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accretion

When gas, dust or other kinds of matter fall towards a compact object (such as a black hole or a neutron star), a disk of infalling matter forms around the central object called the accretion disk.

The energy that matter gains in its fall is transformed into heat energy of the disk matter. Consequently, accretion disks are, as a rule, extremely hot. Their thermal radiation they emit is an important tool for indirect observation of neutron stars and black hole.

Within the disk, matter spirals around and around, coming closer and closer to the central object until at last it falls onto its surface (or, in the case of a black hole, through its event horizon).

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thermal radiation

In a narrow sense: synonym for infrared radiation. In a more general sense: The electromagnetic radiation emitted by every body with non-zero temperature due to the laws of thermodynamics. The properties of this radiation (in particular: its spectrum) depend on the body's temperature - in the simplest case, that of what is called a blackbody, they even depend on nothing but the temperature and some universal constants. For everyday temperatures, such as those of a hotplate, thermal radiation is emitted mainly in the form of infrared radiation. At higher temperatures, significant amounts of visible light are emitted, as well: a hotplate that is very hot indeed looks dark red or even light red; molten metal looks yellow or even white. In more extreme situations, thermal radiation can have energies that are even higher - for instance, the gases in the accretion discs of black holes are so hot that they emit great amounts of thermal radiation in the x-ray region.

surface

Geometric space with two dimensions. Examples include the plane or the surface of a sphere.

stars (star)

A cosmic gas ball that is massive enough for pressure and temperatur in its core to reach values where self-sustained nuclear fusion reactions set in. The energy set free in these reactions makes stars into very bright sources of light and other forms of electromagnetic radiation. Once the nuclear fuel is exhausted, the star becomes a white dwarf, a neutron star or a black hole.

radiation

In a general sense: Collective name for all phenomena in which energy is transported through space in the form of waves or particles. In a more restricted sense, the word is often used synonymously with electromagnetic radiation.

neutron

Particle that is electrically neutral and comparatively massive; the atomic nuclei consist of neutrons and protons. Neutrons are not elementary particles, they are compound particles consisting of quarks that are bound together through the strong nuclear interaction . Collectively, neutrons, protons and a number of similar particles are called baryons. Neutron stars are mainly made of neutrons.

matter

In general relativity: All contents of spacetime that contribute to its curvature: particles, dust, gases, fluids, electromagnetic and other fields. In particle physics: All elementary particles with half-integer spin, such as electrons and quarks, as well as their composites such as protons and neutrons, in contrast with force particles.

gas

In a strict sense: A state of matter in which the atoms and/or molecules wildly careen and collide, without being bound to each other. This movement leads to an inner pressure, while the average kinetic energy of the moving particles is a measure for the temperature of the gas. Compare the other states of matter: solid state, liquid, plasma. In a broader sense, gas is also used to denote other mixtures of freely careening particles, for instance in the case of the electron gas whose pressure stabilizes a white dwarf against further collapse.

energy

Physical quantity with the special property that, in physical processes, energy is neither destroyed nor created, simply transformed from one form of energy to another. Some of the different forms of energy that are defined separately are kinetic energy, thermal energy and the energy carried by electromagnetic radiation. Processes that transform one form of energy into another take place in all machines we use in everyday life, from the engine of a subway train (electrical energy into kinetic energy of the train) to an electric blanket (electrical energy into thermal energy). One important consequence of special relativity is that energy and mass are completely equivalent - two different ways to define what is, on closer inspection, one and the same physical quantity. See the keyword equivalence between mass and energy.

black hole

Region in space where a sufficient amount of mass is concentrated so that it forms a gravitational prison - a region into which matter or light can enter from the outside, but from which nothing that has once entered can ever leave. Basic information on this key phenomenon of Einstein's general theory of relativity can be found in the chapter Black holes & Co. of Elementary Einstein. Selected aspects of the physics of black holes and neutron stars are described in the category Black holes & Co. of our Spotlights on relativity. In Einstein's theory, black holes are truly black, due to the fact that no radiation or light can ever escape them. Once quantum theory is taken into account, this assumption no longer holds - on the contrary, it seems as if black holes should emit so-called Hawking radiation. However, for astrophysical black holes (that typically have more or even much more than one solar mass), that radiation would be undetectable, even if we could transport today's most sensitive sensors into the immediate vicinity of the black hole.

accretion

When gas, dust or other kinds of matter fall towards a compact object (such as a black hole or a neutron star), a disk of infalling matter forms around the central object called the accretion disk.

Within the disk, matter spirals around and around, coming closer and closer to the central object until at last it falls onto its surface (or, in the case of a black hole, through its event horizon).

accretion disk (accretion)

When gas, dust or other kinds of matter fall towards a compact object (such as a black hole or a neutron star), a disk of infalling matter forms around the central object called the accretion disk.

Within the disk, matter spirals around and around, coming closer and closer to the central object until at last it falls onto its surface (or, in the case of a black hole, through its event horizon).

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accretion

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