- (ASTRONOMY) A cloud of gas and
dust that surrounds one or more stars or some other astronomical object.
Young, hot stars often eject envelopes or produce them by ionizing nearby
material. Old stars, in their red giant
phase, shed their outer layers and produce cool envelopes rich in molecules
and dust. When the core of the dying star is exposed, ultraviolet light
from the core makes the envelope luminesce as a planetary
- (MATHEMATICS) A curve
or a surface that touches every member
of a family of lines, curves, planes, or surfaces. For example, the
circle x 2 + y
2 = r 2 is envelope to the family of lines
xcosθ + ysinθ = r.