A nebula is an interstellar cloud of dust,
hydrogen, helium and other ionized gases.
Originally, nebula was a name for any diffuse
astronomical object, including galaxies beyond
the Milky Way. The Andromeda Galaxy, for instance,
was referred to as the Andromeda Nebula before
the true nature of galaxies was confirmed
in the early 20th century by Vesto Slipher,
Edwin Hubble and others. Nebulae are often
star-forming regions, such as in the Eagle
Nebula. This nebula is depicted in one of
NASA's most famous images, the "Pillars of
Creation". In these regions the formations
of gas, dust, and other materials "clump"
together to form larger masses, which attract
further matter, and eventually will become
massive enough to form stars. The remaining
materials are then believed to form planets,
and other planetary system objects.
Observational history
Around 150 AD, Claudius Ptolemaeus 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/Muslim astronomer, Abd al-Rahman al-Sufi,
in his Book of Fixed Stars. 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.
On November 26, 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
wouldn't be performed until 1659 by Christiaan
Huygens, who also believed himself to be 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 in 1746. From
1751–53, Nicolas Louis de Lacaille cataloged
42 nebulae from the Cape of Good Hope, with
most of them being previously unknown. Charles
Messier then compiled a catalog of 103 "nebulae"
by 1781; his interest was detecting comets,
and these were objects that might be mistaken
for them, wasting time.
The number of nebulae was then greatly expanded
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 absorption
spectra 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.
In about 1922, 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 diffuse nebulae, finding
29 that showed emission spectra and 33 had
the continuous spectra of star light. In 1922,
Hubble announced that nearly all nebulae 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 B1 or hotter, while nebulae
with continuous spectra appear with cooler
stars. Both Hubble and Henry Norris Russell
concluded that the nebulae surrounding the
hotter stars are transformed in some manner.
Formation
Many nebulae or stars form from the gravitational
collapse of gas in the interstellar medium
or ISM. As the material collapses under its
own weight, massive stars may form in the
center, and their ultraviolet radiation ionizes
the surrounding gas, making it visible at
optical wavelengths. Examples of these types
of nebulae are the Rosette Nebula and the
Pelican Nebula. The size of these nebulae,
known as HII regions, varies depending on
the size of the original cloud of gas. New
stars are formed in the nebulae. The formed
stars are sometimes known as a young, loose
cluster.
Some nebulae are formed as the result of supernova
explosions, the death throes of massive, short-lived
stars. The materials thrown off from the supernova
explosion are ionized by the energy and the
compact object that it can produce. One of
the best examples of this is the Crab Nebula,
in Taurus. The supernova event was recorded
in the year 1054 and is labelled SN 1054.
The compact object that was created after
the explosion lies in the center of the Crab
Nebula and is a neutron star.
Other nebulae may 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. The nebula is almost 97%
hydrogen and 3% helium, plus trace amounts
of other elements.
Types of nebulae
Classical types
Objects named nebulae belong to four major
groups. Before their nature was understood,
galaxies 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
Dark nebula
Not 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. In visible light
these nebulae may be divided into emission
and reflection nebulae. Emission nebulae emit
spectral line radiation from ionized gas;
they are often called HII regions.
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 form from the gaseous shells
that are ejected from low-mass asymptotic
giant branch stars when they transform into
white dwarfs. They are emission nebulae with
spectra similar to those of emission nebulae
found in star formation regions. Technically
they are HII regions, because most hydrogen
will be ionized, but they are denser and more
compact than the nebulae in star formation
regions. Planetary nebulae were given their
name by the first astronomical observers who
became able to distinguish them from planets,
who tended to confuse them with planets, 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 is an astronomical
object which is at the short-lived episode
during a star's rapid stellar evolution between
the late asymptotic giant branch phase and
the following planetary nebula 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. Collaminated
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 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 is it 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
See also
H I region
H II region
List of diffuse nebulae
Lists of nebulae
Molecular cloud
Magellanic Clouds
Messier object
Nebulae in fiction
Orion Molecular Cloud Complex
Timeline of knowledge about the interstellar
and intergalactic medium
References
External links
Nebulae, SEDS Messier Pages
Fusedweb.pppl.gov
Information on star formation, geocities.com
Clickable table of Messier objects, space-and-telescope.com
