Diffuse interstellar band
Encyclopedia
Diffuse interstellar bands (DIBs) are absorption features seen in the spectra
of astronomical object
s in our galaxy
. They are caused by the absorption of light by the interstellar medium
. More than 200 bands are seen, in ultraviolet
, visible and infrared
wavelengths.
The origin of DIBs was unknown and hotly disputed for many years, and the DIBs were long believed to be due to polycyclic aromatic hydrocarbon
s and other large carbon
-bearing molecules. However, no agreement of the bands could be found with laboratory measurements or with theoretical calculations.
- the light from astronomical object
s dispersed using a prism
or, more usually, a diffraction grating
. A typical stellar spectrum will consist of a continuum
, containing absorption lines, each of which is attributed to a particular atom
ic energy level
transition in the atmosphere of the star.
All astronomical objects are affected by extinction
, the absorption of photon
s by the interstellar medium
. Interstellar absorption predominantly affects the whole spectrum in a continuous way, rather than causing absorption lines, but in 1922 astronomer Mary Lea Heger
first observed a number of line-like absorption features which seemed to be interstellar in origin.
Their interstellar nature was shown by the fact that the strength of the observed absorption was roughly proportional to the extinction, and that in objects with widely differing radial velocities
the absorption bands were not affected by Doppler shifting, implying that the absorption was not occurring in or around the object concerned. The name Diffuse Interstellar Band, or DIB for short, was coined to reflect the fact that the absorption features are much broader than the normal absorption lines seen in stellar spectra.
The first DIBs observed were those at wavelengths 578.0 and 579.7 nanometres. Other strong DIBs are seen at 628.4, 661.4 and 443.0 nm. The 443.0 nm DIB is particularly broad at about 1.2 nm across - typical intrinsic stellar absorption features are 0.1 nm or less across.
Later spectroscopic
studies at higher spectral resolution
and sensitivity revealed more and more DIBs; a catalogue of them in 1975 contained 25 known DIBs, and a decade later the number known had more than doubled. The first detection-limited survey was published by Peter Jenniskens
and Xavier Desert in 1994 (see Figure above), which led to the first conference on The Diffuse Interstellar Bands at the University of Colorado in Boulder on May 16–19, 1994. Today over 300 have been detected.
In recent years, very high resolution spectrograph
s on the world's most powerful telescope
s have been used to observe and analyse DIBs. Spectral resolutions of 0.005 nm are now routine using instruments at observatories such as the European Southern Observatory
at Cerro Paranal
, Chile
, and the Anglo-Australian Observatory
in Australia
, and at these high resolutions, many DIBs are found to contain considerable sub-structure.
s of any ion
or molecule
, and so the material which was responsible for the absorption could not be identified. A large number of theories were advanced as the number of known DIBs grew, and determining the nature of the absorbing material (the 'carrier') became a crucial problem in astrophysics
.
One important observational result is that the strengths of most DIBs are not correlated with each other. This means that there must be many carriers, rather than one carrier responsible for all DIBs. Also significant is that the strength of DIBs is broadly correlated with the extinction. Extinction is caused by dust
in the interstellar medium, and so DIBs are likely to be also due to dust or something related to it.
The existence of sub-structure in DIBs supports the idea that they are caused by molecules. Substructure results from band heads in the rotational band contour and from isotope substitution. In a molecule containing, say, three carbon
atoms, some of the carbon will be in the form of the carbon-13
isotope
, so that while most molecules will contain three carbon-12
atoms, some will contain two C12 atoms and one C13 atom, much less will contain one C12 and two C13s, and a very small fraction will contain three C13 molecules. Each of these forms of the molecule will create an absorption line at a slightly different rest wavelength.
The most likely candidate molecules for producing DIBs are thought to be large carbon-bearing molecules, which are common in the interstellar medium. Polycyclic aromatic hydrocarbon
s, long carbon-chain molecules, and fullerenes are all potentially important.
Astronomical spectroscopy
Astronomical 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...
of astronomical object
Astronomical object
Astronomical 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...
s in our galaxy
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...
. They are caused by the absorption of light by 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...
. More than 200 bands are seen, in 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...
, visible 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...
wavelengths.
The origin of DIBs was unknown and hotly disputed for many years, and the DIBs were long believed to be due to polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
s and other large carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
-bearing molecules. However, no agreement of the bands could be found with laboratory measurements or with theoretical calculations.
Discovery and history
Much astronomical work relies on the study of spectraElectromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
- the light from astronomical object
Astronomical object
Astronomical 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...
s dispersed using a prism
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
or, more usually, a diffraction grating
Diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as...
. A typical stellar spectrum will consist of a continuum
Continuum (theory)
Continuum theories or models explain variation as involving a gradual quantitative transition without abrupt changes or discontinuities. It can be contrasted with 'categorical' models which propose qualitatively different states.-In physics:...
, containing absorption lines, each of which is attributed to a particular 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 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...
transition in the atmosphere of the star.
All astronomical objects are affected by extinction
Extinction (astronomy)
Extinction is a term used in astronomy to describe the absorption and scattering of electromagnetic radiation by matter between an emitting astronomical object and the observer. Interstellar extinction—also called Galactic extinction, when it occurs in the Milky Way—was first...
, the absorption of 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 by 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...
. Interstellar absorption predominantly affects the whole spectrum in a continuous way, rather than causing absorption lines, but in 1922 astronomer Mary Lea Heger
Mary Lea Heger
Mary Lea Shane Born: Mary Lea Heger on July 13, 1897 in Wilmington, Delaware.Married C. Donald Shane in 1920 and was a mother of two.Accredited with founding the Lick Observatory Archives located in the Dean E. McHenry Library...
first observed a number of line-like absorption features which seemed to be interstellar in origin.
Their interstellar nature was shown by the fact that the strength of the observed absorption was roughly proportional to the extinction, and that in objects with widely differing radial velocities
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
the absorption bands were not affected by Doppler shifting, implying that the absorption was not occurring in or around the object concerned. The name Diffuse Interstellar Band, or DIB for short, was coined to reflect the fact that the absorption features are much broader than the normal absorption lines seen in stellar spectra.
The first DIBs observed were those at wavelengths 578.0 and 579.7 nanometres. Other strong DIBs are seen at 628.4, 661.4 and 443.0 nm. The 443.0 nm DIB is particularly broad at about 1.2 nm across - typical intrinsic stellar absorption features are 0.1 nm or less across.
Later spectroscopic
Astronomical spectroscopy
Astronomical 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...
studies at higher spectral resolution
Spectral resolution
The spectral resolution of a spectrograph, or, more generally, of a frequency spectrum, is a measure of its ability to resolve features in the electromagnetic spectrum...
and sensitivity revealed more and more DIBs; a catalogue of them in 1975 contained 25 known DIBs, and a decade later the number known had more than doubled. The first detection-limited survey was published by Peter Jenniskens
Peter Jenniskens
Petrus Matheus Marie Jenniskens is a Dutch astronomer and a senior research scientist at the Carl Sagan Center of the SETI Institute and at NASA Ames Research Center. He is an expert on meteor showers. Jenniskens is the author of the 790 page book "Meteor Showers and their Parent Comets"...
and Xavier Desert in 1994 (see Figure above), which led to the first conference on The Diffuse Interstellar Bands at the University of Colorado in Boulder on May 16–19, 1994. Today over 300 have been detected.
In recent years, very high resolution spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
s on the world's most powerful 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...
s have been used to observe and analyse DIBs. Spectral resolutions of 0.005 nm are now routine using instruments at observatories such as the European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
at Cerro Paranal
Cerro Paranal
Cerro Paranal , also known as Paranal Mountain is a mountain in the Atacama desert of northern Chile that is home to the Paranal Observatory. It is famous for hosting the Very Large Telescope and the VLT Survey Telescope. It is located 120 km west of Antofagasta and 80 km north of Taltal,...
, Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
, and the Anglo-Australian Observatory
Anglo-Australian Observatory
The Australian Astronomical Observatory , formerly the Anglo-Australian Observatory, is an optical/near-infrared astronomy observatory with its headquarters in suburban Sydney, Australia...
in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, and at these high resolutions, many DIBs are found to contain considerable sub-structure.
The nature of the carrier
The great problem with DIBs, apparent from the earliest observations, was that their central wavelengths did not correspond with any known spectral lineSpectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s of any 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...
or molecule
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...
, and so the material which was responsible for the absorption could not be identified. A large number of theories were advanced as the number of known DIBs grew, and determining the nature of the absorbing material (the 'carrier') became a crucial problem in astrophysics
Astrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
.
One important observational result is that the strengths of most DIBs are not correlated with each other. This means that there must be many carriers, rather than one carrier responsible for all DIBs. Also significant is that the strength of DIBs is broadly correlated with the extinction. Extinction is caused by dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
in the interstellar medium, and so DIBs are likely to be also due to dust or something related to it.
The existence of sub-structure in DIBs supports the idea that they are caused by molecules. Substructure results from band heads in the rotational band contour and from isotope substitution. In a molecule containing, say, three carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atoms, some of the carbon will be in the form of the carbon-13
Carbon-13
Carbon-13 is a natural, stable isotope of carbon and one of the environmental isotopes. It makes up about 1.1% of all natural carbon on Earth.- Detection by mass spectrometry :...
isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
, so that while most molecules will contain three carbon-12
Carbon-12
Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons....
atoms, some will contain two C12 atoms and one C13 atom, much less will contain one C12 and two C13s, and a very small fraction will contain three C13 molecules. Each of these forms of the molecule will create an absorption line at a slightly different rest wavelength.
The most likely candidate molecules for producing DIBs are thought to be large carbon-bearing molecules, which are common in the interstellar medium. Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbons , also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH...
s, long carbon-chain molecules, and fullerenes are all potentially important.