談到行星狀星雲的恆星風

the knots and strings of compressed gases--made mainly of
hydrogen, helium, carbon, and oxygen--become visible to astronomers
as the ultraviolet radiation from the core xcites the gas
and causes it to glow
緊密的氣體大部分由氫氦碳氧組成
受到核心發出的紫外線激發而發亮
所以天文學家才觀測的到

我的問題是knots and strings到底是什麼？
因為我從一些行星狀星雲的圖上也看不出有任何knots或strings
拜託知道的人請告訴我，謝謝