Скачать или смотреть What is an H II Region?

Stars are born off of collapsing gas clouds due to gravity. Owing to intense high energy UV radiation of the newly formed stars, the surrounding gas cloud gets ionized. This region of ionized atomic hydrogen is known as an H II region.

‘H II’ (pronounced as H-2) represents singly ionized hydrogen atom. In astronomy, neutral atoms are represented by their atomic symbols followed by I. While the charged species are followed by II, III and so on. For instance,
Neutral Hydrogen (H) - H I,
Singly Ionized Hydrogen (H+) - H II,
Neutral Oxygen (O) - O I,
Doubly Ionized Oxygen (O2+) - O III.

Such regions usually span from one to hundreds of light years, having temperatures around 10,000 K. H II regions give birth to thousands of short-lived blue star clusters, which radiate copious amounts of light - illuminating the nebula. Owing to strong stellar winds, radiation pressure and supernova explosions, such regions get dispersed within a few million years of star formation event.

PLEIADES
Pleiades is one such star cluster, where the surrounding H II region is getting dispersed due to the strong stellar winds of B-type stars.

ORION NEBULA
Another famous example is the Orion Nebula, where the stars of the Trapezium Cluster are ionizing the cloud. The Orion Nebula (M42), located 500 pc away, is a part of the Orion Molecular Cloud Complex.

HORSEHEAD NEBULA
Horsehead Nebula is another active star forming H II region in the Orion Molecular Cloud Complex. Presence of thick dust in front of the stars results in the extinguishing of light. The silhouette of the dark nebula visible against the complex and intricate structure is absolutely breathtaking.

H II regions comprise mostly 90% hydrogen, some helium and traces of other heavy elements. The strongest hydrogen emission line, the H-alpha line at 656.3 nm, gives H II regions their characteristic red colour.

H II regions are mostly found in the arms of spiral galaxies and throughout irregular galaxies.

TARANTULA NEBULA
Tarantula Nebula is the most massive H II region - 2nd largest in the Local Group of Galaxies. It is located 160 kly away in the LMC (a satellite galaxy of Milky Way). Thousands of OB and Wolf-Rayet stars populate this region. In fact, the most massive star known - R136a1 (215 M☉) - is located in the central concentration of this region.

NGC 604
NGC 604 is the largest H II region in the entire Local Group of Galaxies, spanning 1,520 ly in diameter. This H II region is located in the Triangulum Galaxy, 2.7 million ly away.

The young stars in H II regions show evidence for containing planetary systems. The Hubble Space Telescope has revealed hundreds of protoplanetary disks (proplyds) in the Orion Nebula.

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Graphics: NASA, ESA, ESO, Space Engine

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