电离氢区（H II区）是发光的气体和电浆组成的云气，有时会有数百光年的直径，是恒星诞生的场所。从这些气体中诞生的年轻、炙热的蓝色恒星散发出大量的紫外线，使星云环绕在周围的气体游离。

H II区在数百万年的岁月中也许可以诞生成千上万颗的恒星。最后，超新星爆炸和来自星团中质量最大的那些恒星吹出的强烈恒星风，将会吹散掉H II区的气体，留下来的就是像昴宿星团这样的星团。

H II区是因为有大量被游离的氢原子而得名的，天文学家同样的将中性氢的区域称为HI区，而H₂称为分子氢。在宇宙的远处的H II区不会被忽略，也能被看见，对其它星系H II区的观测，在测量距离和化学组成是很重要的研究项目。

An H II region is a region of interstellar hydrogen that is ionized. It is typically a large, low-density cloud of partially ionized gas in which star formation has recently taken place. The short-lived blue stars forged in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regions—sometimes several hundred light-years across—are often associated with giant molecular clouds. The first known H II region was the Orion Nebula, which was examined by telescope in 1610 by Nicolas-Claude Fabri de Peiresc, although its nature was not known.