An H II region is a region of interstellar atomic hydrogen that is ionized. [1] It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre.

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An HI region or H I region (read H one) is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements. (H is the chemical symbol for hydrogen, and "I" is the Roman numeral.

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在天文里，HI, HII 分别是中性氢、电离氢的意思。H就是Hydrogen，I, II表示中性原子、第一电离态原子，以此类推也适用于其他类元素以及III, IV... 等状态。 星系里的HI一般认为是恒星形成的原材料，即星系恒星质量转化的来源 (中间可以转化为分子H2)。而HII则是由HI受电离光子激发后形成，而电离光子一般来源于大质量的O, B型恒星——比较热颜色也比较蓝，因此更容易发射UV光子。

So-named because they contain mostly ionized hydrogen (H+ or HII), HII regions are found throughout the interstellar medium in the Galaxy and in other galaxies. As O and early B-type stars form and turn on within molecular clouds, their UV radiation begins to ionize the surrounding, mostly neutral hydrogen (HI) medium.

2024年1月8日 · H II regions (also spelled as HII regions), formally known as ionized atomic hydrogen regions, represent distinct cosmic zones characterized by the prevalence of ionized atomic hydrogen (H II). These regions owe their ionization to the intense ultraviolet radiation emitted by nearby hot, massive stars.

电离氢区也叫HII区，它们是以氢为主要成分的星际气体云。 若星云附近有比较年轻的炽热恒星，则本来是中性的氢（中性氢区也叫HI区）会被恒星的紫外辐射电离，形成电离氢区——这样的电离氢区就是弥漫的大面积发射星云，激发星云的恒星必须是能发出大量紫外辐射的O型或B型星，这些星的表面温度高达几万度，星云的温度也可以达到上万度。 此外，当星际云之间的密度非常低时，中性氢原子在宇宙线的作用下也会电离。 电子和质子一旦分开，就不容易再复合，从而也 …

H II region, interstellar matter consisting of ionized hydrogen atoms. The energy that is responsible for ionizing and heating the hydrogen in an emission nebula comes from a central star that has a surface temperature in excess of 20,000 K.

Ionized hydrogen in astrophysics is generally indicated by the abbreviation HII or H II (pronounced "H two") if it is a positive ion (i.e., no electron; a more common symbol across science is H+).