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.

Big Bang
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.

Black hole
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.

Blue shift
Shortening of the wavelength of light as a radiating object and an observer move toward each other.

Cepheid variable
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.

Copernican principle
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.

Coriolis effect
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.

Cosmological constant
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 with time.

Cosmological principle
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.

Dark matter
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.

Doppler effect
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.

Event horizon
Boundary around a black hole marking the zone from which light can no longer escape the hole’s gravitational pull.

Gamma-ray burst
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.

Gravitational lensing
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.

Gravitational wave
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 distances.

Greenhouse effect
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.

Habitable zone
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.

Hawking radiation
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.

Neutron star
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 miles across.

Creation of elements within the Big Bang (mostly hydrogen and helium), the interiors of stars (elements as heavy as iron), and supernovas (all heavier elements).

Periodic table
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 waves.

Quantum mechanics
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.

Red giant
Bloated end-stage of the life of a star as it consumes helium within its core and sheds its outer layers into space.

Red shift
Lengthening of the wavelength of light as a radiating object and an observer recede from each other.

Relativity, general
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.

Relativity, special
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.

Solar wind
Stream of high-energy charged particles that blow outward through the solar system from the Sun’s atmosphere.

Standard model
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.

String theory
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.

White dwarf
Compact Earth-sized remnant left behind by the collapse of a star like the Sun after it consumes the nuclear fuel in its core.