H II Region - Observations

Observations

A few of the brightest H II regions are visible to the naked eye. However, none seem to have been noticed before the advent of the telescope in the early 17th century. Even Galileo did not notice the Orion Nebula when he first observed the star cluster within it (previously cataloged as a single star, θ Orionis, by Johann Bayer). French observer Nicolas-Claude Fabri de Peiresc is credited with the discovery of the Orion Nebula in 1610. Since that early observation large numbers of H II regions have been discovered in the Milky Way and other galaxies.

William Herschel observed the Orion Nebula in 1774, and described it later as "an unformed fiery mist, the chaotic material of future suns". Confirmation of this hypothesis had to wait another hundred years, when William Huggins (assisted by his wife Mary Huggins) turned his spectroscope on various nebulae. Some, such as the Andromeda Nebula, had spectra quite similar to those of stars, and turned out to be galaxies consisting of hundreds of millions of individual stars. Others looked very different. Rather than a strong continuum with absorption lines superimposed, the Orion Nebula and other similar objects showed only a small number of emission lines. In planetary nebulae, the brightest of these spectral lines was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known chemical element. At first it was hypothesized that the line might be due to an unknown element, which was named Nebulium—a similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868. However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, Nebulium was not. In the early 20th century, Henry Norris Russell proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions.

Physicists showed in the 1920s that in gas at extremely low densities—even interstellar matter considered dense in an astronomical context is at high vacuum by laboratory standards—electrons can populate excited metastable energy levels in atoms and ions which at higher densities are rapidly de-excited by collisions. Electron transitions from these levels in doubly ionized oxygen give rise to the 500.7 nm line. These spectral lines, which can only be seen in very low density gases, are called forbidden lines. Spectroscopic observations thus showed that planetary nebulae consisted largely of extremely rarefied ionised oxygen gas (OIII).

In HII regions, however, the dominant spectral line has a wavelength of 656.3 nm. This is the well-known H-alpha line emitted by atomic hydrogen. Specifically, a photon of this wavelength is emitted when the electron of a hydrogen atom changes its excitation state from n=3 to n=2. Such state changes happen very frequently when an electron is captured by an ionised hydrogen atom (a proton), and the electron cascades down from some higher excitation state to n=1. Thus, it was concluded that HII regions consist of a mix of electrons and ionised as well as constantly recombining hydrogen atoms.

During the 20th century, observations showed that H II regions often contained hot, bright stars. These stars are many times more massive than the Sun, and are the shortest-lived stars, with total lifetimes of only a few million years (compared to stars like the Sun, which live for several billion years). Therefore it was surmised that H II regions must be regions in which new stars were forming. Over a period of several million years, a cluster of stars will form out of an H II region, before radiation pressure from the hot young stars causes the nebula to disperse. The Pleiades are an example of a cluster which has 'boiled away' the H II region from which it formed. Only a trace of reflection nebulosity remains.