Electromagnetic radiation and optical telescopes

Celestial bodies radiate at different wavelengths: radio, microwave, infrared, visible light (≈ 400–700 nm), ultraviolet, X-rays and gamma rays. All travel in vacuum at c ≈ 300,000 km/s and obey λ · ν = c. The Earth’s atmosphere lets only part through: the optical “window” (≈ 0.3–1 µm) and the radio “window” (≈ 1 mm to 10–20 m) are open, but ultraviolet, X-rays, gamma rays and much of the infrared are absorbed, so they are observed with space telescopes: Hubble (1990), Chandra (X-ray, 1999), Fermi (gamma, 2008) and the James Webb Space Telescope (infrared, launched 25 December 2021, at the L2 point ≈ 1.5 million km from the Earth). Studying one object in several ranges – multiwavelength astronomy – gives a fuller picture: a star hidden in dust shows up in the infrared, hot gas around a black hole in X-rays. Gravitational waves (LIGO, 2015) and neutrinos now serve as additional “messengers”.