Page 321 - NS-2 Textbook

316 NAUTICAL SCIENCES

the pile grows, gradually changmg into heavier ele- and dust cloud surrOlmding a small dense core. This at-
ments. Evenhlally, after a final burst in size and bril- mosphere remains lighted by the core embers. After a
liance, the star either collapses or explodes and disinte- year or two, it often can be photographed through a large
grates, depending on its size. telescope as a gaseous cloud, called a planetary nebula
Most of the bright stars we can see in a clear evening (plural, nebulae).
are stars in the giant or supergiant stages, edging closer
to their last burst of gIOlY. A few are bright new ones, and
others are ordinary ones that are close enough to appear NEBULAE
very bright.
Some nebulae are easily visible through a telescope; thus,
they can be shldied very minutely through the spectro-
NOVAE graph. Nebulae are among the most beautiful of all as-
tronomical phenonlena.
Sometimes a star appears in the sky for the first time in There are three kinds of nebulae. The bright nebula
recorded astronomical history. TI1ese stars have been glows and is easily visible because there is a bright star
called novae (plural form of nova, the Latin word for nearby that illUl11inates it. A dark nebula is composed of
I1ne"w"). Even more rarely this new star is a supernova, the same gas and dust as the bright nebula, but it is visi-
which blazes forth with a luminosity as much as 1 mil- ble only because it is sID10uetted against the stars behind
lion times that of an ordinary star. it; there is no illuminating star in the region of a dark
Records of these Hnew stal'S" appear in accounts as nebula.
far back as 134 B.C., when the ancient Greek astronomer The third kind is the planetary nebula; this is actually
Hipparchus observed one in the constellation Scm·pius. a nova or temporary star with a large cloud of particles
Chinese records tell of a brilliant star appearing in the surrounding it as the result of the stellar explosion. These
daytime sky in A.D. 1054. Tycho Brahe, a German as- nebulae show considerable smface detail, even though
tronomer, found one in the constellation Cassiopeia in they are much less dense than planets.
1572 and observed it until it disappeared in 1574. Others
have been observed throughout history, including one in
1987. BINARIES AND STAR CLUSTERS
Since these stars appear suddenly and disappear rel-
Stars have a tendency to cluster together due to gravi-
atively quickly-after only a few days for supernovae,
tational attraction. Pairs of stars are called binaries, or
and a year or two for other novae-they are nO\l\T more
double stars. Larger groups of stars are referred to as star
correctly called "temporary stars." Novae really are not
elusters.
new stars at all. Actually, they are stars in the very last
stages of life. Due to the instability that develops in their
nuclear core in the end of their life as giants, their nuclear
furnace finally explodes. After the explosion, it is
thought that the star returns to about its original state,
but with a loss of mass. It becomes a huge expanding gas

A globular cluster named Messier 3. Globular clusters contain thou-
The Whirlpool Nebula, a bright spiral nebula. sands of stars. Clusters form systems nearly spherical in shape.