Spectroscopy

[kirkster501]

I read that the lines present in the spectrum indicate the elements present on the remote object emitting the light. But why don't elements in the path of that light on its way towards us contribute to that spectrum and thus skew the result?

Always wondered this....!

Steve

[JulianO]

I read that the lines present in the spectrum indicate the elements present on the remote object emitting the light. But why don't elements in the path of that light on its way towards us contribute to that spectrum and thus skew the result?

Always wondered this....!

Short answer - they do. Telluric lines or telluric contamination. This can be corrected for to a certain extent, and for some things its not really an issue - hydrogen and helium not being significant parts of the atmosphere.

[Ben Ritchie]

Certainly do - attached is a screengrab of the raw, unrectified spectrum of an B0.5 Iabe star taken with the FLAMES spectrograph on the VLT, happen to just be working on it; it illustrates a few of the problems we have. The major features from the star itself are H-alpha in emission and He I 6678 and 7065 in absorption, but there's other issues to deal with

- the complex absorption feature around 6900 angstroms is a telluric (i.e. atmospheric) absorption band from molecular oxygen

- the narrow absorption feature at 6610 angstroms is absorption due to interstellar material (known as a DIB; a diffuse interstellar band)

- the narrow spikes are sky emission lines

so they all need to be removed to get a clean spectrum (by subtracting telluric standards, nearby unreddened standards, and sky spectra respectively). In addition, you'll see strong nebular emission from H-alpha and He I 6678/7065 if there's nebulosity around, in this case there isn't but i'll dig out a spectrum from the SMC to give an example of that.

[Ben Ritchie]

Attached is an object in NGC346 in the SMC, again with VLT/FLAMES. The region is actively forming stars, and looks like this:

The star is a B1.5 giant, so H-alpha is broad and in absorption (it's actually a binary too, so you get a composite H-alpha absorption line, the companion is O9 or thereabouts), but you get a huge H-alpha emission line superimposed from the surrounding nebulosity.

[AlexB67]

It is also perhaps worth pointing out that certain spectroscopic techniques are only sensitive to a specific property, for example infrared spectroscopy only works when a molecule has a dipole moment. In simplee terms, a symmetric molecule like benzene ( a hexagon ) or species that have a symmetric charge distribution do not aborb for this reason, and would not be observed in an infrared spectrum as for common diatomics H2, N2 etc. Other techniques such as Ramam for example work on a different basis, though much weaker, can observe such species.

[AlexB67]

I should have added , for the typical property of interest I was referring to, that are not present in single atoms as given in the examples above, infrared is often used to detect vibrational and rotational modes in a molecule, as a means to fingerprint, or identify the molecule in question, though there are other interesting properties one can calculate. The absorbed radiation will only occur when it matches the frequency of the rovibrational motion in that molecule and when it has a dipole moment since the motions follow the laws of quantum mechanics. The reasons are somewhat involved, though explained in most chemistry, physics textbooks covering the theory of spectroscopy at undergraduate level.