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?無人能道出個所以然來。位于獵戶座分子云團的反射星云NGC 1999是迄今為止天空中神秘的復雜構造之一。圖中該星云被命名為HH-222,伸長的氣體流綿延約10光年并發散出一種不尋常的顏色陣列。其中一種假說認為該氣體長絲是受一顆年輕的恒星風影響附近的分子云所致。上述假說最終沒有得到權威解釋,然而問題是為什么瀑布和微弱氣流均位于弧形構造左上角且聚集在一個明亮的但非上升暖氣流射電源中。另一個假說認則為這種不尋常的射電源源于二進制系統，包含熱白矮星、中子星或黑洞,瀑布只是能量系統在釋放而已。這種系統是典型的強X射線源,但并沒有被X射線探測到。現在這種情況仍然得不到解釋。也許未來精心準備的遠見觀察和聰明的演繹推理會解開這個謎一樣的小精靈的真正起源。