An Etymological Dictionary of Astronomy and Astrophysics
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The speed at which → Alfven waves are propagated along the magnetic field. It is a characteristic velocity at which perturbations of the lines of force travel. Alfvén speed is given by: v_A = B/(μ₀.ρ)^1/2, where B is the → magnetic field strength, μ₀ is the → magnetic permeability, and ρ is the density of the plasma. Alfvén speed plays a role analogous to the sound speed in non-magnetized fluid dynamics. Same as Alfvén velocity.

→ Alfvén wave; → speed.

The speed at which → Alfven waves are propagated along the magnetic field. It is a characteristic velocity at which perturbations of the lines of force travel. Alfvén speed is given by: v_A = B/(μ₀.ρ)^1/2, where B is the → magnetic field strength, μ₀ is the → magnetic permeability, and ρ is the density of the plasma. Alfvén speed plays a role analogous to the sound speed in non-magnetized fluid dynamics. Same as Alfvén velocity.

→ Alfvén wave; → speed.

Same as → orbital velocity.

→ orbital; → speed.

In models of → galactic structure, the → angular velocity of a → spiral arm or a → bar, assumed to be a pattern rotating as a solid body in the galaxy. Pattern speed, Ω_P , is one of the most important parameters of the → density wave theory responsible for the spiral arms. Pattern speed determines the location of the → corotation resonance.

→ pattern; → speed.

Same as → relativistic velocity.

→ relativistic; → speed.

The velocity of propagation of a → longitudinal wave in a medium under specified conditions. Also known as sonic speed, sonic velocity, acoustic velocity, sound velocity, velocity of sound, speed of sound. The speed of sound is a thermodynamic property that relates to the change in pressure and density of the medium and can be expressed as C = (dP/dρ)^1/2, where C is the sound velocity, dP is the change in pressure, and dρ the change in density. It can also be expressed as C = (E/ρ)^1/2, where E is the bulk modulus elasticity. This equation is valid for liquids, solids and gases. The sound travels faster through media with higher → elasticity and/or lower density. If a medium is → incompressible the speed of sound is infinite. For → ideal gases, a simple relationship exists between the sound speed and temperature: C = (γR T)^1/2, where γ is the → specific heat ratio (C_P/C_V), and R is the → gas constant. We see that for ideal gases it the speed is independent of pressure. In air at 0°C it is 332 m/sec. The speed of sound in a gas of hydrogen is 1315 m/s. → Mach number.

→ sound; → speed.

The ratio of the distance covered to the time taken by a moving body. Speed in a specified direction is → velocity.

M.E. spede "good luck, prosperity, rapidity;" O.E. sped "success, prosperity, advancement;" cf. O.S. spod "success," Du. spoed "haste, speed," O.H.G. spuot "success," O.H.G. spuoten "to haste;" from PIE base *spe- "to thrive, prosper" (cf. Skt. sphā- "to increase, become fat;" L. sperare "to hope;" O.C.S. spechu "endeavor;" Lith. speju "to have leisure").

Tondi "speed," from tond "swift, rapid, brisk; fierce, severe" (Mid.Pers. tund "sharp, violent;" Sogdian tund "violent;" cf. Skt. tod- "to thrust, give a push," tudáti "he thrusts;" L. tundere "to thrust, to hit" (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere "to thrust or bore through;" PIE base *(s)teud- "to thrust, to beat") + noun suffix -i.

Same as → velocity of light.

→ speed; → light.