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Spectroscopy: A Powerful Astronomical Tool

By Yagyasha Rastogi


Spectroscopy in colloquial terms is the technique of splitting light (more precisely electromagnetic radiations) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. However, the spectrums produced are not as smooth as a rainbow. The energy levels of electrons in atoms and molecules are quantized, and electromagnetic radiation is only absorbed and emitted at particular wavelengths (Gotame, 2020). As a result, spectra are not smooth but rather fragmented by absorption or emission lines.

Image Source: ESO


The spectra are recorded on a CCD detector and finally saved in computer files for further processing and analysis.

The spectra of various stars reveal the following aspects of the star’s presence:

  1. Constituent elements -Each element in the periodic table can exist as a gas, resulting in a spectrum of bright lines that are unique to that element. Hydrogen will not resemble helium, which will not resemble carbon, which will not resemble iron, and so on. As a result, astronomers can tell what kind of stuff is there in the stars by looking at the lines in the spectrum.

  2. Temperature and density of an element- A star with a lower-pressure photosphere shows narrower spectral lines than a star of the same temperature with a higher-pressure photosphere. The difference is substantial enough that thorough spectral analysis can distinguish which of two stars at the same temperature has a higher pressure than the other.

  3. Speed of the body - If the star is not moving with respect to the Sun, then the wavelength corresponding to each element will be the same as those we measure in a laboratory here on Earth. But if stars are moving toward or away from us, we must consider the Doppler effect. We should see all the spectral lines of moving stars shifted toward the red end of the spectrum if the star is moving away from us, or toward the blue (violet) end if it is moving toward us.(Lumen learning)

  4. Informs about any physical changes -If the lines grow and fade in strength we can learn about the physical changes in the star.

Some other things that the spectra of a star tell us are, magnetic field (if any), if the body is rotating or not, Stellar winds, and helps us estimate the mass and size of the star.


Scientists can use spectroscopy to figure out how an object like a black hole, neutron star, or active galaxy creates light, how fast it moves, and what elements it's made of. One of the most basic instruments used by scientists to examine the Universe is spectroscopy. The enormous quantities of information contained within a single spectrum makes spectroscopy one of the most powerful tools at an astronomer’s disposal.


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