An Etymological Dictionary of Astronomy and Astrophysics
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A thin trail of gas stretching from the → Magellanic System toward our own Galaxy over about 150° on the sky, corresponding to hundreds of thousands of light-years. This gas consists primarily of → neutral hydrogen and is thought to have originated from the Large and Small Magellanic Clouds as a result of tidal interactions with the Milky Way. See, e.g., Fox et al. 2013, arxiv/1304.4240, and references therein.

→ Magellanic; → stream.

An n × n matrix in which every row, column, and diagonal add up to the same number.

→ magic; → square.

Same as → synchrotron radiation.

→ magnetic; → bremsstrahlung.

Same as → magnetic intensity.

→ magnetic; → field; → intensity.

The pressure exerted by a magnetic field on the material that contains the field.

→ magnetic; → pressure.

In a → plasma, a change of → magnetic connectivity of plasma elements due to the presence of a localized → diffusion region. It allows charged particles to move from one → magnetic field line to another. Magnetic reconnection is an important process transforming magnetic energy into heat or/and kinetic energy. Magnetic reconnection events occur in the Earth's → magnetosphere. The process plays an important role in explosive phenomena in the Sun, such as → coronal mass ejections and → solar flares which heat the → solar corona.

→ magnetic; → re-; → connection.

The process by which a magnetic system relaxes to its minimum energy state over time.

→ magnetic; → relaxation.

A phenomenon exhibited by certain atoms whereby they absorb energy at specific (resonant) frequencies when subjected to alternating magnetic fields.

→ magnetic; → resonance.

A change in the Earth's → magnetic field in which the → magnetic north pole is transformed into a → magnetic south pole and the magnetic south pole becomes a magnetic north pole. There are geological proofs indicating that the Earth's magnetic field has undergone numerous reversals of → polarity in the past. In the last 10 million years, there have been, on average, 4 or 5 reversals per million years. At other times, for example during the → Cretaceous era, there have been much longer periods when no reversals occurred. Over the past two centuries, Earth's magnetic field has weakened by 15%. Risks of a weak magnetic field include more deaths from cancer due to increased radiation, electrical grid collapse from severe solar storms, climate change, and temporary ozone holes. See also → geomagnetic excursion.

→ magnetic; → reversal.

A → dimensionless quantity used in → magnetohydrodynamics to describe the relative balance of → magnetic advection to → magnetic diffusion. It is given by: R_m = σμ₀νLU₀, where σ is the → conductivity of the fluid, μ₀ is the → magnetic permeability of the fluid, L is he characteristic length scale of the fluid flow, and U₀ the characteristic velocity of the flow. A typical value for the Earth is R_m ~ 200.

→ magnetic; → Reynolds number.

The region around a celestial body in which the magnetic field of the body dominates the external magnetic field. Each planet with a magnetic field (Earth, Jupiter, Saturn, Uranus, and Neptune) has a magnetopause. The Earth's magnetosphere is a dynamic system that responds to solar variations. It prevents most of the charged particles carried in the → solar wind, from hitting the Earth. Since the solar wind is → supersonic, a → bow shock is formed on the sunward side of the magnetosphere. The solar wind ahead is deflected at a boundary called → magnetopause. The region between the bow shock and the magnetopause is called the → magnetosheath. As the solar wind sweeps past the Earth, the terrestrial magnetic field lines are stretched out toward the night side to form a → magnetotail.

From → magnet + → sphere.

Logic: In a → categorical syllogism, the premise containing the → major term.

→ major; → premisse.

A correction introduced into star counts distributed by apparent magnitude.

→ Malmquist bias; → correction.

A mature female horse or other equine animal.

M.E., variant of mere, O.E. m(i)ere feminine of mearh "horse," (cognates: O.Sax. meriha, O.Norse merr, Du. merrie, O.H.G. meriha, Ger. Mähre "mare"), probably of Gaulish origin (cf. Irish and Gaelic marc, Welsh march, Breton marh "horse").

Mâdiyân, from mâdé "female," → feminine.

1) For → massive stars and → supergiants, the difference between the → spectroscopic mass and the → evolutionary mass. Early studies found that the spectroscopic mass was systematically less than the evolutionary mass by as much as a factor of 2 for supergiants. Improvements in the stellar atmosphere models (taking into account → line blanketing) have decreased or eliminated the size of the discrepancy for Galactic stars. There is still a mass discrepancy for the hottest → O stars in the → Magellanic Clouds (See, e.g. Massey et al. 2009, ApJ 692, 618).
2) For a → cluster of galaxies, the apparent difference between the mass of the cluster obtained by using the → virial theorem, and the mass inferred from the total luminosities of the member galaxies.

→ mass; → discrepancy.

A consequence of the → dynamical relaxation process in a gravitationally → bound system, such as a → star cluster or a → globular cluster, where massive and low-mass members occupy different volumes. Massive members sink toward the center, while less massive members tend to move farther away from the center.

→ mass; → segregation.

The famous equation proposed by Einstein as a consequence of his special theory of relativity describing the equivalence of mass and energy: E = mc², where E is energy, m is the equivalent amount of mass, and c is the velocity of light.

→ mass; → energy; → relation.

A relationship between luminosity and mass for stars that are on the main sequence, specifying how bright a star of a given mass will be. Averaged over the whole main sequence, it has been found that L = M^3.5, where both L and M are in solar units. This means, for example, that if the mass is doubled, the luminosity increases more than 10-fold.

→ mass; → luminosity; → relation.

A correlation between the → stellar mass (or → luminosity) and the → gas metallicity of → star-forming galaxies (Lequeux et al. 1979) according to which massive galaxies have higher gas metallicities. Several large galaxy surveys, such as the → Sloan Digital Sky Survey (SDSS), have confirmed that galaxies at all → redshifts with higher stellar masses retain more metals than galaxies with lower stellar masses. Besides the dependence on stellar mass, other studies have found further dependences of gas metallicity on other physical properties at a given mass, such as → specific star formation rate, → star formation rate, and stellar age. These higher dimensional relations could provide additional constraints to the processes that regulate the metal enrichment in galaxies. In addition to gas metallicity, also the → stellar metallicity of galaxies is found to correlate with the stellar mass, suggesting the mass-metallicity relation already existed at early epochs of galaxy evolution (Lian et al., 2017, MNRAS 474, 1143, and references therein).

→ mass; → metallicity; → relation.

The relation between the → stellar mass and the physical size of a galaxy. Studies show that the sizes increase with stellar mass, but that the relation weakens with increasing → redshift. Separating galaxies by their → star formation rate, model simulations show that → passive galaxies are typically smaller than → active galaxies at a fixed stellar mass. These trends are consistent with those found in observations; the level of agreement between the predicted and observed size-mass relations is of the order of 0.1 dex for redshifts < 1 and 0.2-0.3 dex from redshift 1 to 2. Known also as the → luminosity-size relation (Furlong et al., 2016, MNRAS 465, 722, and references therein).

→ mass; → size; → relation.