Glossary

absolute: something that is the same as measured in each and every frame of reference. Newton thought that space and time were absolute. Einstein said that space and time vary with an observer’s motion.

astronomer: a scientist who uses telescopes to observe cosmic objects or events

astrophysicist: a physicist who specializes in studying cosmic objects and events

atom: a building block of matter. Atoms are made of protons, neutrons, and electrons.

big bang: the scientific theory that says that all space, time, matter, and energy in our universe originated from the violent explosion of a single point of extremely high density and temperature about 13.8 billion years ago

binary system: two objects in orbit around each other. They may be two stars or two black holes or a star and a black hole.

black hole: an object whose mass is concentrated in a point of almost infinite density, with such powerful gravity that nothing can escape from it. Black holes seem to be at the center of galaxies and are the place where stars collapse and are born.

blueshift: a shift of electromagnetic waves from longer (red) to shorter wavelengths on the blue end of the visible electromagnetic spectrum, indicating that an object is moving away from the observer

corpuscle: the word scientists in the seventeenth and eighteenth centuries used for light particles

cosmic ray: a high energy particle that bombards Earth from space

dark energy: a theoretical force that counteracts gravity. Scientists believe it is the best explanation for the accelerating rate of the expansion of the universe. Dark energy makes up about 68 percent of all cosmic matter.

dark matter: matter that exists in the universe but that humans cannot detect because it emits no radiation that we can observe. Scientists believe it exists because it exerts gravitational forces that affect astronomical objects. Dark matter makes up about 27 percent of all cosmic matter.

Doppler shift: the shift of a wave to a higher frequency (shorter wavelength) when the source is moving toward the observer and to a lower frequency (longer wavelength) when the source is moving away from the observer

electromagnetic (EM) radiation: energy that takes the form of electric and magnetic waves, as well as a stream of photons, traveling at the speed of light in a vacuum

electromagnetic spectrum: the entire range of all the various kinds of electromagnetic radiation, including (from short to long wavelengths): gamma rays, X-rays, ultraviolet, optical (visible light), infrared, microwave, and radio waves. The shorter the wavelength, the higher the frequency and energy. The longer the wavelength, the lower the frequency and energy.

electron: a negatively charged elementary particle that exists in a cloud surrounding the outer regions of atoms

elliptical: oval-shaped, like an ellipse

escape velocity: the minimum speed necessary for something to escape the gravitational pull of a celestial body. A rocket launched from Earth must accelerate to about 25,000 miles (40,234 km) to escape Earth’s gravitational pull and travel to another planet. Scientists calculate the escape velocity from a black hole to be greater than the speed of light, which is impossible. This means that nothing can escape a black hole.

ether: a historical term scientists used to talk about the medium or material they thought filled the universe. In the nineteenth century, scientists thought ether was the medium through which light could travel.

event horizon: a black hole’s point of no return. Any light or matter that crosses beyond the event horizon cannot cross back. Although it is not a physical boundary, the event horizon marks the exterior “surface” of a black hole.

field: something that is distributed continuously and smoothly in space, such as an electric field or magnetic field

frequency: the rate at which a wave oscillates, or vibrates. Frequency refers to the number of cycles of oscillations per second. These oscillations are measured from the crest (or peak) of one wave to the crest of the next.

galaxy: a collection of one billion to one trillion stars that all orbit around a common center

gamma rays: electromagnetic waves with extremely short wavelengths and high frequencies

general relativity: Albert Einstein’s theory, published in 1915, in which he described gravity as a curvature of both space and time (space-time)

gravity: a fundamental force that causes two massive objects to be attracted toward each other

infrared radiation: electromagnetic waves with a wavelength a little longer than visible light. We can’t see infrared radiation with our eyes, but we can feel and measure it as heat.

interferometry: the process in which two or more waves interfere with each other. Scientists use interferometry to make very small measurements and to measure tiny distances between objects.

jet: a beam of gas that carries energy outward from a black hole

light-year: the distance that light travels in one year, at the rate of 664 million miles (1.1 billion km) per hour. One light-year is equivalent to 5.9 trillion miles (9.5 trillion km).

mass: a measure of the amount of matter in an object. Mass is always a fixed quantity for any given object and is not the same as weight. Scientists measure mass in kilograms.

matter: anything that has mass

microwave: electromagnetic radiation with a wavelength a little shorter than radio waves

Milky Way: the spiral galaxy in which we live. It is so named because it appears as a faint whitish band stretching across the night sky.

molecule: a group of two or more atoms that are held together by chemical bonds. Molecules do not have an electrical charge.

neutrino: a fundamental particle produced in massive numbers by nuclear reactions in stars. They are very difficult to detect because they interact very weakly with matter. All the same, scientists have found a variety of sophisticated technologies to detect them.

neutron: an electrically neutral subatomic particle. Together with protons, neutrons make up the nucleus of an atom.

neutron star: the compressed core of an exploded star composed almost entirely of neutrons. They have strong gravitational fields. Some emit pulses of energy along their axes and are known as pulsars.

nuclear fusion: the process in which two atomic nuclei merge to form a larger nucleus. The process creates a great deal of energy and is the force scientists use to make hydrogen bombs. In the sun and most stars, hydrogen nuclei fuse together to form helium.

nucleus: in physics, the dense core of an atom

particle: in physics, a particle is a tiny bundle of mass or energy. Particles are the building blocks of matter, including protons, neutrons, and electrons, as well as photons, quarks, and neutrinos.

photon: a particle of light, a packet of electromagnetic radiation

planet: a spherical ball that orbits a star. Planets can be made of rock, gas, or both. Earth, for example, is a rocky planet, while Jupiter is a gas planet.

primordial black hole: a black hole, typically less massive than a solar black hole, thought to have formed shortly after the big bang

proton: a positively charged subatomic particle. Together with neutrons, protons make up the nucleus of an atom.

quantum mechanics: the laws of physics that govern the realm of the very small (atoms and subatomic particles). These laws are also known as quantum theory.

radiation: energy emitted in the form of waves or particles

radio waves: electromagnetic waves of a very low frequency (long wavelength). Radio waves are used to receive and transmit signals between two objects, and astronomers also used naturally occurring radio waves to study certain objects in space.

redshift: a shift of electromagnetic waves from shorter (blue) to longer wavelengths on the red end of the visible EM spectrum, indicating that an object is moving away from the observer

relative: an observation that depends on the observer’s reference frame

rotation: the spin of a body about its axis, or the imaginary line through its center

shock: unusually high pressures that in the universe are produced by the movement of exploding gas

singularity: a one-dimensional point that contains a huge mass in an infinitely small space. In this point, space-time curves infinitely and the laws of physics as we understand them no longer work.

space-time: the mathematical model that combines the three dimensions of space with a fourth dimension, time. A point in space-time is defined by its three space coordinates and its time coordinate. In the general theory of relativity, the curvature of space-time is responsible for the phenomenon we know as gravity.

special relativity: Albert Einstein’s theory, published in 1905, to explain the fundamentals of space, time, and motion, in the absence of gravity. The theory is based on the principle that the laws of physics, including the speed of light, are the same for all non-accelerating observers. It leads to the prediction that objects moving at nearly the speed of light contract and that their time slows down.

speed of light: 186,000 miles (299,338 km) per second, the speed at which electromagnetic radiation, or light, travels through a vacuum

star: a giant ball of hot gas held together by its own gravity that generates nuclear energy in its core through nuclear fusion. The energy released in that reaction is emitted as visible light and other forms of electromagnetic radiation.

stellar black hole: a black hole formed by the gravitational collapse of a massive star

supernova: a gigantic explosion of a dying star; the energy released when a star’s inner core explodes and then powers the explosion of a star’s outer layers

tidal force: the gravitational pull on planetary objects from nearby planets and moons. For example, the movement of Earth’s oceans is impacted by our moon. In a black hole, extreme tidal forces cause the vertical stretching and horizontal compression of objects.

ultraviolet light: electromagnetic radiation whose wavelength is a little shorter than the light on the violet end of the spectrum of visible light

vacuum: a region from which all particles, fields, and energy have been removed or do not exist

wavelength: the distance between the two crests, or peaks, of a wave

white dwarf star: a small white star, about the diameter of Earth, that is the remnant of a star that has exhausted all of its nuclear fuel and is cooling off. The sun will one day become a white dwarf star.

wormhole: a hypothetical tunnel, or connection, that may link two widely separated regions of space-time. Although there is no observational evidence for wormholes, the mathematics of the general theory of relativity predict that they are theoretically possible.

X-rays: electromagnetic waves with a wavelength between that of ultraviolet radiation on the red end of the electromagnetic spectrum and gamma rays on the blue end of the spectrum