Recently the sugar glycoaldehyde was detected in a star system 400 light years from Earth. How exactly are molecules detected in space? I am aware that spectroscopy is used to detect them, but I don't know the details.

  • $\begingroup$ This is a very interesting question. My basic understanding that the molecule has a spectral line within the electromagnetic spectrum that identifies it, like a thumbprint. When a telescope is directed at such star you will receive the spectral line but at a different frequency due to the phenomena of redshift. $\endgroup$
    – AlanZ2223
    Feb 8, 2015 at 2:59
  • $\begingroup$ You may ask this on the astronomy stack exchange and get an answer! $\endgroup$ Feb 8, 2015 at 4:13
  • $\begingroup$ yeah, try astronomy or physics stackexchange $\endgroup$ Feb 8, 2015 at 6:02
  • $\begingroup$ I'm not an astrochemist, but the key technology apparently is infrared spectroscopy. One instrument currently in use is the HIFI (Heterodyne Instrument for the Far Infrared) aboard the Herschel probe. Please have a look at the site of ESA and the JPL for further information. $\endgroup$ Feb 8, 2015 at 8:36

1 Answer 1


The research article corresponding to the observation to which the question is referring is Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA. Note that the molecule is glycolaldehyde rather than glycoaldehyde.

ALMA is the Atacama Large Millimeter/submillimeter Array, a radio telescope array in the Atacama desert. The extremely low humidity of the Atacama desert minimizes water vapor interference.

This observation of glycolaldehyde in space is not the first. It was earlier observed by Greenbank radio telescope, in West Virginia, USA, for example. The Greenbank observation is reported in INTERSTELLAR GLYCOLALDEHYDE: THE FIRST SUGAR. Glycolaldehyde was detected through radio signals in the 71-103 GHz range that are photons emitted when the molecule transitions from an excited rotation state to a lower energy rotational state.

In the ALMA observation, the molecule was detected based upon radio signals near 220 GHz and 690 GHz. These correspond to photons emitted by the molecule when transitioning from an excited rotational state to a lower energy rotational state, but are a different set of transitions from those observed at Greenbank, including some for excited vibrational states.

I'll try to add more information later, there are a wide variety of spectroscopic techniques: absorbance vs. emission; rotation, vibration and electronics transitions, and many different frequency ranges.

  • $\begingroup$ How does rotation and vibration lead to emission. How do you map the frequency of light received uniquely to a molecule. How do you know this frequency of light comes from a molecule and not some energy from a star or other source. I guess I'm concerned with how the actual spectroscopy works. $\endgroup$ Feb 8, 2015 at 16:20
  • $\begingroup$ The rotational states are quantized. Quantum mechanics dictates that a molecule can not rotate arbitrarily, but only in discreet energy amounts. See en.wikipedia.org/wiki/Rotational_spectroscopy On Earth in laboratories libraries of spectra of molecules are compiled such as: spec.jpl.nasa.gov/ftp/pub/catalog/catdir.html Then a comparison is made between the signal from space and the library of spectra $\endgroup$
    – DavePhD
    Feb 8, 2015 at 16:36

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