HII region - Ecchi region

Japanese: ＨII 領域 - えっちつーりょういき

…These emission nebulae, reflection nebulae, and dark nebulae may appear independently, but they are often seen mixed together in the same area. When we reexamine emission nebulae as astronomical phenomena within the Milky Way, they can be divided into planetary nebulae, which are formed when stars release mass in the latter stages of their lives, supernova remnants, which are formed when stars explode and are scattered, and HII regions, which are formed when interstellar gas is ionized by ultraviolet rays emitted from young O-type stars. In fact, emission nebulae and reflection nebulae, excluding planetary nebulae, have come to be collectively called "diffuse nebulae." …

From [Radio Astronomy]

...These are called thermal radio waves because they do not require high energy from electrons, but rather the thermal motion of electrons is sufficient. Emission nebulae (also called HII regions), which shine as the gas around newborn stars heats up, are the source of this type of radio wave. These types of radio wave sources have characteristics such as strong radio waves even at high frequencies and are distributed along the galactic plane, and are used as an indicator of the region where stars are born.

From [Ionized hydrogen region]

...Also called HII region. One of the regions that are easily visible by light or radio waves, it is a physical name for the Orion Nebula, the Omega Nebula, etc.

*Some of the terminology used to refer to "HII regions" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…これらの輝線星雲と反射星雲と暗黒星雲は，それぞれ独立に現れることもあるが，それらは同一の領域に入り乱れて見えている場合が多い。輝線星雲は銀河系内部の天体現象として見直すと，恒星がその生涯の晩年に質量を放出してつくる惑星状星雲，恒星が爆発してとび散ってできる超新星の残骸，若いO型星から放射される紫外線で星間ガスが電離してできるHII領域などに分けられる。実際には惑星状星雲を除く輝線星雲と反射星雲を合わせて〈散光星雲〉と呼ばれてきている。…

【電波天文学】より

…電子に高エネルギーを要せず，電子の熱運動だけで十分なことから熱的電波と呼ばれる。生まれたばかりの星のまわりでガスが熱せられて輝く輝線星雲(HII領域ともいう)がこの種の電波源となる。この種の電波源は高い周波数でも電波が強く，銀河面に沿って分布するなどの特徴を有し，星の誕生する領域を示す指標として使われている。…