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.