10 How the World Began
195
life of Giordano Bruno [1].) Astronomers in the 19th Century rejoiced in
their hard-won emancipation and freedom of speech.
It is this conjecture of an infinite and uniform distribution of stars in
the universe that gives rise to Olbers’ Paradox. If you assume that there are
infinitely many stars scattered everywhere and at all distances, all lines of sight
from your eye terminate sooner or later on the surface of a star. Of course,
the farther you go, the smaller the apparent size of the star. However, even
if the apparent size is reduced to an infinitesimal point, the luminosity will
always be that of the surface of a star. As a result, the sky should always be as
bright as the surface of the sun.
When this argument is proposed, it is generally received with a shake
of the head, and the comment: “but, you are assuming that the skies are
perfectly empty and transparent. Aren’t there dust clouds in space that cut
off the light from stars behind them?” This is certainly true. The central
region of our galaxy, the Milky Way, happens to lie in the southern skies,
and as a consequence, the Milky Way in the Southern Hemisphere is brighter
than in the Northern. Dust clouds are very noticeable against this prevailing
brightness, especially a large one, known as the Coalsack, adjoining the iconic
constellation, Crux, or the Southern Cross.
In Fig. 10.4, the Milky Way is shown as it appears in the southern skies,
with the many dust clouds along its length clearly visible. The Coalsack is
the separate dark patch in the Milky Way at the level of the top of the large
telescope. It lies above and adjoining the two brightest stars of the Southern
Cross. The Coalsack Dark Nebula is located at an approximate distance of
600 light years in the Constellation Crux. Its radius is 30–35 light years.
The distributed dust clouds, not the bright stars themselves, form the
“constellations” of Southern Hemisphere cultures, e.g. the emu of various
Australian Aboriginal peoples. The Coalsack forms the head and beak of
an emu, the elongated dark strip above it is the neck, and the many dark
clouds around the galactic centre form the body of the emu, which in this
photograph is inverted.
These dust clouds clearly obscure the stars behind them, so the objection raised earlier would appear to have merit. However, it stumbles against
the alleged eternity of the universe: if the present age of the cosmos is infinite, dust, radiation, stars, planets, everything, should have had time to reach
thermal equilibrium. A dust grain absorbs radiation having the same temperature as the stars, warms up a bit, then re-emits the absorbed energy in the
infrared (even far infrared) band of frequency (not visible to our eyes). A
similar process occurs when a stone wall becomes warm from the afternoon
sun, and continues to radiate warmth (aka infrared radiation) long after the
195
life of Giordano Bruno [1].) Astronomers in the 19th Century rejoiced in
their hard-won emancipation and freedom of speech.
It is this conjecture of an infinite and uniform distribution of stars in
the universe that gives rise to Olbers’ Paradox. If you assume that there are
infinitely many stars scattered everywhere and at all distances, all lines of sight
from your eye terminate sooner or later on the surface of a star. Of course,
the farther you go, the smaller the apparent size of the star. However, even
if the apparent size is reduced to an infinitesimal point, the luminosity will
always be that of the surface of a star. As a result, the sky should always be as
bright as the surface of the sun.
When this argument is proposed, it is generally received with a shake
of the head, and the comment: “but, you are assuming that the skies are
perfectly empty and transparent. Aren’t there dust clouds in space that cut
off the light from stars behind them?” This is certainly true. The central
region of our galaxy, the Milky Way, happens to lie in the southern skies,
and as a consequence, the Milky Way in the Southern Hemisphere is brighter
than in the Northern. Dust clouds are very noticeable against this prevailing
brightness, especially a large one, known as the Coalsack, adjoining the iconic
constellation, Crux, or the Southern Cross.
In Fig. 10.4, the Milky Way is shown as it appears in the southern skies,
with the many dust clouds along its length clearly visible. The Coalsack is
the separate dark patch in the Milky Way at the level of the top of the large
telescope. It lies above and adjoining the two brightest stars of the Southern
Cross. The Coalsack Dark Nebula is located at an approximate distance of
600 light years in the Constellation Crux. Its radius is 30–35 light years.
The distributed dust clouds, not the bright stars themselves, form the
“constellations” of Southern Hemisphere cultures, e.g. the emu of various
Australian Aboriginal peoples. The Coalsack forms the head and beak of
an emu, the elongated dark strip above it is the neck, and the many dark
clouds around the galactic centre form the body of the emu, which in this
photograph is inverted.
These dust clouds clearly obscure the stars behind them, so the objection raised earlier would appear to have merit. However, it stumbles against
the alleged eternity of the universe: if the present age of the cosmos is infinite, dust, radiation, stars, planets, everything, should have had time to reach
thermal equilibrium. A dust grain absorbs radiation having the same temperature as the stars, warms up a bit, then re-emits the absorbed energy in the
infrared (even far infrared) band of frequency (not visible to our eyes). A
similar process occurs when a stone wall becomes warm from the afternoon
sun, and continues to radiate warmth (aka infrared radiation) long after the