Here is a great piece, link has lots of images with the article;
ottawa.rasc.ca
Spectroscopy: Fingerprint of the Cosmos
by: Al Scott
--------------------------------------------------------------------------------
What is Spectroscopy?
Spectroscopy is the study of the cosmos through the composition of light. Natural processes leave a characteristic ‘fingerprint’ encoded in the light that we observe.
Images tell us where to look for interesting new discoveries. Spectroscopy explains what we are seeing.
The electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency.
In nature there are various different processes which emit electromagnetic radiation. Anywhere charged particles are being accelerated, light is emitted. The frequency tells us about the energy of the source.
Thermal Radiation
Every solid object vibrates at the molecular level. These oscillations result in thermal radiation. The wavelength of the peak emission depends on the temperature of the object.
This is why a ‘white-hot’ object is hotter than a ‘red-hot’ object.
Room temperature objects have a peak emission at ~10 ?m in the infrared.
Types of Spectra
Free floating atoms and molecules in a gas will only vibrate at specific frequencies, giving rise to line spectra: effectively a finger print which is unique to each chemical species. Hot gasses emit while cold gasses absorb.
Stars are classified by their spectra. The OBAFGKM classification scheme breaks down roughly as a function of temperature. The line spectra, superimposed on the stellar thermal emission tell us about the ‘cool’ gaseous elements and molecules on the surface of the star.
The Hertzsprung-Russel Diagram
By plotting temperature against brightness, we can calculate a star’s size even though we can’t resolve it’s disc (stars of identical size fall on the dashed diagonal lines). Most stars appear on the ‘Main Sequence’.
What about dark dust and gas?
Can we somehow measure the fingerprint of dark clouds?
The way that dust ‘reddens’ starlight tells us about the size of the intervening particles. Visible ‘extinction’ is caused by ‘large’ grains ~0.1 ?m.
Can you see how the partially obscured stars are reddened by the dust cloud? Left: Barnard 68 from FORS at the VLT and ESO
The fingerprint of dark interstellar dust and gas can be taken by comparing the spectra of two stars of the same type.
The ‘reddened’ star is the same spectral type, but is partially obscured by a cloud of cold dust and gas.
We know it’s the same type even though the thermal spectrum appears reddened (due to foreground dust). This is because the same stellar absorption lines are present in the spectrum indicating a similar surface temperature.
The strongest DIBs were first noted in the 1920s. These bands in the visible region represent the longest unsolved problem in spectroscopy.
Prior to the ’70s, astronomers used to think that molecules couldn’t survive the harsh UV environment of space. Since then, large radio telescopes have detected the fingerprint of complex molecules in dark clouds, some up to 13 atoms in size.
New laboratory work suggests that the DIBs are associated with complex carbonaceous (organic) molecules and ions.
Complex organic molecules have been identified in space based on their fingerprint! It is likely that these molecules are formed in dense clouds, perhaps through surface-catalyzed reactions on dust grains, where they are protected from the UV flux until they reach sizes where photo-dissociation in diffuse interstellar regions becomes inefficient. Right: The fingerprint of Buckminsterfullerene is found amongst the DIBs. (Foing & Erhenfreund 1997)
Mixture of PAH cations are found in the Orion bar. These PAH molecules are commonly found in tar, coal, auto exhaust and BBQ grills. Left: NASA/Ames Astrochemistry
Pre-biotic chemistry results in a cornucopia of organic molecules in the harsh, low-density environment of space, all recognized by their electromagnetic fingerprint. Right: Ehrenfreund & Charnley, 2000
In the molecular clouds obscuring Sagittarius B2, ethanol has been measured, using large ground-based radio telescopes, at a density of ~1 molecule/m3.
This may not seem like much, but when you consider just one typical cloud with a 5 parsec radius, there’s enough alcohol to supply every man, woman and child on earth with 1 billion pints of ice cold beer each, every second, for the extent of their natural lives!
Unfortunately, there’s also some pretty nasty stuff out there like methanol and formaldehyde which would result in a rotten hangover.
Astrochemists have determined that this is actually grain alcohol. The interstellar gas cannot produce the observed densities faster than they are destroyed by UV light. The alcohol can only be produced on the surface of dust grains at the required rates.
There is also plenty of water in interstellar clouds. The Submillimeter Wave Astronomy Satellite has discovered that the ratio of alcohol to cold water in some regions of Sgr B2 ranges up to 2%, suggesting that any beer one might make, if one were to condense these clouds, would be sold in the US.
|
