Orion Nebula
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Great Orion nebula)
The
Orion Nebula (also known as
Messier 42,
M42, or
NGC 1976) is a
diffuse nebula situated south
[b] of
Orion's Belt in the
constellation of Orion. It is one of the brightest
nebulae, and is visible to the
naked eye in the night sky. M42 is located at a distance of
1,344 ± 20 light years[3][6] and is the closest region of massive
star formation
to Earth. The M42 nebula is estimated to be 24 light years across. It
has a mass of about 2000 times the mass of the Sun. Older texts
frequently refer to the Orion Nebula as the
Great Nebula in Orion or the
Great Orion Nebula.
[7]
The Orion Nebula is one of the most scrutinized and photographed
objects in the night sky, and is among the most intensely studied
celestial features.
[8] The nebula has revealed much about the process of how
stars and
planetary systems are formed from collapsing clouds of gas and dust. Astronomers have directly observed
protoplanetary disks,
brown dwarfs, intense and turbulent motions of the gas, and the
photo-ionizing effects of massive nearby stars in the nebula.
Physical characteristics
Discussing the location of the Orion Nebula, what is seen within the
star-formation region, and the effects of interstellar winds in shaping
the nebula.
Amateur image of the Orion Nebula taken with a DSLR camera.
The constellation of Orion with the Orion Nebula (lower middle).
The nebula is visible with the naked eye even from areas affected by some
light pollution.
It is seen as the middle "star" in the sword of Orion, which are the
three stars located south of Orion's Belt. The star appears fuzzy to
sharp-eyed observers, and the nebulosity is obvious through
binoculars or a small
telescope. The peak surface brightness of the central region is about 17 Mag/arcsec
2 and the outer bluish glow has a peak surface brightness of 21.3 Mag/arcsec
2.
[9]
The Orion Nebula contains a very young
open cluster, known as the
Trapezium due to the
asterism of its primary four stars. Two of these can be resolved into their component binary systems on nights with good
seeing, giving a total of six stars. The stars of the Trapezium, along with many other stars, are still in their
early years. The Trapezium may be a component of the much larger
Orion Nebula Cluster,
an association of about 2,000 stars within a diameter of 20 light
years. Two million years ago this cluster may have been the home of the
runaway stars AE Aurigae,
53 Arietis, and
Mu Columbae, which are currently moving away from the nebula at velocities greater than 100 km/s.
[10]
Coloration
Observers have long noted a distinctive greenish tint to the nebula,
in addition to regions of red and of blue-violet. The red hue is a
result of the
Hα recombination line
radiation at a
wavelength of 656.3
nm. The blue-violet coloration is the reflected radiation from the massive
O-class stars at the core of the nebula.
The green hue was a puzzle for astronomers in the early part of the 20th century because none of the known
spectral lines
at that time could explain it. There was some speculation that the
lines were caused by a new element, and the name "nebulium" was coined
for this mysterious material. With better understanding of
atomic physics, however, it was later determined that the green spectrum was caused by a low-probability
electron transition in doubly
ionized oxygen, a so-called "
forbidden transition".
This radiation was all but impossible to reproduce in the laboratory
because it depended on the quiescent and nearly collision-free
environment found in deep space.
[11]
History
Messier's drawing of the Orion Nebula in his 1771 memoir, Mémoires de l'Académie Royale
There has been speculation that the
Mayans of
Central America
may have described the nebula within their "Three Hearthstones"
creation myth; if so, the three would correspond to two stars at the
base of Orion,
Rigel and
Saiph, and another,
Alnitak at the tip of the "belt" of the imagined hunter, the vertices of a nearly perfect equilateral triangle
[vague] with
Orion's Sword (including the Orion Nebula) in the middle of the triangle
[vague] seen as the smudge of smoke from
copal
incense in a modern myth, or, in (the translation it suggests of) an
ancient one, the literal or figurative embers of a fiery creation.
[12][13]
Neither
Ptolemy's
Almagest nor
Al Sufi's
Book of Fixed Stars noted this nebula, even though they both listed patches of nebulosity elsewhere in the night sky; nor did
Galileo mention it, even though he also made telescopic observations surrounding it in 1610 and 1617.
[14] This has led to some speculation that a flare-up of the illuminating stars may have increased the brightness of the nebula.
[15]
The first discovery of the diffuse nebulous nature of the Orion Nebula is generally credited to French astronomer
Nicolas-Claude Fabri de Peiresc, on 26 November 1610 when he made a record of observing it with a
refracting telescope purchased by his patron
Guillaume du Vair.
[14]
The first published observation of the nebula was by the Jesuit mathematician and astronomer
Johann Baptist Cysat of
Lucerne in his 1619 monograph on the comets (describing observations of the nebula that may date back to 1611).
[16] He made comparisons between it and a bright
comet seen in 1618 and described how the nebula appeared through his telescope as:
- "one sees how in like manner some stars are compressed into a
very narrow space and how round about and between the stars a white
light like that of a white cloud is poured out"[17]
His description of the center stars as different from a comet's head
in that they were a "rectangle" may have been an early description of
the
Trapezium Cluster[14][17][18] (The first detection of three of the four stars of this cluster is credited to
Galileo Galilei
in a February 4, 1617 although he did not notice the surrounding nebula
— possibly due to the narrow field of vision of his early telescope.
[19])
The nebula was independently discovered by several other prominent astronomers in the following years, including, in 1656,
Christiaan Huygens (whose sketch was the first published, in 1659).
Charles Messier
first noted the nebula on March 4, 1769, and he also noted three of the
stars in Trapezium. Messier published the first edition of his catalog
of deep sky objects in 1774 (completed in 1771).
[20] As the Orion Nebula was the 42nd object in his list, it became identified as M42.
Henry Draper's 1880 photograph of the Orion Nebula, the first ever taken.
One of Andrew Ainslie Common's 1883 photograph of the Orion Nebula, the
first to show that a long exposure could record new stars and nebulae
invisible to the human eye.
In 1865 English
amateur astronomer William Huggins used his visual
spectroscopy method to examine the nebula showing it, like other nebulae he had examined, was made up of "luminous gas".
[21] On September 30, 1880
Henry Draper used the new
dry plate photographic process with an 11-inch (28 cm)
refracting telescope to make a 51-minute exposure of the Orion Nebula, the first instance of
astrophotography
of a nebula in history. Another set of photographs of the nebula in
1883 saw breakthrough in astronomical photography when amateur
astronomer
Andrew Ainslie Common used the dry plate process to record several images in exposures up to 60 minutes with a 36-inch (91 cm)
reflecting telescope
that he constructed in the backyard of his home in Ealing, outside
London. These images for the first time showed stars and nebula detail
too faint to be seen by the human eye.
[22]
In 1902,
Vogel and Eberhard discovered differing velocities within the nebula and by 1914 astronomers at
Marseilles
had used the interferometer to detect rotation and irregular motions.
Campbell and Moore confirmed these results using the spectrograph,
demonstrating turbulence within the nebula.
[23]
In 1931,
Robert J. Trumpler noted that the fainter stars near the
Trapezium
formed a cluster, and he was the first to name them the Trapezium
cluster. Based on their magnitudes and spectral types, he derived a
distance estimate of 1,800 light years. This was three times farther
than the commonly accepted distance estimate of the period but was much
closer to the modern value.
[24]
In 1993, the
Hubble Space Telescope
first observed the Orion Nebula. Since then, the nebula has been a
frequent target for HST studies. The images have been used to build a
detailed model of the nebula in three dimensions.
Protoplanetary disks have been observed around most of the newly formed stars in the nebula, and the destructive effects of high levels of
ultraviolet energy from the most massive stars have been studied.
[25]
In 2005, the
Advanced Camera for Surveys instrument of the
Hubble Space Telescope finished capturing the most detailed image of the
nebula yet taken. The image was taken through 104 orbits of the
telescope, capturing over 3,000 stars down to the 23rd magnitude,
including infant
brown dwarfs and possible brown dwarf
binary stars.
[26]
A year later, scientists working with the HST announced the first ever
masses of a pair of eclipsing binary brown dwarfs, 2MASS
J05352184–0546085. The pair are located in the Orion Nebula and have
approximate masses of 0.054
M☉ and 0.034 M
☉
respectively, with an orbital period of 9.8 days. Surprisingly, the
more massive of the two also turned out to be the less luminous.
[27]
Structure
Optical images reveal clouds of gas and dust in the Orion Nebula; an
infrared image (right) reveals the new stars shining within.
The entirety of the Orion Nebula extends across a 1° region of the sky, and includes
neutral clouds of gas and dust,
associations of stars,
ionized volumes of gas, and
reflection nebulae.
The Nebula is part of a much larger nebula that is known as the
Orion Molecular Cloud Complex. The Orion Molecular Cloud Complex extends throughout the
constellation of
Orion and includes
Barnard's Loop, the
Horsehead Nebula,
M43,
M78, and the
Flame Nebula.
Stars are forming throughout the Orion Nebula, and due to this
heat-intensive process the region is particularly prominent in the
infrared.
The nebula forms a roughly spherical cloud that peaks in density near the core.
[28] The cloud has a temperature ranging up to 10,000 K, but this temperature falls dramatically near the edge of the nebula.
[28]
Unlike the density distribution, the cloud displays a range of
velocities and turbulence, particularly around the core region. Relative
movements are up to 10 km/s (22,000 mi/h), with local variations of up
to 50 km/s and possibly more.
The current astronomical model for the nebula consists of an ionized region roughly centered on
Theta1 Orionis C, the star responsible for most of the
ultraviolet ionizing radiation. (It emits 3-4 times as much photoionizing light as the next brightest star, Theta
2 Orionis A.)
[29]
This is surrounded by an irregular, concave bay of more neutral,
high-density cloud, with clumps of neutral gas lying outside the bay
area. This in turn lies on the perimeter of the Orion Molecular Cloud.
Observers have given names to various features in the Orion Nebula.
The dark lane that extends from the north toward the bright region is
called the "Fish's Mouth". The illuminated regions to both sides are
called the "Wings". Other features include "The Sword", "The Thrust",
and "The Sail".
[30]
Star formation
Star Formation Fireworks in Orion.
The Orion Nebula is an example of a
stellar nursery
where new stars are being born. Observations of the nebula have
revealed approximately 700 stars in various stages of formation within
the nebula.
Recent observations with the
Hubble Space Telescope have yielded the major discovery of
protoplanetary disks within the Orion Nebula, which have been dubbed
proplyds.
[31] HST has revealed more than 150 of these within the nebula, and they are considered to be systems in the earliest stages of
solar system formation. The sheer numbers of them have been used as evidence that the formation of star systems is fairly common in our
universe.
Stars form when clumps of
hydrogen and other gases in an
H II region
contract under their own gravity. As the gas collapses, the central
clump grows stronger and the gas heats to extreme temperatures by
converting
gravitational potential energy to
thermal energy. If the temperature gets high enough,
nuclear fusion will ignite and form a
protostar. The protostar is 'born' when it begins to emit enough radiative energy to balance out its gravity and halt
gravitational collapse.
Typically, a cloud of material remains a substantial distance from
the star before the fusion reaction ignites. This remnant cloud is the
protostar's protoplanetary disk, where planets may form. Recent
infrared observations show that dust grains in these protoplanetary disks are growing, beginning on the path towards forming
planetesimals.
[32]
Once the protostar enters into its
main sequence
phase, it is classified as a star. Even though most planetary disks can
form planets, observations show that intense stellar radiation should
have destroyed any proplyds that formed near the Trapezium group, if the
group is as old as the low mass stars in the cluster.
[25]
Since proplyds are found very close to the Trapezium group, it can be
argued that those stars are much younger than the rest of the cluster
members.
[c]
Stellar wind and effects
Once formed, the stars within the nebula emit a stream of charged particles known as a
stellar wind.
Massive stars and
young stars have much stronger stellar winds than the
Sun.
[33]
The wind forms shock waves or hydrodynamical instabilities when it
encounters the gas in the nebula, which then shapes the gas clouds. The
shock waves from stellar wind also play a large part in stellar
formation by compacting the gas clouds, creating density inhomogeneities
that lead to gravitational collapse of the cloud.
There are three different kinds of shocks in the Orion Nebula. Many are featured in
Herbig–Haro objects:
[35]
- Bow shocks
are stationary and are formed when two particle streams collide with
each other. They are present near the hottest stars in the nebula where
the stellar wind speed is estimated to be thousands of kilometers per
second and in the outer parts of the nebula where the speeds are tens of
kilometers per second. Bow shocks can also form at the front end of
stellar jets when the jet hits interstellar particles.
- Jet-driven shocks are formed from jets of material sprouting off newborn T Tauri stars.
These narrow streams are traveling at hundreds of kilometers per
second, and become shocks when they encounter relatively stationary
gases.
- Warped shocks appear bow-like to an observer. They are produced when
a jet-driven shock encounters gas moving in a cross-current.
- The interaction of the stellar wind with the surrounding cloud also
forms "waves" which are believed to be due to the hydrodynamical Kelvin-Helmholtz instability.[36]
The dynamic gas motions in M42 are complex, but are trending out through the opening in the bay and toward the Earth.
[28] The large neutral area behind the ionized region is currently contracting under its own gravity.
There are also
supersonic "bullets" of gas piercing the hydrogen clouds of the Orion Nebula. Each bullet is ten times the diameter of
Pluto's
orbit and tipped with iron atoms glowing bright blue. They were
probably formed one thousand years ago from an unknown violent event.
[37]
Evolution
Panoramic image of the center of the nebula, taken by the Hubble
Telescope. This view is about 2.5 light years across. The Trapezium is
at center left.
Interstellar clouds like the Orion Nebula are found throughout
galaxies such as the
Milky Way.
They begin as gravitationally bound blobs of cold, neutral hydrogen,
intermixed with traces of other elements. The cloud can contain hundreds
of thousands of
solar masses
and extend for hundreds of light years. The tiny force of gravity that
could compel the cloud to collapse is counterbalanced by the very faint
pressure of the gas in the cloud.
Whether due to collisions with a spiral arm, or through the shock wave emitted from
supernovae,
the atoms are precipitated into heavier molecules and the result is a
molecular cloud. This presages the formation of stars within the cloud,
usually thought to be within a period of 10-30 million years, as regions
pass the
Jeans mass
and the destabilized volumes collapse into disks. The disk concentrates
at the core to form a star, which may be surrounded by a protoplanetary
disk. This is the current stage of evolution of the nebula, with
additional stars still forming from the collapsing molecular cloud. The
youngest and brightest stars we now see in the Orion Nebula are thought
to be less than 300,000 years old,
[38] and the brightest may be only 10,000 years in age.
Some of these collapsing stars can be particularly massive, and can emit large quantities of ionizing
ultraviolet
radiation. An example of this is seen with the Trapezium cluster. Over
time the ultraviolet light from the massive stars at the center of the
nebula will push away the surrounding gas and dust in a process called
photo evaporation.
This process is responsible for creating the interior cavity of the
nebula, allowing the stars at the core to be viewed from Earth.
[8] The largest of these stars have short life spans and will evolve to become supernovae.
Within about 100,000 years, most of the gas and dust will be ejected.
The remains will form a young open cluster, a cluster of bright, young
stars surrounded by wispy filaments from the former cloud.
[39] The
Pleiades is a famous example of such a cluster.
Gallery
-
An
APEX view of star formation in the Orion Nebula.
-
Orion Nebula was captured using the Wide Field Imager camera on the MPG/ESO 2.2-metre telescope.
-
Orion Nebula Complex including M42,
M43,
Running Man Nebula (NGC 1973, 1975, and 1977) and much of the surrounding nebulosity.
-
Panoramic image of the Orion Nebula, taken by Ioannidis Panos with an 8 Inch Newtonian telescope and a Nikon D70 camera.
-
Infant stars, image from NASA's Spitzer Space Telescope.
-
The Orion Nebula imaged with the 2.2m ESO/MPG telescope.
-
The central part of the Orion Nebula.
-
-
-
The Orion Nebula's biggest stars.
-
An infrared image showing fledgling stars located in the Orion Nebula.
-
Core detail of the nebula with all the stars identified.
-
A shot of the core details of the Orion Nebula.
, temperature 
, total pressure (matter plus radiation) 
, luminosity 
, and energy generation rate per unit mass 
in a spherical shell of a thickness 
at a distance 
from the center of the star. The star is assumed to be in local thermodynamic equilibrium (LTE) so the temperature is identical for matter and photons.
Although LTE does not strictly hold because the temperature a given
shell "sees" below itself is always hotter than the temperature above,
this approximation is normally excellent because the photon mean free path, 
, is much smaller than the length over which the temperature varies considerably, i. e. 
.,
is the cumulative mass inside the shell at 
) to the radius of the star (
) yields the total mass of the star.,
is the luminosity produced in the form of neutrinos
(which usually escape the star without interacting with ordinary
matter) per unit mass. Outside the core of the star, where nuclear
reactions occur, no energy is generated, so the luminosity is constant.
is the opacity of the matter, 
is the Stefan-Boltzmann constant, and the Boltzmann constant is set to one.
is the adiabatic index, the ratio of specific heats in the gas. (For a fully ionized ideal gas, 
.)
When the convection is not adiabatic, the true temperature gradient is
not given by this equation. For example, in the Sun the convection at
the base of the convection zone, near the core, is adiabatic but that
near the surface is not. The mixing length theory contains two free
parameters which must be set to make the model fit observations, so it
is a phenomelogical theory rather than a rigorous mathematical formulation.[6]
, meaning the pressure at the surface of the star is zero; 
, there is no mass inside the center of the star, as required if the mass density remains finite; 
, the total mass of the star is the star's mass; and 
, the temperature at the surface is the effective temperature of the star.
