An Etymological Dictionary of Astronomy and Astrophysics
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The act or fact of stopping by means of or as if by means of a brake. See: → magnetic braking; → radiative braking; → tidal braking; → braking index.

Verbal noun of → brake.

A parameter indicating the rate at which a → pulsar slows down. Neutron stars are powered by → rotational energy and lose energy by accelerating particle → winds and by emitting → electromagnetic radiation. The → rotation frequency, Ω, thus decreases with time and this slowdown is usually described by the relation Ω^. = - kΩn, where k is a positive constant which depends on the → moment of inertia and the → magnetic dipole moment of the → neutron star and n is the braking index. Conventionally, the braking index is derived by differentiation of the above equation, yielding n = ΩΩ^.. / Ω^.2. In a highly simplified model in which the spin-down torque arises from dipole radiation at the rotation frequency, one expects n = 3 (Johnston, S., Galloway, D., 1999, arXiv:astro-ph/9905058).

→ braking; → index.

The process whereby a star which loses mass slows down under the action of its → magnetic field. The stellar material follows the → magnetic field lines extending well beyond the stellar surface. The material gain → angular momentum and the underlying object is slowed down. Magnetic braking is an efficient mechanism for removing angular momentum from the the rotating object. See also → disk locking.

→ magnetic; → braking.

The failure of numerical star formation calculations to produce rotationally supported → Keplerian disks because of the → magnetic braking effect, when → magnetic fields of strengths comparable to those observed in → molecular clouds are accounted for. The formation and early evolution of disks is a long-standing fundamental problem in → star formation models. Early work in the field had concentrated on the simpler problem of disk formation from the → collapse of a rotating dense core in the absence of a magnetic field. However, dense star-forming cores are observed to be significantly magnetized. There is increasing theoretical evidence that disk formation is greatly modified, perhaps even suppressed, by a dynamically important magnetic field. This has been found in analytic studies, axisymmetric numerical models and in 3D calculations using → ideal magnetohydrodynamics. By contrast, recent observations suggest the presence of massive, 50-100 AU disks and evidence for associated → outflows in the earliest (→ class 0) stages of star formation around both low and high mass stars. Two primary solutions have been proposed: → turbulence and → non-ideal magnetohydrodynamics. Calculations of the collapse of a massive 100 Msun core have shown that 100 AU scale disk formation in the presence of strong magnetic fields was indeed possible, with some argument over whether this is caused by turbulent reconnection or another mechanism. Studies, using simulations of collapsing 5 Msun cores, have found that turbulence diffuses the strong magnetic field out of the inner regions of the core, and that the non-zero → angular momentum of the turbulence causes a misalignment between the rotation axis and the magnetic field. Both of these effects reduce the magnetic braking, and allow a massive disk to form (Wurster et al. 2016, arxiv/1512.01597 and references therein).

→ magnetic; → braking; → catastrophe.

The slowing down of a star's rotation due to radiative momentum transfer caused by emission of electromagnetic radiation.

→ radiative; → braking.

The physical process that slows the → Earth's rotation rate due to → tidal friction. The → Earth rotates faster than the → Moon orbits the Earth (24 hours compared to 27 days). The → friction between the ocean and the solid Earth below drags the → tidal bulge ahead of the line joining the Earth and the Moon. The → gravitational attraction of the Moon on the bulge provides a braking action on the Earth and decelerates its rotation. Tidal braking lengthens the day by 0.002 seconds every century. Because the total → angular momentum of the → Earth-Moon system in conserved, the loss in the angular momentum of the Earth is compensated by the orbital angular momentum of the Moon. Hence, the Moon moves away from Earth at a rate of about 3 cm per year. This process must continue until Earth's → day and → month are equal, at which point the Moon will never seem to move in Earth's sky and Earth is said to be tidally locked to the Moon (→ tidal locking).

→ tidal; → braking.