In astronomy, an envelope is 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 nebula.
In mathematics, an envelope is 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.