Types of nebulae
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Types of Nebulae: A Comprehensive Overview
Planetary Nebulae: Classification and Characteristics
Two Types of Planetary Nebulae
Planetary nebulae can be categorized into two distinct types based on their Doppler expansion velocity and the brightness difference between the nebula and its central star. These differences also extend to the spectral types of the central stars and their chemical compositions1.
Morphological Types: Bipolar and Elliptical
Planetary nebulae can also be classified morphologically into bipolar (B) and elliptical (E) types. Studies have shown that these types do not exhibit strong evidence for non-random orientations within the Galaxy, suggesting a more homogeneous distribution than previously thought2.
Peimbert Classification System
The Peimbert classification system divides planetary nebulae into five types (I, IIa, IIb, III, and IV) based on their kinematic properties, spatial distribution, chemical composition, and morphologies. This system helps in understanding the diverse characteristics of planetary nebulae and their progenitor stars3. For instance, Type I nebulae are characterized by high nitrogen and helium abundances and filamentary structures, while Type III nebulae have lower abundances and belong to the Galactic thick disc6.
Turbulent Planetary Nebulae Around (WC)-Type Stars
Planetary nebulae with (WC)-type central stars exhibit finite turbulent velocities superimposed on a constant expansion velocity pattern. This turbulence is likely triggered or enhanced by stellar wind inhomogeneities, which are common in Wolf-Rayet nuclei8.
Other Types of Nebulae
Emission Nebulae
Emission nebulae are clouds of ionized gas that emit light of various colors. The most common type of emission nebula is the H II region, where hydrogen gas is ionized by nearby hot stars.
Reflection Nebulae
Reflection nebulae do not emit their own light but reflect the light of nearby stars. They are usually blue because blue light is scattered more efficiently than red light.
Dark Nebulae
Dark nebulae are dense clouds of gas and dust that block the light from objects behind them. They are visible as dark patches against the brighter background of stars or emission nebulae.
Supernova Remnants
Supernova remnants are the remains of exploded stars. They are characterized by their complex structures and high-energy emissions, which result from the interaction of the ejected material with the surrounding interstellar medium4.
Proto-Planetary Nebulae: SOLE and DUPLEX Types
Proto-planetary nebulae, the precursors to planetary nebulae, can be classified into two types based on their morphology: SOLE (star-obvious low-level-elongated) and DUPLEX (dust-prominent longitudinally extended). SOLE nebulae have a bright central star embedded in a faint nebulosity, while DUPLEX nebulae exhibit a bipolar structure with a partially or completely obscured central star. This morphological dichotomy is attributed to differences in the optical thickness of the circumstellar dust/gas shell7.
Conclusion
Nebulae are diverse astronomical objects with various types and classifications. Planetary nebulae alone exhibit a wide range of characteristics, from their chemical compositions to their morphological structures. Understanding these differences helps astronomers gain insights into the life cycles of stars and the dynamics of the interstellar medium. Other types of nebulae, such as emission, reflection, dark nebulae, and supernova remnants, each have unique properties and mechanisms that contribute to the rich tapestry of our universe.
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Most relevant research papers on this topic
Planetary Nebulae: Two Types
Two distinct types of planetary nebulae can be observed based on Doppler expansion velocity, nebula brightness, and central star spectral types, with the distinction also extending to differences between planetaries with differing chemical compositions.
Orientation of planetary nebulae within the Galaxy
There is no strong evidence for non-random orientations of planetary nebulae within the Galaxy, contrary to previous work in this field.
Bayesian posterior classification of planetary nebulae according to the Peimbert types
A larger number of classificatory parameters is needed for a more reliable physical classification of planetary nebulae, as ambiguities in the Peimbert types are linked to difficulties in defining sharp boundaries for progenitor star mass.
Abundances of southern type I planetary nebulae
Southern planetary nebulae have 8 probable Type I nebulae, with He/H and N/O abundance ratios indicating they are likely to be a subtype of the Sun.
The mean properties of planetary nebulae as a function of Peimbert class
Type I planetary nebulae have the highest progenitor masses, but also the lowest rms densities and 5-GHz brightness temperatures, contradicting recent modeling results.
A Hubble Space Telescope Snapshot Survey of Proto-Planetary Nebula Candidates: Two Types of Axisymmetric Reflection Nebulosities
The Hubble Space Telescope survey reveals two types of axisymmetric reflection nebulosities in proto-planetary nebulae, SOLE and DUPLEX, with SOLE nebulae showing a bright central star and DUPLEX nebulae showing bipolar
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