terminal velocity تندای ِ پایانی tondâ-ye pâyâni
Fr.: vitesse terminale 1) 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.
2) The velocity acquired at the end of a body's motion.
3) 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). → terminal; → velocity. |
