A nebula is a diffuse cloud of gas—primarily hydrogen and helium—and interstellar dust that occupies regions between stars and forms part of the Interstellar Medium, typically with extremely low average densities compared with Earth’s atmosphere. Today the term is used for interstellar clouds within a galaxy; earlier usage also included distant "spiral nebulae" that are now known to be galaxies. Encyclopaedia Britannica;
Encyclopaedia Britannica.
Etymology and historical usage
- –The English word derives from Latin nebula, meaning mist or cloud, a sense attested in early modern scientific English before its astronomical usage.
Merriam-Webster Dictionary.
- –In the late 19th and early 20th centuries, many diffuse objects were classed as "nebulae"; by the 1920s, Edwin Hubble’s identification of Cepheid variables in the Andromeda "Nebula" (M31) established that spiral nebulae are external galaxies, narrowing the technical meaning of nebula to interstellar clouds.
Encyclopaedia Britannica;
Classification and examples
- –Emission nebulae (H II regions) glow because nearby hot O- or B-type stars ionize hydrogen; typical gas temperatures are around 8,000–10,000 K and densities range from roughly 10 to 100,000 particles per cm³. H II region
- –Reflection nebulae shine by scattering starlight from nearby, relatively cooler luminous stars; their characteristic bluish color arises from preferential scattering of shorter wavelengths by fine dust grains.
NASA Hubble;
- –Dark (absorption) nebulae are dense, cold clouds—often molecular—that block background starlight at visible wavelengths; small isolated examples are known as Bok globules.
- –Planetary nebulae are expanding shells of ionized gas expelled by low- to intermediate-mass stars at the end of the red-giant phase; typical radii are a few tenths of a light-year and lifetimes are up to ~30,000 years, after which the gas disperses into the interstellar medium. The label is historical and unrelated to planets. Planetary Nebula
European Space Agency.
- –Supernova remnants are expanding shock-heated debris and swept-up interstellar gas from stellar explosions, radiating from radio to X-rays and sometimes powered by a central pulsar wind nebula (e.g., the Crab Nebula); shock speeds can reach thousands of km/s.
Chandra X-ray Observatory;
Physical properties and composition
- –In spiral galaxies, interstellar gas and dust (collectively the medium from which nebulae are condensed) constitute a few percent of the galaxy’s mass overall and a larger fraction in spiral arms; about ~1% of the interstellar medium’s mass is dust, which strongly affects how nebulae absorb and scatter light.
- –Dust grains both heat and cool gas and are essential to the cold, dense molecular environments in which stars form; absorption is greater at shorter wavelengths, allowing near-infrared and radio observations to penetrate dusty regions more effectively than optical light.
- –H II regions commonly exhibit electron temperatures of ~8,000 K and densities from tens to 10^5 cm⁻³, while molecular clouds can reach densities ≳10^3 H2 molecules per cm³ and temperatures as low as ~10 K; intercloud hot gas can reach ~10^6–10^7 K at very low densities.
Formation, evolution, and role in the galactic ecosystem
- –Star formation proceeds in cold, dense molecular clouds; ultraviolet radiation and winds from massive young stars carve cavities, compress surrounding gas, and can trigger further star formation, as observed in regions like the Orion Nebula.
- –Low- and intermediate-mass stars shed their outer layers to create planetary nebulae; massive stars end in Supernova explosions whose remnants enrich the medium with heavy elements and dust, sustaining the cycle of gas, dust, and star formation.
Spectra and diagnostic lines
- –Nebular spectra contain strong emission lines that trace physical conditions; historically, bright green lines seen in planetary and diffuse nebulae were attributed to a hypothetical element "nebulium" until Ira S. Bowen showed in 1927 that they are forbidden transitions of common ions such as O III in very low-density gas.
Nature (Bowen, 1927).
- –Forbidden lines (denoted by square brackets, e.g., O III 500.7 nm) arise where collisions are rare; they are prominent diagnostics of temperature, density, and ionization in nebulae.
Oxford Reference;
Observation across the spectrum
- –Neutral hydrogen in and around nebulae is mapped via the 21‑cm hyperfine transition, which penetrates dust and reveals large-scale structure in galactic disks.
- –Infrared observations trace dust and embedded protostars in obscured regions (e.g., Spitzer/WISE imaging of Orion), while optical narrowband imaging maps ionized gas through Hα and O III filters.
NASA/JPL;
- –X-ray and radio observations probe shock-heated plasma and synchrotron emission in supernova remnants, providing insights into shock physics and element distribution.
Notable nebulae
- –The Orion Nebula (M42) is a nearby, bright H II region and stellar nursery containing the Trapezium cluster and hundreds of young stars and protoplanetary disks.
- –The Crab Nebula (M1) is a filamentary supernova remnant powered by a central pulsar; the 1054 CE explosion was recorded historically, and the remnant emits from radio to gamma rays.
Internal cross-references: Milky Way; Molecular Cloud; H II region; Planetary Nebula; Supernova; Orion Nebula; Crab Nebula; James Webb Space Telescope.