Overview
A reflection nebula is an interstellar cloud of dust that becomes visible because it scatters light from one or more nearby stars. The visible light from a reflection nebula is starlight redirected by microscopic dust grains rather than light produced by hot, ionized gas. For general context about the field that studies these objects, see astronomy overview. The stars that illuminate a reflection nebula are often ordinary or young stars; see illuminating stars.
Physical cause and color
The characteristic blue appearance of many reflection nebulae arises because scattering by interstellar dust is more efficient at shorter (bluer) wavelengths. This wavelength dependence is related to the same basic physics that makes the sky appear blue and causes reddening of sunlight at sunsets. Scattering in nebulae can be described by a mixture of regimes, often with behavior between simple Rayleigh scattering and larger-particle (Mie-like) scattering depending on grain size. Because the light is scattered rather than generated locally, the nebular spectrum largely reflects the spectrum of the illuminating stars, modified by the scattering and extinction properties of the dust.
Distinction from emission and dark nebulae
Reflection nebulae differ from emission nebulae, which glow because energetic photons from very hot stars ionize surrounding interstellar gas and produce emission lines. Reflection nebulae do not show strong emission-line spectra because the local radiation field is not energetic enough to cause significant ionization. They also differ from dark nebulae, which block background light and appear as obscuring silhouettes. In many star-forming regions these types occur together: bright emission zones, blue reflection regions and dark dust lanes can be adjacent, creating complex, composite structures often called diffuse nebulae.
Dust properties and diagnostics
Observations of reflection nebulae provide valuable information about interstellar dust. Polarization measurements are particularly important because scattered light is partially polarized; the degree and orientation of polarization constrain grain sizes, shapes and alignment in magnetic fields. Multiwavelength observations from the ultraviolet through the infrared help determine the size distribution and composition of dust particles, which typically include silicates, carbonaceous compounds and icy mantles in cold regions. Studies of scattered light also allow astronomers to map the distribution of obscuring material and to estimate optical depths and albedos of grains.
Environment, formation and role in star formation
Reflection nebulae are commonly found within or near molecular clouds where new stars form. Dust produced in these dense environments scatters light from embedded or nearby young stars, making the cloud visible against the background. Because reflection nebulae often trace the locations of dense dust and young stellar objects, they are useful indicators of recent or ongoing star formation. In cases where nearby stars are energetic enough, surrounding zones may transition from reflection-dominated regions to emission-dominated H II regions as ionization increases.
Observation and scientific importance
Astronomers study reflection nebulae across optical and infrared bands and sometimes in the ultraviolet. Imaging, spectroscopy and polarimetry are combined to separate scattered starlight from any faint nebular emission and foreground or background sources. Analyses of reflection nebulae contribute to understanding the early stages of stellar evolution, the physical and chemical properties of interstellar dust, and the interaction between young stars and their natal environment. Instrumental advances in sensitivity and angular resolution have improved the ability to study fine structure and to connect nebular features with specific illuminating stars.
Examples and further reading
Classic nearby examples include the blue nebulosity around the Pleiades cluster and other well-known objects such as the Iris Nebula and various reflection-dominated regions in complex star-forming clouds. Parts of the Orion nebula complex display both reflection and emission components, illustrating how the two processes can coexist. For more introductory material on scattering and how it affects astronomical observations, see resources on scattering and on the behavior of light such as reflection. Background on the interstellar medium and the roles of gas and dust are discussed in general treatments of interstellar gas and ionization. To explore practical observational techniques and educational summaries, consult pages about astronomy methods and guides to observing stars.
