Different wavelengths of electromagnetic radiation are produced by different physical processes and reveal different aspects of distant objects, and modern astronomy is organized around this fact. Cool objects emit primarily in the infrared and radio; hot objects in the ultraviolet, X-ray, and gamma-ray. Most of the universe is invisible at any single wavelength, and a complete picture requires observing the same object across multiple bands — what astronomers call multi-wavelength astronomy. The history of the field since the mid-twentieth century is, in significant part, the history of opening up new bands: radio in the 1930s, infrared in the 1960s, X-ray and ultraviolet via space telescopes in the 1970s, gamma-ray in the 1990s, gravitational waves in 2015.
Radio (wavelengths > ~1 mm) reveals the neutral-hydrogen 21-cm line that maps the gas distribution of the Milky Way, molecular emission from carbon monoxide and water in cold clouds, AGN jets, and pulsar pulses; the Very Large Array in New Mexico, ALMA in Chile (millimeter wavelengths), and MeerKAT in South Africa (an SKA precursor) are the most-used current facilities. Infrared (~1–300 µm) reveals cool stars, dust-shrouded star-forming regions, exoplanet atmospheres, and the redshifted optical emission of the highest-redshift galaxies — Spitzer (2003–2020) and JWST (2022+) are the canonical IR observatories. Visible (~400–700 nm) is the most-developed regime; Hubble and the ground-based 8–10 m class (Keck, VLT, Subaru, Gemini) are the workhorses. Ultraviolet reveals hot young stars, stellar flares, and active galactic nuclei. X-ray reveals accretion onto compact objects and hot plasma in galaxy clusters (Chandra, 1999+). Gamma-ray — the most energetic photons — is the regime of gamma-ray bursts and AGN (Fermi, 2008+). Gravitational waves are a different channel altogether: LIGO, Virgo, and KAGRA together have detected dozens of compact-binary mergers since 2015. The diffraction limit — angular resolution scales as wavelength over aperture — ties the bands to specific instruments: matching 10 m optical resolution at radio wavelengths requires a synthesized aperture of kilometers, which is why all serious radio astronomy uses interferometry, combining many small dishes into the equivalent of one giant dish. The Event Horizon Telescope combined dishes from Hawaii to Antarctica into a planet-scale aperture and produced the first images of supermassive-black-hole shadows. Earth's atmosphere is opaque or distorting at most wavelengths, which is why Hubble, Chandra, and JWST exist on space platforms.
The next decade is multi-wavelength by design. The Vera Rubin Observatory (Chile, first light 2024) will image the entire visible southern sky every few nights for a decade, producing an unprecedented time-domain dataset that will reshape how transient events (supernovae, tidal disruptions, microlensing) are discovered and followed up. The Square Kilometre Array (Australia and South Africa, science around 2030) will be the most sensitive radio facility ever built. Athena (X-ray) and LISA (millihertz gravitational waves) are planned space missions for the 2030s. Each new band reveals classes of objects and physics the older bands could not.