Almost thirty years ago the Infrared Astronomy Satellite,
IRAS, discovered that the universe contained many fabulously luminous galaxies,
some of them more than a thousand times brighter than our own galaxy, but which
are practically invisible at optical wavelengths. The reason for their optical
dimness is that their bright light comes not from stars, which can be seen in
the visible, but from dust that is warmed by bursts of star formation to
temperatures of about 70 kelvin (about 200 degrees below zero Celsius) where
infrared radiation predominates. Luminous galaxies not only shed light on how
galaxies evolve and form stars, they act as lanterns that can be seen across cosmological
distances, thereby helping scientists study the relatively early universe.