Smallest unit of matter for which a
chemical element retains its identity.
Composed of protons and neutrons
in a compact nucleus surrounded by
a cloud of electrons.
Theory that an explosion 13 billion
years ago created all matter and energy
in the universe. Observed expansion
of the cosmos and a faint glow of
microwave light that fills space
support the theory.
A region of space with a gravitational
field so intense that the fabric of space
curves back upon itself, preventing
everything, including light, from
escaping. Born during the collapse of
very high mass stars and often within
the matter-rich centers of galaxies.
Shortening of the wavelength of light
as a radiating object and an observer
move toward each other.
Pulsating star that flickers regularly at
a rate that depends on its luminosity.
Used to gauge distances to other
galaxies; played a key role in the
discovery that the universe expands.
Conservation of energy
Principle that the total amount of
energy within a system remains
constant unless an outside force acts
upon it. Disguised when energy shifts
from one form to another, including
kinetic (the energy of motion),
potential (the capacity of an object
to move, such as under the influence
of gravity), and heat.
Conservation of momentum
Principle that the total amount of
momentum (an object’s mass times its
velocity) remains constant for a system
of objects unless an outside force acts
upon it. Related is the concept of
conservation of angular momentum,
which takes into account the spins
and orbital motions of objects.
Proposition credited to Nicolaus
Copernicus that Earth and its
inhabitants exist in an ordinary
place and time rather than at the
center of the cosmos.
An apparent force acting on objects
moving north or south across the surface
of a spinning body, caused by east-west
motion that is faster near the equator than
near the poles.
Cosmic microwave background
Faint radiation, principally in microwaves
and radio waves, that pervades the universe
at just a few degrees above absolute
zero. Represents leftover warmth from
the Big Bang.
A part of Einstein’s equations that calls
for a repulsive pressure that may arise
from the springiness of the vacuum of
space, counteracting gravity and causing
the universe to expand more quickly
Postulate that, on average, large-scale
properties of the universe are the same
everywhere and that the same laws of
physics apply throughout the cosmos.
Invisible and unknown material that
accounts for more than 90 percent of the
gravity in the universe. Has pronounced
effects on the formation and evolution
of galaxies and clusters of galaxies.
Change in frequency of waves as an
object and an observer approach or
recede from one another. Used to gauge
how quickly celestial bodies move.
One of four fundamental forces of nature,
arising from simultaneous motions of electric
and magnetic fields through space.
Responsible for attraction and repulsion
between charged and magnetized objects,
and for propagation of light waves.
Relentless tendency of an isolated system
of objects to become increasingly
disordered with time.
Boundary around a black hole marking
the zone from which light can no longer
escape the hole’s gravitational pull.
Titanic release of energy, especially
high-energy gamma rays, from massive
objects in distant parts of the universe.
Their origin remains a mystery, but
they are thought to arise from the
birth of black holes in large supernovas
or from collisions of neutron stars.
Bending of light along curved or
multiple paths through space as a
result of the gravity of a massive
object, such as a star or a cluster of
galaxies, lying between an observer
and a distant light source.
Subtle ripple in the four-dimensional
fabric of space-time caused by the sudden
motion of a massive object. Moves
through space at the speed of light.
Attraction between two objects based
solely on their mass and the distance
between them. Although the weakest
by far of the four fundamental forces
of nature, it extends over the greatest
Warming of the surface and lower
atmosphere of a planet by a blanket
of carbon dioxide, water vapor, and
other gases that prevent infrared
energy from escaping into space.
Region around a star in which liquid
water can exist on or beneath the
surface of a planet or moon, thereby
providing a possible habitat for life as
we know it.
Quantum-mechanical process which
enables black holes to evaporate slowly
over time until they vanish in a burst
of gamma rays and subatomic particles.
Tendency of a moving object to keep
moving in a straight line until some
external force, such as friction, makes
it change. Equivalently, the tendency
of a stationary object to remain at rest
until an external force acts upon it.
Hypothesis that the universe expanded exponentially for a tiny fraction of a
second immediately after the Big
Bang. May have smoothed the cosmos
to the uniformity we see today while
imprinting subtle fluctuations that led
to galaxies and clusters of galaxies.
Region of space surrounding a planet
in which the planet’s magnetic field
deflects the solar wind, shielding the
planet from charged particles.
Fundamental particle produced by
radioactive decay, fusion in the
cores of stars, and energetic events
such as supernova explosions.
Contains little or no mass and rarely
interacts with other matter.
Ultracompact object left behind at
the core of many supernovas.
Consists entirely of neutrons and
packs slightly more than the mass of
the Sun into a sphere about a dozen
Creation of elements within the Big
Bang (mostly hydrogen and helium),
the interiors of stars (elements as
heavy as iron), and supernovas (all
Chart that arranges the known
chemical elements in rows and
columns according to their properties,
which arise from the fundamental
rules of quantum mechanics.
Energy-carrying particle of electromagnetism
with no mass, acting
simultaneously as a particle and a
wave. Commonly thought of as the
basic particle of light.
Charged gas in which electrons or
ions (atoms that have lost or gained
electrons) can move freely, carrying
electric currents through space.
Change in the direction of the spin
axis of a rotating body caused by
force that acts on an equatorial
bulge or other nonspherical aspect
of the body. Makes Earth’s axis trace a
cone in space once every 26,000 years as
a result of combined gravitational tugs
from the Sun and the Moon.
Rapidly spinning neutron star that emits
beams of radiation on sweeping paths
through space. Usually detected in radio
Twentieth-century theory of matter and
energy that describes light and elementary
particles as both particles
and waves. Introduces uncertainty and
probability into physics by limiting
electrons to certain “energy levels”
around atoms and preventing physicists
from simultaneously measuring the
position and momentum of any particle.
Basic constituent of matter that makes up
protons and neutrons in sets of three and
other subatomic particles in sets of two.
Nuclei of some galaxies near the
fringes of the observable universe that
emit powerful streams of x-rays, radio
waves, and visible light. Probably
powered by supermassive black holes
consuming nearby stars and gas.
Spontaneous decay of an unstable atom
into another atom through the capture
or emission of subatomic particles.
Bloated end-stage of the life of a star as
it consumes helium within its core and
sheds its outer layers into space.
Lengthening of the wavelength of light
as a radiating object and an observer
recede from each other.
Extension of Albert Einstein’s special
theory of relativity to include the
effects of acceleration and gravity.
Explains gravitational attraction as
dimples in the four-dimensional fabric
of space-time around massive objects.
Theory advanced by Einstein in which
a beam of light is measured to move
at a constant speed regardless of the
motion of the observer. Results in
altered measurements of time, length,
and mass for a rapidly moving object
relative to a stationary observer.
Stream of high-energy charged particles
that blow outward through the solar
system from the Sun’s atmosphere.
Current model of the universe that
serves to account for the behavior and
interaction of all known particles. Holds
that the basic units of matter are quarks,
electrons, and neutrinos, which interact
through gravity, electromagnetism, and
the strong and weak nuclear forces.
Candidate for a “theory of everything”
that combines all nature’s basic forces
into a coherent description of the
universe. Depicts fundamental particles
as tiny vibrating loops, not points.
Strong nuclear force
Fundamental force of nature that binds
protons and neutrons within atomic
nuclei. Strongest of all forces, but acts
only at subatomic distances.
Direct transformation of a solid into a
gas, as in the tail of a comet.
Cataclysmic explosion of a massive star
that has started to accumulate iron in its
core, triggered by a shock wave when
the outer layers of the star rush inward.
Forges elements heavier than iron and
blasts them into space.
Distortions raised in the body of a
celestial object by an outside gravitational
field which pulls with different
strengths on the near and far sides of
the first body.
Weak nuclear force
Fundamental force of nature that mediates
the radioactive decay of atomic nuclei.
Compact Earth-sized remnant left
behind by the collapse of a star like the
Sun after it consumes the nuclear fuel
in its core.