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Planetary nebulae, represented here by the Ring Nebula, are examples of emission nebulae. An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star .
A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae.
There are three main types of nebula: absorption, reflection, and emission nebulae. Absorption (or dark) nebulae are made of dust and gas in such quantities that they obscure the starlight behind them, making photometry difficult. Reflection nebulae, as their name suggest, reflect the light of nearby stars.
Diffuse nebulae can be divided into emission nebulae, reflection nebulae and dark nebulae. Visible light nebulae may be divided into emission nebulae, which emit spectral line radiation from excited or ionized gas (mostly ionized hydrogen ); [ 25 ] they are often called H II regions , H II referring to ionized hydrogen), and reflection nebulae ...
In the case of emission nebulae, it is common to look at the ratio of two emission lines which should not be affected by the temperature and density in the nebula. For example, the ratio of hydrogen-alpha to hydrogen-beta emission is always around 2.85 under a wide range of conditions prevailing in nebulae. A ratio other than 2.85 must ...
Barnard 68 is a molecular cloud, dark absorption nebula or Bok globule, towards the southern constellation Ophiuchus and well within the Milky Way galaxy at a distance of about 125 parsecs (407 lightyears). [2] It is both close and dense enough that stars behind it cannot be seen from Earth.
In astronomy, collisional excitation gives rise to spectral lines in the spectra of astronomical objects such as planetary nebulae and H II regions.. In these objects, most atoms are ionised by photons from hot stars embedded within the nebular gas, stripping away electrons.
They are mainly used for nebulae observation. Emission nebulae mainly radiate the doubly ionized oxygen in the visible spectrum, which emits near 500 nm wavelength. These nebulae also radiate weakly at 486 nm, the Hydrogen-beta line. There are two main types of Narrowband filters: Ultra-high contrast (UHC), and specific emission line(s) filters.