H II region
Ionized hydrogen regions where new stars are born.
An H II region is a region of interstellar atomic hydrogen that is ionized, typically found in molecular clouds where star formation has recently occurred. These regions range in size from one to hundreds of light years and in density from a few to about a million particles per cubic centimetre. They are important because they are sites of ongoing star formation and can be observed at considerable distances, aiding in the study of galactic distances and chemical composition.
- 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
The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc using a telescope, the first such object discovered.
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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