COMETS--THE HAIRY STARS
Comets--hairy stars, as the origin of the name would indicate--are the
freaks of the heavens. Of great variety in shape, some with heads and
some without, some with tails and some without, moving very slowly at
one time and with exceedingly high velocity at another, in orbits at all
possible angles of inclination to the general plane of the planetary
paths round the sun, their antics and irregularities were the wonder and
terror of the ancient world, and they are keenly dreaded by
superstitious people even to the present day.
Down through the Middle Ages the advent of a comet was regarded as:
Threatening the world with famine, plague and war;
To princes, death; to kingdoms, many curses;
To all estates, inevitable losses;
To herdsmen, rot; to plowmen, hapless seasons;
To sailors, storms; to cities, civil treasons.
Comets appeared to be marvelous objects, as well as sinister, chiefly
because they bid apparent defiance to all law. Kepler had shown that the
moon and the planets travel in regular paths--slightly elliptical to be
sure, but nevertheless unvarying. None of the comets were known to
follow regular paths till the time of Halley late in the seventeenth
century, when, as we have before told, a fine comet made its
appearance, and Halley calculated its orbit with much precision.
Comparing this with the orbits of comets that had previously been seen,
he found its path about the sun practically identical with that of at
least two comets previously observed in 1531 and 1607.
So Halley ventured to think all these comets were one and the same body,
and that it traveled round the sun in a long ellipse in a period of
about seventy-five or seventy-six years. We have seen how his prediction
of its return in 1758 was verified in every particular. On the comet's
return in 1910, Crowell and Crommelin of Greenwich made a thorough
mathematical investigation of the orbit, indicating that the year 1986
will witness its next return to the sun.
There is a class of astronomers known as comet-hunters, and they pass
hours upon hours of clear, sparkling, moonless nights in search for
comets. They are equipped with a peculiar sort of telescope called a
comet-seeker, which has an object glass usually about four or five
inches in diameter, and a relatively short length of focus, so that a
larger field of view may be included. Regions near the poles of the
heavens are perhaps the most fruitful fields for search, and thence
toward the sun till its light renders the sky too bright for the finding
of such a faint object as a new comet usually is at the time of
discovery. Generally when first seen it resembles a small circular patch
of faint luminous cloud.
When a suspect is found, the first thing to do is to observe its
position accurately with relation to the surrounding stars. Then, if on
the next occasion when it is seen the object has moved, the chances are
that it is a comet; and a few days' observation will provide material
from which the path of the comet in space can be calculated. By
comparing this with the complete lists of comets, now about 700 in
number, it is possible to tell whether the comet is a new one, or an old
one returning. The total number of comets in the heavens must be very
great, and thousands are doubtless passing continually undetected,
because their light is wholly overpowered by that of the sun. Of those
that are known, perhaps one in twelve develops into a naked-eye comet,
and in some years six or seven will be discovered. With sufficiently
powerful telescopes, there are as a rule not many weeks in the year when
no comet is visible. Brilliant naked-eye comets are, however,
infrequent.
Comets, except Halley's, generally bear the name of their discoverer, as
Donati (1858), and Pons-Brooks (1893). Pons was a very active discoverer
of comets in France early in the nineteenth century: he was a doorkeeper
at the observatory of Marseilles, and his name is now more famous in
astronomy than that of Thulis, then the director of the Observatory, who
taught and encouraged him. Messier was another very successful
discoverer of comets in France, and in America we have had many: Swift,
Brooks, and Barnard the most successful.
How bright a comet will be and how long it will be visible depends upon
many conditions. So the comets vary much in these respects. The first
comet of 1811 was under observation for nearly a year and a half, the
longest on record till Halley's in 1910. In case a comet eludes
discovery and observation until it has passed its perihelion, or nearest
point to the sun, its period of visibility may be reduced to a few
weeks only. The brightest comets on record were visible in 1843 and
1882: so brilliant were they that even the effulgence of full daylight
did not overpower them. In particular the comet of 1843 was not only
excessively bright, but at its nearest approach to the earth its tail
swept all the way across the sky from one horizon to the other. It must
have looked very much like the straight beam of an enormous searchlight,
though very much brighter.
The tails of comets are to the naked eye the most compelling thing about
them, and to the ancient peoples they were naturally most terrifying.
Their tails are not only curved, but sometimes curved with varying
degrees of curvature, and this circumstance adds to their weirdness of
appearance. If we examine the tail of a comet with a telescope, it
vanishes as if there were nothing to it: as indeed one may almost say
there is not. Ordinarily, only the head of the comet is of interest in
the telescope. When first seen there is usually nothing but the head
visible, and that is made up of portions which develop more or less
rapidly, presenting a succession of phenomena quite different in
different comets.
When first discovered a comet is usually at a great distance from the
sun, about the distance of Jupiter; and we see it, not as we do the
planets, by sunlight reflected from them, but by the comet's own light.
This is at that time very faint, and nearly all comets at such a
distance look alike: small roundish hazy patches of faint, cloudlike
light, with very often a concentration toward the center called the
nucleus, on the average about 4,000 miles in diameter. Approach toward
the sun brightens up the comet more and more, and the nucleus usually
becomes very much brighter and more starlike. Then on the sunward side
of the nucleus, jetlike streamers or envelopes appear to be thrown off,
often as if in parallel curved strata, or concentrically. As they expand
and move outward from the nucleus, these envelopes grow fainter and are
finally merged in the general nebulosity known as the comet's head,
which is anywhere from 30,000 to 100,000 miles in diameter. As a rule,
this is an orderly development which can be watched in the telescope
from hour to hour and from night to night; but occasionally a cometary
visitor is quite a law to itself in development, presenting a
fascinating succession of unpredictable surprises.
Then follows the development of the comet's tail, perhaps more striking
than anything that has preceded it. Here a genuine repulsion from the
sun appears to come into play. It may be an electrical repulsion. Much
of the material projected from the comet's nucleus, seems to be driven
backward or repelled by the sun, and it is this that goes to form the
tail. The particles which form the tail then travel in modified paths
which nevertheless can be calculated. The tail is made up of these
luminous particles and it expands in space much in the form of a hollow,
horn-shaped cone, the nucleus being near the tip of the horn.
Some comets possess multiple tails with different degrees of curvature,
Donati's for example. Usually there is a nearly straight central dark
space, marking the axis of the comet, and following the nucleus. But
occasionally this is replaced by a thin light streak very much less in
breadth than the diameter of the head. Cometary tails are sometimes 100
million miles in length.
Three different types of cometary tails are recognized. First, the long
straight ones, apparently made up of matter repelled by the sun twelve
to fifteen times more powerfully than gravitation attracts it. Such
particles must be brushed away from the comet's head with a velocity of
perhaps five miles a second, and their speed is continually increasing.
Probably these straight tails are due to hydrogen. The second type tails
are somewhat curved, or plume-like, and they form the most common type
of cometary tail. In them the sun's repulsion is perhaps twice its
gravitational attraction, and hydrocarbons in some form appear to be
responsible for tails of this character. Then there is a third type,
much less often seen, short and quickly curving, probably due to heavier
vapors, as of chlorine, or iron, or sodium, in which the repulsive force
is only a small fraction of that of gravitation.
Many features of this theory of cometary tails are borne out by
examination of their light with the spectroscope, although the
investigation is as yet fragmentary. It is evident that the tail of a
comet is formed at the expense of the substance of the nucleus and head;
so that the matter repelled is forever dissipated through the regions of
space which the comet has traveled. Comets must lose much of their
original substance every time they return to perihelion. Comets actually
age, therefore, and grow less and less in magnitude of material as well
as brightness, until they are at last opaque, nonluminous bodies which
it becomes impossible to follow with the telescope.