H II region
Encyclopedia
An H II region is a large, low-density cloud of partially ionized gas
in which star formation
has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet
light, ionizing the surrounding gas. H II regions—sometimes several hundred light-year
s across—are often associated with giant molecular clouds in which star formation takes place, and from which the stars that produce the H II region were born. The first known H II region was the Orion Nebula
, which was discovered in 1610 by Nicolas-Claude Fabri de Peiresc
.
H II regions are named for the large amount of ion
ised atom
ic hydrogen
they contain, referred to as H II by astronomers
(H I region
being neutral
atomic hydrogen, and H2 being molecular
hydrogen). They have extremely diverse morphologies, because the distribution of the stars and gas inside them is inhomogeneous. They often appear clumpy and filamentary, sometimes showing bizarre shapes like the Horsehead Nebula
. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova
explosions and strong stellar wind
s from the most massive stars in the resulting star cluster
will disperse the gases of the H II region, leaving behind a cluster such as the Pleiades
.
H II regions can be seen out to considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distance and chemical composition of other galaxies
. Spiral
and irregular
galaxies contain a lot of H II regions, while elliptical
galaxies are almost devoid of them. In the spiral galaxies, including the Milky Way, H II regions are concentrated in the spiral arms, while in the irregular galaxies they are distributed chaotically. Some galaxies contain huge H II regions, which may contain tens of thousands of stars. Examples include the 30 Doradus region in the Large Magellanic Cloud
and NGC 604
in the Triangulum Galaxy
.
A few of the brightest H II regions are visible to the naked eye
. However, none seem to have been noticed before the advent of the telescope
in the early 17th century. Even Galileo
did not notice the Orion Nebula
when he first observed the star cluster
within it (previously cataloged as a single star, θ Orionis, by Johann Bayer
). French observer Nicolas-Claude Fabri de Peiresc
is credited with the discovery of the Orion Nebula in 1610. Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies.
William Herschel
observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns". Confirmation of this hypothesis had to wait another hundred years, when William Huggins
(assisted by his wife Mary Huggins
) turned his spectroscope on various nebulae. Some, such as the Andromeda Nebula
, had spectra quite similar to those of star
s, and turned out to be galaxies
consisting of hundreds of millions of individual stars. Others looked very different. Rather than a strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines. In planetary nebulae, the brightest of these spectral lines was at a wavelength
of 500.7 nanometre
s, which did not correspond with a line of any known chemical element
. At first it was hypothesized that the line might be due to an unknown element, which was named Nebulium—a similar idea had led to the discovery of helium
through analysis of the Sun
's spectrum in 1868. However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, Nebulium was not. In the early 20th century, Henry Norris Russell
proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions.
Physicists showed in the 1920s that in gas at extremely low densities
, electron
s can populate excited metastable energy level
s in atom
s and ion
s which at higher densities are rapidly de-excited by collisions. Electron transitions from these levels in doubly ionized
oxygen
give rise to the 500.7 nm line. These spectral lines, which can only be seen in very low density gases, are called forbidden lines. Spectroscopic observations thus showed that planetary nebulae consisted largely of extremely rarefied ionised oxygen gas (OIII).
In HII regions, however, the dominant spectral line has a wavelength of 656.3 nm. This is the well-known H-alpha
line emitted by atomic hydrogen. Specifically, a photon
of this wavelength is emitted when the electron of a hydrogen atom changes its excitation state
from n=3 to n=2. Such state changes happen very frequently when an electron
is captured by an ionised hydrogen atom (a proton
), and the electron cascades down from some higher excitation state to n=1. Thus, it was concluded that HII regions consist of a mix of electrons and ionised as well as constantly recombining hydrogen atoms.
During the 20th century, observations showed that H II regions often contained hot, bright stars
. These stars are many times more massive than the Sun
, and are the shortest-lived stars, with total lifetimes of only a few million years (compared to stars like the Sun, which live for several billion years). Therefore it was surmised that H II regions must be regions in which new stars were forming. Over a period of several million years, a cluster of stars will form out of an H II region, before radiation pressure
from the hot young stars causes the nebula to disperse. The Pleiades
are an example of a cluster which has 'boiled away' the H II region from which it formed. Only a trace of reflection nebulosity
remains.
The precursor to an H II region is a giant molecular cloud (GMC). A GMC is a cold (10–20 K
) and dense cloud consisting mostly of molecular hydrogen. GMCs can exist in a stable state for long periods of time, but shock waves due to supernova
e, collisions between clouds, and magnetic interactions can all trigger the collapse of part of the cloud. When this happens, via a process of collapse and fragmentation of the cloud, stars are born (see stellar evolution
for a lengthier description).
As stars are born within a GMC, the most massive will reach temperatures hot enough to ionise the surrounding gas. Soon after the formation of an ionising radiation field, energetic photon
s create an ionisation front, which sweeps through the surrounding gas at supersonic
speeds. At greater and greater distances from the ionising star, the ionisation front slows, while the pressure of the newly ionised gas causes the ionised volume to expand. Eventually, the ionisation front slows to subsonic
speeds, and is overtaken by the shock front caused by the expansion of the nebula. The H II region has been born.
The lifetime of an H II region is of the order of a few million years. Radiation pressure from the hot young stars will eventually drive most of the gas away. In fact, the whole process tends to be very inefficient, with less than 10 per cent of the gas in the H II region forming into stars before the rest is blown away. Also contributing to the loss of gas are the supernova explosions of the most massive stars, which will occur after only 1–2 million years.
Stars form in clumps of cool molecular gas that hide the nascent stars. It is only when the radiation pressure from a star drives away its 'cocoon' that it becomes visible. The hot, blue stars that are powerful enough to ionize significant amounts of hydrogen and form H II regions will do this quickly, and light up the region in which they just formed. The dense regions which contain younger or less massive forming stars and have not blown away the material from which they formed are often seen in silhouette against the rest of the ionised nebula—these dark patches are known as Bok globule
s, after astronomer Bart Bok
, who proposed in the 1940s that they might be stellar birthplaces, a hypothesis which was shown to be correct in 1990. The presence of the hot, young stars means that these globules will begin to dissipate, as the radiation from the stars powering the H II region drive the material away. In this sense, the stars which generate H II regions act to destroy stellar nurseries. In doing so, however, one last burst of star formation may be triggered, as radiation pressure and mechanical pressure from supernova
may act to squeeze globules, thereby enhancing the density inside the globule.
The young stars in H II regions show evidence for containing planetary systems. The Hubble Space Telescope
has revealed hundreds of protoplanetary disks (proplyds) in the Orion Nebula. At least half the young stars in the Orion Nebula appear to be surrounded by disks of gas and dust, thought to contain many times as much matter as would be needed to create a planetary system like the Solar System
.
or less across, to giant H II regions several hundred light-years across. Their size is also known as the Stromgren radius and essentially depends on the intensity of the source of ionising photons and the density of the region. Their densities range from over a million particles per cm³ in the ultra-compact H II regions to only a few particles per cm³ in the largest and most extended regions. This implies total masses between perhaps 102 and 105 solar masses.
There are also "ultra-dense HII" regions (UDHII).
Depending on the size of an H II region there may be several thousand stars within it. This makes H II regions more complicated than planetary nebula
e, which have only one central ionising source. Typically H II regions reach temperatures of 10,000 K. They are mostly ionised gases with weak magnetic field
s with strengths of several nanoteslas. Nevertheless H II regions are almost always associated with a cold molecular gas, which originated from the same parent GMC. Magnetic fields are produced by these weak moving electric charges in the ionised gas, suggesting that H II regions might contain electric field
s.
Chemically, H II regions consist of about 90% hydrogen. The strongest hydrogen emission line at 656.3 nm gives H II regions their characteristic red colour. Most of the rest of an H II region consists of helium
, with trace amounts of heavier elements. Across the galaxy, it is found that the amount of heavy elements in H II regions decreases with increasing distance from the galactic centre. This is because over the lifetime of the galaxy, star formation rates have been greater in the denser central regions, resulting in greater enrichment of the interstellar medium
with the products of nucleosynthesis
.
like the Milky Way and irregular galaxies
. They are rarely seen in elliptical galaxies
. In irregular galaxies, they may be found throughout the galaxy, but in spirals they are most abundant within the spiral arms. A large spiral galaxy may contain thousands of H II regions.
The reason H II regions rarely appear in elliptical galaxies is that ellipticals are believed to form through galaxy mergers. In galaxy cluster
s, such mergers are frequent. When galaxies collide, individual stars almost never collide, but the GMCs and H II regions in the colliding galaxies are severely agitated. Under these conditions, enormous bursts of star formation are triggered, so rapid that most of the gas is converted into stars rather than the normal 10% or less.
Galaxies undergoing such rapid star formation are known as starburst galaxies
. The post-merger elliptical galaxy has a very low gas content, and so H II regions can no longer form. Twenty first century observations have shown that a very small number of H II regions exist outside galaxies altogether. These intergalactic H II regions may be the remnants of tidal disruptions of small galaxies, and in some cases they may represent a new generation of stars in a galaxy's most recently accreted gas.
results in complex shapes. Supernova explosions may also sculpt H II regions. In some cases, the formation of a large star cluster within an H II region results in the region being hollowed out from within. This is the case for NGC 604
, a giant H II region in the Triangulum Galaxy
.
Notable Galactic H II regions include the Orion Nebula, the Eta Carinae Nebula, and the Berkeley 59 / Cepheus OB4 Complex. The Orion Nebula, which lies at a distance of about 500 pc (1,500 light-years) is part of a GMC (called OMC-1) which, if it were visible, would fill most of the constellation of Orion
. The Horsehead Nebula
and Barnard's Loop
are two other illuminated parts of this cloud of gas. The Orion Nebula is actually a thin layer of ionised gas on the outer border of the OMC-1 cloud. The stars in the Trapezium cluster and especially θ1 Orionis are responsible for this ionisation.
The Large Magellanic Cloud
, a satellite galaxy of the Milky Way
at about 50 kpc from the Sun, contains a giant H II region called the Tarantula Nebula
. Measuring at about this nebula is the most massive and second largest H II region in the Local group
. It is much bigger than the Orion Nebula, and is forming thousands of stars, some with masses of over 100 times that of the sun—OB
and Wolf-Rayet star
s. If the Tarantula Nebula was as close to Earth as the Orion Nebula, it would shine about as brightly as the full moon in the night sky. The supernova SN 1987A
occurred in the outskirts of the Tarantula Nebula.
Another giant H II region—NGC 604
is located in M33
spiral galaxy, which is about 817 kpc (2.66 million light years) away from the Sun. Measuring at approximately across NGC 604 is the second most massive H II region in the Local group after the Tarantula Nebula, however, it is slightly larger than the latter. It contains around 200 hot OB and Wolf-Rayet stars, which heat the gas inside it to millions of degrees producing the bright X-ray
emissions. The total mass of the hot gas in NGC 604 is about 6,000 Solar masses.
in H II regions are subject to some uncertainty. There are two different ways of determining the abundance of metals (that is, elements other than hydrogen and helium) in nebulae, which rely on different types of spectral lines, and large discrepancies are sometimes seen between the results derived from the two methods. Some astronomers put this down to the presence of small temperature fluctuations within H II regions; others claim that the discrepancies are too large to be explained by temperature effects, and hypothesise the existence of cold knots containing very little hydrogen to explain the observations.
The full details of massive star formation within H II regions are not yet well known. Two major problems hamper research in this area. First, the distance from Earth to large H II regions is considerable, with the nearest H II (California Nebula
) region being over 300 pc (1,000 light-years) away; other H II regions are several times that distance away from Earth. Secondly, the formation of these stars is deeply obscured by dust, and visible light observations are impossible. Radio
and infrared
light can penetrate the dust, but the youngest stars may not emit much light at these wavelength
s.
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
in which star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
has recently taken place. The short-lived, blue stars forged in these regions emit copious amounts of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light, ionizing the surrounding gas. H II regions—sometimes several hundred light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s across—are often associated with giant molecular clouds in which star formation takes place, and from which the stars that produce the H II region were born. The first known H II region was the Orion Nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. 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 and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
, which was discovered in 1610 by Nicolas-Claude Fabri de Peiresc
Nicolas-Claude Fabri de Peiresc
Nicolas-Claude Fabri de Peiresc was a French astronomer, antiquary and savant who maintained a wide correspondence with scientists and was a successful organizer of scientific inquiry...
.
H II regions are named for the large amount of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ised atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
ic hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
they contain, referred to as H II by astronomers
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
(H I region
H I region
An H I region is an interstellar cloud composed of neutral atomic hydrogen , in addition to the local abundance of helium and other elements. These regions are non-luminous, save for emission of the 21-cm region spectral line. This line has a very low transition probability, so requires large...
being neutral
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
atomic hydrogen, and H2 being molecular
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
hydrogen). They have extremely diverse morphologies, because the distribution of the stars and gas inside them is inhomogeneous. They often appear clumpy and filamentary, sometimes showing bizarre shapes like the Horsehead Nebula
Horsehead Nebula
The Horsehead Nebula is a dark nebula in the constellation Orion. The nebula is located just to the south of the star Alnitak, which is farthest east on Orion's Belt, and is part of the much larger Orion Molecular Cloud Complex. The Horsehead Nebula is approximately 1500 light years from Earth...
. H II regions may give birth to thousands of stars over a period of several million years. In the end, supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
explosions and strong stellar wind
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
s from the most massive stars in the resulting star cluster
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
will disperse the gases of the H II region, leaving behind a cluster such as the Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
.
H II regions can be seen out to considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distance and chemical composition of other galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
. Spiral
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
and irregular
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
galaxies contain a lot of H II regions, while elliptical
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
galaxies are almost devoid of them. In the spiral galaxies, including the Milky Way, H II regions are concentrated in the spiral arms, while in the irregular galaxies they are distributed chaotically. Some galaxies contain huge H II regions, which may contain tens of thousands of stars. Examples include the 30 Doradus region in the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
and NGC 604
NGC 604
NGC 604 is a H II region inside the Triangulum Galaxy. It was discovered by William Herschel on September 11, 1784. It is one of the largest H II regions in the Local Group of galaxies; at the galaxy's estimated distance of 2.7 million light-years its longest diameter is roughly 1500 light-years ,...
in the Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...
.
Observations
Naked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
. However, none seem to have been noticed before the advent of the telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
in the early 17th century. Even Galileo
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...
did not notice the Orion Nebula
Orion Nebula
The Orion Nebula is a diffuse nebula situated south of Orion's Belt. 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 and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
when he first observed the star cluster
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
within it (previously cataloged as a single star, θ Orionis, by Johann Bayer
Johann Bayer
Johann Bayer was a German lawyer and uranographer . He was born in Rain, Bavaria, in 1572. He began his study of philosophy in Ingolstadt in 1592, and moved later to Augsburg to begin work as a lawyer. He grew interested in astronomy during his time in Augsburg...
). French observer Nicolas-Claude Fabri de Peiresc
Nicolas-Claude Fabri de Peiresc
Nicolas-Claude Fabri de Peiresc was a French astronomer, antiquary and savant who maintained a wide correspondence with scientists and was a successful organizer of scientific inquiry...
is credited with the discovery of the Orion Nebula in 1610. Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies.
William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...
observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns". Confirmation of this hypothesis had to wait another hundred years, when William Huggins
William Huggins
Sir William Huggins, OM, KCB, FRS was an English amateur astronomer best known for his pioneering work in astronomical spectroscopy.-Biography:...
(assisted by his wife Mary Huggins
Margaret Lindsay Huggins
Margaret Lindsay, Lady Huggins , born Margaret Lindsay Murray, was an Irish scientific investigator and amateur astronomer. With her husband William Huggins she was a pioneer in the field of spectroscopy....
) turned his spectroscope on various nebulae. Some, such as the Andromeda Nebula
Andromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
, had spectra quite similar to those of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s, and turned out to be galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
consisting of hundreds of millions of individual stars. Others looked very different. Rather than a strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines. In planetary nebulae, the brightest of these spectral lines was at a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of 500.7 nanometre
Nanometre
A nanometre is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- with the parent unit name metre .The nanometre is often used to express dimensions on the atomic scale: the diameter...
s, which did not correspond with a line of any known chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
. At first it was hypothesized that the line might be due to an unknown element, which was named Nebulium—a similar idea had led to the discovery of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
through analysis of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's spectrum in 1868. However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, Nebulium was not. In the early 20th century, Henry Norris Russell
Henry Norris Russell
Henry Norris Russell was an American astronomer who, along with Ejnar Hertzsprung, developed the Hertzsprung–Russell diagram . In 1923, working with Frederick Saunders, he developed Russell–Saunders coupling which is also known as LS coupling.-Biography:Russell was born in 1877 in Oyster Bay, New...
proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions.
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
, electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s can populate excited metastable energy level
Energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
s in atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s and ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s which at higher densities are rapidly de-excited by collisions. Electron transitions from these levels in doubly ionized
Doubly ionized oxygen
Doubly ionized oxygen is a forbidden line of the ion O2+. It is significant in that it emits light in the green part of the spectrum primarily at the wavelength 500.7 nanometres and secondarily at 495.9 nm. Concentrated levels of [O III] are found in diffuse and planetary nebulae...
oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
give rise to the 500.7 nm line. These spectral lines, which can only be seen in very low density gases, are called forbidden lines. Spectroscopic observations thus showed that planetary nebulae consisted largely of extremely rarefied ionised oxygen gas (OIII).
In HII regions, however, the dominant spectral line has a wavelength of 656.3 nm. This is the well-known H-alpha
H-alpha
H-alpha is a specific red visible spectral line created by hydrogen with a wavelength of 656.28 nm, which occurs when a hydrogen electron falls from its third to second lowest energy level...
line emitted by atomic hydrogen. Specifically, a photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
of this wavelength is emitted when the electron of a hydrogen atom changes its excitation state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
from n=3 to n=2. Such state changes happen very frequently when an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
is captured by an ionised hydrogen atom (a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
), and the electron cascades down from some higher excitation state to n=1. Thus, it was concluded that HII regions consist of a mix of electrons and ionised as well as constantly recombining hydrogen atoms.
During the 20th century, observations showed that H II regions often contained hot, bright stars
OB star
OB stars are hot, massive stars of spectral types O or B which form in loosely organized groups called OB associations. They are short lived, and thus don't move very far from where they were formed within their life. During their lifetime, they will emit copious amounts of ultraviolet radiation...
. These stars are many times more massive than the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, and are the shortest-lived stars, with total lifetimes of only a few million years (compared to stars like the Sun, which live for several billion years). Therefore it was surmised that H II regions must be regions in which new stars were forming. Over a period of several million years, a cluster of stars will form out of an H II region, before radiation pressure
Radiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
from the hot young stars causes the nebula to disperse. The Pleiades
Pleiades (star cluster)
In astronomy, the Pleiades, or Seven Sisters , is an open star cluster containing middle-aged hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky...
are an example of a cluster which has 'boiled away' the H II region from which it formed. Only a trace of reflection nebulosity
Reflection nebula
In Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust...
remains.
Origin and lifetime
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
) and dense cloud consisting mostly of molecular hydrogen. GMCs can exist in a stable state for long periods of time, but shock waves due to supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
e, collisions between clouds, and magnetic interactions can all trigger the collapse of part of the cloud. When this happens, via a process of collapse and fragmentation of the cloud, stars are born (see stellar evolution
Stellar evolution
Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...
for a lengthier description).
As stars are born within a GMC, the most massive will reach temperatures hot enough to ionise the surrounding gas. Soon after the formation of an ionising radiation field, energetic photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s create an ionisation front, which sweeps through the surrounding gas at supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
speeds. At greater and greater distances from the ionising star, the ionisation front slows, while the pressure of the newly ionised gas causes the ionised volume to expand. Eventually, the ionisation front slows to subsonic
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
speeds, and is overtaken by the shock front caused by the expansion of the nebula. The H II region has been born.
The lifetime of an H II region is of the order of a few million years. Radiation pressure from the hot young stars will eventually drive most of the gas away. In fact, the whole process tends to be very inefficient, with less than 10 per cent of the gas in the H II region forming into stars before the rest is blown away. Also contributing to the loss of gas are the supernova explosions of the most massive stars, which will occur after only 1–2 million years.
Destruction of stellar nurseries
Bok globule
Bok globules are dark clouds of dense cosmic dust and gas in which star formation sometimes takes place. Bok globules are found within H II regions, and typically have a mass of about 2 to 50 solar masses contained within a region about a light year or so across...
s, after astronomer Bart Bok
Bart Bok
Bart Jan Bok was a Dutch-American astronomer.He was born in the Netherlands, but spent a good deal of his childhood days growing up in what was then known as the Dutch East Indies. He was educated at the Leiden and Groningen Universities. In 1929 he married fellow astronomer Dr...
, who proposed in the 1940s that they might be stellar birthplaces, a hypothesis which was shown to be correct in 1990. The presence of the hot, young stars means that these globules will begin to dissipate, as the radiation from the stars powering the H II region drive the material away. In this sense, the stars which generate H II regions act to destroy stellar nurseries. In doing so, however, one last burst of star formation may be triggered, as radiation pressure and mechanical pressure from supernova
Type II supernova
A Type II supernova results from the rapid collapse and violent explosion of a massive star. A star must have at least 9 times, and no more than 40–50 times the mass of the Sun for this type of explosion. It is distinguished from other types of supernova by the presence of hydrogen in its spectrum...
may act to squeeze globules, thereby enhancing the density inside the globule.
The young stars in H II regions show evidence for containing planetary systems. The Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
has revealed hundreds of protoplanetary disks (proplyds) in the Orion Nebula. At least half the young stars in the Orion Nebula appear to be surrounded by disks of gas and dust, thought to contain many times as much matter as would be needed to create a planetary system like the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
.
Physical properties
H II regions vary greatly in their physical properties. They range in size from so-called ultra-compact (UCHII) regions perhaps only a light-yearLight-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
or less across, to giant H II regions several hundred light-years across. Their size is also known as the Stromgren radius and essentially depends on the intensity of the source of ionising photons and the density of the region. Their densities range from over a million particles per cm³ in the ultra-compact H II regions to only a few particles per cm³ in the largest and most extended regions. This implies total masses between perhaps 102 and 105 solar masses.
There are also "ultra-dense HII" regions (UDHII).
Depending on the size of an H II region there may be several thousand stars within it. This makes H II regions more complicated than planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
e, which have only one central ionising source. Typically H II regions reach temperatures of 10,000 K. They are mostly ionised gases with weak magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s with strengths of several nanoteslas. Nevertheless H II regions are almost always associated with a cold molecular gas, which originated from the same parent GMC. Magnetic fields are produced by these weak moving electric charges in the ionised gas, suggesting that H II regions might contain electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
s.
Chemically, H II regions consist of about 90% hydrogen. The strongest hydrogen emission line at 656.3 nm gives H II regions their characteristic red colour. Most of the rest of an H II region consists of helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, with trace amounts of heavier elements. Across the galaxy, it is found that the amount of heavy elements in H II regions decreases with increasing distance from the galactic centre. This is because over the lifetime of the galaxy, star formation rates have been greater in the denser central regions, resulting in greater enrichment of the interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
with the products of nucleosynthesis
Nucleosynthesis
Nucleosynthesis is the process of creating new atomic nuclei from pre-existing nucleons . It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma from the Big Bang as it cooled below two trillion degrees...
.
Numbers and distribution
H II regions are found only in spiral galaxiesSpiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
like the Milky Way and irregular galaxies
Irregular galaxy
An irregular galaxy is a galaxy that does not have a distinct regular shape, like a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a...
. They are rarely seen in elliptical galaxies
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
. In irregular galaxies, they may be found throughout the galaxy, but in spirals they are most abundant within the spiral arms. A large spiral galaxy may contain thousands of H II regions.
The reason H II regions rarely appear in elliptical galaxies is that ellipticals are believed to form through galaxy mergers. In galaxy cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
s, such mergers are frequent. When galaxies collide, individual stars almost never collide, but the GMCs and H II regions in the colliding galaxies are severely agitated. Under these conditions, enormous bursts of star formation are triggered, so rapid that most of the gas is converted into stars rather than the normal 10% or less.
Galaxies undergoing such rapid star formation are known as starburst galaxies
Starburst galaxy
A starburst galaxy is a galaxy in the process of an exceptionally high rate of star formation, compared to the usual star formation rate seen in most galaxies. Galaxies are often observed to have a burst of star formation after a collision or close encounter between two galaxies...
. The post-merger elliptical galaxy has a very low gas content, and so H II regions can no longer form. Twenty first century observations have shown that a very small number of H II regions exist outside galaxies altogether. These intergalactic H II regions may be the remnants of tidal disruptions of small galaxies, and in some cases they may represent a new generation of stars in a galaxy's most recently accreted gas.
Morphology
H II regions come in an enormous variety of sizes. They are usually clumpy and inhomogeneous on all scales from the smallest to largest. Each star within an H II region ionises a roughly spherical region—known as a Strömgren sphere—of the gas surrounding it, but the combination of ionisation spheres of multiple stars within an H II region and the expansion of the heated nebula into surrounding gases with sharp density gradientsDensity Gradient
Density gradient is a spatial variation in density over an area. The term is used in the natural sciences to describe varying density of matter, but can apply to any quantity whose density can be measured...
results in complex shapes. Supernova explosions may also sculpt H II regions. In some cases, the formation of a large star cluster within an H II region results in the region being hollowed out from within. This is the case for NGC 604
NGC 604
NGC 604 is a H II region inside the Triangulum Galaxy. It was discovered by William Herschel on September 11, 1784. It is one of the largest H II regions in the Local Group of galaxies; at the galaxy's estimated distance of 2.7 million light-years its longest diameter is roughly 1500 light-years ,...
, a giant H II region in the Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...
.
Notable H II regions
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
. The Horsehead Nebula
Horsehead Nebula
The Horsehead Nebula is a dark nebula in the constellation Orion. The nebula is located just to the south of the star Alnitak, which is farthest east on Orion's Belt, and is part of the much larger Orion Molecular Cloud Complex. The Horsehead Nebula is approximately 1500 light years from Earth...
and Barnard's Loop
Barnard's Loop
Barnard's Loop is an emission nebula in the constellation of Orion. It is part of a Orion Molecular Cloud Complex which also contains the bright Horsehead and Orion nebulae. The loop takes the form of a large arc centred approximately on the Orion Nebula...
are two other illuminated parts of this cloud of gas. The Orion Nebula is actually a thin layer of ionised gas on the outer border of the OMC-1 cloud. The stars in the Trapezium cluster and especially θ1 Orionis are responsible for this ionisation.
The Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
, a satellite galaxy of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
at about 50 kpc from the Sun, contains a giant H II region called the Tarantula Nebula
Tarantula Nebula
The Tarantula Nebula is an H II region in the Large Magellanic Cloud . It was originally thought to be a star, but in 1751 Nicolas Louis de Lacaille recognized its nebular nature....
. Measuring at about this nebula is the most massive and second largest H II region in the Local group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
. It is much bigger than the Orion Nebula, and is forming thousands of stars, some with masses of over 100 times that of the sun—OB
OB star
OB stars are hot, massive stars of spectral types O or B which form in loosely organized groups called OB associations. They are short lived, and thus don't move very far from where they were formed within their life. During their lifetime, they will emit copious amounts of ultraviolet radiation...
and Wolf-Rayet star
Wolf-Rayet star
Wolf–Rayet stars are evolved, massive stars , which are losing mass rapidly by means of a very strong stellar wind, with speeds up to 2000 km/s...
s. If the Tarantula Nebula was as close to Earth as the Orion Nebula, it would shine about as brightly as the full moon in the night sky. The supernova SN 1987A
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...
occurred in the outskirts of the Tarantula Nebula.
Another giant H II region—NGC 604
NGC 604
NGC 604 is a H II region inside the Triangulum Galaxy. It was discovered by William Herschel on September 11, 1784. It is one of the largest H II regions in the Local Group of galaxies; at the galaxy's estimated distance of 2.7 million light-years its longest diameter is roughly 1500 light-years ,...
is located in M33
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...
spiral galaxy, which is about 817 kpc (2.66 million light years) away from the Sun. Measuring at approximately across NGC 604 is the second most massive H II region in the Local group after the Tarantula Nebula, however, it is slightly larger than the latter. It contains around 200 hot OB and Wolf-Rayet stars, which heat the gas inside it to millions of degrees producing the bright X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
emissions. The total mass of the hot gas in NGC 604 is about 6,000 Solar masses.
Current issues
As with planetary nebulae, determinations of the abundance of elementsChemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
in H II regions are subject to some uncertainty. There are two different ways of determining the abundance of metals (that is, elements other than hydrogen and helium) in nebulae, which rely on different types of spectral lines, and large discrepancies are sometimes seen between the results derived from the two methods. Some astronomers put this down to the presence of small temperature fluctuations within H II regions; others claim that the discrepancies are too large to be explained by temperature effects, and hypothesise the existence of cold knots containing very little hydrogen to explain the observations.
The full details of massive star formation within H II regions are not yet well known. Two major problems hamper research in this area. First, the distance from Earth to large H II regions is considerable, with the nearest H II (California Nebula
California Nebula
The California Nebula is an emission nebula located in the constellation Perseus. It is so named because it appears to resemble the outline of the US State of California on long exposure photographs. It is almost 2.5° long on the sky and, because of its very low surface brightness, it is extremely...
) region being over 300 pc (1,000 light-years) away; other H II regions are several times that distance away from Earth. Secondly, the formation of these stars is deeply obscured by dust, and visible light observations are impossible. Radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
and infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
light can penetrate the dust, but the youngest stars may not emit much light at these wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s.
See also
- Emission nebulaEmission nebulaAn emission nebula is a cloud of ionized gas emitting light of various colors. The most common source of ionization is high-energy photons emitted from a nearby hot star...
- Reflection nebulaReflection nebulaIn Astronomy, reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. The energy from the nearby star, or stars, is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust...
- Astronomical objectAstronomical objectAstronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
- H I regionH I regionAn H I region is an interstellar cloud composed of neutral atomic hydrogen , in addition to the local abundance of helium and other elements. These regions are non-luminous, save for emission of the 21-cm region spectral line. This line has a very low transition probability, so requires large...
- Planetary nebulaPlanetary nebulaA planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
- Protoplanetary nebulaProtoplanetary nebulaA protoplanetary nebula or preplanetary 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 subsequent planetary nebula phase. A PPN emits strongly in infrared radiation, and is a...
- Astronomical spectroscopyAstronomical spectroscopyAstronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
- Interstellar mediumInterstellar mediumIn astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...