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Diffuse interstellar band

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Relative strengths of observed diffuse interstellar bands
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   Relative strengths of observed diffuse interstellar bands

   Diffuse interstellar bands (DIBs) are absorption features seen in the
   spectra of astronomical objects in the Galaxy. They are caused by the
   absorption of light by the interstellar medium. About 100 bands are
   seen, in ultraviolet, visible and infrared wavelengths.

   The origin of DIBs was unknown and hotly disputed for many years, but
   currently they seem likely to be due mostly to polycyclic aromatic
   hydrocarbons and other large carbon-bearing molecules..

Discovery and history

   Much astronomical work relies on the study of spectra - the light from
   astronomical objects 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 atomic energy level transition in the atmosphere of the
   star.

   All astronomical objects are affected by extinction, the absorption of
   photons by the interstellar medium. Interstellar absorption
   predominantly affects the whole spectrum in a continuous way, rather
   than causing absorption lines, but in 1921, astronomers 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. Today over 300 have been detected, but none - identified.

   In recent years, very high resolution spectrographs on the world's most
   powerful telescopes 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.

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 lines 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. 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 C^12 atoms and one C^13 atom, much less will contain one
   C^12 and two C^13s, and a very small fraction will contain three C^13
   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 hydrocarbons, long carbon-chain molecules,
   and fullerenes are all potentially important. However, fully
   identifying the molecules responsible for each DIB is still very much a
   work in progress.
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