A

A for Andromeda

A BBC television science fiction series, written by cosmologist Fred Hoyle and author and TV producer John Elliot. It was broadcast in seven parts in 1961 and published as a novel the following year.1 A For Andromeda was inspired by seminal developments in SETI (>> Morrison-Cocconi Conjecture) and the construction of the first giant radio telescope at Jodrell Bank; it aired in the year following Project Ozma. The story begins with the accidental discovery of an extraterrestrial signal containing a complex message in binary (>> mathematics, as a universal language) - instructions, as it turns out, for making an immensely powerful computer. A similar theme was explored later by James Gunn in The Listeners and Carl Sagan in Contact.

1. Hoyle, Fred and Elliot, John. A For Andromeda. New York: Harper & Row (1962).

abiogenesis

The development of living organisms from non-living precursors. The term was first applied by Thomas Huxley and comes from the Greek a "without" and bios "life." Gradual abiogenesis by chemical and biochemical evolution on the surface of the young Earth is the standard model of how terrestrial life arose (>> life, origin of), although evidence is accumulating that at least some of the early stages of abiogenesis may have taken place in space. By contrast, some advocates of the panspermia hypothesis have maintained that there are no compelling reasons to believe that life originally came from non-living matter. The old theory of instantaneous abiogenesis, known as spontaneous generation, was discredited over a period of two centuries by the experimental work of Francesco Redi, Cagniard de La Tour, Lazzaro Spallanzani, Theodor Schwann, Louis Pasteur, John Tyndall, and others.

abiotic

Nonbiological in origin.

ablation

1. Removal of the surface layers of a meteorite during its passage through the atmosphere.

2. Controlled degradation of the leading surface of a spacecraft during atmospheric re-entry or passage through a dusty medium in space, such as the tail of a comet.

absolute magnitude

A measure of the true brightness of an object in space. Specifically, it is the apparent magnitude of a star, or other luminous object, when seen from a standard distance of 10 parsecs (32.6 light years). Absolute magnitude is directly related to luminosity.

absorption

The capture of photons by atoms, molecules, or ions, which results in a decrease in the intensity of light, or other electromagnetic radiation, as it passes through a substance. Absorption occurs in the atmospheres of stars, planets, and moons, and in the interstellar medium, giving rise to absorption lines or absorption bands in a spectrum.

absorption band

A series of very closely spaced absorption lines resulting from the absorption of light by molecules. Bands caused by substances such as titanium dioxide and carbon compounds occur in the spectra of low temperature stars (>> M-type star), including red giants. So-called diffuse interstellar bands, most likely due to complex carbon molecules, are observed in the spectra of remote stars.

absorption line

A dark line in a continuous spectrum that corresponds to the absorption of light, or some other form of electromagnetic radiation, at a well-defined wavelength. If a light source with a continuous spectrum is viewed through a cool gas then dark lines appear in the spectrum. The wavelengths of the lines are identical to the wavelengths of emission lines from the same gas when heated. The pattern of absorption lines in a spectrum is diagnostic of the types of atoms and molecules present, for example, in the surface layers of a star or the atmosphere of a planet.

absorption nebula

A dark interstellar cloud of gas and dust, so opaque that it prevents light from stars or bright nebulae from passing through it. Such nebulae appear as "holes" in the sky. They range in size from globules a few thousand AU across to sprawling regions of starless darkness such as the Coalsack.

abundance of elements

Biological abundance

In all terrestrial organisms, carbon, hydrogen, oxygen, and nitrogen are the primary constituents, making up 96.3% of the body mass of human beings and a similarly large proportion of all other organisms on Earth. Several factors contribute to their predominance: (a) They can combine with one another through covalent bonds (chemical bonds involving the sharing of electrons) to form molecules. (b) These bonds can be broken at temperatures compatible with life, so that the atoms can be rearranged into an enormous variety of other molecules. (c) Carbon is exceptional in its ability to form the basis of macromolecules. (d) Water, which is composed of oxygen and hydrogen atoms, plays a central role in the biochemistry of terrestrial life. About 90% of the atoms (75% by weight) of the atoms in terrestrial organisms are oxygen and hydrogen. (e) Among the simpler molecules formed by the four main biological elements are water vapor and gases that are soluble in water. As water vapor in the Earth's primordial atmosphere condensed and fell as rain, it brought the other dissolved substances into the early oceans. Subsequently, it is generally assumed that the interaction of small molecules led to more complex substances and eventually to life itself (>> life, origin of).

Terrestrial abundance

The proportion of elements within the Earth changes dramatically with depth, heavier elements, notably iron and nickel, having sunk to the center of the planet during its early stages of formation. The table below ranks the elements according to their preponderance in the lithosphere, which includes the crust and the upper part of the mantle.

Cosmic abundance

In the universe as a whole, hydrogen is by far the commonest element followed by helium and more distantly by heavier elements such as oxygen and carbon. Locally, the proportion of heavy elements varies from one star system to another and is an influential factor in the formation of planets.

Some elemental abundances in the Universe. (The elemental abundance of the Sun can be used as an approximate elemental abundance for the Universe.)

Atom

Sun

Earth

Earth's crust

Earth's atmosph.

Ocean

Microbe

Hydrogen

91

< 0.1

0.14

< 0.001

11

 63

Oxygen

  0.08

50

46

21

86

 26

Carbon

  0.03

< 0.1

0.02

  0.04

  0.003

   6.4

Nitrogen

  0.010

< 0.1

0.002

78

5 x 10-5

   1.4