THE PHYSICAL NATURE OF THE STARS.
Star Spectroscopes--Classification of Stellar Spectra--Type I.,
with very Few Absorption Lines--Type II., like the Sun--Type III.,
with Strongly Marked Dark Bands--Distribution of these Classes over
the Heavens--Motion in the Line of Sight--Orbital Motion Discovered
with the Spectroscope: New Class of Binaries--Spectra of Temporary
Stars--Nature of these Bodies.
We have frequently in the previous chapters had occasion to refer to the
revelations of the spectroscope, which form an important chapter in the
history of modern science. By its aid a mighty stride has been taken in
our attempt to comprehend the physical constitution of the sun. In the
present chapter we propose to give an account of what the spectroscope
tells us about the physical constitution of the fixed stars.
Quite a new phase of astronomy is here opened up. Every improvement in
telescopes revealed fainter and fainter objects, but all the telescopes
in the world could not answer the question as to whether iron and other
elements are to be found in the stars. The ordinary star is a mighty
glowing globe, hotter than a Bessemer converter or a Siemens furnace; if
iron is in the star, it must be not only white-hot and molten, but
actually converted into vapour. But the vapour of iron is not visible in
the telescope. How would you recognise it? How would you know if it
commingled with the vapour of many other metals or other substances? It
is, in truth, a delicate piece of analysis to discriminate iron in the
glowing atmosphere of a star. But the spectroscope is adequate to the
task, and it renders its analysis with an amount of evidence that is
absolutely convincing.
That the spectra of the moon and planets are practically nothing but
faint reproductions of the spectrum of the sun was discovered by the
great German optician Fraunhofer about the year 1816. By placing a prism
in front of the object glass of a small theodolite (an instrument used
for geodetic measurements) he was able to ascertain that Venus and Mars
showed the same spectrum as the sun, while Sirius gave a very different
one. This important observation encouraged him to procure better
instrumental means with which to continue the work, and he succeeded in
distinguishing the chief characteristics of the various types of stellar
spectra. The form of instrument which Fraunhofer adopted for this work,
in which the prism was placed outside the object glass of the telescope,
has not been much used until within the last few years, owing to the
difficulty of obtaining prisms of large dimensions (for it is obvious
that the prism ought to be as large as the object glass if the full
power of the latter is to be made use of), but this is the simplest form
of spectroscope for observing spectra of objects of no sensible angular
diameter, like the fixed stars. The parallel rays from the stars are
dispersed by the prism into a spectrum, and this is viewed by means of
the telescope. But as the image of the star in the telescope is nothing
but a luminous point, its spectrum will be merely a line in which it
would not be possible to distinguish any lines crossing it laterally
such as those we see in the spectrum of the sun. A cylindrical lens is,
therefore, placed before the eye-piece of the telescope, and as this has
the effect of turning a point into a line and a line into a band, the
narrow spectrum of the star is thereby broadened out into a luminous
band in which we can distinguish any details that exist. In other forms
of stellar spectroscope we require a slit which must be placed in the
focus of the object glass, and the general arrangement is similar to
that which we have described in the chapter on the sun, except that a
cylindrical lens is required.
The study of the spectra of the fixed stars made hardly any progress
until the principles of spectrum analysis had been established by
Kirchhoff in 1859. When the dark lines in the solar spectrum had been
properly interpreted, it was at once evident that science had opened
wide the gates of a new territory for human exploration, of the very
existence of which hardly anyone had been aware up to that time. We have
seen to what splendid triumphs the study of the sun has led the
investigators in this field, and we have seen how very valuable results
have been obtained by the new method when applied to observations of
comets and nebulae. We shall now give some account of what has been
learned with regard to the constitution of the fixed stars by the
researches which were inaugurated by Sir William Huggins and continued
and developed by him, as well as by Secchi, Vogel, Pickering, Lockyer,
Duner, Scheiner and others. Here, as in the other modern branches of
astronomy, photography has played a most important part, not only
because photographed spectra of stars extend much farther at the violet
end than the observer can follow them with his eye, but also because the
positions of the lines can be very accurately measured on the
photographs.
The first observer who reduced the apparently chaotic diversity of
stellar spectra to order was Secchi, who showed that they might all be
grouped according to four types. Within the last thirty years, however,
so many modifications of the various types have been found that it has
become necessary to subdivide Secchi's types, and most observers now
make use of Vogel's classification, which we shall also for convenience
adopt in this chapter.
_Type I._--In the spectra of stars of this class the metallic lines,
which are so very numerous and conspicuous in the sun's violet spectrum,
are very faint and thin, or quite invisible, and the blue and white
parts are very intensely bright. Vogel subdivides the class into three
groups. In the first (I.a) the hydrogen lines are present, and are
remarkably broad and intense; Sirius, Vega, and Regulus are examples of
this group. The great breadth of the lines probably indicates that these
stars are surrounded by hydrogen atmospheres of great dimensions. It is
generally acknowledged that stars of this group must be the hottest of
all, and support is lent to this view by the appearance in their
spectra of a certain magnesium line, which, as Sir Norman Lockyer showed
many years ago, by laboratory experiments, does not appear in the
ordinary spectrum of magnesium, but is indicative of the presence of the
substance at a very high temperature. In the spectra of stars of Group
I.b the hydrogen lines and the few metallic lines are of equal breadth,
and the magnesium line just mentioned is the strongest of all. Rigel and
several other bright stars in Orion belong to this group, and it is
remarkable that helium is present at least in some of these stars, so
that (as Professor Keeler remarks) the spectrum of Rigel may almost be
regarded as the nebular spectrum reversed (lines dark instead of
bright), except that the two chief nebular lines are not reversed in the
star. This fact will doubtless eventually be of great importance to our
understanding the successive development of a star from a nebula; and a
star like Rigel is no doubt also of very high temperature. This is
probably not the case with stars of the third subdivision of Type I.
(I.c), the spectra of which are distinguished by the presence of bright
hydrogen lines and the bright helium line D3. Among the stars having
this very remarkable kind of spectrum is a very interesting variable
star in the constellation Lyra (b) and the star known as g
Cassiopeiae, both of which have been assiduously observed, their spectra
possessing numerous peculiarities which render an explanation of the
physical constitution of the stars of this subdivision a very difficult
matter.
Passing to _Type II._, we find spectra in which the metallic lines are
strong. The more refrangible end of the spectrum is fainter than in the
previous Class, and absorption bands are sometimes found towards the red
end. In its first subdivision (II.a) are contained spectra with a large
number of strong and well-defined lines due to metals, the hydrogen
lines being also well seen, though they are not specially conspicuous.
Among the very numerous stars of this group are Capella, Aldebaran,
Arcturus, Pollux, etc. The spectra of these stars are in fact
practically identical with the spectrum of our own sun, as shown, for
instance, by Dr. Scheiner, of the Potsdam Astrophysical Observatory, who
has measured several hundred lines on photographs of the spectrum of
Capella, and found a very close agreement between these lines and
corresponding ones in the solar spectrum. We can hardly doubt that the
physical constitution of these stars is very similar to that of our sun.
This cannot be the case with the stars of the second subdivision (II.b),
the spectra of which are very complex, each consisting of a continuous
spectrum crossed by numerous dark lines, on which is superposed a second
spectrum of bright lines. Upwards of seventy stars are known to possess
this extraordinary spectrum, the only bright one among them being a star
of the third magnitude in the southern constellation Argus. Here again
we have hydrogen and helium represented by bright lines, while the
origin of the remaining bright lines is doubtful. With regard to the
physical constitution of the stars of this group it is very difficult to
come to a definite conclusion, but it would seem not unlikely that we
have here to do with stars which are not only surrounded by an
atmosphere of lower temperature, causing the dark lines, but which,
outside of that, have an enormous envelope of hydrogen and other gases.
In one star at least of this group Professor Campbell, of the Lick
Observatory, has seen the F line as a long line extending a very
appreciable distance on each side of the continuous spectrum, and with
an open slit it was seen as a large circular disc about six seconds in
diameter; two other principal hydrogen lines showed the same appearance.
As far as this observation goes, the existence of an extensive gaseous
envelope surrounding the star seems to be indicated.
_Type III._ contains comparatively few stars, and the spectra are
characterised by numerous dark bands in addition to dark lines, while
the more refrangible parts are very faint, for which reason the stars
are more or less red in colour. This class has two strongly marked
subdivisions. In the first (III.a) the principal absorption lines
coincide with similar ones in the solar spectrum, but with great
differences as to intensity, many lines being much stronger in these
stars than in the sun, while many new lines also appear. These
dissimilarities are, however, of less importance than the peculiar
absorption bands in the red, yellow, and green parts of the spectrum,
overlying the metallic lines, and being sharply defined on the side
towards the violet and shading off gradually towards the red end of the
spectrum. Bands of this kind belong to chemical combinations, and this
appears to show that somewhere in the atmospheres of these distant suns
the temperature is low enough to allow stable chemical combinations to
be formed. The most important star of this kind is Betelgeuze or a
Orionis, the red star of the first magnitude in the shoulder of Orion;
but it is of special importance to note that many variable stars of long
period have spectra of Type III.a. Sir Norman Lockyer predicted in 1887
that bright lines, probably of hydrogen, would eventually be found to
appear at the maximum of brightness, when the smaller swarm is supposed
to pass through the larger one, and this was soon afterwards confirmed
by the announcement that Professor Pickering had found a number of
hydrogen lines bright on photographs, obtained at Harvard College
Observatory, of the spectrum of the remarkable variable, Mira Ceti, at
the time of maximum. Professor Pickering has since then reported that
bright lines have been found on the plates of forty-one previously known
variables of this class, and that more than twenty other stars have been
detected as variables by this peculiarity of their spectrum; that is,
bright lines being seen in them suggested that the stars were variable,
and further photometric investigations corroborated the fact.
The second subdivision (III.b) contains only comparatively faint stars,
of which none exceed the fifth magnitude, and is limited to a small
number of red stars. The strongly marked bands in their spectra are
sharply defined and dark on the red side, while they fade away gradually
towards the violet, exactly the reverse of what we see in the spectra of
III.a. These bands appear to arise from the absorption due to
hydrocarbon vapours present in the atmospheres of these stars; but there
are also some lines visible which indicate the presence of metallic
vapours, sodium being certainly among these. There can be little doubt
that these stars represent the last stage in the life of a sun, when it
has cooled down considerably and is not very far from actual extinction,
owing to the increasing absorption of its remaining light in the
atmosphere surrounding it.
The method employed for the spectroscopic determination of the motion of
a star in the line of sight is the same as the method we have described
in the chapter on the sun. The position of a certain line in the
spectrum of a star is compared with the position of the corresponding
bright line of an element in an artificially produced spectrum, and in
this manner a displacement of the stellar line either towards the violet
(indicating that the star is approaching us) or towards the red
(indicating that it is receding) may be detected. The earliest attempt
of this sort was made in 1867 by Sir William Huggins, who compared the F
line in the spectrum of Sirius with the same line of the spectrum of
hydrogen contained in a vacuum tube reflected into the field of his
astronomical spectroscope, so that the two spectra appeared side by
side. The work thus commenced and continued by him was afterwards taken
up at the Greenwich Observatory; but the results obtained by these
direct observations were never satisfactory, as remarkable discrepancies
appeared between the values obtained by different observers, and even by
the same observer on different nights. This is not to be wondered at
when we bear in mind that the velocity of light is so enormous compared
with any velocity with which a heavenly body may travel, that the change
of wave length resulting from the latter motion can only be a very
minute one, difficult to perceive, and still more difficult to measure.
But since photography was first made use of for these investigations by
Dr. Vogel, of Potsdam, much more accordant and reliable results have
been obtained, though even now extreme care is required to avoid
systematic errors. To give some idea of the results obtainable, we
present in the following table the values of the velocity per second of
a number of stars observed in 1896 and 1897 by Mr. H.F. Newall with the
Bruce spectrograph attached to the great 25-inch Newall refractor of the
Cambridge Observatory, and we have added the values found at Potsdam by
Vogel and Scheiner. The results are expressed in kilometres (1 km. =
0.62 English mile). The sign + means that the star is receding from
us,-that it is approaching.
Newall. Vogel. Scheiner.
Aldebaran + 49.2 + 47.6 + 49.4
Betelgeuze + 10.6 + 15.6 + 18.8
Procyon - 4.2 - 7.2 - 10.5
Pollux - 0.7 + 1.9 + 0.4
g Leonis - 39.9 - 36.5 - 40.5
Arcturus - 6.4 - 7.0 - 8.3
These results have been corrected for the earth's orbital motion round
the sun, but not for the sun's motion through space, as the amount of
the latter is practically unknown, or at least very uncertain; so that
the above figures really represent the velocity per second of the
various stars relative to the sun. We may add that the direction and
velocity of the sun's motion may eventually be ascertained from
spectroscopic measures of a great number of stars, and it seems likely
that the sun's velocity will be much more accurately found in this way
than by the older method of combining proper motions of stars with
speculations as to the average distances of the various classes of
stars. This has already been attempted by Dr. Kempf, who from the
Potsdam spectrographic observations found the sun's velocity to be 18.6
kilometres, or 11.5 miles per second, a result which is probably not far
from the truth.
But the spectra of the fixed stars can also tell us something about
orbital motion in these extremely distant systems. If one star revolved
round another in a plane passing through the sun, it must on one side of
the orbit move straight towards us and on the other side move straight
away from us, while it will not alter its distance from us while it is
passing in front of, or behind, the central body. If we therefore find
from the spectroscopic observations that a star is alternately moving
towards and away from the earth in a certain period, there can be no
doubt that this star is travelling round some unseen body (or, rather,
round the centre of gravity of both) in the period indicated by the
shifting of the spectral lines. In Chapter XIX. we mentioned the
variable star Algol in the constellation Perseus, which is one of a
class of variable stars distinguished by the fact that for the greater
part of the period they remain of unaltered brightness, while for a very
short time they become considerably fainter. That this was caused by
some sort of an eclipse--or, in other words, by the periodic passage of
a dark body in front of the star, hiding more or less of the latter from
us--was the simplest possible hypothesis, and it had already years ago
been generally accepted. But it was not possible to prove that this was
the true explanation of the periodicity of stars like Algol until
Professor Vogel, from the spectroscopic observations made at Potsdam,
found that before every minimum Algol is receding from the sun, while it
is approaching us after the minimum. Assuming the orbit to be circular,
the velocity of Algol was found to be twenty-six miles per second. From
this and the length of the period (2d. 22h. 48m. 55s.) and the time of
obscuration it was easy to compute the size of the orbit and the actual
dimensions of the two bodies. It was even possible to go a step further
and to calculate from the orbital velocities the masses of the two
bodies,[41] assuming them to be of equal density--an assumption which is
no doubt very uncertain. The following are the approximate elements of
the Algol system found by Vogel:--
Diameter of Algol 1,054,000 miles.
Diameter of companion 825,000 miles.
Distance between their centres 3,220,000 miles.
Orbital velocity of Algol 26 miles per sec.
Orbital velocity of companion 55 miles per sec.
Mass of Algol 4/9 of sun's mass.
Mass of companion 2/9 of sun's mass.
The period of Algol has been gradually decreasing during the last
century (by six or seven seconds), but whether this is caused by the
motion of the pair round a third and very much more distant body, as
suggested by Mr. Chandler, has still to be found out.
We have already mentioned that in order to produce eclipses, and thereby
variations of light, it is necessary that the line of sight should lie
nearly in the plane of the orbit. It is also essential that there should
be a considerable difference of brightness between the two bodies. These
conditions must be fulfilled in the fifteen variable stars of the Algol
class now known; but according to the theory of probability, there must
be many more binary systems like that of Algol where these conditions
are not fulfilled, and in those cases no variations will occur in the
stars' brightness. Of course, we know many cases of a luminous star
travelling round another, but there must also be cases of a large
companion travelling round another at so small a distance that our
telescopes are unable to "divide" the double star. This has actually
been discovered by means of the spectroscope. If we suppose an extremely
close double star to be examined with the spectroscope, the spectra of
the two components will be superposed, and we shall not be aware that we
really see two different spectra. But during the revolution of the two
bodies round their common centre of gravity there must periodically come
a time when one body is moving towards us and the other moving from us,
and consequently the lines in the spectrum of the former will be subject
to a minute, relative shift towards the violet end of the spectrum, and
those of the other to a minute shift towards the red. Those lines which
are common to the two spectra will therefore periodically become double.
A discovery of this sort was first made in 1889 by Professor Pickering
from photographs of the spectrum of Mizar, or z Ursa Majoris, the
larger component of the well-known double star in the tail of the Great
Bear. Certain of the lines were found to be double at intervals of
fifty-two days. The maximum separation of the two components of each
line corresponds to a relative velocity of one star as compared with the
other of about a hundred miles per second, but subsequent observations
have shown the case to be very complicated, either with a very eccentric
elliptic orbit or possibly owing to the presence of a third body. The
Harvard College photographs also showed periodic duplicity of lines in
the star b Aurigae, the period being remarkably short, only three
days and twenty-three hours and thirty-seven minutes. In 1891 Vogel
found, from photographs of the spectrum of Spica, the first magnitude
star in Virgo, that this star alternately recedes from and approaches to
the solar system, the period being four days. Certain other
"spectroscopic binaries" have since then been found, notably one
component of Castor, with a period of three days, found by M.
Belopolsky, and a star in the constellation Scorpio, with a period of
only thirty-four hours, detected on the Harvard spectrograms.
Quite recently Mr. H.F. Newall, at Cambridge, and Mr. Campbell, of the
Lick Observatory, have shown that a Aurigae, or Capella, consists
of a sun-like star and a Procyon-like star, revolving in 104 days.
At first sight there is something very startling in the idea of two suns
circling round each other, separated by an interval which, in comparison
with their diameters, is only a very small one. In the Algol system, for
instance, we have two bodies, one the size of our own sun and the other
slightly larger, moving round their common centre of gravity in less
than three days, and at a distance between their surfaces equal to only
twice the diameter of the larger one. Again, in the system of Spica we
have two great suns swinging round each other in only four days, at a
distance equal to that between Saturn and his sixth satellite. But
although we have at present nothing analogous to this in our solar
system, it can be proved mathematically that it is perfectly possible
for a system of this kind to preserve its stability, if not for ever, at
any rate for ages, and we shall see in our last chapter that there was
in all probability a time when the earth and the moon formed a peculiar
system of two bodies revolving rapidly at a very small distance compared
to the diameters of the bodies.
It is possible that we have a more complicated system in the star known
as b Lyrae. This is a variable star of great interest, having a
period of twelve days and twenty-two hours, in which time it rises from
magnitude 4-1/2 to a little above 3-1/2, sinks nearly to the fourth
magnitude, rises again to fully 3-1/2, and finally falls to magnitude
4-1/2. In 1891 Professor Pickering discovered that the bright lines in
the spectrum of this star changed their position from time to time,
appearing now on one side, now on the other side of corresponding dark
lines. Obviously these bright lines change their wave length, the
light-giving source alternately receding from and approaching to the
earth, and the former appeared to be the case during one-half of the
period of variation of the star's light, the latter during the other
half. The spectrum of this star has been further examined by Belopolsky
and others, who have found that the lines are apparently double, but
that one of the components either disappears or becomes very narrow from
time to time. On the assumption that these lines were really single (the
apparent duplicity resulting from the superposition of a dark line),
Belopolsky determined the amount of their displacement by measuring the
distances from the two edges of a line of hydrogen (F) to the artificial
hydrogen line produced by gas glowing in a tube and photographed along
with the star-spectrum. Assuming the alternate approach and recession to
be caused by orbital revolution, Belopolsky found that the body emitting
the light of the bright lines moved with an orbital velocity of
forty-one miles. He succeeded in 1897 in observing the displacement of a
dark line due to magnesium, and found that the body emitting it was also
moving in an orbit, but while the velocities given by the bright F line
are positive after the principal minimum of the star's light, those
given by the dark line are negative. Therefore, during the principal
minimum it is a star giving the dark line which is eclipsed, and during
the secondary minimum another star giving the bright line is eclipsed.
This wonderful variable will, however, require more observatioens before
the problem of its constitution is finally solved, and the same may be
said of several variable stars, _e.g._ e Aquilae and d Cephei,
in which a want of harmony has been found between the changes of
velocity and the fluctuations of the light.
There are some striking analogies between the complicated spectrum of
b Lyrae and the spectra of temporary stars. The first "new star"
which could be spectroscopically examined was that which appeared in
Corona Borealis in 1866, and which was studied by Sir W. Huggins. It
showed a continuous spectrum with dark absorption lines, and also the
bright lines of hydrogen; practically the same spectrum as the stars of
Type II.b. This was also the case with Schmidt's star of 1876, which
showed the helium line (D3) and the principal nebula line in addition
to the lines of hydrogen; but in the autumn of 1877, when the star had
fallen to the tenth magnitude, Dr. Copeland was surprised to find that
only one line was visible, the principal nebula line, in which almost
the whole light of the star was concentrated, the continuous spectrum
being hardly traceable. It seemed, in fact, that the star had been
transformed into a planetary nebula, but later the spectrum seems to
have lost this peculiar monochromatic character, the nebula line having
disappeared and a faint continuous spectrum alone being visible, which
is also the case with the star of 1866 since it sank down to the tenth
magnitude. A continuous spectrum was all that could be seen of the new
star which broke out in the nebula of Andromeda in 1885, much the same
as the spectrum of the nebula itself.
When the new star in Auriga was announced, in February, 1892,
astronomers were better prepared to observe it spectroscopically, as it
was now possible by means of photography to study the ultra-violet part
of the spectrum which to the eye is invisible. The visible spectrum was
very like that of Nova Cygni of 1876, but when the wave-lengths of all
the bright lines seen and photographed at the Lick Observatory and at
Potsdam were measured, a strong resemblance to the bright line spectrum
of the chromosphere of the sun became very evident. The hydrogen lines
were very conspicuous, while the iron lines were very numerous, and
calcium and magnesium were also represented. The most remarkable
revelation made by the photographs was, however, that the bright lines
were in many cases accompanied, on the side next the violet, by broad
dark bands, while both bright and dark lines were of a composite
character. Many of the dark lines had a thin bright line superposed in
the middle, while on the other hand many of the bright lines had two or
three points maxima of brightness. The results of the measures of motion
in the line of sight were of special importance. They showed that the
source of light, whence came the thin bright lines within the dark ones,
was travelling towards the sun at the enormous rate of 400 miles per
second, and if the bright lines were actual "reversals" of the dark
ones, then the source of the absorption spectrum must have been endowed
with much the same velocity. On the other hand, if the two or three
maxima of brightness in the bright lines really represent two or three
separate bodies giving bright lines, the measures indicate that the
principal one was almost at rest as regards the sun, while the others
were receding from us at the extraordinary rates of 300 and 600 miles
per second. And as if this were not sufficiently puzzling, the star on
its revival in August, 1892, as a tenth magnitude star had a totally
different spectrum, showing nothing but a number of the bright lines
belonging to planetary nebulae! It is possible that the principal ones of
these were really present in the spectrum from the first, but that their
wave lengths had been different owing to change of the motion in the
line of sight, so that the nebula lines seen in the autumn were
identical with others seen in the spring at slightly different places.
Subsequent observations of these nebula lines seemed to point to a
motion of the Nova towards the solar system (of about 150 miles per
second) which gradually diminished.
But although we are obliged to confess our inability to say for certain
why a temporary star blazes up so suddenly, we have every cause to think
that these strange bodies will by degrees tell us a great deal about the
constitution of the fixed stars. The great variety of spectra which we
see in the starry universe, nebula spectra with bright lines, stellar
spectra of the same general character, others with broad absorption
bands, or numerous dark lines like our sun, or a few absorption lines
only--all this shows us the universe as teeming with bodies in various
stages of evolution. We shall have a few more words to say on this
matter when we come to consider the astronomical significance of heat;
but we have reached a point where man's intellect can hardly keep pace
with the development of our instrumental resources, and where our
imagination stands bewildered when we endeavour to systematise the
knowledge we have gained. That great caution will have to be exercised
in the interpretation of the observed phenomena is evident from the
recent experience of Professor Rowland, of Baltimore, from which we
learn that spectral lines are not only widened by increased pressure of
the light-giving vapour, but that they may be bodily shifted thereby.
Dr. Zeeman's discovery, that a line from a source placed in a strong
magnetic field may be both widened, broadened, and doubled, will also
increase our difficulties in the interpretation of these obscure
phenomena.