The element that never was: how glowing star nurseries fooled astronomers
Nineteenth-century spectroscopes revealed a bright green line at 500.7 nanometres in some nebulae that matched no known element. Astronomers called the mystery substance nebulium. It took until the 1920s to learn the truth: ordinary oxygen, stripped of two electrons, glowing in gas so thin that no laboratory on Earth could reproduce it.
Clouds of hydrogen lit up by young stars are known as H II regions, pronounced H two. The Roman numeral marks the atom's state: I for neutral, II for missing one electron. Inside, short-lived blue giants pour out ultraviolet light that strips electrons from surrounding hydrogen and makes it glow. The Orion Nebula is the best-known example; Nicolas-Claude Fabri de Peiresc spotted it through a telescope in 1610, while Galileo, looking at the same stars, missed the cloud entirely. William Herschel later called it a fiery mist, the raw material of future suns.
They begin in giant molecular clouds, cold reservoirs at 10 to 20 degrees above absolute zero. A nearby supernova, a collision between clouds or magnetic effects can make one collapse and fragment into stars. The heaviest newborns ionise the gas around them, sending a front racing outward faster than sound. Heated to about 10,000 degrees, a region can range from under a light-year across to several hundred, with anything from a few to over a million particles per cubic centimetre.
These nurseries are wasteful and short-lived. Within a few million years radiation and stellar winds blow most gas away, and fewer than one in ten parts ever becomes stars; the biggest stars explode after just one or two million years. Small dark knots called Bok globules, which Bart Bok flagged in the 1940s and which were confirmed as stellar birthplaces in 1990, get squeezed and scattered in the process. Meanwhile, the Hubble Space Telescope has found hundreds of dusty discs around young Orion stars, probable seeds of planets.
Spiral galaxies carry H II regions along their arms, irregular galaxies scatter them randomly and ellipticals have almost none. Some are enormous, such as 30 Doradus in the Large Magellanic Cloud, and because they shine so brightly they help astronomers gauge the distances and chemistry of other galaxies.
Source: H II region
