<P> The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule making a transition from a high energy state to a lower energy state . The photon energy of the emitted photon is equal to the energy difference between the two states . There are many possible electron transitions for each atom, and each transition has a specific energy difference . This collection of different transitions, leading to different radiated wavelengths, make up an emission spectrum . Each element's emission spectrum is unique . Therefore, spectroscopy can be used to identify the elements in matter of unknown composition . Similarly, the emission spectra of molecules can be used in chemical analysis of substances . </P> <P> In physics, emission is the process by which a higher energy quantum mechanical state of a particle becomes converted to a lower one through the emission of a photon, resulting in the production of light . The frequency of light emitted is a function of the energy of the transition . Since energy must be conserved, the energy difference between the two states equals the energy carried off by the photon . The energy states of the transitions can lead to emissions over a very large range of frequencies . For example, visible light is emitted by the coupling of electronic states in atoms and molecules (then the phenomenon is called fluorescence or phosphorescence). On the other hand, nuclear shell transitions can emit high energy gamma rays, while nuclear spin transitions emit low energy radio waves . </P> <P> The emittance of an object quantifies how much light is emitted by it . This may be related to other properties of the object through the Stefan--Boltzmann law . For most substances, the amount of emission varies with the temperature and the spectroscopic composition of the object, leading to the appearance of color temperature and emission lines . Precise measurements at many wavelengths allow the identification of a substance via emission spectroscopy . </P> <P> Emission of radiation is typically described using semi-classical quantum mechanics: the particle's energy levels and spacings are determined from quantum mechanics, and light is treated as an oscillating electric field that can drive a transition if it is in resonance with the system's natural frequency . The quantum mechanics problem is treated using time - dependent perturbation theory and leads to the general result known as Fermi's golden rule . The description has been superseded by quantum electrodynamics, although the semi-classical version continues to be more useful in most practical computations . </P>

How would astronomers use an emission spectrum to identify the composition of a gas