Origins: Fourteen Billion Years of Cosmic Evolution (29 page)

Kelvin (absolute) temperature scale:
The
temperature
scale named for Lord Kelvin (William Thomson, 1824–1907) and created during the mid-nineteenth century, for which the coldest possible temperature is, by definition, zero degrees. The temperature intervals on this scale (denoted by K) are the same as those on the
Celsius (Centigrade) temperature scale
, so that on the Kelvin scale, water freezes at 273.16 degrees and boils at 373.16 degrees.

kilogram
: A unit of mass in the metric system, consisting of 1,000 grams.

kilohertz
: A unit of
frequency
that describes 1,000 vibrations or oscillations per second.

kilometer
: A unit of length in the metric system, equal to 1,000 meters and approximately 0.62 mile.

kinetic energy:
The
energy
that an object possesses by virtue of its motion, defined as one half of the object’s mass times the square of the object’s speed. Thus a more massive object, such as a truck, has more kinetic energy than a less massive object, such as a tricycle, that moves at the same speed.

Kuiper Belt
: The material in orbit around the Sun at distances extending from about 40 A.U. (Pluto’s average distance) out to several hundred A.U., almost all of which is debris left over from the Sun’s
protoplanetary disk
. Pluto is one of the largest objects in the Kuiper Belt.

Large Magellanic Cloud
: The larger of the two irregular satellite
galaxies
of the
Milky Way
.

latitude:
On Earth, the coordinate that measures north and south by specifying the number of degrees from the Equator (zero degrees) toward the North Pole (90º north) or the South Pole (90º south).

life
: A property of matter characterized by the abilities to reproduce and to
evolve
.

light
(
visible light
):
Electromagnetic radiation
that consists of photons whose
frequencies
and
wavelengths
fall within the band denoted as visible light, between
infrared
and
ultraviolet
.

light-year
: The distance that light or other forms of
electromagnetic radiation
travel in one year, equal to approximately 10 trillion kilometers or 6 trillion miles.

Local Group:
The name given to the two dozen or so
galaxies
in the immediate vicinity of the
Milky Way
galaxy. The Local Group includes the
Large and Small
Magellanic Clouds
and the
Andromeda galaxy
.

logarithmic scale:
A method for plotting data whereby tremendous ranges of numbers can fit on the same piece of paper. In official terms, the logarithmic scale increases exponentially (e.g., 1, 10, 100, 1,000, 10,000) rather than arithmetically (e.g., 1, 2, 3, 4, 5).

longitude:
On Earth, the coordinate that measures east or west by specifying the number of degrees from the arbitrarily defined “prime meridian,” the north-south line passing through Greenwich, England. Longitudes range from zero to 180 degrees east or 180 degrees west of Greenwich, thus including the 360 degrees that span Earth’s surface.

luminosity
: The total amount of
energy
emitted each second by an object in all types of
electromagnetic radiation
.

mass:
A measure of an object’s material content, not to be confused with weight, which measures the amount of
gravitational force
on an object. For objects at Earth’s surface, however, mass and weight vary in direct proportion.

mass extinction
: An event in the history of life on Earth, in some cases as the result of a massive impact, during which a significant fraction of all species of organisms become extinct within a geologically short interval of time.

megahertz
: A unit of
frequency
, equal to 1 million vibrations or oscillations per second.

metabolism
: The totality of an organism’s chemical processes, measured by the rate at which the organism uses
energy
. A high-metabolism animal must consume energy (food) much more frequently to sustain itself.

meteor
: A luminous streak of light produced by the heating of a
meteoroid
as it passes through Earth’s atmosphere.

meteor shower
: A large number of
meteors
observed to radiate from a specific point on the sky, the result of Earth’s crossing the orbits of a large number of
meteoroids
within a short time.

meteorite
: A
meteoroid
that survives its passage through Earth’s atmosphere.

meteoroid
: An object of rock or metal, or a metal-rock mixture, smaller than an
asteroid
, moving in an orbit around the Sun, part of the debris left over from the formation of the solar system or from collisions between solar-system objects.

meter
: The fundamental unit of length in the metric system, equal to approximately 39.37 inches.

Milky Way
: The
galaxy
that contains the Sun and approximately 300 billion other stars, as well as interstellar gas and dust and a huge amount of dark matter.

model
: A mental construct, often created with the aid of pencil and paper or of high-speed computers, that represents a simplified version of reality and allows scientists to attempt to isolate and to understand the most important processes occurring in a specific situation.

modified Newtonian dynamics (MOND):
A variant theory of gravity proposed by the Israeli physicist Mordehai Milgrom.

molecule
: A stable grouping of two or more
atoms
.

mutation
: A change in an organism’s
DNA
that can be inherited by descendants of that organism.

natural selection
: Differential success in reproduction among organisms of the same species, the driving force behind the
evolution
of life on Earth.

nebula
: A diffuse mass of gas and dust, usually lit from within by young, highly luminous stars that have recently formed from this material.

neutrino
: An
elementary particle
with no
electric charge
and a mass much smaller than an
electron
’s mass, characteristically produced or absorbed in reactions among elementary particles governed by
weak forces
.

neutron
: An
elementary particle
with no
electric charge
; one of the two basic components of an atomic
nucleus
.

neutron star:
The tiny remnants (less than twenty miles in diameter) of the core of a
supernova
explosion, composed almost entirely of
neutrons
and so dense that its matter effectively crams two thousand ocean liners into each cubic inch of space.

nitrogen
: The element made up of
atoms
whose
nuclei
each have seven
protons
, and whose different
isotopes
have nuclei with six, seven, eight, nine, or ten neutrons. Most nitrogen nuclei have seven neutrons.

nuclear fusion
: The joining of two
nuclei
under the influence of
strong forces
, which occurs only if the nuclei approach one another at a distance approximately the size of a proton (10
-13
centimeter).

nucleic acid
: Either
DNA
or
RNA
.

nucleotide
: One of the cross-linking molecules in
DNA
and
RNA
. In DNA, the four nucleotides are adenine, cytosine, guanine, and thymine; in RNA, uracil plays the role that thymine does in DNA.

nucleus
(pl.
nuclei
): (1) the central region of an
atom
, composed of one or more
protons
and zero or more
neutrons
. (2) The region within a
eukaryotic
cell that contains the cell’s genetic material in the form of chromosomes. (3) The central region of a
galaxy
.

Oort cloud
: The billions or trillions of
comets
that orbit the Sun, which formed first as the
protosun
began to contract, almost all of which move in orbits thousands or even tens of thousands of times larger than Earth’s orbit.

organic
: Referring to chemical compounds with
carbon atoms
as an important structural element; carbon-based molecules. Also, having properties associated with life.

organism
: An object endowed with the property of being alive.

oxidation
: Combination with
oxygen atoms
, typified by the rusting of metals upon exposure to oxygen in Earth’s atmosphere.

oxygen
: The element whose
nuclei
each have eight
protons
, and whose different
isotopes
each have seven, eight, nine, ten, eleven, or twelve neutrons in each nucleus. Most oxygen nuclei have eight neutrons to accompany their eight protons.

ozone
(O
3
): Molecules made of three
oxygen atoms
, which, at high altitudes in the Earth’s atmosphere, shield Earth’s surface against
ultraviolet
radiation.

panspermia
: The hypothesis that life from one locale can be transferred to another, e.g., from planet to planet within the solar system; also called cosmic seeding.

photon
: An elementary particle with no mass and no
electric charge
, capable of carrying
energy
. Streams of photons form
electromagnetic radiation
and travel through space at the speed of light, 299,792 kilometers per second.

photosynthesis
: The use of
energy
in the form of
visible light
or
ultraviolet
photons
to produce
carbohydrate
molecules from
carbon dioxide
and water. In some organisms, hydrogen sulfide (H
2
S) plays the same role that water (H
2
O) does in most photosynthesis on Earth.

planet
: An object in orbit around another star that is not another star and has a size at least as large as Pluto, which ranks either as the Sun’s smallest planet or as a
Kuiper Belt
object too small to be a planet.

planetesimal
: An object much smaller than a planet, capable of building planets through numerous mutual collisions.

plate tectonics
: Slow motions of plates of the crust of Earth and similar planets.

primitive atmosphere
: The original atmosphere of a planet.

prokaryote
: A member of one of the three domains of life, consisting of single-celled organisms in which the genetic material does not reside within a well-defined
nucleus
of the cell.

protein
: A long-chain
molecule
made of one or more chains of
amino acids
.

proton:
An
elementary particle
with one unit of positive
electric charge
found in the
nucleus
of every
atom
. The number of protons in an atom’s
nucleus
defines the elemental identity of that atom. For example, the
element
that has one proton is
hydrogen
, the one with two protons is
helium
, and the element with ninety-two protons is uranium.

proton-proton cycle
: The chain of three
nuclear
fusion
reactions by which most stars fuse
protons
into
helium
nuclei and convert
energy
of mass into
kinetic energy
.

protoplanet:
A planet during its later stages of formation.

protoplanetary disk
: The disk of gas and dust that surrounds a
star
as it forms, from and within which individual planets may form.

protostar
: A
star
in formation, contracting from a much larger cloud of gas and dust as the result of its self-gravitation.

pulsar
: An object that emits regularly spaced pulses of radio
photons
(and often of higher-energy photons as well) as the result of the rapid rotation of a
neutron star
, which produces
radiation
as charged particles accelerate in the intense magnetic field associated with the neutron star.

quantum mechanics
: The description of particles’ behavior at the smallest scales of size, hence of the structure of
atoms
and their interaction with other atoms and
photons
, as well as the behavior of atomic
nuclei
.

quasar
(quasi-stellar radio source)
: An object almost starlike in appearance, but whose
spectrum
show a large
red shift
, as a result of the object’s immense distance from the
Milky Way
.

radiation:
Short for
electromagnetic radiation
.
In this nuclear age, the term has also come to mean any particle or form of light that is bad for your health.