| Nebulae A nebula is a localized cloud of dust and gases
(mostly hydrogen) in intergalactic space. Nebulae are
often responsible for star formation. Gravity causes a
portion of a nebula's dust and gases to contract into a
much smaller and denser mass. Pressure and temperature
build up within the mass as contraction continues through
millions of years. In time, the mass becomes hot enough
to shine -- a star is born.
Types of Nebulae
 Emission
Nebulae are so-called because they generate
and emit their own light. This type of nebula is always
associated with an extremely hot, bright star. The intense ultraviolet light from the star
energizes the gas atoms in the cloud, but since gases do
not like being energized the atoms get rid of the extra
energy by emitting it in the form of light. Most emission
nebulae appear red because hydrogen is the most prominent
gas in the universe and hydrogen atoms generate red light
when energized.
Left: The Orion Nebula, which makes
up part of the sword in the constellation Orion, is the
most well-known example of an emission nebula.
 A Reflection
Nebula is nebula near a cool star that does
not emit enough ultraviolet light to energize the gas
atoms in the cloud. Such a nebula is incapable of
emitting its own light, but the dust particles can still
reflect visible light, hence the name. This kind of
nebula looks blue in color when viewed from Earth because
the dust scatters the blue light while allowing
the rest of the light spectrum to pass through
undisturbed (the same phenomenon that gives our sky its
blue color).
Right: A reflection nebula surrounds the Pleiades
Cluster.
 Planetary Nebulae
are so-called because many of them superficially resemble
planets when viewed through a telescope. They are formed
when a red giant or supergiant star begins to shed its
outer layers. When such a star begins to run out of fuel
it will expand. After millions of years it will begin to
shrink down to its normal size again, and as it shrinks
much of its surface layers are shed, leaving a visible
ring (sometimes multiple rings) behind. A planetary
nebula is only visible from Earth for about 50,000 years.
Over time the stellar material will mix into the
surrounding space and the ring will become too thin to
see.
Left: The Cat's-Eye Nebula, in the constellation
Draco, is a spectacular example of a planetary nebula.
 A Supernova
Nebula is, as the name implies, a nebula
formed by the remnants of a supernova. Nebulae of this
type are strong emitters of radio waves, as the result of
the explosions that formed them and the pulsar remnants
of the original star.
Right: The Crab Nebula, in the constellation
Taurus, is the remnant of a supernova that was seen on
Earth in 1054.
 If a nebula is in an area that has no
nearby stars, it can neither emit nor reflect enough
light to be visible on its own. This kind of nebula can
only be seen as a dark cloud because the dust particles
blot out the light from stars behind them and is,
therefore, known as a Dark Nebula.
Left: The Horsehead Nebula is a dark nebula that
is only visible because it partially blocks light from
the Orion Nebula.
Questions or comments about this
page?
|
