A nebula (Latin for "cloud" or "fog"; pl.
nebulae, nebulæ, or nebulas) is an interstellar
cloud of dust, hydrogen, helium and other
ionized gases. Originally, the term was used
to describe any diffuse astronomical object,
including galaxies beyond the Milky Way. The
Andromeda Galaxy, for instance, was once referred
to as the Andromeda Nebula (and spiral galaxies
in general as "spiral nebulae") before the
true nature of galaxies was confirmed in the
early 20th century by Vesto Slipher, Edwin
Hubble and others.
Most nebulae are of vast size; some are hundreds
of light years in diameter. A nebula that
is barely visible to the human eye from Earth
would appear larger, but no brighter, from
close by. The Orion Nebula, the brightest
nebula in the sky and occupying an area twice
the diameter of the full Moon, can be viewed
with the naked eye but was missed by early
astronomers. Although denser than the space
surrounding them, most nebulae are far less
dense than any vacuum created on Earth – a
nebular cloud the size of the Earth would
have a total mass of only a few kilograms.
Many nebulae are visible due to fluorescence
caused by embedded hot stars, while others
are so diffuse they can only be detected with
long exposures and special filters. Some nebulae
are variably illuminated by T Tauri variable
stars.
Nebulae are often star-forming regions, such
as in the "Pillars of Creation" in the Eagle
Nebula. In these regions the formations of
gas, dust, and other materials "clump" together
to form denser regions, which attract further
matter, and eventually will become dense enough
to form stars. The remaining material is then
believed to form planets and other planetary
system objects.
== Observational history ==
Around 150 AD, Claudius Ptolemaeus (Ptolemy)
recorded, in books VII–VIII of his Almagest,
five stars that appeared nebulous. He also
noted a region of nebulosity between the constellations
Ursa Major and Leo that was not associated
with any star. The first true nebula, as distinct
from a star cluster, was mentioned by the
Persian astronomer Abd al-Rahman al-Sufi,
in his Book of Fixed Stars (964). He noted
"a little cloud" where the Andromeda Galaxy
is located. He also cataloged the Omicron
Velorum star cluster as a "nebulous star"
and other nebulous objects, such as Brocchi's
Cluster. The supernova that created the Crab
Nebula, the SN 1054, was observed by Arabic
and Chinese astronomers in 1054.In 1610, Nicolas-Claude
Fabri de Peiresc discovered the Orion Nebula
using a telescope. This nebula was also observed
by Johann Baptist Cysat in 1618. However,
the first detailed study of the Orion Nebula
was not performed until 1659, by Christiaan
Huygens, who also believed he was the first
person to discover this nebulosity.In 1715,
Edmund Halley published a list of six nebulae.
This number steadily increased during the
century, with Jean-Philippe de Cheseaux compiling
a list of 20 (including eight not previously
known) in 1746. From 1751 to 1753, Nicolas
Louis de Lacaille cataloged 42 nebulae from
the Cape of Good Hope, most of which were
previously unknown. Charles Messier then compiled
a catalog of 103 "nebulae" (now called Messier
objects, which included what are now known
to be galaxies) by 1781; his interest was
detecting comets, and these were objects that
might be mistaken for them.The number of nebulae
was then greatly increased by the efforts
of William Herschel and his sister Caroline
Herschel. Their Catalogue of One Thousand
New Nebulae and Clusters of Stars was published
in 1786. A second catalog of a thousand was
published in 1789 and the third and final
catalog of 510 appeared in 1802. During much
of their work, William Herschel believed that
these nebulae were merely unresolved clusters
of stars. In 1790, however, he discovered
a star surrounded by nebulosity and concluded
that this was a true nebulosity, rather than
a more distant cluster.Beginning in 1864,
William Huggins examined the spectra of about
70 nebulae. He found that roughly a third
of them had the emission spectrum of a gas.
The rest showed a continuous spectrum and
thus were thought to consist of a mass of
stars. A third category was added in 1912
when Vesto Slipher showed that the spectrum
of the nebula that surrounded the star Merope
matched the spectra of the Pleiades open cluster.
Thus the nebula radiates by reflected star
light.About 1923, following the Great Debate,
it had become clear that many "nebulae" were
in fact galaxies far from our own.
Slipher and Edwin Hubble continued to collect
the spectra from many different nebulae, finding
29 that showed emission spectra and 33 that
had the continuous spectra of star light.
In 1932, Hubble announced that nearly all
nebula are associated with stars, and their
illumination comes from star light. He also
discovered that the emission spectrum nebulae
are nearly always associated with stars having
spectral classifications of B or hotter (including
all O-type main sequence stars), while nebulae
with continuous spectra appear with cooler
stars. Both Hubble and Henry Norris Russell
concluded that the nebulae surrounding the
hotter stars are transfomed in some manner.
== Formation ==
There are a variety of formation mechanisms
for the different types of nebulae. Some nebulae
form from gas that is already in the interstellar
medium while others are produced by stars.
Examples of the former case are giant molecular
clouds, the coldest, densest phase of interstellar
gas, which can form by the cooling and condensation
of more diffuse gas. Examples of the latter
case are planetary nebulae formed from material
shed by a star in late stages of its stellar
evolution.
Star-forming regions are a class of emission
nebula associated with giant molecular clouds.
These form as a molecular cloud collapses
under its own weight, proceeding stars. Massive
stars may form in the center, and their ultraviolet
radiation ionizes the surrounding gas, making
it visible at optical wavelengths. The region
of ionized hydrogen surrounding the massive
stars is known as an H II region while the
shells of neutral hydrogen surrounding the
H II region are known as photodissociation
region. Examples of star-forming regions are
the Orion Nebula, the Rosette Nebula and the
Omega Nebula. Feedback from star-formation,
in the form of supernova explosions of massive
stars, stellar winds or ultraviolet radiation
from massive stars, or outflows from low-mass
stars may disrupt the cloud, destroying the
nebula after several million years.
Other nebulae form as the result of supernova
explosions; the death throes of massive, short-lived
stars. The materials thrown off from the supernova
explosion are then ionized by the energy and
the compact object that its core produces.
One of the best examples of this is the Crab
Nebula, in Taurus. The supernova event was
recorded in the year 1054 and is labeled SN
1054. The compact object that was created
after the explosion lies in the center of
the Crab Nebula and its core is now a neutron
star.
Still other nebulae form as planetary nebulae.
This is the final stage of a low-mass star's
life, like Earth's Sun. Stars with a mass
up to 8–10 solar masses evolve into red
giants and slowly lose their outer layers
during pulsations in their atmospheres. When
a star has lost enough material, its temperature
increases and the ultraviolet radiation it
emits can ionize the surrounding nebula that
it has thrown off. Our Sun will produce a
planetary nebula and its core will remain
behind in the form of white dwarf.
== Types of nebulae ==
=== 
Classical types ===
Objects named nebulae belong to 4 major groups.
Before their nature was understood, galaxies
("spiral nebulae") and star clusters too distant
to be resolved as stars were also classified
as nebulae, but no longer are.
H II regions, large diffuse nebulae containing
ionized hydrogen
Planetary nebulae
Supernova remnant (e.g., Crab Nebula)
Dark nebulaNot all cloud-like structures are
named nebulae; Herbig–Haro objects are an
example.
=== Diffuse nebulae ===
Most nebulae can be described as diffuse nebulae,
which means that they are extended and contain
no well-defined boundaries. Diffuse nebulae
can be divided into emission nebula, reflection
nebulae and "dark nebulae." Visible light
nebulae may be divided into emission nebulae
that emit spectral line radiation from excited
or ionized gas (mostly ionized hydrogen);
they are often called HII regions (the term
"HII" refers to ionized hydrogen). Reflection
nebulae are visible primarily due to the light
they reflect. Reflection nebulae themselves
do not emit significant amounts of visible
light, but are near stars and reflect light
from them. Similar nebulae not illuminated
by stars do not exhibit visible radiation,
but may be detected as opaque clouds blocking
light from luminous objects behind them; they
are called "dark nebulae".Although these nebulae
have different visibility at optical wavelengths,
they are all bright sources of infrared emission,
chiefly from dust within the nebulae.
=== Planetary nebulae ===
Planetary nebulae are the remnants of the
final stages of stellar evolution for lower-mass
stars. Evolved asymptotic giant branch stars
expel their outer layers outwards due to strong
stellar winds, thus forming gaseous shells,
while leaving behind the star's core in the
form of a white dwarf. The hot white dwarf
illuminates the expelled gases producing emission
nebulae with spectra similar to those of emission
nebulae found in star formation regions. Technically
they are HII regions, because most hydrogen
are ionized, but are denser and more compact
than nebulae found in star formation regions.
Planetary nebulae were given their name by
the first astronomical observers who were
initially unable to distinguish them from
planets, and who tended to confuse them with
planets, which were of more interest to them.
Our Sun is expected to spawn a planetary nebula
about 12 billion years after its formation.
==== Protoplanetary nebula ====
A protoplanetary nebula (PPN) is an astronomical
object at the short-lived episode during a
star's rapid stellar evolution between the
late asymptotic giant branch (LAGB) phase
and the following planetary nebula (PN) phase.
During the AGB phase, the star undergoes mass
loss, emitting a circumstellar shell of hydrogen
gas. When this phase comes to an end, the
star enters the PPN phase.
The PPN is energized by the central star,
causing it to emit strong infrared radiation
and become a reflection nebula. Collimated
stellar winds from the central star shape
and shock the shell into an axially symmetric
form, while producing a fast moving molecular
wind. The exact point when a PPN becomes a
planetary nebula (PN) is defined by the temperature
of the central star. The PPN phase continues
until the central star reaches a temperature
of 30,000 K, after which it is hot enough
to ionize the surrounding gas.
=== Supernova remnants ===
A supernova occurs when a high-mass star reaches
the end of its life. When nuclear fusion in
the core of the star stops, the star collapses.
The gas falling inward either rebounds or
gets so strongly heated that it expands outwards
from the core, thus causing the star to explode.
The expanding shell of gas forms a supernova
remnant, a special diffuse nebula. Although
much of the optical and X-ray emission from
supernova remnants originates from ionized
gas, a great amount of the radio emission
is a form of non-thermal emission called synchrotron
emission. This emission originates from high-velocity
electrons oscillating within magnetic fields.
== Notable named nebulae ==
=== Nebula catalogs ===
Gum catalog
RCW Catalogue
Sharpless catalog
Messier Catalogue
Caldwell Catalogue
Abell Catalog of Planetary Nebulae
== 
See also ==
H I region
H II region
List of diffuse nebulae
Lists of nebulae
Molecular cloud
Magellanic Clouds
Messier object
Nebulae in fiction
Nebular hypothesis
Orion Molecular Cloud Complex
Timeline of knowledge about the interstellar
and intergalactic medium
