H II region
Ionized hydrogen regions where new stars are born.
An H II region is a volume of interstellar space where atomic hydrogen has been ionized, typically embedded within a molecular cloud of partially ionized gas where star formation has recently taken place. These regions vary greatly in size, from one to hundreds of light-years across, and in density, ranging from a few particles to about a million particles per cubic centimetre. Their shapes are irregular, as the distribution of stars and gas within them is uneven. The short-lived, massive blue stars born in these regions emit intense ultraviolet light that ionizes the surrounding gas, creating the characteristic glow. H II regions often appear clumpy and filamentary, sometimes forming intricate shapes like the Horsehead Nebula, and can give rise to thousands of stars over several million years. Eventually, supernova explosions and strong stellar winds from the most massive stars disperse the remaining gas. The Orion Nebula, first observed through a telescope in 1610 by Nicolas-Claude Fabri de Peiresc, was the first such object discovered. In the 18th century, William Herschel described it as "the chaotic material of future suns." Spectroscopic work in the 19th century by William and Mary Huggins revealed that these nebulae emit only a few spectral lines, unlike stars. A prominent line at 500.7 nanometres was initially attributed to a hypothetical element called nebulium, but in the 20th century it was identified as a forbidden line from doubly ionized oxygen in extremely low-density gas. Observations later confirmed that H II regions contain hot, bright stars with lifetimes of only a few million years, establishing them as sites of active star formation. These regions can be observed at great distances, and studying extragalactic examples like NGC 604 and 206 is crucial for determining the distances and chemical compositions of galaxies. Spiral and irregular galaxies contain many H II regions, concentrated in spiral arms or distributed chaotically, while elliptical galaxies have almost none. Some galaxies host enormous H II regions containing tens of thousands of stars, such as 30 Doradus in the Large Magellanic Cloud.
- field
- Astronomy
- known_for
- Ionized hydrogen regions where star formation occurs
- first_discovered
- Orion Nebula, observed in 1610 by Nicolas-Claude Fabri de Peiresc
- typical_size
- One to hundreds of light years
- typical_density
- A few to about a million particles per cubic centimetre
Lore & Background
H II regions are expansive clouds of ionized hydrogen, typically found within molecular clouds where star formation has recently occurred. Their size ranges from one to hundreds of light-years across, with densities varying from a few to about a million particles per cubic centimeter. The shape of an H II region is irregular, as the distribution of stars and gas within it is uneven. These regions often appear clumpy and filamentary, sometimes taking on intricate forms like the Horsehead Nebula. They are frequently associated with giant molecular clouds and may give birth to thousands of stars over several million years. The short-lived, massive blue stars within them emit intense ultraviolet light that ionizes the surrounding gas. Eventually, supernova explosions and strong stellar winds from the most massive stars disperse the remaining gas. H II regions are visible at great distances across the universe, and studying extragalactic examples, such as NGC 604 and 206, helps determine the distances and chemical compositions of galaxies. Spiral and irregular galaxies contain many H II regions, while elliptical galaxies have almost none. In spiral galaxies like the Milky Way, they are concentrated in the spiral arms; in irregular galaxies, they are chaotically distributed. Some galaxies host enormous H II regions containing tens of thousands of stars, such as 30 Doradus in the Large Magellanic Cloud and NGC 604 in the Triangulum Galaxy.
Reader's Guide
H II regions are significant because they are the primary sites of star formation in galaxies. They are often associated with giant molecular clouds and can give birth to thousands of stars over several million years. Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are almost devoid of them. The lifetime of an H II region is a few million years, after which radiation pressure from hot young stars and supernova explosions disperse the remaining gas. Less than 10 percent of the gas in an H II region typically forms into stars before the rest is blown off. The term 'H II' uses Roman numeral II to denote singly-ionized hydrogen, consisting of free protons.
Did You Know?
- The Orion Nebula, an H II region, was first observed in 1610 by Nicolas-Claude Fabri de Peiresc.
- H II regions can be several hundred light-years across and are often associated with giant molecular clouds.
- Less than 10 percent of the gas in an H II region forms into stars before the rest is blown off.
Frequently Asked Questions
What is an H II region?
An H II region is a cloud of ionized atomic hydrogen that sits inside a larger molecular cloud where star formation has recently taken place. It is essentially a glowing stellar nursery powered by the ultraviolet radiation of the young, hot stars it contains.
Where was the first H II region observed?
The Orion Nebula, the most iconic example, was first recorded by Nicolas-Claude Fabri de Peiresc in 1610. That observation made it one of the earliest deep-sky objects ever catalogued by a human eye.
How large and dense are H II regions?
They stretch from roughly one light year up to several hundred light years across. Their particle density runs from just a few particles per cubic centimetre up to about a million, depending on where you look within the cloud.
Why do astronomers care about H II regions?
They are active sites of ongoing star formation, so studying them reveals how and where new stars are born. Because they are bright enough to be detected at great distances, they also serve as natural distance markers and probes of a galaxy's chemical makeup.
What is the lifecycle, or 'story arc,' of an H II region?
A region ignites when a cluster of hot young stars strips electrons from the surrounding hydrogen, making it glow. Over millions of years the gas is pushed outward or recombines, the glow fades, and the material may eventually seed the next generation of stars elsewhere in the cloud.
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