The Electromagnetic Spectrum in Astronomy

The night sky the human eye evolved to read shows almost none of what is actually out there. The cold hydrogen threading the galaxy, the black holes feeding at the centers of distant galaxies, the hundred-million-degree plasma filling galaxy clusters — none of it registers in visible light, because different physical processes radiate at different wavelengths, and the eye is tuned to one narrow band. For most of astronomy's history we studied that thin slice and called it the universe. A complete picture demands observing the same object across many bands — radio, infrared, ultraviolet, X-ray, gamma-ray — what astronomers call multi-wavelength astronomy. Cool objects glow in the infrared and radio; hot ones in the ultraviolet, X-ray, and gamma. Most of the cosmos is invisible at any single wavelength, which means a 'complete' optical image is, for most of physics, a near-total blind spot.

The history of modern astronomy is in large 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 and ALMA in Chile are the most-used 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 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.