Nebulosa

A nebulosa, or nebula, is a vast cloud of gas and dust in interstellar space. Most lie within galaxies, including the Milky Way, and range from a few to hundreds of light-years across. They are primarily composed of hydrogen and helium, with dust grains that can absorb or scatter light. The term originates from the Latin nebula, meaning a cloud, and in Spanish and Portuguese astronomy nebulosa is the common word for nebula.

Nebulae come in several major types. Emission nebulae glow because their gas is ionized by nearby hot stars, producing characteristic spectral lines, especially hydrogen alpha. Reflection nebulae shine by reflecting the light of nearby stars and often appear blue due to scattering. Dark nebulae are dense dust clouds that obscure background stars and the light behind them. Planetary nebulae are shells ejected by aging low- to intermediate-mass stars and typically measure a few light-years across; examples include the Ring Nebula and the Helix Nebula. Supernova remnants are the expanding debris of exploded stars, such as the Crab Nebula, and can span tens of light-years.

Nebulae form within molecular clouds under gravity, initiating star formation in their dense regions. They also participate in galactic chemical evolution, returning enriched material to the interstellar medium through winds and supernovae. Observations across the electromagnetic spectrum—from optical to infrared and radio—reveal different components: ionized gas, warm dust, and molecular gas. Studying nebulae helps scientists understand how stars and planetary systems originate and how galaxies recycle material.