nebulas

A nebula is a giant cloud of gas and dust that exists in interstellar space. Most of its material is hydrogen and helium, with traces of heavier elements. Nebulae are formed from gas ejected by dying stars, stellar winds, or the remnants of supernovae, and they are the birthplaces of new stars.

Nebulae are categorized by how they emit or reflect light. Emission nebulae glow when ultraviolet radiation from nearby hot stars ionizes their gas, producing characteristic emission lines. Reflection nebulae shine by reflecting light from nearby stars and do not emit much light of their own. Dark nebulae are dense clouds that obscure light from stars behind them. Planetary nebulae are relatively small, expanding shells ejected by dying low- to intermediate-mass stars. Supernova remnants are the expanding shells left after massive stars explode.

Observationally, nebulae span a wide range of sizes and distances, from nearby star-forming regions within our galaxy to distant clouds in other galaxies. They can be a few light-years across, contain substantial masses of gas and dust, and have temperatures that vary from a few tens to tens of thousands of kelvin. Dust within nebulae absorbs visible light and radiates in the infrared, making infrared and radio telescopes crucial for study.

Nebulae play a central role in the study of stellar evolution and the chemical evolution of galaxies. They are the nurseries where new stars and planetary systems form, and their gas and dust contribute to the galactic medium after stars conclude their lifecycles.

Notable examples include the Orion Nebula (M42), the Eagle Nebula (M16) with the Pillars of Creation, the Crab Nebula (M1), and the Ring Nebula (M57).