Coming of Age in the Milky Way (63 page)

Materialism.
Belief that material objects and their interactions constitute the
complete reality of all phenomena, including such seemingly insubstantial phenomena as thoughts and dreams. Compare
spiritualism
.

Matter waves.
Characteristic by virtue of which matter, like energy, displays the qualities of waves as well as of particles. See
wave-particle duality
.

Mechanics.
The study, in physics, of the influence of
forces
.

Megaparsec.
One million (10
⁶
)
parsecs
.

Mesons.
See
hadrons
.

Metals.
In astrophysics, all elements heavier than helium. MeV. One million (10
⁶
)
electron volts
.

Micrometry.
The measurement of the apparent sizes and separations of astronomical objects by use of knife blades or crosshairs in the eyepiece of a telescope. If the distance of an object is known, its size can be determined through micrometry.

Microwave background.
See
cosmic background radiation
. Microwaves.
Radio
radiation with wavelengths of about 10
^•4
to 1 meter, equal to 10
⁹
to 10
¹³
hertz
.

Mile.
The mile employed in this book is the statute mile, equal to 5,280 feet. Milky Way. A softly glowing band of light that bisects the skies of Earth, produced by light from stars and nebulae in the
galactic disk
. Milky Way galaxy. The spiral galaxy in which the sun resides. Million. A thousand thousand (10
⁶
). Minor planets. See
asteroids
. Missing matter. Alternate term for
dark matter
. MIT. Massachusetts Institute of Technology, in Cambridge, Mass. Molecules. The smallest units of a chemical compound. A molecule is composed of two or more atoms, linked by interactions of their
electrons
. Monopole. See
magnetic monopole
.

Muon.
Short-lived elementary particle with negative electrical charge. Muons are
leptons
. They resemble
electrons
, but are 207 times more massive.

Natural philosophy.
A term widely employed in the seventeenth century to mean what today is encompassed in the word
science
.

Natural selection.
Tendency of individuals better suited to their environment to survive and perpetuate their species, leading to changes in the genetic makeup of the species and, eventually, to the origin of new species. See
evolution
.

Nautical mile.
Equals 1.15 statute
miles
.

Nebulae.
Indistinct, nonterrestrial objects visible in the night sky. “Bright” nebulae glow with light emitted by the gas of which they are composed (“emission” nebulae) or by reflected starlight (“reflection” nebulae) or both. “Dark” nebulae consist of clouds of gas and dust that are not so illuminated. “Planetary” nebulae are shells of gas ejected by stars.
Spiral nebulae
are
galaxies
.

Nebular hypothesis.
Hypothesis, maintained in the nineteenth and early twentieth century, that the
spiral nebulae
are not galaxies but are instead whirlpools of gas from which new systems of stars and planets are condensing. Compare
island universe
theory.

Neutrinos.
Electrically neutral, massless particles that respond to the
weak nuclear force
but not the
strong nuclear
and
electromagnetic
forces.

Neutrons.
Electrically neutral, massive particles found in the nuclei of
atoms
. Each neutron is composed of one up
quark
and two down quarks; its mass is 939.6 MeV, slightly more than that of the proton. Stable within the nucleus, the neutron if isolated decays, with a
half-life
of fifteen minutes.

Neutron stars.
Stars with gravitational fields so intense that most of their matter has been compressed into neutrons. They are formed when massive stars run out of nuclear fuel and collapse. Many rotate rapidly and generate radio pulses; when detected by
radiotelescopes
, they are known as
pulsars
.

NGC.
Designates entries in the New General/Catalog of nonstellar objects.

Noneuclidean geometry.
See
geometry
.

Nova.
A star that brightens suddenly and to an unprecedented degree, creating the impression that a new star has appeared where none was before. Hence the name, from
nova
for “new.” See
supernova
. Nuclei. (1) The central part of atoms, composed of
protons
and
neutrons
(which are made of
quarks)
and containing nearly all of each atom’s
mass
. (2) The central region of a galaxy. Nucleons. Protons and neutrons, the constituents of atomic
nuclei
. Nucleosynthesis, nucleogenesis. The fusion of
nucleons
to create the nuclei of new atoms. Nucleosynthesis takes place in stars, and, at an accelerated rate, in
supernovae
.

Observational cosmology.
The application of observational data to the study of the universe as a whole. Open clusters. See
star clusters
.

Omega.
Index of the matter density of the universe, defined as the ratio between its actual density and the “critical density” required to “close” the universe and eventually halt its expansion. If omega equals less than 1, the universe is “open” and will expand forever. See
cosmic matter density
.

Oort Cloud.
Home of most solar system
comets
.

Open universe.
Cosmological model in which the universe continues to expand forever; its space-time geometry is hyperbolic, or “open.” Optics. The science of light.

Oscillating universe.
Cosmological model in which the universe is “closed” and its expansion is destined to stop, to be succeeded by collapse and “then” (if ordinary temporal terms may be said to apply) by a rebound into a new expansion phase.

Panstellar.
Of or pertaining to more than one star.

Paradox.
A self-contradictory proposition. Paradoxes are most useful when they seem most likely to be true, for it is then that they best serve to expose flaws in the data or reasoning that led to their appearance.

Parallax.
The apparent displacement in the position of a star or planet occasioned by its being viewed from two different locations—e.g., by observing it from two widely separated stations on Earth, or at intervals of six months, when the earth is at either extreme of its orbit around the sun. The resulting angle can be used, by
triangulation
, to determine the distance of the star or planet.

Parsec.
Astronomical unit of distance, equal to 3.26 light-years. Particle accelerator. See
accelerator
.

Particle physics.
The branch of science that deals with the smallest known structures of matter and energy. As their experimental investigation usually involves the application of considerable energy, particle physics overlaps with
high-energy physics
.

Particles.
Fundamental units of matter and energy. All may be classed
as fermions
, which have half-integral spin and obey the
exclusion principle
, and
bosons
, which have integral spin and do not obey the exclusion principle. The term
particle
is metaphoric, in that all subatomic particles also evince aspects of wave-like behavior.

Pauli exclusion principle.
See
exclusion principle
.

Period-luminosity function.
The relationship between the absolute
magnitude
and period of variability of
Cepheid variable
stars.

Phase transition.
An abrupt change in the equilibrium state of a system, as evoked by the cooling of the early universe as it expanded.

Photon decoupling.
The release of
photons
from constant collisions with massive particles as the universe expanded and its matter density diminished. See
decoupling
.

Photons.
The
quanta
of the
electromagnetic force
. The name comes from the fact that light is a form of electromagnetism. Photons have zero rest mass and can therefore travel infinitely far.

Physics.
The scientific study of the interactions of matter and energy.

Planck epoch, Planck time.
The first instant following the beginning of the expansion of the universe, when the
cosmic matter density
was still so high that
gravitational force
acted as strongly as the other fundamental forces on the subatomic scale.

Planck’s constant.
The fundamental quantity of action in quantum mechanics.

Planet.
An astronomical object more massive than an
asteroid
but less so than a
star
. Planets shine by reflected light; stars generate light of their own.

Plasma.
A state in which matter consists of electrons and other subatomic particles without any structure of an order higher than that of atomic
nuclei
.

Platonic solids.
The five regular polyhedrons—the tetrahedron, octahedron, hexahedron, icosahedron, and dodecahedron—esteemed by Plato as embodying aesthetic and rational ideals.

Plurality of worlds.
Hypothesis that the universe contains inhabited planets other than Earth.

Pole star.
The star—Polaris—that lies near the direction in the sky toward which the north pole of the earth points. Positron. The
antimatter
twin of the electron.

Post hoc fallacy.
The erroneous assumption that, because B follows A, B therefore was caused by A. More strictly, the fallacy of calculating, in retrospect, the odds of B’s having occurred by adding up a long sequence of such putative causes.

Precession.
The slow (once per twenty-six thousand years) gyration of the earth’s axis.

Proper motion.
Individual drifting of stars through space. Protogalaxy. A galaxy in the process of formation. None are observed nearby, indicating that all or most galaxies formed long ago.

Proton.
A massive
particle
with positive electrical charge found in the nuclei of atoms. Composed of two up
quarks
and one down quark. The proton’s mass is 938.3 MeV, slightly less than that of the
neutron
. Proton decay. Spontaneous disintegration of the proton, predicted by
grand unified theory
but never observed experimentally. Proton-proton reaction. An important
nuclear fusion
reaction that occurs in stars. It begins with the fusion of two hydrogen
nuclei
, each of which consists of a single
proton
. Pulsars. See
neutron stars
. QCD. See
quantum chromodynamics
. QED. See
quantum electrodynamics
.

Quadrant.
An instrument, based on a quarter of a circle, employed to measure the altitude above the horizon of astronomical bodies. Eventually replaced by the
sextant
.

Quanta.
Fundamental units of energy.

Quantum chromodynamics.
The quantum theory of the
strong nuclear force
, which it envisions as being conveyed by quanta called
gluons
. The name derives from the assignment of a quantum number called
color
to designate how
quarks
function in response to the strong force.