Nebula

A nebula is an interstellar cloud of gas, dust, or both. It can glow, obscure background light, or provide material from which stars and planets form.

Interstellar medium: The matter and radiation occupying the space between stars in a galaxy. Nebulae are denser regions within this diffuse galactic material.
Photoionization: The removal of electrons from atoms or molecules by absorbed photons. Ultraviolet photons from hot stars ionize gas, producing emission from nebulae.
Orion Nebula: A nearby, bright star-forming nebula in the constellation Orion. It is a prominent example of an H II region with active star formation.
Charles Messier: An eighteenth-century French astronomer who compiled a catalog of nebulae and star clusters. His catalog preserved many nebulae as named observing targets.
Dark nebula: A dense cloud of interstellar dust that blocks visible light from objects behind it. Unlike bright emission nebulae, it is seen mainly as a silhouette against background stars.
Interstellar dust: Tiny solid grains between stars, composed mainly of silicates, carbon compounds, and ice. Dust grains darken some nebulae and scatter or reradiate their light.
H II region expansion: The growth of an ionized hydrogen region as heated gas pushes into surrounding material. This expansion reshapes star-forming nebulae and can compress nearby gas.
Carina Nebula: A vast star-forming nebula in the southern constellation Carina. Its massive stars and sculpted gas show how stellar feedback transforms clouds.
William Herschel: An eighteenth- and nineteenth-century astronomer who surveyed the sky and studied nebulae. His observations helped establish nebulae as varied objects rather than one uniform class.
Reflection nebula: A dust cloud that shines by scattering light from nearby stars. Its light is reflected starlight, rather than emission from ionized gas.

Linked from 12 pages