-lighted day by the greenish
rays of the secondary sun. Compared to the wonders and beauties of
the heavens in such a system, our own well-regulated and orderly
planet family, undisturbed by the exciting proximity of a rival sun,
seems to pale into insignificance. Yet we have every good reason to
be content with the ordering of affairs within our own solar system,
and to feel relief rather than regret at the absence of a secondary
sun. In a planet world revolving about one member of a double star
system, we may imagine the dread rather than pleasure with which the
periodic near-approach of a rival sun would be hailed, and even the
possible hurried migration from exposed to sheltered portions of the
planetary world to escape the rapidly increasing heat and intensity
of light from the approaching sun. In such systems the coming and
going of the seasons might indeed be a matter of life and death to
the inhabitants of satellite worlds!