Nebula types
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Types of Nebulae: An Overview
Nebulae are fascinating celestial objects that come in various forms, each with unique characteristics and formation mechanisms. This article explores the different types of nebulae, including planetary nebulae, emission nebulae, reflection nebulae, dark nebulae, and supernova remnants.
Planetary Nebulae: Characteristics and Classification
Types of Planetary Nebulae
Planetary nebulae are the remnants of stars that have shed their outer layers. They are classified into two main types based on their Doppler expansion velocity and the brightness difference between the nebula and its central star2. Additionally, planetary nebulae can be further categorized into Peimbert types I, IIa, IIb, III, and IV, which are distinguished by their kinematic properties, spatial distribution, chemical composition, and morphologies3.
Morphological Types
Planetary nebulae exhibit various morphologies, including bipolar and elliptical shapes. Studies have shown that these shapes do not have a strong orientation pattern within the Galaxy, suggesting a random distribution4. The chemical composition of planetary nebulae, such as the He/H and N/O ratios, also plays a role in their classification, with Type I nebulae being significantly enriched in nitrogen and helium5 6.
Turbulence and Central Stars
The velocity fields of planetary nebulae can vary depending on the type of central star. For instance, nebulae around Wolf-Rayet (WC) stars exhibit finite turbulent velocities, while those around O-type stars do not show significant turbulence7. This turbulence is likely influenced by stellar wind inhomogeneities.
Binary Systems
Recent studies suggest that the diverse morphologies of planetary nebulae are often the result of binary star systems. The interaction between binary stars, particularly during the common envelope phase, plays a crucial role in shaping these nebulae8.
Emission Nebulae
Emission nebulae are clouds of ionized gas that emit light of various colors. The ionization is typically caused by high-energy photons from nearby hot stars. These nebulae are often sites of active star formation and can be observed in regions like the Orion Nebula.
Reflection Nebulae
Reflection nebulae do not emit their own light but instead reflect the light of nearby stars. They are usually blue because blue light is scattered more efficiently than red light. An example of a reflection nebula is the Pleiades cluster.
Dark Nebulae
Dark nebulae are dense clouds of gas and dust that block the light from objects behind them. They are often seen as dark patches against the backdrop of brighter regions. The Horsehead Nebula is a well-known example of a dark nebula.
Supernova Remnants
Supernova remnants are the remains of massive stars that have exploded in supernova events. These remnants are composed of ejected material and the shock waves from the explosion. They play a significant role in enriching the interstellar medium with heavy elements.
Conclusion
Nebulae are diverse and complex objects that offer valuable insights into the life cycles of stars and the dynamics of the interstellar medium. Understanding the different types of nebulae, from planetary nebulae to supernova remnants, helps astronomers piece together the intricate puzzle of our universe.
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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.
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.
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.
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.
CLASSIFICATION OF THE PLANETARY NEBULA YM 29 (=S 274 = A 21).
The filamentary planetary nebula YM29 is a Peimbert type I planetary nebula, with significantly enriched nitrogen and helium content, and is classified as a Peimbert type I planetary nebula.
Turbulent planetary nebulae around (WC)-type stars ⋆
Turbulence in planetary nebulae around (WC)-type stars is likely related to long-lasting momentum-driven phase bubbles, while nebulae around O-type stars show no significant turbulence and their expansion velocity increases outwards.
Binary stars as the key to understanding planetary nebulae
Recent evidence suggests that planetary nebulae are shaped by binary evolution, challenging the single star scenario and revealing surprises in binary evolution.
Highly CitedIdentification of PN G232.0+05.7 as a new halo planetary nebula and of IRAS 19336-0400 as a new type III planetary nebula
PNG 232.0+05.7 is a new halo planetary nebula and IRAS 19336-0400 is likely a type III planetary nebula, with a better estimation of their distance needed.
A RELATION BETWEEN DISTANCE AND RADIAL VELOCITY AMONG EXTRA-GALACTIC NEBULAE.
This paper suggests a correlation between distance and radial velocity among extra-galactic nebulae, using absolute-luminosity criteria for stars, and suggests a uniform upper limit for absolute luminosity in late-type spirals and irregular nebulae.
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