Nebula
A nebula (
Most nebulae are of vast size; some are hundreds of light-years in diameter. A nebula that is visible to the human eye from Earth would appear larger, but no brighter, from close by.[6] The Orion Nebula, the brightest nebula in the sky and occupying an area twice the angular diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers.[7] Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth (105 to 107 molecules per cubic centimeter) – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. Earth's air has a density of approximately 1019 molecules per cubic centimeter; by contrast, the densest nebulae can have densities of 104 molecules per cubic centimeter. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffused that they can be detected only with long exposures and special filters. Some nebulae are variably illuminated by T Tauri variable stars.
Originally, the term "nebula" was used to describe any diffused
Observational history
Around 150 AD,
In 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was also observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659 by Christiaan Huygens, who also believed he was the first person to discover this nebulosity. [11]
In 1715,
The number of nebulae was then greatly increased by the efforts of William Herschel and his sister, Caroline Herschel. Their Catalogue of One Thousand New Nebulae and Clusters of Stars[16] was published in 1786. A second catalog of a thousand was published in 1789, and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars. In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity rather than a more distant cluster.[15]
Beginning in 1864,
In 1923, following the Great Debate, it became clear that many "nebulae" were in fact galaxies far from the Milky Way.
Slipher and
Formation
There are a variety of formation mechanisms for the different types of nebulae. Some nebulae form from gas that is already in the
Other nebulae form as the result of supernova explosions; the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces. One of the best examples of this is the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is labeled SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and its core is now a neutron star.
Still other nebulae form as
Types
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The Omega Nebula, an example of an emission nebula
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The Horsehead Nebula, an example of a dark nebula.
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The Cat's Eye Nebula, an example of a planetary nebula.
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The Red Rectangle Nebula, an example of a protoplanetary nebula.
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The delicate shell of SNR B0509-67.5
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Southern Ring Nebula, Planetary Nebula
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Ring Nebula in the northern constellation of Lyra
Classical types
Objects named nebulae belong to four major groups. Before their nature was understood, galaxies ("spiral nebulae") and star clusters too distant to be resolved as stars were also classified as nebulae, but no longer are.
- H II regions, large diffuse nebulae containing ionized hydrogen
- Planetary nebulae
- Supernova remnant (e.g., Crab Nebula)
- Dark nebula
Not all cloud-like structures are named nebulae; Herbig–Haro objects are an example.
Flux Nebula
Diffuse nebulae
Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries.[22] 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);[23] they are often called H II regions, H II referring to ionized hydrogen), and reflection nebulae which are visible primarily due to the light they reflect.
Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them.[23] Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called dark nebulae.[23]
Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within the nebulae.[23]
Planetary nebulae
Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars (varying in size between 0.5-~8 solar masses). Evolved asymptotic giant branch stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells while leaving behind the star's core in the form of a white dwarf.[23] Radiation from the hot white dwarf excites the expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in star formation regions.[23] They are H II regions, because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions.[23]
Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, and who tended to confuse them with planets, which were of more interest to them. The Sun is expected to spawn a planetary nebula about 12 billion years after its formation.[24]
Protoplanetary nebula
A protoplanetary nebula (PPN) is an astronomical object at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the following planetary nebula (PN) phase.[25] During the AGB phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas. When this phase comes to an end, the star enters the PPN phase.
The PPN is energized by the central star, causing it to emit strong infrared radiation and become a reflection nebula. Collimated stellar winds from the central star shape and shock the shell into an axially symmetric form, while producing a fast moving molecular wind.[26] The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star. The PPN phase continues until the central star reaches a temperature of 30,000 K, after which it is hot enough to ionize the surrounding gas.[27]
Supernova remnants
A
Examples
Catalogs
- Gum catalog
- RCW Catalogue
- Sharpless catalog
- Messier Catalogue
- Caldwell Catalogue
- Abell Catalog of Planetary Nebulae
See also
- H I region
- H II region
- List of largest nebulae
- List of diffuse nebulae
- Lists of nebulae
- Molecular cloud
- Magellanic Clouds
- Messier object
- Nebular hypothesis
- Orion molecular cloud complex
- Timeline of knowledge about the interstellar and intergalactic medium
References
- ^ Harper, Douglas. "nebula". Online Etymology Dictionary.
- ^ American Heritage Dictionary of the English Language, Fifth Edition. S.v. "nebula." Retrieved November 23, 2019, from https://thefreedictionary.com/nebula
- ^ Collins English Dictionary – Complete and Unabridged, 12th Edition 2014. S.v. "nebula." Retrieved November 23, 2019, from https://thefreedictionary.com/nebula
- ^ Random House Kernerman Webster's College Dictionary. S.v. "nebula." Retrieved November 23, 2019, from https://thefreedictionary.com/nebula
- ^ The American Heritage Dictionary of Student Science, Second Edition. S.v. "nebula." Retrieved November 23, 2019, from https://thefreedictionary.com/nebula
- ^ Howell, Elizabeth (2013-02-22). "In Reality, Nebulae Offer No Place for Spaceships to Hide". Universe Today.
- ISBN 9780521361552.
- ^ "What is a nebula?". Space Center Houston. March 19, 2020. Retrieved June 27, 2021.
- Bibcode:1987Msngr..49...42K, retrieved 2009-10-31
- ^ ISBN 0-521-37079-5.
- ^ Bibcode:1984QJRAS..25...65H.
- doi:10.1086/123101.
- Bibcode:1939ASPL....3..145M.
- Philosophical Transactions. XXXIX: 390–92.
- ^ S2CID 161558679.
- ^ Philosophical Transactions. T.N. 1786. p. 457.
- ^ Watts, William Marshall; Huggins, Sir William; Lady Huggins (1904). An introduction to the study of spectrum analysis. Longmans, Green, and Co. pp. 84–85. Retrieved 2009-10-31.
- ^ Bibcode:1937PA.....45....9S.
- Bibcode:1912LowOB...2...26S.
- doi:10.1086/142713.
- ^ "A stellar sneezing fit". ESA/Hubble Picture of the Week. Retrieved 16 December 2013.
- ^ "The Messier Catalog: Diffuse Nebulae". SEDS. Archived from the original on 1996-12-25. Retrieved 2007-06-12.
- ^ ISBN 0-935702-05-9.
- ISBN 0-13-733916-X.
- S2CID 1790433.
- S2CID 7727592.
- doi:10.1086/167597.
External links
- Nebulae, SEDS Messier Pages
- Fusedweb.pppl.gov
- Historical pictures of nebulae, digital library of Paris Observatory