What are H II Regions?
H II regions, also known as emission nebulae, are glowing clouds of gas and plasma up to several hundred light-years across. H II regions are named after the large amounts of ionized atomic hydrogen they contain — in astronomical parlance, H I regions refer to neutral atomic hydrogen, H II refers to ionized atomic hydrogen, and H2 refers to molecular hydrogen, the primary form in which hydrogen takes when synthesized in labs. H II regions are well known as nebulae where star formation is taking place. The light that H II regions emit comes from all the ionized gas within. The cause of the ionization are young, hot blue stars which emit large quantities of heat and light. H II regions are among the most fascinating targets in astronomy because of the star formation taking place in them. Unfortunately, the star formation itself is concealed from us in the form of Bok globules — pockets of gas within H II regions so dense that they obscure their contents. Typically, a Bok globule contai
Near what kinds of stars are they found? Why do only these kinds of stars give rise to H II regions? These are regions in which the interstellar hydrogen is mostly ionized (in the state H II). Such a state of ionization requires an abundance of ionizing ultraviolet radiation which is emitted with sufficient intensity only by O- and B-type stars. Therefore at least one such star (usually several) is embedded in or near each H II region. Other spectral types are not hot enough to emit sufficient ionizing radiation to produce H II regions. 20-12. Describe the energy source that causes a protostar to shine. How does this source differ from the energy source inside a main-sequence star? Gravitational energy is released by the contraction of a forming protostar. This is the energy source that heats up the star to a temperature sufficient to enable it to shine visibly. The energy radiated by a main sequence star is energy released in the thermonuclear fusion of hydrogen to form helium, deep i
Near what kinds of stars are they found? Why do only these kinds of stars give rise to H II regions? These are regions in which the interstellar hydrogen is mostly ionized (in the state H II). Such a state of ionization requires an abundance of ionizing ultraviolet radiation which is emitted with sufficient intensity only by O- and B-type stars. Therefore at least one such star (usually several) is embedded in or near each H II region. Other spectral types are not hot enough to emit sufficient ionizing radiation to produce H II regions. 18-13. Describe the energy source that causes a protostar to shine. How does this source differ from the energy source inside a main-sequence star? Gravitational energy is released by the contraction of a forming protostar. This is the energy source that heats up the star to a temperature sufficient to enable it to shine visibly. The energy radiated by a main sequence star is energy released in the thermonuclear fusion of hydrogen to form helium, deep i
