A stream of matter in two opposing directions from a central
object, usually a star. Bipolar outflows represent significant periods of
mass loss in a star's life. They tend to occur during the protostar
and pre-main-sequence phase and, again, during the red
giant phase just before the production of a planetary
nebula (for example, see Ant Nebula).
It isn't certain if the bipolar flow is caused by a lack of material being
ejected at other stellar latitudes or if something, such as an accretion
disk, blocks the material in the equatorial regions and allows only
that which is ejected at the poles to escape. It has also been suggested
that magnetic fields may constrain the outflowing material. The outflows
carve out cavities in the surrounding interstellar medium and result in
the formation of bipolar nebulae.