What is the meaning of these different types of stars—the blue, the
yellow, and the red? and can a star change its colour and type? Yes,
we believe that all the blue stars, if they follow the normal course,
will in countless ages become red, and though the life of the whole
human race may perhaps not be long enough to witness a single instance,
we feel confident that this process is going on. One walk through a
forest is enough to teach us that a tree grows from seed to sapling,
from maturity to decay, because we see trees around us in all these
stages. So with the stars. Here the process of development is far
more difficult to understand, and we are still ignorant concerning
the birth and death of stars; but it is clear that we see a series of
stages which pass gradually into one another, and that the cause of a
star’s growing old is a gradual loss of heat by radiation. The blue
Orion stars are the brightest and hottest of all those about which we
have spoken, and are generally thought to have reached the climax of a
star’s career. They are great globes of thin gas, much less dense than
water, but glowing hot through and through. The outer layers naturally
part with their heat first, and in cooling they absorb more and more
light from the radiant centre. As the blue rays are always first and
most powerfully affected, the blue tint is soon lost, and the colour
slowly changes through white, yellow, and orange to deep-red. At the
same time changes take place which cause different gases to become most
conspicuous in the star’s spectrum at different stages.
But what are the early stages which precede the brilliance of an Orion
star? Arguments have been brought forward to show that young stars also
are comparatively cool, but even less dense than Orion stars, that as
they grow denser and smaller through gravity, which causes all the
particles of a star to condense towards its centre, they must at first
grow gradually hotter and brighter until a certain point is reached,
after which they will grow cooler and fainter again until they become
quite cold and dark; and so there is a double progression, viz. from
red towards blue, and back again to red, some stars changing in one
direction and some in the other.
On the other hand, the stage which immediately precedes the Orion
type somewhat resembles a peculiar class of nebulae, so the upward
progression of the Orion stars may have been from nebulae through this
stage.
Let us see what these stages are.
At one end of the series we find stars like Gamma in the Sails of the
Ship Argo. When its light is spread out by prisms into a coloured band
it is a wonderful sight, for not only are there dark lines and dark
flutings, but wide bright bands mingle with these and stand out against
the fainter background. We are fortunate to possess γ Velorum in the
southern hemisphere, for it is the only bright star of its class. The
type is known as WOLF-RAYET STARS, from the name of their discoverer.
Only a few are known, and because of their faintness not very much has
been discovered about them, but it is remarkable that all of them lie
near the middle line of the Milky Way.
The bright lines in their spectra and some other features of Wolf-Rayet
stars point (as we have said) to a connection with gaseous nebulae,
and it is possible that they developed from nebulae not very long
ago—as time is reckoned in astronomy. Though this past is doubtful, the
future of these stars is clearly indicated: they are destined to become
Orion stars, for in stars like τ Canis majoris we see an intermediate
stage between the two types.
ORION STARS sometimes have bright lines in their spectra also,
especially the very blue ones found near nebulae, but the most striking
and characteristic feature is the series of dark helium lines, from
which they are often called “helium stars.” The three in Orion’s belt
are typical of this class; several first-magnitude stars belong to it,
as we have seen, and it contains others which are very bright, such as
β and δ Crucis, β Scorpii, α Lupi, α Pavonis, α Gruis, α Sagittarii.
Spica is a distinctly blue Orion star; Rigel and γ Gruis have already
lost the blue tint and are approaching the next stage.
For the Orion class passes by gradual transition into the SIRIAN STARS,
which may be called “hydrogen stars,” from the wide hazy lines of
hydrogen which are the most conspicuous feature in their spectrum. They
are also very hot and bright, though less so than the Orion stars.
Besides Sirius, a large number of bright southern stars belong to this
class, among which we may mention Fomalhaut, γ Centauri, β Carinae,
δ Velorum, and β Pavonis. Among stars visible to the naked eye this
is by far the most numerous class, partly, no doubt, because they are
intrinsically bright and therefore visible at distances where redder
stars could not be seen. This cannot however be the whole explanation,
or the Orion stars would be still more numerous instead of being
comparatively rare.
The blue end of the spectrum begins to be darkened as a star reaches
the stage of sun-like or SOLAR STARS, and although the hydrogen lines
grow narrower and less intense, an immense number of fine lines cross
the bright band and absorb much of the light. α Centauri, as we have
seen, is a replica of the sun, and another star which almost exactly
resembles it, in spectrum if not in mass, is β Hydri. ζ Gruis and one
star of the naked-eye double in Grus, δ¹ Gruis, are also solar stars.
This class may be subdivided into three: the Sirian-solar, which is
typified by Canopus, and also by η Crucis and α Hydri; the solar,
closely resembling the sun; and the red-solar, which tend towards the
red stars. The temperatures apparently are lower and lower in these
three divisions in the order given, and all solar stars are cooler than
those of the preceding classes. Among these red-solar stars is the
beautiful ε Carinae, the foot of the False Cross,[5] which is a rich
reddish orange even to the naked eye, and more brilliantly coloured in
a binocular. Others are α Toucani, α Trianguli australi, ε Crucis (the
little fifth star of the Southern Cross), the two brightest stars of
Indus (α and β), almost all the bright stars of the Phoenix (α, β, δ,
ε), and ε Scorpii which is beautifully coloured. This class is also
extremely numerous among naked-eye stars, although they cannot be so
bright intrinsically as the whiter solar stars: the significance of the
fact is not easy to understand.
[5] The stars ε and ι Carinae, κ and δ Velorum, form a cross much like
the Southern Cross, but less bright, and this is called the False Cross.
The deep-red stage is reached in suns like Antares, whose spectrum
shows not only lines like those in the sun but also a series of broad
bands or flutings which absorb much of its light, and in photographs
a great part of the violet end is wholly cut off. Were it not such
an enormous size, this would be a very dim star. ANTARIAN STARS are
almost all very remote. They are often very faint, and many of them are
unstable in their light, as if fluctuating towards extinction. β and
δ² Gruis and γ Hydri are among them, and Mira, the wonderful variable
star. Compare the ruddy γ in the Cross with pale δ to see the contrast
between stars of this class and the Orion-type. Because of the want of
blue rays in the former it makes so little impression on a photographic
plate compared with α, β, and δ, which are all Orion stars, that one
can scarcely recognise the form of the Cross in a photograph.
The brightest of these red stars are in the northern hemisphere, viz.
Betelgueux in the shoulder of Orion, and Aldebaran, the eye of the
Bull: both are slightly brighter than Antares.
Nearly all the stars we know have a place in this series, though there
are individual peculiarities, but there is one class which seems to lie
outside it. Stars of this class are red and have spectra crossed by
dark bands, but they are unlike the Antarian bands and resemble instead
those seen in the spectra of comets and of candle-flames. They are due
to carbon compounds, so these stars may be called CARBON STARS. Most of
them are extremely remote, and all are so faint that among the very
brightest is U Hydrae, just visible to the naked eye a little east of
Alphard. They are probably aged stars, but no links between them and
the other types have yet been discovered to enable us to place them in
the series.
Strange and interesting discoveries have been made by grouping large
numbers of stars into their classes and comparing the average motions,
distances, &c., of the groups. It is found that the redder stars are on
an average moving more rapidly and in a more random fashion than blue
and white stars. Thus, Wolf-Rayet and Orion stars have a low average
speed, and both are very much more numerous in and near the Milky Way
than elsewhere; Sirian stars are travelling a good deal faster, show a
marked tendency to congregate in two streams, and move chiefly parallel
with the plane of the Milky Way; solar stars, including our own sun,
move more rapidly still and show less preference for the Galaxy and the
two streams: Antarians have the most rapid motions of all, but these
appear to be haphazard, and the stars are scattered all over the sky in
every direction.
These facts are very unexpected and very difficult to explain. It
looks as if the Milky Way were the birthplace of the stars, and that
as they develop they gradually scatter through space; but how are we
to explain the fact that speed and direction of movement differ for
different types? There seems to be no reason why a cooler star should
move more quickly than a hotter one, and none of the theories yet
advanced can be considered final.
V