An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The mass per unit area of the ring material, integrated through the thickness of the ring. Sometimes called → surface density (Ellis et al., 2007, Planetary Ring Systems, Springer).

→ mass; → density.

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.

The energy (E) associated with a mass (m), as specified by the → mass-energy equivalence  E = mc², where c is the → speed of light. For a moving body the total energy of the particle is expressed by: E² = m²c⁴ + p²c², where m is → rest mass and p → momentum.

→ mass; → energy.

An event in the history of life on Earth in which large numbers of species (sometimes more than 90% of some species) vanish in a relatively short period of time. In spite of controversy, it is generally recognized that there have been at least six major mass extinctions. These occurred in the late Cambrian (500 million years ago), in the late Ordovician (440 million years ago), in the late Devonian (365 million years ago), at the end of the Permian (245 million years ago), in the late Triassic (208 million years ago), and at the end of the Cretaceous (65 million years ago).

→ mass; → extinction.

The mass of a fluid that passes a specified unit area in a unit amount of time.

→ mass; → flow.

An → equation expressing the → atomic mass of a → nuclide as a function of its → mass number and the → atomic mass unit.

→ mass; → formula.

The fractional amount (by mass) of a given → chemical element or → nuclide in a given → chemical composition. In chemical composition studies of astrophysical objects the mass fractions of → hydrogen, → helium, and all the remaining chemical elements are usually denoted by the parameter X, Y, and Z, respectively. Their sum is defined as X + Y + Z = 1. The parameter Z is usually referred to as → heavy elements or → metals.

→ mass; → fraction.

1) The number of a class of objects as a function of their mass. → initial mass function (IMF); → present-day mass function (PDMF).
2) A numerical relation between the masses of the two components of a → spectroscopic binary that provides information on the relative masses of the two stars when the spectral lines of only one component can be seen. If M_p is the mass of primary (whose spectrum is known), M_s is the mass of secondary, and i the → angle of inclination of the orbit, the mass function is given by: (M_s³. sin³i) / (M_p + M_s)².

→ mass; → function.

The outpouring of particles and gas from a star, occurring at varying rates and by a variety of processes throughout a star's lifetime. → Bipolar flows are believed to be due to mass loss by forming → protostars, while → massive stars lose their mass through powerful → stellar winds.

→ mass; → loss.

The rate with which the → mass loss process takes place, usually expressed in → solar mass per year. → radiation-driven mass loss. The mass loss rate and the → terminal velocity are anti-correlated, since the → wind momentum is constant, → bi-stability jump.

→ mass loss; → rate.

The total number of → protons and → neutrons in the → atomic nucleus (symbol A). The mass number is written either after the → chemical element name or as a superscript to the left of an element's symbol. For example, the most common isotope of oxygen is oxygen-16, or ¹⁶O, which has 8 protons and 8 neutrons.

→ mass; → number.

The flowing out of mass through various processes from an object, for example in a star forming region or in a close binary.

→ mass; → outflow.

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 portion of the isotope shift which results from the difference between the nuclear masses of different isotopes.

→ mass; → shift.

An analytical technique for identification of chemical structures, determination of mixtures, and quantitative elemental analysis, in which ions are separated according to the mass/charge ratio and detected by a suitable detector.

→ mass; → spectrometry.

A spectrum of charged particles, arranged in order of mass or mass-to-charge ratios. → mass spectrometry.

→ mass; → spectrum.

The process in which the evolved member of a close binary system passes gaseous material to its companion star.

→ mass; → transfer.

In fluid mechanics, the motion of a given amount of material carried by a fluid from one point to another.

→ mass; → transfer.

The principle of interconversion of mass and energy, described by the → mass-energy relation.

→ mass; → energy; → equivalence.

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.