FUSE Spectroscopy - An Astronomer's Best Friend

Astronomers use a technique called spectroscopy to analyze light from distant astronomical sources. A spectrograph is a device used to disperse light into its component wavelengths (colors). The dispersed light is known as a spectrum. The resolving power of a spectrograph is a measure of how fine the wavelengths of light, or colors, can be separated.

Each chemical element has a unique spectral "fingerprint", a set of spectral features defined by the energy levels of atoms of that element. The wavelength range observable by FUSE is extremely rich in these fingerprints. However, this also results in complexity since many spectral features of different elements appear to overlap (are "blended") unless they are observed at high resolution. One can make a direct analogy to looking at a set of overlapping fingerprints - it is difficult to resolve the whorls and lines of the individual prints unless a magnifying glass is used.

FUSE is a high-resolution spectrograph with a resolving power of 30,000, meaning that the divisions of light observable with FUSE are 30,000 times smaller than the wavelengths of the light itself. For comparison, this is equivalent to being able to identify the edge of an individual sheet of paper in a stack of paper 60 reams (about 10 feet) tall.

Astronomers can learn a lot from a spectrum. The patterns of spectral features tell them what elements are present in the gas how many atoms of each element are present. Shifts in the patterns tell them how fast the atoms are moving and whether they are approaching us or moving away. Spectral analysis of light provides a powerful tool for understanding the properties of astronomical objects in ways simple pictures cannot.