偶极外向流意指两股从一颗恒星的两极持续向外流动的气体。偶极外向流可能与原恒星（年轻、郑在形成的恒星）相关联，或与演化的后渐近巨星分支（通常是双极星云的形式）相关联。

在年轻恒星的情况，偶极外向流是由高密度、直直的喷流驱动的。 喷流比向外流狭窄，因此很难直接观察到。然而，超音速激波前缘沿着喷流将周围的气体加热至数千度。这些袖珍的热气体辐射出红外线的波长，因此可以被像英国红外线望远镜（UKIRT）这种望远镜检测到。他们经常沿着喷流的梁呈现分离不连续的结或是弧。因为这些结通常弯曲成弓形，如同船舶前方的波，因此也被称为分子弓形冲击波（弓形震波）。

A bipolar outflow represents two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars (young, forming stars), or with evolved post-AGB stars (often in the form of bipolar nebulae).

In the case of a young star, the bipolar outflow is driven by a dense, collimated jet. These astrophysical jets are narrower than the outflow and very difficult to observe directly. However, supersonic shock fronts along the jet heat the gas in and around the jet to thousands of degrees. These pockets of hot gas radiate at infrared wavelengths and thus can be detected with telescopes like the United Kingdom Infrared Telescope (UKIRT). They often appear as discrete knots or arcs along the beam of the jet. They are usually called molecular bow shocks, since the knots are usually curved like the bow wave at the front of a ship.