Spectroscopy involves spreading light out into a spectrum in order to analyze the intensity (or brightness) of individual colors, or wavelengths. During a spectroscopic observation, light from space is directed through a spectrograph, which spreads the light out into its component wavelengths. The light then strikes the detectors, which measure the intensity of each individual wavelength of light. Spectral data are typically plotted on a graph of intensity vs. wavelength. Differences in brightness with wavelength, and the presence or absence of specific wavelengths, provide information about temperature, composition, density, motion, and distance. All four of Webb’s scientific instruments (MIRI, NIRCam, NIRSpec, and NIRISS) have spectroscopy modes.
Wide-Field Slitless Spectroscopy involves capturing the overall spectrum of a wide field of view—a field of stars, part of a nearby galaxy, or many galaxies at once. (NIRCam and NIRISS)
Single-Object Slitless Spectroscopy involves capturing the spectrum of a single bright object like a star in a field of view. (MIRI and NIRISS)
Slitted Spectroscopy provides the ability to capture the spectrum of a single object—a single star, a single exoplanet, or a single distant galaxy—in a wide field of view. Single slit spectroscopy is also used to analyze the spectrum of a small area of an object that is large in the field of view, such as a galaxy or planet. (NIRSpec and MIRI)
Multi-Object Spectroscopy involves using a microshutter array to capture individual spectra of up to 100 objects or locations in space at one time. Multi-object spectroscopy is important for efficiency, in particular when observing very distant and dim targets, such as ancient galaxies, which require hundreds of hours of observation time. (NIRSpec)
Integral Field Unit Spectroscopy (IFU) involves a combination of imaging and spectroscopy. During an IFU observation, the instrument captures an image of the field of view along with individual spectra of each pixel in the field of view. IFU observations allow astronomers to investigate how properties—such as composition, temperature, and motion—vary between different objects such as stars in a crowded star field, or from place to place over a large region of space such as a galaxy or nebula. (NIRSpec and MIRI)
Time-Series Spectroscopy involves capturing the spectrum of an object or region of space at regular intervals in order to observe how the spectrum changes over time. Time-series spectroscopy is used to study planets as they transit their stars. (NIRCam, NIRSpec, and MIRI)
Credits
Illustration
NASA, ESA, Andi James (STScI)
