The observer pattern is a software design pattern in which an object, named the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
It is mainly used for implementing distributed event handling systems, in "event driven" software. In those systems, the subject is usually named a "stream of events" or "stream source of events", while the observers are called "sinks of events". The stream nomenclature alludes to a physical setup where the observers are physically separated and have no control over the emitted events from the subject/stream-source. This pattern then perfectly suits any process where data arrives from some input that is not available to the CPU at startup, but instead arrives "at random" (HTTP requests, GPIO data, user input from keyboard/mouse/..., distributed databases and blockchains, ...). Most modern programming-languages comprise built-in "event" constructs implementing the observer-pattern components. While not mandatory, most 'observers' implementations would use background threads listening for subject-events and other support mechanisms provided by the kernel (Linux epoll, ...).
Strong vs. weak reference[edit]
The observer pattern can cause memory leaks, known as the lapsed listener problem, because in a basic implementation, it requires both explicit registration and explicit deregistration, as in the dispose pattern, because the subject holds strong references to the observers, keeping them alive. This can be prevented by the subject holding weak references to the observers.
https://en.wikipedia.org/wiki/Observer_pattern
In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the various chunks that make them up.
Asteroids Bennu and Ryugu have a measured bulk density which suggests a rubble pile internal structure.[1][2] Many comets and most smaller minor planets are thought to be composed of coalesced rubble.[3]
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Suspected rubble piles:
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https://en.wikipedia.org/wiki/Rubble_pile
https://en.wikipedia.org/wiki/Water_hammer
https://en.wikipedia.org/wiki/Helium_flash
https://en.wikipedia.org/wiki/Accumulator
https://en.wikipedia.org/wiki/Accretion_disk
https://en.wikipedia.org/wiki/Planetesimal
https://en.wikipedia.org/wiki/Chondrite
https://en.wikipedia.org/wiki/Chondrule
https://en.wikipedia.org/wiki/Accumulation
Friday, September 24, 2021
A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars (between 0.8 solar masses (M☉) and 2.0 M☉[1]) during their red giant phase (the Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence). A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars.
Low-mass stars do not produce enough gravitational pressure to initiate normal helium fusion. As the hydrogen in the core is exhausted, some of the helium left behind is instead compacted into degenerate matter, supported against gravitational collapse by quantum mechanical pressure rather than thermal pressure. This increases the density and temperature of the core until it reaches approximately 100 million kelvin, which is hot enough to cause helium fusion (or "helium burning") in the core.
However, a fundamental quality of degenerate matter is that increases in temperature do not produce an increase in volume of the matter until the thermal pressure becomes so very high that it exceeds degeneracy pressure. In main sequence stars, thermal expansion regulates the core temperature, but in degenerate cores this does not occur. Helium fusion increases the temperature, which increases the fusion rate, which further increases the temperature in a runaway reaction. This produces a flash of very intense helium fusion that lasts only a few minutes, but briefly emits energy at a rate comparable to the entire Milky Way galaxy.
In the case of normal low-mass stars, the vast energy release causes much of the core to come out of degeneracy, allowing it to thermally expand, however, consuming as much energy as the total energy released by the helium flash, and any left-over energy is absorbed into the star's upper layers. Thus the helium flash is mostly undetectable to observation, and is described solely by astrophysical models. After the core's expansion and cooling, the star's surface rapidly cools and contracts in as little as 10,000 years until it is roughly 2% of its former radius and luminosity. It is estimated that the electron-degenerate helium core weighs about 40% of the star mass and that 6% of the core is converted into carbon.[2]
https://en.wikipedia.org/wiki/Helium_flash
Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the centre of gravity.[1] Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter will collapse to form pockets of higher density, typically creating a hierarchy of condensed structures such as clusters of galaxies, stellar groups, stars and planets.
A star is born through the gradual gravitational collapse of a cloud of interstellar matter. The compression caused by the collapse raises the temperature until thermonuclear fusion occurs at the center of the star, at which point the collapse gradually comes to a halt as the outward thermal pressure balances the gravitational forces. The star then exists in a state of dynamic equilibrium. Once all its energy sources are exhausted, a star will again collapse until it reaches a new equilibrium state.
https://en.wikipedia.org/wiki/Gravitational_collapse
Thus the thermal pressure from fusion is no longer sufficient to counter the gravitational collapse and create the hydrostatic equilibrium found in most stars. This causes the star to start contracting and increasing in temperature until it eventually becomes compressed enough for the helium core to become degenerate matter. This degeneracy pressure is finally sufficient to stop further collapse of the most central material but the rest of the core continues to contract and the temperature continues to rise until it reaches a point (≈1×108 K) at which the helium can ignite and start to fuse.[4][5][6]
https://en.wikipedia.org/wiki/Helium_flash
The explosive nature of the helium flash arises from its taking place in degenerate matter. Once the temperature reaches 100 million–200 million kelvin and helium fusion begins using the triple-alpha process, the temperature rapidly increases, further raising the helium fusion rate and, because degenerate matter is a good conductor of heat, widening the reaction region.
However, since degeneracy pressure (which is purely a function of density) is dominating thermal pressure (proportional to the product of density and temperature), the total pressure is only weakly dependent on temperature. Thus, the dramatic increase in temperature only causes a slight increase in pressure, so there is no stabilizing cooling expansion of the core.
This runaway reaction quickly climbs to about 100 billion times the star's normal energy production (for a few seconds) until the temperature increases to the point that thermal pressure again becomes dominant, eliminating the degeneracy. The core can then expand and cool down and a stable burning of helium will continue.[7]
A star with mass greater than about 2.25 M☉ starts to burn helium without its core becoming degenerate, and so does not exhibit this type of helium flash. In a very low-mass star (less than about 0.5 M☉), the core is never hot enough to ignite helium. The degenerate helium core will keep on contracting, and finally becomes a helium white dwarf.
The helium flash is not directly observable on the surface by electromagnetic radiation. The flash occurs in the core deep inside the star, and the net effect will be that all released energy is absorbed by the entire core, causing the degenerate state to become nondegenerate. Earlier computations indicated that a nondisruptive mass loss would be possible in some cases,[8] but later star modeling taking neutrino energy loss into account indicates no such mass loss.[9][10]
In a one solar mass star, the helium flash is estimated to release about 5×1041 J,[11] or about 0.3% of the energy release of a 1.5×1044 J type Ia supernova,[12] which is triggered by an analogous ignition of carbon fusion in a carbon–oxygen white dwarf.
Shell helium flashes are a somewhat analogous but much less violent, nonrunaway helium ignition event, taking place in the absence of degenerate matter. They occur periodically in asymptotic giant branch stars in a shell outside the core. This is late in the life of a star in its giant phase. The star has burnt most of the helium available in the core, which is now composed of carbon and oxygen. Helium fusion continues in a thin shell around this core, but then turns off as helium becomes depleted. This allows hydrogen fusion to start in a layer above the helium layer. After enough additional helium accumulates, helium fusion is reignited, leading to a thermal pulse which eventually causes the star to expand and brighten temporarily (the pulse in luminosity is delayed because it takes a number of years for the energy from restarted helium fusion to reach the surface[13]). Such pulses may last a few hundred years, and are thought to occur periodically every 10,000 to 100,000 years.[13] After the flash, helium fusion continues at an exponentially decaying rate for about 40% of the cycle as the helium shell is consumed.[13] Thermal pulses may cause a star to shed circumstellar shells of gas and dust.[citation needed]
https://en.wikipedia.org/wiki/Helium_flash
Complete spatial randomness (CSR) describes a point process whereby point events occur within a given study area in a completely random fashion. It is synonymous with a homogeneous spatial Poisson process.[1] Such a process is modeled using only one parameter , i.e. the density of points within the defined area. The term complete spatial randomness is commonly used in Applied Statistics in the context of examining certain point patterns, whereas in most other statistical contexts it is referred to the concept of a spatial Poisson process.[1]
https://en.wikipedia.org/wiki/Complete_spatial_randomness
A plane mirror is a mirror with a flat (planar) reflective surface.[1][2] For light rays striking a plane mirror, the angle of reflection equals the angle of incidence.[3] The angle of the incidence is the angle between the incident ray and the surface normal (an imaginary line perpendicular to the surface). Therefore, the angle of reflection is the angle between the reflected ray and the normal and a collimated beam of light does not spread out after reflection from a plane mirror, except for diffraction effects.
A plane mirror makes an image of objects in front of the mirror; these images appear to be behind the plane in which than mirror lies. A straight line drawn from part of an object to the corresponding part of its image makes a right angle with, and is bisected by, the surface of the plane mirror. The image formed by a plane mirror can either be virtual (meaning that the light rays do not actually come from the image) or real (meaning that the light rays do actually come from the image). But it is always upright, and of the same shape and size as the object it is reflecting. A virtual image is a copy of an object formed at the location from which the light rays appear to come. Actually, the image formed in the mirror is a perverted image (Perversion), there is a misconception among people about having confused with perverted and laterally-inverted image. If a person is reflected in a plane mirror, the image of his right hand appears to be the left hand of the image.
Plane mirrors are the only type of mirror for which a object always produces an image that is virtual, erect and of the same size as the object. however. The focal length of a plane mirror is infinity;[4] its optical power is zero.
Using the mirror formula:
1/u +1/v =1/f
1/u=-1/v since [1/f=0]
=> u=-v
Concave and Convex mirrors (spherical mirrors)[5] are also able to produce virtual images similar to a plane mirror. However, the images formed by them are not of the same size as the object like they are in a plane mirror. In a convex mirror, the virtual image formed is always diminished, whereas in a concave mirror when the object is placed between the focus and the pole, an enlarged virtual image is formed. Therefore, in applications where a virtual image of the same size is required, a plane mirror is preferred over spherical mirrors.
https://en.wikipedia.org/wiki/Plane_mirror
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