An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

A central, tightly bound stellar subsystem observed in some elliptical galaxies which rotates in the opposite direction with respect to the main body of the → elliptical galaxy. Elliptical galaxies are thought to be the result of the → merger of two or more sizable galaxies. A plausible scenario for how counter-rotating cores could form in such a merger is as follows. If at least one of the galaxies has a core region that is fairly tightly bound by the galaxy's gravity, and the direction in which the two galaxies orbit each other before merging is opposite to the direction of rotation of stars in that tightly bound core, it is likely that, after the merger, the tightly bound core will end up as the core of the new, larger galaxy, while retaining its original sense of rotation. The surrounding stars, on the other hand, will rotate in a different way dictated by the orbital motion of the galaxies around each other, before the merger. While this is a plausible scenario, it can only explain some of the counter-rotating cores. Recently A. Tsatsi et al. (2015, ApJ 802, L3) have shown that although the two → progenitor galaxies are initially following a → prograde orbit, strong reactive forces during the merger can cause a short-lived change of their orbital spin; the two progenitors follow a → retrograde orbit right before their final coalescence. This results in a central kinematic decoupling and the formation of a large-scale (~2 kpc radius) counter-rotating core at the center of the final elliptical-like merger remnant, while its outer parts keep the rotation direction of the initial orbital spin.

→ kinematical; → decouple; → core.

The upper zone of the → Earth's core, just below the → mantle, extending from a depth of about 2900 km to 5100 km. It is presumed to be → liquid because it sharply reduces → compressional wave velocities and does not transmit → shear waves. Its density is from 9 to 11 g/cm³. The → temperature ranges from 4400 °C in the outer areas to 6100 °C near the → inner core. Since shear waves do not propagate through a fluid, the Earth's outer core is considered to be liquid because the shear wave velocity is zero. Convection motion within the outer core, along with the rotation of the Earth creates an effect that maintains the Earth's → magnetic field.

→ outer; → core.

A precursor of a small, loosely bound → star cluster (→ bound cluster) as well as an → OB association, with masses ranging from about 10 to 1000 → solar masses or more.

→ pre-; → cluster; → core.

A small, gravitationally unstable molecular → clump of typical size of less than 0.1 pc resulting from → gravitational collapse and → fragmentation of a larger → molecular cloud. It is a centrally concentrated structure which evolves into a → class 0 object, where eventually a single star or a stellar system is formed. Core masses range between 0.5 and 5 solar masses, with a mean number density of at least 10⁴-10⁵ cm^-3, and a temperature as low as about 10 K. A pre-stellar core evolves into a → Class 0 object. Also called dense core.

→ pre-stellar; → core.

The part of a → nuclear reactor in which → nuclear fission takes place and huge quantities of heat energy are generated.

→ reactor; → core.

A hydrostatic object predicted to result from the → second collapse of a → molecular cloud in an early stage of star formation.

→ second; → core.

A star formation scenario whereby → massive stars form from gravitationally bound → pre-stellar cores, which are supersonically → turbulent and in approximate pressure equilibrium with the surrounding protocluster medium. The high → accretion rates that characterize such media allow accretion to overcome the radiation pressure due to the luminosity of the star. The core is assumed to → collapse via an → accretion disk to form a single star or binary. The core density structure adopted is ρ ∝ r^-k, with k = 1.5 set from observations. This choice affects the evolution of the accretion rate, which increases linearly with time. The high densities in regions of massive-star formation lead to typical time scales for the formation of a massive star of about 10⁵ years (McKee & Tan 2003, ApJ 585, 850).

→ turbulent; → core.