Explanation: What created the Waterfall Nebula? No one knows. The structure seen in the region of NGC 1999 in the Great Orion Molecular Cloud complex is one of the more mysterious structures yet found on the sky. Designated HH-222, the elongated gaseous stream stretches about ten light years and emits an unusual array of colors. One hypothesis is that the gas filament results from the wind from a young star impacting a nearby molecular cloud. That would not explain, however, why the Waterfall and fainter streams all appear to converge on a bright but unusual non thermal radio source located toward the upper left of the curving structure. Another hypothesis is that the unusual radio source originates from a binary system containing a hot white dwarf, neutron star, or black hole, and that the Waterfall is just a jet from this energetic system. Such systems, though, are typically strong X-rays emitters, and no X-rays have been detected. For now, this case remains unsolved. Perhaps well-chosen future observations and clever deductive reasoning will unlock the true origin of this enigmatic wisp in the future.

是什么形成了瀑布星云?无人能道出个所以然来。位于猎户座分子云团的反射星云NGC 1999是迄今为止天空中神秘的复杂构造之一。图中该星云被命名为HH-222,伸长的气体流绵延约10光年并发散出一种不寻常的颜色阵列。其中一种假说认为该气体长丝是受一颗年轻的恒星风影响附近的分子云所致。上述假说最终没有得到权威解释,然而问题是为什么瀑布和微弱气流均位于弧形构造左上角且聚集在一个明亮的但非上升暖气流射电源中。另一个假说认则为这种不寻常的射电源源于二进制系统，包含热白矮星、中子星或黑洞,瀑布只是能量系统在释放而已。这种系统是典型的强X射线源,但并没有被X射线探测到。现在这种情况仍然得不到解释。也许未来精心准备的远见观察和聪明的演绎推理会解开这个谜一样的小精灵的真正起源。