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terminal velocity
تندای ِ پایانی
tondâ-ye pâyâni
Fr.: vitesse terminale
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The constant maximum velocity reached by a body falling under gravity through a
liquid or gas, especially the atmosphere. The body ceases
to accelerate downward because the force of gravity is equal
to the opposing force of resistance by the medium.
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The velocity acquired at the end of a body’s motion.
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The velocity attained by → stellar wind
at very large distance from the star.
The material that escapes from the outer layers of the stars
is accelerated outward from a small radial velocity at the
→ photosphere of the star,
to some high velocity at large distance from the star.
The theory of → radiation-driven winds predicts that the
terminal velocity scales with the → escape velocity as:
v∞ ≅ 2-3 vesc.
The winds of hot → O stars experience fast acceleration
and reach 80% of their
terminal velocity near the star (some 3 R*).
The terminal velocity ranges from about 10 km s-1 for a cool
→ supergiant star to 3000 km s-1 for a luminous hot star.
Terminal velocities are quite accurately measured from the violet trough of
saturated → P Cygni line profiles in the ultraviolet
(mainly N V λ1239, C IV λ1548, Si IV λ1394).
See also: → terminal; → velocity.
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