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In astrophysics, accretion is the accumulation of particles into a massive object by gravitationally attracting more matter, typically gaseous matter, into an accretion disk. [ 1 ] [ 2 ] Most astronomical objects , such as galaxies , stars , and planets , are formed by accretion processes.
Pebble accretion is the accumulation of particles, ranging from centimeters up to meters in diameter, into planetesimals in a protoplanetary disk that is enhanced by aerodynamic drag from the gas present in the disk. This drag reduces the relative velocity of pebbles as they pass by larger bodies, preventing some from escaping the body's gravity.
The most prominent accretion disks are those of active galactic nuclei and of quasars, which are thought to be massive black holes at the center of galaxies. As matter enters the accretion disc, it follows a trajectory called a tendex line, which describes an inward spiral. This is because particles rub and bounce against each other in a ...
In astrophysics, the Bondi accretion (also called Bondi–Hoyle–Lyttleton accretion), named after Hermann Bondi, is spherical accretion onto a compact object traveling through the interstellar medium. It is generally used in the context of neutron star and black hole accretion. To achieve an approximate form of the Bondi accretion rate ...
An X-ray pulsar is a type of binary star system consisting of a typical star (stellar companion) in orbit around a magnetized neutron star.The magnetic field strength at the surface of the neutron star is typically about 10 8 Tesla, over a trillion times stronger than the strength of the magnetic field measured at the surface of the Earth (60 μT).
Accretion (coastal management), the process where coastal sediments return to the visible portion of the beach following storm erosion; Accretion (geology), the increase in size of a tectonic plate by addition of material along a convergent boundary; Accretionary wedge
His studies of the distribution of quasars challenged the now-rejected steady state theory. [28] He was one of the first to propose that enormous black holes power quasars, [ 29 ] and that superluminal astronomical observations can be explained as an optical illusion caused by an object moving partly in the direction of the observer.
The runaway accretion lasts between 10,000 and 100,000 years and ends when the largest bodies exceed approximately 1,000 km in diameter. [20] Slowing of the accretion is caused by gravitational perturbations by large bodies on the remaining planetesimals. [20] [65] In addition, the influence of larger bodies stops further growth of smaller ...