GALEX

GALEX was designed to measure how galaxies formed stars over much of the history of the universe. NASA launched the spacecraft on 28 April 2003 as an Explorer-class mission, placing it in low Earth orbit with a 50-centimeter ultraviolet telescope and a wide circular field of view.¹ Its detector system recorded two bands simultaneously: far-ultraviolet light at approximately 135–175 nanometers and near-ultraviolet light at approximately 175–280 nanometers.²

The telescope used a dichroic beam splitter to send incoming ultraviolet light to separate detectors, allowing color information to be collected for the same sources. Ultraviolet emission is especially sensitive to short-lived, massive stars, so GALEX could identify comparatively recent star formation even when a galaxy appeared faint or quiescent at visible wavelengths. The mission was led by the California Institute of Technology for NASA and completed its science operations in 2013.¹

GALEX combined an exceptionally broad sky survey with deeper targeted observations. Its All-Sky Imaging Survey mapped more than 29,000 square degrees, while medium- and deep-imaging programs examined selected fields to fainter ultraviolet limits.² The resulting catalogues supplied ultraviolet measurements for millions of sources and became a major foundation for studies of galaxies, stars, quasars, and the diffuse sky.

The mission established ultraviolet luminosity functions and star-formation-rate measurements for nearby galaxies, helping astronomers quantify the evolution of cosmic star formation.³ GALEX also revealed ultraviolet features that are inconspicuous in optical images, including extended ultraviolet disks around some nearby galaxies and ultraviolet-bright regions associated with tidal interactions. Joint analyses with optical surveys such as the Sloan Digital Sky Survey and infrared missions such as Spitzer made it possible to compare young stars, older stellar populations, dust, and active galactic nuclei within the same galaxies.⁴

GALEX data remain valuable because ultraviolet observations connect directly to stellar ages and recent star formation while covering a much larger area than many pointed ultraviolet programs. The mission's public image products and source catalogues are distributed through NASA's Mikulski Archive for Space Telescopes, where researchers can retrieve calibrated observations and combine them with data from other observatories.⁵

Researchers use GALEX measurements to study nearby galaxy evolution, low-level star formation, stellar populations in galaxy halos, ultraviolet excesses in old stellar systems, and the environments of quasars and active galactic nuclei. Its large, homogeneous survey also supports statistical work: rather than concentrating only on famous objects, astronomers can compare rare systems with ordinary galaxies across broad samples. The ultraviolet source counts and background measurements have additionally been used to investigate the diffuse extragalactic ultraviolet background, although such analyses must account carefully for foreground emission and instrumental effects.²

GALEX's unusual strength was its combination of ultraviolet sensitivity, wide coverage, and two-band imaging rather than extreme angular resolution. That design made it possible to discover ultraviolet structures extending beyond the conventionally defined optical edges of galaxies, including outer disks where star formation was sparse but detectable.⁶

The mission also contributed to time-domain astronomy. Repeated visits to portions of the sky revealed ultraviolet variability from active galactic nuclei, transient events, variable stars, and explosive stellar phenomena. GALEX observations have been used retrospectively to identify ultraviolet counterparts or early emission from some supernovae and tidal-disruption candidates. Another less visible legacy is methodological: its catalogue and calibration work established widely reused ultraviolet photometric standards and showed how space-based ultraviolet surveys could be merged with optical and infrared databases to infer galaxy properties without relying on a single wavelength regime.²

Far-ultraviolet (FUV)

GALEX's shorter-wavelength imaging band, approximately 135–175 nanometers, especially sensitive to hot young stars.

Near-ultraviolet (NUV)

GALEX's longer-wavelength imaging band, approximately 175–280 nanometers, useful for tracing young and intermediate-age stellar populations.

All-Sky Imaging Survey

GALEX's broadest survey program, designed to image most of the sky accessible to the spacecraft at relatively shallow depth.

Ultraviolet luminosity function

A statistical description of how many galaxies occur at different ultraviolet luminosities, used to study star formation across cosmic time.