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Understanding the H II Region

written by: Nick Oza•edited by: RC Davison•updated: 8/28/2011

Clouds of ionized hydrogen are commonly found in spiral and irregular galaxies. These fascinating regions in space consist of thousands of stars. Find out what they are, how they are formed and what their eventual fate is.

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    What are These Regions?

    HII Region 

    HII regions are massive formations of clouds of gas, primarily hydrogen, that have been ionized due to the stripping of electrons from the surrounding gas. In a sense, these regions are like stellar nurseries where hot, young stars are born. The new stars emit ultraviolet radiation which is responsible for ionizing the hydrogen gas in these clouds, hence giving it the name HII. These regions are found around extremely hot stars and in turn are also very hot with temperatures approaching 10,000° Kelvin. They are most commonly found in spiral and irregular shaped galaxies. Since elliptical galaxies are made up of an aging star population and have very little going on in terms of star formation, these regions are nearly absent in elliptical galaxies. They vary greatly in shape and structure due to the non-uniform distribution of stars and gas within them.

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    Nebulae of a Different Kind

    These regions are often classified as emission nebulae since they contain ionized gases. Generally, within an emission nebula, hot young stars are formed and the gas within them is ionized. These types of nebulae emit light in varying colors depending on the degree of ionization of the various gases within them. Within HII regions, gases are ionized and emit light at different frequencies, which makes them emission nebulae. Planetary nebulae are also another good example of these types of nebulae.

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    Formation Process

    The regions form from what are known as giant molecular clouds (GMCs). A GMC is a very massive cloud of gas consisting mostly of molecular hydrogen (H2). When GMCs become unstable due to a shock-wave from a stellar explosion or by collisions among the clouds, the clouds within the GMC begin to collapse upon themselves. This in turn triggers star formation to occur and new hot, young stars are formed. It is the intense radiation from the hottest stars that causes the gas in the GMC to get ionized resulting in them becoming HII regions.

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    Emission Lines and Bremsstrahlung Radiation

    Ionization of the gas in these regions creates a very rich spectrum. The various physical conditions of the gas can be interpreted by the emission lines. For instance, the [O III] and [S II] lines are sensitive to temperature whereas the [O II] and [S II] lines are sensitive to electron density. A different kind of radiation known as Bremsstrahlung radiation is observed within the clouds at radio frequencies. This radiation is produced by the deceleration of a charged particle by another charged particle. An example of this would be the deceleration of the electron by the atomic nucleus. Bremsstrahlung radiation is a superb indicator of the temperatures and electron density within the clouds.

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    Other Characteristics

    Some of these regionTarantula Nebula s are ultra-compact. They can range in size from about one light-year across to several hundred light-years across. The ionized region within the clouds is known as the Stromgren sphere and the radius is known as the Stromgren radius. Particle densities vary from millions of particles per cm3 in the ultra-compact regions to only a few particles per cm3 in the larger regions. The majority of the clouds are made up of hydrogen. The remainder consists of helium with a trace amount of other heavier elements. Within spiral galaxies, these regions are most common in the spiral arms of the galaxy and in irregular galaxies they are sporadically found all over. The Eta Carinae Nebula, the Orion Nebula and the Tarantula Nebula in the Large Magellanic Cloud are good examples of these regions.

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    Demise of the Regions

    These regions last for a period of a few million years. Generally, the radiation pressure from hot, young stars will drive away most of the gas from the surrounding regions into the depths of space. Another way in which these clouds might dissipate is by stellar explosions such as supernovae that can cause the gas in these regions to disperse. Supernova explosions within these regions tend to occur about 2 million years after they form. This means that many of the regions we see now are probably less than 2 million years old. However, some of these regions can exist for more than 2 million years if supernova explosions have not occurred.

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    In Conclusion

    These are some of the most spectacular star forming regions in the Universe. A close examination of them is imperative for scientists as studying them reveals new details about stellar evolution and the nature of the Universe in general. With more scientists getting interested in this particular aspect of astronomy, new facts will be revealed about the mystery of star formation and the secrets of the cosmos.


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  • Image Credits:WikimediaCommons/RCW49 by NASA/Tarantula Nebula Detail by The Hubble Heritage Team (AURA / STScI / NASA)
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