Photometry and Spectroscopy

Just about all of the information we have about the Universe and the objects within it comes from gathering and studying light. Astronomers collect light to produce the beautiful images of the cosmos that we are all familiar with. These images provide useful information about the structure of objects in space, but to get a more detailed view of what is going on in the Universe, light has to be studied in much more detail. This is where photometry and spectroscopy come in. These are the two most common techniques used by astronomers to study the cosmos.


Photometry is a way to measure how much light we receive from objects in space. It is a measure of the relative amount of light each color (or wavelength range) has. Instruments used for the measurement of light intensity, called photometers, compare an unknown intensity with a standard or known intensity. Different filters are used to let different wavelengths of light through to the detectors. Light detectors such as CCDs (charged-couple devices), electronically measure the amount of light entering the detector at each wavelength range and this information is fed directly into a computer for analysis. Often imaging (creating pictures) and photometry are done at the same time.

The Multiband Imaging Photometer for the Spitzer Space Telescope


Spectroscopy is the detailed study of the light from an object. Spectrometers are instruments which spread light out into its wavelengths, creating a spectra. Within this spectra, astronomers can study emission and absorption lines which are the fingerprints of atoms and molecules. An emission line occurs when an electron drops down to a lower orbit around the nucleus of an atom and loses energy. An absorption line occurs when electrons move to a higher orbit by absorbing energy. Each atom has a unique spacing of orbits and can emit or absorb only certain energies or wavelengths. This is why the location and spacing of spectral lines is unique for each atom.

A detailed spectra of the Sun

Astronomers can learn a great deal about an object in space by studying its spectrum. By identifying the atomic and molecular fingerprints in a spectra, we can learn about an object's composition (what its made of). The intensity and width of spectral lines tell us about an object's temperature and density. The wavelengths at which the spectral lines of atoms and molecules appear tell us about its motion (both its rotation as well as how fast it is moving towards or away from us).

Carbon II spectral line intensity profiles showing that carbon is abundant in many regions of space.

The Spitzer Space Telescope will perform both spectroscopy and photometry, using three instruments, to gather new information about the Universe. Spitzer's Infrared Array Camera (IRAC) will provide imaging capabilities at near- and mid-infrared wavelengths. It is a general-purpose camera that will be used by scientists for a wide variety of astronomical research programs. Spitzer's Infrared Spectrograph (IRS) will provide both high- and low-resolution spectroscopy at mid-infrared wavelengths. Spitzer's Multiband Imaging Photometer (MIPS) will provide imaging and limited spectroscopic data at far-infrared wavelengths.