THE UNIVERSE OF STARS
Our consideration of the solar system hitherto has kept us quite at home
in the universe. The outer known planets, Uranus and Neptune, are indeed
far removed from the sun, and a few of the comets that belong to our
family travel to even greater distances before they begin to retrace
their steps sunward. When we come to consider the vast majority of the
glistening points on the celestial sphere--all in fact except the five
great planets, Mercury, Venus, Mars, Jupiter, and Saturn--we are dealing
with bodies that are self-luminous like the sun, but that vary in size
quite as the bodies of the solar system do, some stars being smaller
than the sun and others many hundred fold larger than he is; some being
"giants," and others "dwarfs." But the overwhelming remoteness of all
these bodies arrests our attention and even taxes our credulity
regarding the methods that astronomers have depended on to ascertain
their distances from us.
Their seeming countlessness, too, is as bewildering as are the
distances; though, if we make actual counts of those visible to the
naked eye within a certain area, in the body of the "Great Bear," for
example, the great surprise will be that there are so few. And if the
entire dome of the sky is counted, at any one time, a clear, moonless
sky would reveal perhaps 2,500, so that in the entire sky, northern and
southern, we might expect to find 5,000 to 6,000 lucid stars, or stars
visible to the naked eye.
But when the telescope is applied, every accession of power increases
the myriads of fainter and fainter stars, until the number within
optical reach of present instruments is somewhere between 400 and 500
millions. But if we were to push the 100-inch reflector on Mount Wilson
to its limit by photography with plates of the highest sensitiveness,
millions upon millions of excessively faint stars would be plainly
visible on the plates which the human eye can never hope to see directly
with any telescope present or future, and which would doubtless swell
the total number of stars to a thousand millions. Recent counts of stars
by Chapman and Melotte of Greenwich tend to substantiate this estimate.
What have astronomers done to classify or catalogue this vast array of
bodies in the sky? Even before making any attempt to estimate their
number, there is a system of classification simply by the amount of
light they send us, or by their apparent stellar magnitudes--not their
actual magnitudes, for of those we know as yet very little. We speak of
stars of the "first magnitude," of which there are about 20, Sirius
being the brightest and Regulus the faintest. Then there are about 65 of
the second, or next fainter, magnitude, stars like Polaris, for example,
which give an amount of light two and a half times less than the average
first magnitude star. Stars of the third magnitude are fainter than
those of the second in the same ratio, but their number increases to
200; fourth magnitude, 500; fifth magnitude, 1,400; sixth magnitude,
5,000, and these are so faint that they are just visible on the best
nights without telescopic aid.
Decimals express all intermediate graduations of magnitude. Astronomers
carry the telescopic magnitudes much farther, till a magnitude beyond
the twentieth is reached, preserving in every case the ratio of two and
one-half for each magnitude in relation to that numerically next to it.
Even Jupiter and Venus, and the sun and moon, are sometimes calculated
on this scale of stellar magnitude, numerically negative, of course,
Venus sometimes being as bright as magnitude -4.3, and the sun -26.7.
Knowing thus the relation of sun, moon, and stars, and the number of the
stars of different magnitudes, it is possible to estimate the total
light from the stars. This interesting relation comes out this way: that
the stars we cannot see with the naked eye give a greater total of light
than those we can because of their vastly greater numbers. And if we
calculate the total light of all the brighter stars down to magnitude
nine and one-half, we find it equal to 1/80th of the light of the
average full moon.
Many stars show marked differences in color, and strictly speaking the
stars are now classified by their colors. The atmosphere affects star
colors very considerably, low altitudes, or greater thickness of air,
absorbing the bluish rays more strongly and making the stars appear
redder than they really are. Aldebaran, Betelgeuse and Antares are
well-known red stars, Capella and Alpha Ceti yellowish, Vega and Sirius
blue, and Procyon and Polaris white. Among the telescopic stars are many
of a deep blood-red tint, variable stars being numerous among them.
Double stars, too, are often complementary in color. There is evidence
indicating change of color of a very few stars in long periods of time;
Sirius, for example, two thousand years ago was a red star, now it is
blue or bluish white. But the meaning of color, or change of color in a
star is as yet only incompletely ascertained. It may be connected with
the radiative intensity of the star, or its age, or both.
The late Professor Edward C. Pickering was famous for his life-long
study and determination of the magnitudes of the stars. Standards of
comparison have been many, and have led to much unnecessary work.
Pickering chose Polaris as a standard and devised the meridian
photometer, an ingenious instrument of high accuracy, in which the light
of a star is compared directly with that of the pole star by reflection.
All the bright stars of both the northern and the southern skies are
worked into a standard system of magnitudes known as HP, or the Harvard
Photometry.
Astronomers make use of several different kinds of magnitude for the
stars: the apparent magnitude, as the eye sees it, often called the
visual magnitude; the photographic magnitude, as the photographic plate
records it, and these are now determined with the highest accuracy; the
photovisual magnitude, quite the same as the visual, but determined
photographically on an isochromatic plate with a yellow screen or
filter, so that the intensity is nearly the same as it appears to the
eye. The difference between the star's visual or photovisual magnitude
and its photographic magnitude is called its color-index, and is often
used as a measure of the star's color. Light of the shorter wave
lengths, as blue and violet, affects the photographic plate more rapidly
than the reds and yellows of longer wave length by which the eye mainly
sees; so that red stars will appear much fainter and blue stars much
brighter on the ordinary photographic plate than the eye sees them.
So great are the differences of color in the stars that well-known
asterisms, with which the eye is perfectly familiar, are sometimes quite
unrecognizable on the photographic plate, except by relative positions
of the stars composing them. White stars affect the eye and the plate
about equally, so that their visual or photovisual and photographic
magnitudes are about equal. The studies of the colors of the stars, the
different methods of determining them, and the relations of color to
constitution have been made the subject of especial investigation by
Seares of Mount Wilson and many other astronomers.
Centuries of the work of astronomers have been faithfully devoted to
mapping or charting the stars and cataloguing them. Just as we have
geographical maps of countries, so the heavens are parceled out in
sections, and the stars set down in their true relative positions just
as cities are on the map. Recent years have added photographic charts,
especially of detailed regions of the sky; but owing to spectral
differences of the stars, their photographic magnitudes are often quite
different from their visual magnitudes. From these maps and charts the
positions of the stars can be found with much precision; but if we want
the utmost accuracy, we must go to the star catalogues--huge volumes
oftentimes, with stellar positions set down therein with the last degree
of precision.
First there will be the star's name, and in the next column its
magnitude, and in a third the star's right ascension. This is its
angular distance eastward around the celestial sphere starting from the
vernal equinox, and it corresponds quite closely to the longitude of a
place which we should get from a gazetteer, if we wished to locate it on
the earth. Then another column of the catalogue will give the star's
declination, north or south of the equator, just as the gazetteer will
locate a city by its north or south latitude.