Nebulae

Nebulae are vast clouds of gas and dust located in interstellar space. They span a wide range of temperatures, densities, and luminosities and play a central role in the life cycle of stars. The term nebula comes from the Latin for cloud. Some nebulae glow brightly or reflect starlight, while others appear dark by silhouetting background stars. They occur within our galaxy and in other galaxies, serving as stellar nurseries or as remnants of stellar evolution.

Types of nebulae include emission nebulae, which glow when ultraviolet radiation from nearby hot stars ionizes the gas and produces characteristic emission lines such as hydrogen alpha; reflection nebulae, which shine by scattering the light of nearby stars off dust; and dark nebulae, dense clouds that obscure background light. Planetary nebulae are glowing shells of gas ejected by dying low- to intermediate-mass stars, while supernova remnants are the expanding debris of exploded stars. H II regions are a subset of emission nebulae surrounding newly formed star clusters and are often bright in optical images.

Composition and structure: Nebulae consist mainly of hydrogen and helium, with traces of heavier elements and interstellar dust. The observed light depends on the excitation mechanism; spectroscopy reveals temperatures, motions, and chemical abundances. Sizes vary widely: planetary nebulae are typically about 0.1 to 1 light-year across, while emission regions and supernova remnants can span several to tens of light-years. Giant molecular clouds, many dozens of light-years across, are sites of future star formation.

Observations across optical, infrared, radio, and X-ray wavelengths uncover the physical conditions and dynamics within nebulae. Notable examples include the Orion Nebula (emission nebula), the Crab Nebula (supernova remnant), the Ring Nebula, the Ring, Eagle, and Helix Nebulae, each illustrating different aspects of nebular phenomena.