The brighter of the two Pointers to the Cross, Alpha Centauri,
ranks very high among first-magnitude stars, though it is excelled
in brightness by Sirius and Canopus; but its greatest claim to our
interest is that among all the host of stars it is our nearest
neighbour. Yet, should we decide to pay a visit and travel with the
speed of light (11 million miles a minute), it would be four years
and three months before we could reach our destination—so great are
the spaces which separate our sun with his family of planets from the
nearest of his brother suns.
Alpha Centauri is also interesting as a double star. The two components
are nearly equal in brightness, and are seen as a brilliant yellow pair
in quite a small telescope. It was one of the first doubles known,
being discovered as such in 1689; and Herschel calls it a “superb
double star, beyond all comparison the most striking object of the kind
in the heavens.” The two stars he describes as “both of a high ruddy or
orange colour, though that of the smaller is of a somewhat more sombre
and brownish cast.”
All stars are suns, but they vary so much in brightness and size and
quality of light that it is as difficult to match two stars in the
sky as two trees in a wood. Yet one of this pair is an almost exact
counterpart of our sun. The pair is approaching us, and at the same
time revolving one about the other in a period of about eighty years.
The other Pointer, Beta Centauri, is a star very different from our
sun. It belongs to a type called “Orion stars” because so many are
found in that constellation. Instead of shining with a yellow light,
they are blue or bluish-white, and their glowing atmospheres contain
quantities of helium, the gas which was only recently discovered on
earth though it had been known for many years in the sun. Oxygen and
nitrogen and some yet unknown elements are also present.
Alpha Crucis, the brightest star of the Cross, is also a blue Orion
star, and the telescope shows it to be triple. There are two bright
twin stars, and a fainter one which would be just visible to the naked
eye if alone, but being overpowered by the brilliance of the bright
pair can only be seen in a large telescope. Quite a small telescope
will separate the twins.
Achernar, the Last-of-the-River, is also an Orion star, 75 light-years
distant from us—_i.e._ light takes 75 years to come from it to us. The
sun if removed to a third of this distance would be barely visible to
the naked eye.
Rigel, Orion’s foot, is the brightest star of this class. Its intrinsic
brilliancy and its size must be enormous, for its distance is certainly
more than 360 light-years, and may be greatly more. And the distance
is increasing at the rate of 39 miles a second. Rigel is also a double
star, but its companion is very faint. In an 8-inch telescope the
bright star looks white or pale lemon-yellow, contrasting beautifully
with the little companion which is blue.
Spica, the Ear of Corn in the hand of the Virgin, is immeasurably
distant from us, and its companion is invisible even in the most
powerful telescopes. It was discovered by means of the spectroscope,
for, when the light of the star is drawn out into a long
rainbow-coloured ribbon crossed by dark lines, it is found that there
are two ribbons, one bright, the other very faint, and that the lines
of these two spectra draw apart and then come together again once in
every four days, showing that there are two stars close together and
revolving round one another in this short time. The joint mass of the
pair is two and a half times that of the sun.
Spica is one of the first stars whose invisible companion was
discovered in this way, and it is also connected with another
discovery, made nearly twenty centuries earlier by Hipparchus in the
island of Rhodes. He was patiently plotting the positions of all the
visible stars, when he found a slight discrepancy between the places
given to Spica by himself and another Greek astronomer, who had
observed about a hundred and fifty years earlier. Examining into this
led him to discover that all stars change their apparent positions
very slowly, completing a cycle of change in nearly 26,000 years, so
that 240 centuries more must elapse before any astronomer sees Spica
in exactly the same place as where Hipparchus saw her. It remained for
modern astronomers to discover that the apparent change is due to a
slow nodding motion of the Earth’s axis.
Brightest of all stars in the whole sky is Sirius, the Dog-Star. It was
worshipped by the ancient Egyptians, and the day on which it rose just
before the sun was counted as the first day of their year. The Arabs,
when they learned the astronomy of more ancient nations, were forbidden
to adopt their star-worship, hence the saying in the Koran, often
quoted by Arab writers: “The Highest saith, He is lord of Sirius.”
Sirius is moving rapidly through space, not uniformly but with an
oscillating movement, and Bessel in 1844 “founded the astronomy of the
invisible” by showing that these irregularities might be caused by a
dark disturbing companion. Eight years later, Mr. Alvan Clark, wishing
to test a large lens just made by his firm, turned it on Sirius, and
lo! there was the satellite in the position required to explain the
vagaries of Sirius. It is not therefore wholly dark, but it shines with
so feeble a light that, if it were brought as near to us as our own
sun, it would appear only one-hundredth as bright as he is, even though
it is a somewhat more massive body. Sirius itself is only about two
and a half times as massive as our sun, but immensely more brilliant.
It is the typical star of the “Sirian” class, to which belong many
of the brightest stars in the heavens, white stars in whose spectra
broad hydrogen lines form the most striking feature, indicating a very
extensive atmosphere of glowing hydrogen. Bright Sirius and his dim
companion revolve round their common centre of gravity in fifty years.
Fomalhaut, the mouth of the Fish, belongs to the Sirian type of stars,
and is also very brilliant, giving out fourteen and a half times as
much light as our sun. Its distance is 25 light-years.
Canopus, the rudder of the Ship Argo, must be a giant sun, for its
distance is altogether beyond reach of measurement and it is steadily
receding from us, yet it shines as the brightest star in the sky except
Sirius.
In Southern India it is called Agastya, after a Brahmin rishi who
led an early Aryan colony to the south, and before whom the Vindhya
Mountains prostrated themselves as he passed.
Greek astronomers noticed that this star rose only just above the
horizon of Rhodes, but 7½ degrees above it at Alexandria, from which
Poseidonius calculated that the circumference of the whole earth,
_i.e._ 360°, must be 240,000 stadia. This is equal to nearly 23,500
miles, a value surprisingly near the correct figure, considering
how difficult it must have been to measure the distance over the sea
between Rhodes and Alexandria.
Antares, the brightest star in the Scorpion, was so named by the Greeks
because it rivals the red planet Mars (Greek Ares) in colour. It is red
because a dense atmosphere shuts out most of its blue rays. Like most
red stars it is very distant, and its light takes 155 years to reach
us. Nevertheless it shines 2000 times as brightly as our sun would
do at the same distance, hence it must be of an enormous size. This
immense red star is accompanied by a little green satellite, and there
is also a very close companion which can never be seen, but is known to
exist through the shifting of lines in the spectrum, like that of Spica.
What an amazing variety among these ten stars! Though the eye can only
decide that all are much brighter than the average, and that they
differ somewhat in colour among themselves, science tells us that
they vary enormously in many ways. Half belong to the class of blue
Orion stars, others to the white Sirian, yellow solar, red Antarian
classes; more than half are known to be double or multiple, and among
these we find twins, while others have faint companions differing
from themselves in colour. In one case the pair is widely separated,
and revolves in a period of half a century, while another pair is so
close that the circuit is performed in four days. Again, while one
star is the same size as the sun, others are much greater, and their
distances from us vary all the way from four light-years to spaces we
are powerless to plumb.