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An astrophysical plasma is a plasma (highly ionized gas) that occurs beyond the solar system. It is studied as part of astrophysics and is a commonly observed phenomenon in space. The accepted view is that much of the baryonic matter in the universe exists in this state.
Sufficient heating of atoms and molecules causes matter to become ionized and form a plasma. This energetic process breaks them into their constituent particles as negatively-charged electrons and positively-charged ions: either as protons or atomic nuclei. These electrically-charged particles are susceptible to influences by local electromagnetic fields: both strong fields generated by stars or weak fields that exist in star forming regions and the interstellar and intergalactic mediums. Similarly, electric fields are observed in some stellar astrophysical phenomena, but they are inconsequential in very low density gaseous mediums.
Astrophysical plasma is often made distinct from the associated term space plasma (as space physics). This usually includes observing behaviours of strongly ionised plasma on the Sun and in its atmosphere, or the solar wind interactions with Earth's magnetic field which causes the aurora.. This can also be extended to the Earth's upper atmosphere, other planetary atmospheres and their magnetospheres (aeronomy), or when monitoring interplanetary space weather.
Observing and studying astrophysical plasma
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Plasmas in stars can both generate and interact with magnetic fields, resulting in a variety of dynamic astrophysical phenomena. These are sometimes observed in spectra by the Zeeman effect. Other forms of astrophysical plasmas can all be influenced by preexisting weak magnetic fields, whose interactions may only be determined directly by polarimetry or other indirect methods. In particular, the intergalactic medium, the interstellar medium, the interplanetary medium and solar winds are all mainly diffuse plasmas. The cross-scale energy transport between particle heating across the boundary separating the shocked solar wind and magnetospheric plasma is a compelling and fundamental problem of plasma physics. Stars, however, are made of dense plasma.
Astrophysical plasma may also be studied in a variety of ways as they emit electromagnetic radiation across a wide range of the electromagnetic spectrum. Because astrophysical plasmas are generally hot, often meaning that they are highly ionized, electrons in the plasmas are continually emitting X-rays through the process called bremsstrahlung, when electrons nearly collide with atomic nuclei. This radiation may be detected with X-ray telescopes, performed in the upper atmosphere or space, such as the Chandra X-ray Observatory. Astrophysical plasmas also emit radio waves and gamma rays.
There is much interest in active galactic nuclei because astrophysical plasmas could be directly related to the plasmas studied in the laboratory, or with fusion power experiments.[dubious ] Many of these galaxy phenomena seemingly exhibit an array of complex magnetohydrodynamic behaviors, such as turbulence and instabilities. Although these phenomena may occur on astronomical scales as large as the galactic core, many astrophysicists suggest that they do not significantly involve plasma effects but are caused by matter consumed by super massive black holes (SMBH).
In the Big Bang cosmology, the entire universe was in a plasma state prior to recombination. Afterwards, much of the universe reionized after the first quasars formed and emitted radiation, which largely remains in plasma form. It is assumed that very little of this baryonic matter is neutral.
Studying astrophysical plasmas is part of mainstream academic astrophysics. Though plasma processes are part of the standard cosmological model, current theories indicate that they might have only a minor role to play in forming the very largest structures, such as voids, galaxy clusters and superclusters.
Norwegian explorer and physicist Kristian Birkeland predicted that space is filled with plasma. He wrote in 1913: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system through its evolution throws off electric corpuscles into space." From this, he assumed that most of the mass in the universe should be found in "empty" space. In 1937, plasma physicist Hannes Alfvén argued that if plasma pervaded the universe, then it could generate a galactic magnetic field. During the 1940s and 1950s, Alfvén developed magnetohydrodynamics (MHD) which enables plasmas to be modelled as waves in a fluid, for which Alfvén won the 1970 Nobel Prize for physics. Alfvén later proposed this as the possible basis of plasma cosmology, although this theory is now openly rejected as the hypothesised galactic and intergalactic magnetic field strengths have never been observed.
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