How do astronomers use spectrographs ?

Hint: To understand the applications of spectrographs, and how they are being used by astronomers, we must know about the working principle of spectrographs, and some basic concepts of spectroscopy. Spectrographs are used to obtain and measure the spectrum of the sample by shining a beam of electromagnetic radiation.

Complete answer:
 Spectrographs are an optical instrument which belong to the class of spectrometers and are used to obtain and record an astronomical spectrum. It disperses the light from the object under consideration into its component wavelengths, so that it can be recorded and then analyzed. Now, there are various ways through which dispersion of light is achieved which includes diffraction grating, prism, etc.

Now, different celestial objects produce different types of spectra, and these spectra can of emission or absorption type. In the obtained astronomical spectrum there will be some spikes or spectral lines which are used by astronomers to determine about the element of a star or planet, its temperature, density of its matter and magnetic field around it.

Accurate measurement of these spectral lines can tell us about velocities and redshifts also. Various shapes of emission or absorption lines are used to learn about the pressure, density, and rotation of the object. If the intensity of these lines grow or fade then we can learn about the physical changes in the star or the object under observation. The radiation from stuff in between the stars allows astronomers to study the interstellar medium which is not empty but consists of dust or gaseous particles.

Note: Spectroscopy is one of the fundamental tools which scientists use to study the Universe. Spectroscopy is a technique in which we are concerned with the spectrum of the sample obtained after shining electromagnetic radiation upon it. Spectrum is obtained because of the interaction of matter with radiation. Various types of spectroscopic methods are available, for example depending upon the absorption or emission of radiation, spectroscopic methods available includes visible, infrared, ultraviolet, and radio (nuclear magnetic resonance, NMR).