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

An intrinsic quantum mechanical characteristic of a particle that has no classical counterpart but may loosely be likened to the classical → angular momentum of a particle arising from rotation about its own axis. The magnitude of spin angular momentum is given by the expression S = ħ √ s(s + 1), where s is the → spin quantum number. As an example, the spin of an electron is s = 1/2; this means that its spin angular momentum is (ħ /2) √ 3 or 0.91 x 10^-34 J.s. In addition, the projection of an angular momentum onto some defined axis is also quantized, with a z-component S_z = m_sħ. The only values of m_s (magnetic quantum number) are ± 1/2. See also → Stern-Gerlach experiment.

→ spin; → angular; → momentum.

The magnetic moment associated with the → spin angular momentum of a charged particle. The direction of the magnetic moment is opposite to the direction of the angular momentum. The magnitude of the magnetic moment is given by: μ = -g(q / 2m)J, where q is the charge, m is the mass, and J the angular momentum. The parameter g is a characteristic of the state of the atom. It would be 1 for a pure orbital moment, or 2 for a spin moment, or some other number in between for a complicated system like an atom. The quantity in the parenthesis for the electron is the → Bohr magneton. The electron spin magnetic moment is important in the → spin-orbit interaction which splits atomic energy levels and gives rise to → fine structure in the spectra of atoms. It is also a factor in the interaction of atom with external fields, → Zeeman effect.

→ spin; → magnetic moment.

An integer or half-integer on which the magnitude of a particle's → spin angular momentum depends. It is expressed in units of → Planck's constant divided by 2π. Called also spin, denoted s. The spin of a particle can only have a value that is zero or a multiple of 1/2. Particles with half-integer spins, 1/2, 3/2, 5/2, ..., are → fermions. Particles with integer spin (0, 1, 2, ...) are called → bosons.

→ spin; → quantum; → number.

The → excitation temperature of the → hyperfine structure levels of the → neutral hydrogen→ 21-centimeter line.

→ spin; → temperature.

A phenomenon in which the rotation period of a pulsar steadily decreases with the pulsar age. The cause of the spin-down is magnetic torque due to the strong fields threading out from the pulsar. The magnetic energy is being converted to high-energy particles and radiation from the nebula. Observed spin-down rates range from about 10^-5 seconds/year for the youngest pulsars to about 10^-12 seconds/year for recycled pulsars. The Crab pulsar is slowing down at a rate of about 10^-5 seconds/year. Knowing the rotation period and the lengthening rate of a pulsar leads to its age.

→ spin; down, M.E.; O.E. ofdune "downward," from dune "from the hill."

Kond-carxi, from kond "slow; dull" + carx→ rotate + -i noun suffix.

Quantum mechanics: The scattering of a particle that reverses the spin direction.

→ spin; flip, from flip-flap; → scattering.

Parâkaneš, → scattering; bâ "with;" vâruni, noun from vârun, → inverse; espin, → spin.

1) Astro.: A relationship between the orbital period of one body around another and its rotational period on its axis. The relationship results from tidal forces between the two bodies. For example, the rotation period of the Moon equals its revolution period around the Earth.
2) Quantum mechanics: The interaction between a particle's → spin angular momentum and its → orbital angular momentum.

→ spin; → orbit; → coupling.

1) A rounded rod, usually of wood, tapering toward each end, used in hand-spinning to twist into thread the fibers drawn from the mass on the distaff, and on which the thread is wound as it is spun (Dictionary.com).
2) → Spindle Galaxy.

M.E. spindel, O.E. spin(e)l, from spinnan, → spin.

Duk "spindle," variants dêk, dik, ultimately from Proto-Ir. *dau- "to run;" cf. Pers. dow-, davidan "to run" (Cheung 2007).

Same as → NGC 5866.

→ spindle; → galaxy.

1) In 3D → magnetic reconnection models of solar plasma, a field line crossing the → fan at the → magnetic null point. See also → fan (Lau & Finn. 1990, ApJ 350, 672; Parnell et al. 1996, Physics of Plasmas 3, 759).
2) A very narrow line of light extending back from the coma into the tail of some → comets.

M.E., from O.Fr. espine, from L. spina "backbone," originally "thorn, prickle," cf. L. spica "ear of corn," O.N. spikr "nail;" from PIE *spei- "sharp point."

Xâr "spine, thorn," related to xal-, xalidan "to prick, to pierce," xâridan "to scratch, itch;" Av. x^vara- "wound, sore."

A mineral, MgAl₂O₄, occurring in various colors, used as a gem, the most valuable being red. The famous "Black Prince's Ruby" which forms part of the Crown Jewels of England, is, in fact, a red spinel. Spinel has often been confounded with → ruby. The most famous source of spinel is the historic region of Badakhshan (today northeastern Afghanistan and southeastern Tajikistan). The Badakhshan mines were mentioned by Persian writers as early as the 10th century. According to a Persian tradition, these mines were first disclosed when the mountain was broken open by an earthquake.

From Fr. spinelle, from It. spinella of unknown origin.  

Lâl, la'l "spinel; red," originally "red" (cf. Tabari âl "red"); cf. Av. raoidita- "red, reddish;" Skt. rudhirá- "red, bloody;" L. ruber "red;" Gk. erythros "red;" akin to E. → red.

A toy that with a quick or vigorous twist spins around its symmetry axis and balances on a point. Suppose a top is perfectly fashioned so that its → rotation axis passes through its → center of mass. If it is spun carefully such that it remains perfectly upright while rotating, it will spin at a steady → angular velocity almost indefinitely in the absence of → friction. Rotation creates an → angular momentum which is directed upward along the rotation axis, opposite to the → gravity vector. However, a slight mismatch between the rotation axis and the center of mass causes gravity to exert a → torque on the top due to its weight, acting through the center of mass. The torque gives rise to a time rate of change of angular momentum, so the top experiences → precession about its point of contact. The tip of the angular momentum vector can be perceived as precessing about the → vertical, thus describing the → precessional circle. The top's precession period is given by: T_p = (4π²I)/(mgrT_s), where I is the → moment of inertia, m the mass of the top, g gravity, r the distance between the center of mass and the contact point, and T_s is the spinning period of the top. Precession is accompanied by another oscillatory phenomenon, called → nutation. Nutation is less influenced by the gravity torque and is determined by the inertia forces acting on the spinning body.

→ spin; → -ing; top M.E., from O.E. top, maybe related to Fr. toupie.

Farmuk, ferferé "spinning top" (Dehxodâ), two words of unknown etymology.

A hypothetical, very rapidly → rotating star formed in the → metal-deficient conditions of the primordial → interstellar medium. The → first stars were probably spinstars, because the lack of metals leads to faster rotation velocities. Indeed → metal-poor stars are more compact than → metal-rich ones. Stars formed from a gas whose → metallicity is below 1/2000 of the → solar metallicity could attain rotation velocities of 500-800 km s^-1 (see also → Population III star). Rotation triggers → mixing processes inside the star, leading to the production of important quantities of ¹⁴N, ¹³C, and ²²Ne (Maeder & Meynet 2012, and references therein). The production of primary ²²Ne has an important impact on the → s-process  → nucleosynthesis in spinstars compared to non-rotating stars. This increases by orders of magnitude the s-process → yields of → heavy elements. Spinstars would therefore have strongly influenced the properties and appearance of the first galaxies that formed in the → Universe (See G. Meynet et al. 2009, arXiv:0709.2275; C. Chiappini, 2013, Astron. Nachr. /AN 334, No. 6, 595 and references therein).

→ spin; → star.

A new area of science and technology which exploits the intrinsic → spin of electrons and its associated → magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. In brief, spin-based electronics. For example, information could be transported or stored through the spin-up or spin-down states of electrons. Spintronics techniques are capable of much higher speed while requiring less power than the conventional method of using electron charges to represent data. The first use of spintronics was in the late 1980s with the development of → giant magnetoresistance (GMR) read heads for disk drives

Short for → spin + → electronics.

Running continuously around a fixed point or center while constantly receding from or approaching it.

From M.Fr. spiral, from M.L. spiralis "winding, coiling," from L. spira "coil," from Gk. speira "coil, twist, wreath."

Mârpic, literally "snake coil," from mâr "snake, serpent," → Serpens, + pic, present stem of picidan "to twist, entwine, coil," ultimately Proto-Iranian *paticā-citanai- "to coil;" cf. Av. paitica "inversely; back" and ci- (caē-, caii-) "to heap up, gather" (Nyberg 1974).

The region in a → spiral galaxy that contains concentrations of → gas, → dust, and → massive stars. Spiral arms are created by → density waves. See also → density wave theory.

→ spiral; → arm.

A galaxy with a prominent nuclear → bulge and luminous → spiral arms of gas, dust, and young stars that wind out from the nucleus. Masses span the range from 10¹⁰ to 10¹²→ solar masses.

→ spiral; → galaxy.

An obsolete term used to describe nebular objects with spiral shape before it was understood that they are independent galaxies lying outside our → Milky Way galaxy. Now called → spiral galaxy.

→ spiral; → nebula.

The morphology of a galaxy which displays → spiral arms.

→ spiral; → structure.

The principle of conscious life; the vital principle in humans, animating the body or mediating between body and soul (Dictionary.com).

M.E., from L. spiritus "a breathing, breath; breath of life," related to spirare "to blow, breathe."

Minu "spirit;" Mid.Pers. mênôg "spirit;" Av. mainyu- "mind, mentality, mental force, inspiration," from mān- "to think," → mind.