Origins: Fourteen Billion Years of Cosmic Evolution (28 page)

elements:
The basic components of matter, classified by the number of
protons
in the
nucleus
. All ordinary matter in the
universe
is composed of ninety-two elements that range from the smallest
atom
,
hydrogen
(with one proton in its nucleus), to the largest naturally occurring element, uranium (with ninety-two protons in its nucleus). Elements heavier than uranium have been produced in laboratories.

elementary particle
: A fundamental particle of nature, normally indivisible into other particles.
Protons
and
neutrons
are usually designated as elementary particles although they each consist of three particles called
quarks
.

ellipse
: A closed curve defined by the fact that the sum of the distances from any point on the curve to two interior fixed points, called foci, has the same value.

elliptical galaxy
: A galaxy with an ellipsoidal distribution of stars, containing almost no interstellar gas or dust, whose shape seems elliptical in a two-dimensional projection.

energy
: The capacity to do work; in physics, “work” is specified by a given amount of
force
acting through a specific distance.

energy of mass
: The
energy
equivalent of a specific amount of mass, equal to the mass times the square of the speed of light.

energy of motion
: See
kinetic energy
.

enzyme
: A type of
molecule
, either a
protein
or
RNA
, that serves as a site at which molecules can interact in certain specific ways, and thus acts as a
catalyst
, increasing the rate at which particular molecular reactions occur.

escape velocity
: For a projectile or spacecraft, the minimum speed required for an outbound object to leave its point of launching and never return to the object, despite the object’s
gravitational force
.

Eukarya
: The totality of organisms classified as
eukaryotes
.

eukaryote
: An organism, either single-celled or multicellular, that keeps the genetic material in each of its cells within a membrane-bounded nucleus.

Europa
: One of Jupiter’s four large satellites, notable for its icy surface that may cover a worldwide ocean.

event horizon:
The poetic name given to an object’s
black hole radius
: the distance from a
black hole
’s center that marks the point of no return, because nothing can escape from the black hole’s
gravitational force
after passing inward through the event horizon. The event horizon may be considered to be the “edge” of a black hole.

evolution
: In biology, the ongoing result of
natural selection
, which under certain circumstances causes groups of similar organisms, called species, to change over time so that their descendants differ significantly in structure and appearance; in general, any gradual change of an object into another form or state of development.

exosolar
(also
extrasolar
): Pertaining to objects beyond the
solar system
. We prefer “exo” for its correspondence with exobiology, the study of life forms with origins beyond Earth.

exosolar planet
(also
extrasolar planet)
: A
planet
that orbits a
star
other than the Sun.

extremophile:
Organisms that thrive at high
temperatures
, typically between 70 and 100 degrees Celsius.

Fahrenheit temperature scale:
The
temperature
scale named for the German-born physicist Gabriel Daniel Fahrenheit (1686–1736), who introduced it in 1724, according to which water freezes at 32 degrees and boils at 212 degrees.

fission:
The splitting of a larger atomic
nucleus
into two or more smaller nuclei. The fission of nuclei larger than iron releases energy. This fission (also called atomic fission) is the source of energy in all present-day nuclear power plants.

force
: The capacity to do work or to produce a physical change; an influence that tends to
accelerate
an object in the direction that the force is applied to the object.

fossil
: A remnant or trace of an ancient organism.

frequency
: Of
photons
, the number of oscillations or vibrations per second.

fusion:
The combining of smaller
nuclei
to form larger ones. When nuclei smaller than iron fuse, energy is released. Fusion provides the primary energy source for the world’s nuclear weapons, and for all stars in the universe. Also called
nuclear fusion
and
thermonuclear fusion
.

galaxy
: A large group of stars, numbering from several million up to many hundred billion, held together by the stars’ mutual gravitational attraction, and also usually containing significant amounts of gas and dust.

galaxy cluster
: A large group of
galaxies
, usually accompanied by gas and dust and by a much greater amount of
dark matter
, held together by the mutual gravitational attraction of the material forming the galaxy cluster.

Galileo
spacecraft
: The spacecraft sent by NASA to Jupiter in 1990, which arrived in December 1995, dropped a probe into Jupiter’s atmosphere, and spent the next few years in orbit around the giant planet, photographing the planet and its large satellites.

gamma rays
: The highest-
energy
, highest-
frequency
, and shortest-
wavelength
type of
electromagnetic radiation
.

gene
: A section of a
chromosome
that specifies, by means of the genetic code, the formation of a specific chain of
amino acids
.

genetic code
: The set of “letters” in
DNA
or
RNA
molecules, each of which specifies a particular
amino acid
and consists of three successive molecules like those that form the cross-links between the twin spirals of DNA molecules.

genome
: The total complement of an organism’s
genes
.

general theory of relativity:
Introduced in 1915 by Albert Einstein, forming the natural extension of
special relativity theory
into the domain of
accelerating
objects, this is a modern theory of gravity that successfully explains many experimental results not otherwise explainable in terms of Newton’s theory of gravity. Its basic premise is the “equivalence principle,” according to which a person in a spaceship, for example, cannot distinguish whether the spaceship is accelerating through space, or whether it is stationary in a gravitational field that would produce the same acceleration. From this simple yet profound principle emerges a completely reworked understanding of the nature of gravity. According to Einstein, gravity is not a
force
in the traditional meaning of the word. Gravity is the curvature of space in the vicinity of a mass. The motion of a nearby object is completely determined by its velocity and the amount of curvature that is present. As counterintuitive as this sounds, general relativity theory explains all known behavior of gravitational systems ever studied and it predicts a myriad of even more counterintuitive phenomena that are continually verified by controlled experiment. For example, Einstein predicted that a strong gravity field should warp space and noticeably bend light in its vicinity. It was later shown that starlight passing near the edge of the Sun (as seen during a total solar eclipse) is found to be displaced from its expected position by an amount precisely matching Einstein’s predictions. Perhaps the grandest application of the general theory of relativity involves the description of our expanding universe where all of space is curved from the collected gravity of hundreds of billions of galaxies. An important and currently unverified prediction is the existence of “gravitons”—particles that carry gravitational forces and communicate abrupt changes in a gravitational field like those expected to arise from a supernova explosion.

giant planet
: A planet similar in size and composition to Jupiter, Saturn, Uranus, or Neptune, consisting of a solid core of rock and ice surrounded by thick layers of mainly
hydrogen
and
helium
gas, with a mass ranging from a dozen or so Earth masses up to many hundred times the mass of Earth.

gravitational forces
: One of the four basic types of
forces
, always attractive, whose strength between any two objects varies in proportion to the product of the objects’ masses, divided by the square of the distance between their centers.

gravitational lens
: An object that exerts sufficient
gravitational force
on passing light rays to bend them, often focusing them to produce a brighter image than an observer would see without the gravitational lens.

gravitational radiation
(gravity waves)
:
Radiation
, quite unlike
electromagnetic radiation
except for traveling at the speed of light, produced in relatively large amounts when massive objects move past one another at high speeds.

greenhouse effect
: The trapping of
infrared
radiation by a planet’s atmosphere, which raises the temperature on and immediately above the planet’s surface.

habitable zone
: The region surrounding a star within which the star’s heat can maintain one or more
solvents
in a liquid state, hence a spherical shell around the star with an inner and an outer boundary.

halo
: The outermost regions of a galaxy—occupying a volume much larger than the visible galaxy does—within which most of a galaxy’s
dark matter
resides.

helium
: The second lightest and second most abundant
element
, whose nuclei all contain two
protons
and either one or two
neutrons
. Stars generate energy through the
fusion
of
hydrogen
nuclei (
protons
) into helium nuclei.

hertz
: A unit of
frequency
, corresponding to one vibration per second.

Hubble’s constant
: The constant that appears in
Hubble’s law
and relates galaxies’ distances to their recession velocities.

Hubble’s law
: The summary of the
universe
’s expansion as observed today, which states that the recession velocities of faraway galaxies equals a constant times the galaxies’ distances from the Milky Way.

Hubble Space Telescope
: The space-borne telescope launched in 1991 that has secured marvelous
visible light
images of a host of astronomical objects, owing to the fact that the telescope can observe the cosmos free from the blurring and absorbing effects inevitably produced by Earth’s atmosphere.

hydrogen
: The lightest and most abundant
element
, whose
nuclei
each contain one
proton
and a number of
neutrons
equal to zero, one, or two.

infrared
:
Electromagnetic radiation
consisting of
photons
whose
wavelengths
are all somewhat longer, and whose
frequencies
are all somewhat higher, than those of the photons that form visible light.

initial singularity
: The moment at which the expansion of the
universe
began, also called the
big bang
.

inner planets
: The Sun’s planets Mercury, Venus, Earth, and Mars, all of which are small, dense, and rocky in comparison to the
giant planets
.

interstellar cloud
: A region of interstellar space considerably denser than average, typically spanning a diameter of several dozen
light-years
, with densities of matter that range from ten atoms per cubic centimeter up to millions of molecules per cubic centimeter.

interstellar dust
: Dust particles, each made of a million or so
atoms
, probably ejected into interstellar space from the atmospheres of highly rarefied
red-giant stars
.

interstellar gas
: Gas within a
galaxy
not part of any stars.

ion
: An
atom
that has lost one or more of its
electrons
.

ionization
: The process of converting an
atom
into an
ion
by stripping the atom of one or more
electrons
.

irregular galaxy
: A
galaxy
whose shape is irregular, that is, neither
spiral
(disklike) nor
elliptical
.

isotope
:
Nuclei
of a specific
element
, all of which contain the same number of
protons
but different numbers of
neutrons
.

JWST
(
James Webb Space Telescope
): The space-borne telescope, planned to begin operations during the 2010 decade, that will supersede the
Hubble Space Telescope
, carrying a larger mirror and more advanced instruments into space.