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A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
A nebula is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in the Eagle Nebula. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter and eventually become dense enough to form stars. The remaining material is then thought to form planets and other planetary system objects.
A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
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An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable matter in the universe is either hydrogen or helium, and astronomers use the word "metals" as convenient shorthand for "all elements except hydrogen and helium". This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting solid. Stars and nebulae with relatively high abundances of heavier elements are called "metal-rich" in when discussing metallicity, even though many of those elements are called nonmetals in chemistry.
A pulsar wind nebula, sometimes called a plerion, is a type of nebula sometimes found inside the shell of a supernova remnant (SNR), powered by winds generated by a central pulsar. These nebulae were proposed as a class in 1976 as enhancements at radio wavelengths inside supernova remnants. They have since been found to be infrared, optical, millimetre, X-ray and gamma ray sources.
NGC 3109 is a small barred Magellanic type spiral or irregular galaxy around 4.35 Mly away in the direction of the constellation of Hydra. NGC 3109 is believed to be tidally interacting with the dwarf elliptical galaxy Antlia Dwarf. It was discovered by John Herschel on March 24, 1835 while he was in what is now South Africa.
An HI region or H I region (read H one) is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements. (H is the chemical symbol for hydrogen, and "I" is the Roman numeral. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionized—HII is H+ in other sciences—III for doubly-ionized, e.g. OIII is O++, etc.) These regions do not emit detectable visible light (except in spectral lines from elements other than hydrogen) but are observed by the 21-cm (1,420 MHz) region spectral line. This line has a very low transition probability, so it requires large amounts of hydrogen gas for it to be seen. At ionization fronts, where HI regions collide with expanding ionized gas (such as an H II region), the latter glows brighter than it otherwise would. The degree of ionization in an HI region is very small at around 10−4 (i.e. one particle in 10,000). At typical interstellar pressures in galaxies like the Milky Way, HI regions are most stable at temperatures of either below 100 K or above several thousand K; gas between these temperatures heats or cools very quickly to reach one of the stable temperature regimes. Within one of these phases, the gas is usually considered isothermal, except near an expanding H II region. Near an expanding H II region is a dense HI region, separated from the undisturbed HI region by a shock front and from the H II region by an ionization front.
Cosmic dust – also called extraterrestrial dust, space dust, or star dust – is dust that occurs in outer space or has fallen onto Earth. Most cosmic dust particles measure between a few molecules and 0.1 mm (100 μm), such as micrometeoroids and meteoroids. Cosmic dust can be further distinguished by its astronomical location: intergalactic dust, interstellar dust, interplanetary dust, and circumplanetary dust. There are several methods to obtain space dust measurement.
A megamaser is a type of astrophysical maser, which is a naturally occurring source of stimulated spectral line emission. Megamasers are distinguished from other astrophysical masers by their large isotropic luminosity. Megamasers have typical luminosities of 103 solar luminosities (L☉), which is 100 million times brighter than masers in the Milky Way, hence the prefix mega. Likewise, the term kilomaser is used to describe masers outside the Milky Way that have luminosities of order L☉, or thousands of times stronger than the average maser in the Milky Way, gigamaser is used to describe masers billions of times stronger than the average maser in the Milky Way, and extragalactic maser encompasses all masers found outside the Milky Way. Most known extragalactic masers are megamasers, and the majority of megamasers are hydroxyl (OH) megamasers, meaning the spectral line being amplified is one due to a transition in the hydroxyl molecule. There are known megamasers for three other molecules: water (H2O), formaldehyde (H2CO), and methine (CH).
A champagne flow is an astrophysical event whereby an HII region inside a molecular cloud expands outward until it reaches the interstellar medium. At that point, the ionized hydrogen gas bursts outward like an uncorked champagne bottle. This event is also sometimes called a Blister. An HII region is created by ionization from a recently formed star inside a molecular cloud.
NGC 2366 is a Magellanic barred irregular dwarf galaxy located in the constellation Camelopardalis.
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The Antlia-Sextans Group is a small Galaxy group in the constellations Hydra, Sextans, Antlia and Leo. It is, on average, approximately 4.3 million light-years away from the Milky Way. It is generally considered to be at the very edge of the Local Group and thus part of it. However, other researchers indicate it is an independent Galaxy group, unlikely to be gravitationally bound to the Local Group due to probably lying outside the Local Group's Zero-velocity surface, and thus the nearest Galaxy group to the Local Group rather than a subgroup within the Local Group. Nonetheless—this possible independence may disappear as the Milky Way continues coalescing with Andromeda due to the increased mass, and density thereof, plausibly widening the radius of the Zero-velocity surface of the Local Group.
Gary Joseph Ferland is an American astrophysicist. He is a professor of Physics and Astronomy at The University of Kentucky. He is best known for developing the astrophysical simulation code Cloudy, for his work on physical processes in ionized plasmas, and investigations of the chemical evolution of the cosmos.
Sh 2-185 is an H II region centered on the massive star system Gamma Cassiopeiae in the northern constellation of Cassiopeia. It was included in the second Catalogue of H II Regions, published in 1959 by Stewart Sharpless. Sh 2-185 is located at a distance of approximately 10,500 ly (3.23 kpc) from the Sun. The region is surrounded by a dust shell, and displays several infrared point sources that are a characteristics of young stellar objects.
References
- 1 2 Hollenbach, D.J.; Tielens, A.G.G.M. (1999). "Photodissociation regions in the interstellar medium of galaxies". Reviews of Modern Physics. 71 (1): 173–230. Bibcode:1999RvMP...71..173H. doi:10.1103/RevModPhys.71.173.
- ↑ Tielens, A.G.G.M. (1993). "Photodissociation regions and planetary nebulae". Planetary Nebulae: Proceedings of the 155 Symposium of the International Astronomical Union. 155: 155–162. Bibcode:1993IAUS..155..155T. doi: 10.1017/S0074180900170330 .
- 1 2 Hollenbach, D. J.; Tielens, A. G. G. M. (1997). "Dense photodissociation regions". Annual Review of Astronomy and Astrophysics. 35: 179–215. Bibcode:1997ARA&A..35..179H. doi:10.1146/annurev.astro.35.1.179.
- ↑ Eiermann, Jacob M.; et al. (April 2024). "The 3D geometry of reflection nebulae IC 59 and IC 63 with their illuminating star gamma Cas". Monthly Notices of the Royal Astronomical Society. 529 (2): 1680–1687. arXiv: 2401.06941 . Bibcode:2024MNRAS.529.1680E. doi:10.1093/mnras/stae102.
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