NEBULA,
in astronomy, a localized conglomerate of the gaseous and
finely divided dust particles that are spread throughout interstellar
space. Before the invention of the telescope, the term nebula (Lat., “cloud”)
was applied to all celestial objects of a diffuse appearance. As
a result, many objects now known to be star clusters or galaxies
were called nebulae.
Nebulae exist within other galaxies as well as in our own
Milky Way galaxy. They are classified as planetary nebulae, supernova
remnants, and diffuse nebulae, including reflecting, emission, and
dark nebulae. Small, very bright nebulae called Herbig-Haro objects
are found in dense interstellar clouds and are probably the products
of gas jets expelled by new stars in the process of formation.
Planetary nebulae, or planetaries, are so called because many
of them superficially resemble planets through telescopes. They
are actually shells of material that an old average star sheds during
a late, red giant stage in its evolution, before becoming a white
dwarf. The Ring nebula of the constellation Lyra, a typical planetary,
has a rotational period of 132,900 years and a mass calculated to
be about 14 times that of the earth’s sun. Several thousand
planetaries have been discovered in the Milky Way. More spectacular
but fewer in number are nebulae that are the fragments of supernova
explosions, perhaps the most famous of which is the Crab nebula
in Taurus, now fading at the rate of about 0.4 percent per year.
Nebulae of this kind are strong emitters of radio waves, as a result
of the explosions that formed them and the probable pulsar remnants
of the original star.
Diffuse nebulae are extremely large structures, often many
light-years wide, that have no definite outline and a tenuous, cloudlike
appearance. They are either luminous or dark. The former shine as
a result of the light of neighboring stars. They include some of
the most striking objects in the sky, such as the Great nebula in
Orion (the middle “star” in the sword). The tremendous
streams of matter in the diffuse nebulae are intermingled in violent,
chaotic currents. Many thousands of luminous nebulae are known.
Spectral studies show that light emanating from them consists of
reflected light from stars and also, in so-called emission nebulae,
of stimulated radiation of ionized gases and dust from the nebulae
themselves.
Dark, diffuse nebulae are observed as nonluminous clouds or
faintly luminous, obscuring portions of the Milky Way and too distant
from the stimulation of neighboring stars to reflect or emit much
light of their own. One of the most famous dark nebulae is the Horsehead
nebula in Orion, so named for the silhouette of the dark mass in
front of a more luminous nebular region. The longest dark rift observed
on photographic plates of the star clouds of the Milky Way is a
succession of dark nebulae. Both dark nebulae and luminous nebulae are
considered likely sites for the processes of dust-cloud condensation
and the formation of new stars.