Here's how:

* Composition: When light from a star passes through a spectrograph, it is separated into its different wavelengths (colors). Each element absorbs and emits light at specific wavelengths, creating unique "fingerprints" in the spectrum. By analyzing these fingerprints, astronomers can identify the elements present in the star.

* Temperature: The peak wavelength of a star's emitted light is related to its temperature. Hotter stars emit more blue light, while cooler stars emit more red light. By measuring the peak wavelength, astronomers can estimate the star's temperature.

* Motion: The Doppler effect causes the wavelengths of light to shift slightly depending on whether the object is moving towards or away from us. Astronomers can analyze the shifts in a star's spectrum to determine its radial velocity (speed towards or away from us).

In short, spectrographs are powerful tools that allow astronomers to learn a great deal about the universe by analyzing the light emitted by celestial objects.