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