# How are molecules detected in space?

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. Please explain the methods involved.

• – user2449 Feb 8 '15 at 5:29

This is a broad question. The energy levels occupied by molecules consist of a mixture of rotational and vibrational energy states. The values of the energies of these states are characteristic of particular molecules - they can be measured in the laboratory or occasionally need to be calculated if the molecules can only be formed in conditions found in space. A molecule may make a transition from a higher to a lower energy level. When it does so, the energy difference is emitted as a photon with a frequency corresponding to the energy level difference: $\nu = (E_2 - E_1)/h$.