Nebulae: Gas & Dust Condensed Tweet

Nebulae: Gas & Dust Condensed

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We think of galaxies as collections of stars, but irregular galaxies and spiral galaxies, like our own, contain gas as well. Some of the gas is in form of hydrogen leftover from the original formation of each galaxy, fresh from the Big Bang. Some has been recycled from dying stars back into space, by the persistent blowing of a stellar wind, by the sporadic puf of a quietly dying white dwarf or by the cataclysmic explosion of a massive star as it goes supernova. The recycled gas thus contains cosmic pollution, generated by the nuclear reactions inside the stars.

The interstellar gas is mostly in the form of individual atoms, approximately one atom per cubic centimeter in the sun's neighborhood. One kind, the hydrogen atom, can be detected by its ratio emission at a wavelength of 21 cm. Others, for instance calcium and sodium, absorb particular wavelengths from the light of distant stars, as the light traverses interstellar space.

In the cold of interstellar space molecules which would be destroyed by the heat of a star can survive. A remarkable variety has been detected by radio astronomers. The list of discoveries includes organic molecules of the type involved in the chemistry of life: Water, ammonia, formaldehyde, and ethanol are some of the more complicated molecules which have been detected. These discoveries have sparked new discussions on the origin of life, and give credence to those who claim that life is a universal phenomenon. (In words of a famous astronomer: "The cosmos is also within us. We're made of star-stuff." - Carl Sagan)

Interstellar space also contain dust. From its effects on the light of distant stars, astronomers know that a typical interstellar dust grain is of size one micron (one-25-thousandth of an inch), and that there is on average one grain in each 12 meter cube of space near to the Sun.

The effects of the interstellar dust and gas are subtle, except where they lie in thick clouds near bright stars. This combination is in fact quite likely, since stars are even now forming from dust and gas clouds. The dust and gas leftover from the formation of these newly born stars is illuminated and heated by them. To the astronomers of the 18 century the illuminated dust and gas had the form of clouds and they named them in Latin: Nebulae.

There are two kinds of bright nebula. Dusty nebulae merely reflect the light of any star which lies nearby. The emission nebulae, however, are those which generate their own light and do not simply reflect light from somewhere else. To shine as an emission nebula, gas must lie near particularly hot stars which emit plenty of ultraviolet light. The ultraviolet light ionizes hydrogen atoms in the nearby gas, splitting them into individuals protons and electrons which come together again to reform their hydrogen atoms. In the process of recombining the hydrogen atoms predominantly emits the red hydrogen-alpha light which reveals the existence of the nebulae on photographs and gives them their generally red appearance.

Catalogue of the Universe p. 102 - 103

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