"This [image at the right] is the first strong evidence of water plumes erupting off Europa's surface."
"Hubble didn't photograph plumes, but spectroscopically detected auroral emissions from oxygen and hydrogen. The aurora is powered by Jupiter's magnetic field. This is only the second moon in the solar system found ejecting water vapor from the frigid surface. The image of Europa is derived from a global surface map generated from combined observations taken by NASA's Voyager and Galileo space probes."
Def. matter "that can be contained only if it is fully surrounded by a solid (or in a bubble of liquid) (or held together by gravitational pull); it can condense into a liquid, or can (rarely) become a solid directly" is called a gas.
Def. relating "to, or existing as, gas" is called gaseous.
Molecular hydrogen gas is excited in the discharge tube shown on the right. When an electron returns to a lower energy orbital state the purple color is observed.
Theoretical gaseous objects
"[T]he evolution of star accretion onto a supermassive gaseous object in the central region of an active galactic nucleus [may be addressed using] a gaseous model of relaxing dense stellar systems".
is called a fragment.
Def. the act or process of producing a fragment is called fragmentation.
With respect to low-mass star formation, "fragmentation to form a binary star [may be] most simply achieved if collapse is initiated by an external impulse." "On its own, [the] process under which a dense molecular cloud core can collapse to form a binary, or multiple, star system would produce wide binaries". "[C]lose binaries [may be] formed because of mutual interactions between the protostellar discs surrounding the various fragments." "[T]he most likely collision which has an effect on the core is the one for which [the velocity change imparted by the impulse,] Δv ~ cs [(the internal sound speed),] induced by a clump of mass ~0.1Mʘ." "[T]he impulsive collapse of the cloud cores [requires] that they are not primarily magnetically supported in their central regions."
"The sources are separated by 17", or 4200 AU. Binary separations of this order are consistent with early fragmentation in a relatively dense cloud ("prompt initial fragmentation," e.g., Pringle 1989; Looney et al. 2000), in which case the individual sources would have distinct protostellar envelopes."
"In a binary formed via gravitational fragmentation, we would expect the separation to correspond to the local Jeans length (Jeans 1928):"
"where cs is the local sound speed, and μp = 2.33 and n are the mean molecular weight and mean particle density, respectively. A Jeans length of 4200 AU would require a relatively high density (n ~ 6 x 105 cm−3, assuming cs = 0.2 km s−1). The mean density of the Per-Bolo 102 core, measured within an aperture of 104 AU, is 4 x 105 cm−3, close to the required value."
The "fragmentation of the first gaseous objects [to stars] is a well-posed physics problem with well specified initial conditions, for a given power-spectrum of primordial density fluctuations. This problem is ideally suited for three-dimensional computer simulations, since it cannot be reliably addressed in idealized 1D or 2D geometries."
Attempted "detailed 3D simulations of the formation process of the first stars in a halo of ~ 106 M⊙ [can be accomplished] by following the dynamics of both the dark matter and the gas components, including H2 chemistry and cooling."
For cooling "rates as a function of temperature for a primordial gas composed of atomic hydrogen and helium, as well as molecular hydrogen, in the absence of any external radiation [assume] a hydrogen number density nH = 0.045 cm-3, corresponding to the mean density of virialized halos at z = 10."
"The collapsing region forms a disk which fragments into many clumps. The clumps have a typical mass ~ 102 - 103 M⊙. This mass scale corresponds to the Jeans mass for a temperature of ~ 500K and the density ~ 104 cm-3 where the gas lingers because its cooling time is longer than its collapse time at that point [...] Each clump accretes mass slowly until it exceeds the Jeans mass and collapses at a roughly constant temperature (isothermally) due to H2 cooling that brings the gas to a fixed temperature floor. The clump formation efficiency is high in this simulation".
The simulated collapse "of one of the [first-formed clumps] with ~ 1000 M⊙ [...] demonstrated that it does not tend to fragment into sub-components. Rather, the clump core of ~ 100 M⊙ free-falls towards the center leaving an extended envelope behind with a roughly isothermal density profile. At very high gas densities, three-body reactions become important in the chemistry of H2. Omukai & Nishi (1998)  have included these reactions as well as radiative transfer and followed the collapse in spherical symmetry up to stellar densities. Radiation pressure from nuclear burning at the center is unlikely to reverse the infall as the stellar mass builds up. These calculations indicate that each clump may end as a single massive star; however, it is conceivable that angular momentum may eventually halt the collapsing cloud and lead to the formation of a binary stellar system instead."
The image on the left precedes the one on the right. "A hydrodynamic simulation of the collapse and fragmentation of a turbulent molecular cloud in the present-day Universe [is shown left and right, respectively]. The cloud has a mass of 50 M⊙. The panels show the column density through the cloud, and span a scale of 0.4 pc across. Left: The initial phase of the collapse. The turbulence organizes the gas into a network of filaments, and decays thereafter through shocks. Right: A snapshot taken near the end of the simulation, after 1.4 initial free-fall times of 2 × 105 yr. Fragmentation has resulted in ~ 50 stars and brown dwarfs. The star formation efficiency is ~ 10% on the scale of the overall cloud, but can be much larger in the dense sub-condensations. This result is in good agreement with what is observed in local star-forming regions."
Depending primarily upon gas temperature, the presence of gas may be used to determine the composition of the gas object observed, at least the outer layer. Early spectroscopy of the Sun using estimates of "the line intensities of several lines by eye [to derive] the abundances of ... elements ... [concluded] that the Sun [is] largely made of hydrogen."
The right image is the first X-ray image ever made of Venus. It "shows a half crescent due to the relative orientation of the Sun, Earth and Venus. The X-rays from Venus are produced by fluorescent radiation from oxygen and other atoms in the atmosphere between 120 and 140 kilometers above the surface of the planet." The fluorescent source of the X-rays places Venus in the gas dwarf category even though a rocky object lies some 120 km beneath this layer.
- Gaseous objects in the interstellar medium may assume any shape.
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