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Definition
| The sun and all the material that orbits it, including planets, dwarf planets, and small solar system bodies. Although the term solar system technically refers only to our own star system, it is often applied to other star systems. |
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Definition
| A great island of stars in space, containing from a few hundred million to a trillion or more stars, all held together by gravity and orbiting a common center. |
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Definition
| The group of about 40 galaxies to which the Milky Way belongs. |
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Definition
| A gigantic region of space where many individual galaxies and and many groups and clusters of galaxies are packed more closely together than elsewhere in the universe. |
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Definition
| The sum total of all matter and energy - that is, all galaxies and everything in between them. |
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| The name given to the event thought to mark the birth of the universe. |
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Definition
| The process in which two (or more) smaller nuclei slam together and make one large nucleus. |
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Definition
| A large, glowing ball of gas that generates heat and light through nuclear fusion in its core. |
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Definition
| A moderately large object that orbits a star and shines primarily through by reflecting light from its star. According to a definition approved in 2006, an object can be considered a planet only if: 1. orbits a star, 2. is large enough for its own gravity to make it round, and 3. has cleared most other objects from its orbital path. An object that meets the first two criteria but has not cleared its orbital path, like Pluto, is designated a dwarf planet. |
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Definition
| An object that orbits a planet. The term satellite can refer to any object orbiting another object. |
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Definition
| A relatively small and rocky object that orbits a star. |
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Definition
| A relatively small and ice-rich object that orbits a star. |
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Definition
| An asteroid, comet, or other object that orbits a star but is too small to qualify as a planet or dwarf planet. |
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Definition
| A star (sometimes more than one star) and any planets and other materials that orbit it. |
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Definition
| (or group of galaxies) A collection of galaxies bound together by gravity. Small collections (up to a few dozen galaxies) are generally called groups, while larger collections are called clusters. |
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Definition
| The portion of the entire universe that can be seen from Earth, at least in principle. The observable universe is probably only a tiny portion of the universe. |
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Definition
| The average distance between Earth and the Sun, which is about 150 million kilometers. More technically, 1 AU is the length of the semimajor axis of Earth's orbit. |
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Definition
| The distance that light can travel in 1 year, which is about 9.46 trillion kilometers. |
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Definition
| The spinning of an object around its axis, which is an imaginary line connecting the North Pole and the South Pole through the center of Earth. |
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Term
| expansion of (the universe) |
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Definition
| The increase in the average distance between galaxies as time progresses. |
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Term
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Definition
| The orbital motion of one object around another. For example, Earth orbits around the sun once a year. |
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Definition
| The plane of Earth's orbit around the sun. |
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Term
| axis tilt (of a planet in our solar system) |
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Definition
| The amount by which a planet's axis is tilted with respect to a line perpendicular to the ecliptic plane. |
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Definition
| A representation of some aspect of nature that can be used to explain and predict real phenomena without invoking myth, magic, or the supernatural. |
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Definition
| Any of the ancient Greek models that were used to predict planetary positions under the assumption that Earth lay in the center of the universe. |
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Definition
| A geocentric model of the universe developed by Ptolemy in about 150 A.D. |
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Definition
| The dramatic change, initiated by Copernicus, that occurred when we learned that Earth is a planet orbiting the Sun rather than the center of the universe. |
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Definition
| A type of oval that happens to be the shape of bound orbits. An ellipse can be drawn by moving a pencil along a string whose ends are tied to two tacks; the locations of the tacks are the foci (singular:focus) of the ellipse. |
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Term
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Definition
| Half the distance across the long axis of an ellipse, in this text, it is usually referred to as the average distance of an orbiting object, abbreviated a in the formula for Kepler's third law. |
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Term
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Definition
| A measure of how much an ellipse deviates from a perfect circle; defined as the center-to-focus distance divided by the length of the semimajor axis. |
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Term
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Definition
| Law stating that the orbit of each planet about the Sun is an ellipse with the Sun at one focus. |
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Term
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Definition
| The principle that, as a planet moves around its orbit, it sweeps out equal areas in equal times. This tells us that a planet moves faster when it is closer to the Sun (near perihelion) than when it is farther from the Sun (aphelion) an its orbit. |
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Term
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Definition
| The principle that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun (semimajor axis), which tells us that more distance planets move more slowly in their orbits; in its original form, written p2=a3. |
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Definition
| Something that purports to be science or may appear to be scientific but that does not adhere to the testing and verification requirements of the scientific method. |
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Definition
| A general pattern of thought that tends to shape scientific study during a particular time period. |
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Definition
| A model of some aspect of nature that has been rigorously tested and has passed all tests to date. |
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Definition
| The rate at which an object moves. It's units are distance divided by time, such as m/s or km/hr. |
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Term
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Definition
| The combination of speed and direction of motion; it can be stated as a speed in a particular direction, such as 100km/h due north. |
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Term
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Definition
| The rate at which an object's velocity changes. Its standard units are m/s2. |
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Term
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Definition
| The acceleration of a falling object. On Earth, the acceleration of gravity, designated by g, is 9.8 m/s2. |
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Term
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Definition
| The product of an object's mass and velocity. |
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Term
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Definition
| Anything that can cause a change in momentum. |
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Term
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Definition
| The overall force to which an object responds; the net force is equal to the rate of change in the object's momentum, or equivalently to the object's mass x acceleration. |
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Term
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Definition
| Momentum attributable to rotation or revolution. The angular momentum of an object moving in a circle of radius r is the product m x v x r. |
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Definition
| A twisting force that can cause a change in an object's angular momentum. |
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Definition
| A measure of the amount of matter in an object. |
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Definition
| The net force that an object applies to its surroundings; in the case of a stationary body on the surface of the Earth, it equals mass x acceleration of gravity. |
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Term
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Definition
| The condition in which an object is falling without resistance, objects are weightless when in free-fall. |
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Term
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Definition
| A weight of zero as occurs during free-fall. |
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Term
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Definition
| Principle that, in the absence of net force, an object moves with constant velocity. |
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Term
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Definition
| Law stating how a net force affects an object's motion. Specifically, force = rate of change in momentum, or force = mass x acceleration. |
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Term
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Definition
| Principle that, for any force, there is always an equal and opposite reaction force. |
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Term
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Definition
| The principle that, in the absence of net force, the total momentum of a system remains constant. |
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Term
| conservation of angular momentum |
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Definition
| The principle that, in the absence of net torque, the total angular momentum of a system remains constant. |
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Term
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Definition
| The principle that energy (including mass-energy) can be neither created nor destroyed, but can only change from one form to the other. |
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Term
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Definition
| Energy of of motion, given by the formula 1/2mv^2. |
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Term
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Definition
| Energy carried by light; the energy of a photon is Planck's constant times its frequency, or h x f. |
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Term
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Definition
| Energy stored for later conversion into kinetic energy; includes gravitational potential energy, electrical potential energy, and chemical potential energy. |
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Definition
| The international unit of energy, equivalent to about 1/4000 of a Calorie. |
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Term
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Definition
| The collective kinetic energy, as measured by temperature, of the many individual particles moving within a substance. |
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Term
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Definition
| A measure of the average kinetic energy of particles in a substance. |
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Term
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Definition
| The most commonly used temperature scale in science, defined such that absolute zero is 0k and water freezes at 273.15k. |
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Term
| gravitational potential energy |
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Definition
| Energy that an object has by virtue of its position in a gravitational field; an object has more gravitational potential energy when it has a greater distance that it can potentially fall. |
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Term
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Definition
| The potential energy of mass, which has an amount E=MC^2 |
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Term
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Definition
| Orbits on which an object travels repeatedly around another object; bound orbits are elliptical in shape. |
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Term
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Definition
| Orbits on which an object comes in towards a large body only once, never to return; unbound orbits may be parabolic or hyperbolic in shape. |
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Term
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Definition
| The point of at which two or more orbiting objects would balance if they were somehow connected; it is the point around which the orbiting objects actually orbit. |
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Term
| Newton's version of Kepler's Law |
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Definition
| A generalization of Kepler's third law used to calculate the masses of orbiting objects from measurements of orbital period and distance, usually written as "refer to formula" |
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Term
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Definition
| The sum of an orbiting object's kinetic and gravitational potential energies. |
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Term
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Definition
| An encounter in which two (or more) objects pass near enough so that each can feel the effects of the other's gravity and they can therefore exchange energy. |
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Term
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Definition
| The speed necessary for an object to completely escape the gravity of a large body such as a moon, planet, or star. |
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Term
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Definition
| A force that occurs when the gravity pulling on one side of an object is larger than that on the other side, causing the object to stretch. |
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Term
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Definition
| Friction within an object that is caused by tidal force. |
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Definition
| The rotation of an object that always shows the same face to an object that it is orbiting because its rotation period and orbital period are equal. |
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Definition
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Definition
| The state of energy usage, usually measured in watts. (1 watt=1 joule/s) |
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Definition
| The standard unit of power in science. |
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Definition
| A light bulb emits visible light; the energy of the light comes from electrical potential energy supplied to the light bulb. |
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Term
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Definition
| When you place your hand near an incandescent light bulb, your hand absorbs some of the light, and this absorbed energy warms your hand. |
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Term
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Definition
| Some forms of matter, such as glass or air, transmit light, which means allowing it to pass through. |
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Term
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Definition
| Light can bounce off matter, leading to what we call reflection (when bouncing is all in the same general direction) or scattering (when the bouncing is more random.) |
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Term
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Definition
| The distance between adjacent peaks or troughs between waves. |
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Term
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Definition
| The rate at which peaks of a wave pass by a point, measured in units of 1/s, often cycles per second or hertz. |
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Term
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Definition
| The rate at which peaks of a wave pass by a point, measured in units of 1/s, often cycles per second or hertz. |
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Term
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Definition
| The standard unit of frequency for light waves; equivalent to units of 1/s. |
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Term
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Definition
| An abstract concept used to describe how a particle would interact with a force. For example, the idea of a gravitational field describes how a particle would react to the local strength of gravity, and the idea of an electromagnetic field describes how a charged particle would respond to forces from the other charged particles. |
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Definition
| A synonym for light, which consists of waves of electric and magnetic fields. |
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Definition
| The speed at which light travels, which is about 300,000 km/s. |
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Term
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Definition
| An individual particle of light, characterized by the wavelength and a frequency. |
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Term
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Definition
| The light our eyes can see, ranging in wavelength from about 400 to 700nm. |
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Term
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Definition
| The complete spectrum of light, including radio waves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays. |
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Term
| electromagnetic radiation |
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Definition
| Another name for all types of light, from radio waves through gamma rays. |
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Term
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Definition
| Light with wavelength that falls in the portion of the electromagnetic spectrum between radio waves and visible light. |
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Term
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Definition
| Light with very long wavelengths (and hence low frequencies) - longer than those of infrared light. |
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Term
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Definition
| Light with the wavelengths in the range of micrometers to millimeters. Microwaves are generally considered to be a subset of the radio wave portion of the electromagnetic spectrum. |
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Term
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Definition
| Light with wavelengths that fall in the portion of the electromagnetic spectrum between visible light and X rays. |
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Term
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Definition
| Light with very short wavelengths (and hence high frequencies) - shorter than those of X rays. |
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Term
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Definition
| Light with wavelengths that fall in the portion of the electromagnetic spectrum between ultraviolet light and gamma rays. |
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Term
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Definition
| Consists of a nucleus made from protons and neutrons, surrounded by a cloud of electrons. |
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Term
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Definition
| A substance made from individual atoms of a particular atomic number. |
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Definition
| Particles found in atomic nuclei with positive electrical charge, built from three quarks. |
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Term
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Definition
| Particles with no electrical charges found in atomic nuclei, built from three quarks. |
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Term
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Definition
| Fundamental particles with negative electric charge; the distribution of electrons in an atom gives an atom its size. |
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Term
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Definition
| The compact center of an atom made from protons and neutrons. |
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Term
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Definition
| A fundamental property of matter that is described by its amount and as either positive or negative, more technically, a measure of how a particle responds to the electromagnetic force. |
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Term
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Definition
| The number of protons in an atom. |
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Term
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Definition
| The combined number of protons and neutrons in an atom. |
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Term
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Definition
| Forms of an element that have the same number of protons but a different number of electrons. |
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Term
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Definition
| Technically, the smallest unit of chemical element or compound; in this text, the term refers only to combinations of two or more atoms held together by chemical bonds. |
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Term
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Definition
| A substance made from molecules consisting of two or more atoms with different atomic numbers. |
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Definition
| The phase of matter in which atoms or molecules are held tightly in place. |
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Term
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Definition
| The phase of matter in which atoms or molecules are held together but move relatively freely. |
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Definition
| The phase of matter in which atoms or molecules can move essentially independently of one another. |
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Definition
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Definition
| The process by which atoms or molecules escape into the gas phase from a liquid |
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Term
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Definition
| The process by which atoms or molecules escape into the gas phase from a solid. |
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Term
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Definition
| The process by which a molecule splits into its component atoms. |
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Definition
| Atoms with a positive or negative charge. |
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Term
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Definition
| The process of stripping an electron from an atom. |
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Definition
| A gas consisting of ions and electrons. |
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Definition
| The force (per unit of area) pushing on an object. In astronomy, we are generally interested in pressure applied by gas (or plasma). Ordinarily, such pressure is related to the temperature of the gas (see thermal pressure). In objects such as white dwarfs and neutron stars, pressure may arise from a quantum effect. Light can also exert pressure - radiation pressure. |
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Definition
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Definition
| The process of obtaining spectra from astronomical objects. |
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Term
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Definition
| A spectrum of light that spans a broad range of wavelengths without interruption by emission or absorption lines. |
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Term
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Definition
| The process by which matter emits energy in the form of light. |
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Definition
| A spectrum that contains emission lines. |
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Term
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Definition
| The process by which matter absorbs radiative energy. |
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Term
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Definition
| A spectrum that contains absorption lines. |
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Term
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Definition
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Term
| thermal radiation spectrum |
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Definition
| The spectrum of radiation produced by an opaque object that depends only on the object's temperature; sometimes called blackbody radiation. |
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Term
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Definition
| A Doppler shift in which spectral features are shifted to shorter wavelengths, observed when an object is moving toward the observer. |
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Term
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Definition
| A Doppler shift in which spectral features are shifted to longer wavelengths, observed when an object is moving away from the observer. |
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Term
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Definition
| The wavelength of a spectral feature in the absence of any Doppler shift or gravitational redshift. |
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Term
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Definition
| A picture of an object made by focusing light. |
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Term
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Definition
| The place where an image created by a lens or mirror is in focus. |
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Definition
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Term
| CCDs (charge coupled device) |
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Definition
| A type of electronic light detector that has largely replaced photographic film in astronomical research. |
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Term
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Definition
| An individual "picture element" on a CCD. |
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Term
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Definition
| The area of the primary mirror or lens that collects light in a telescope. |
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Term
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Definition
| The smallest angular separation that two pointlike objects can have and still be seen as distinct points of light. |
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Term
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Definition
| The angular resolution that a telescope could achieve if it were limited only by the interference of light waves; it is smaller (i.e. better angular resolution) for larger telescopes. |
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Term
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Definition
| A telescope that uses mirrors to focus light. |
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Term
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Definition
| A graph of an object's intensity against time. |
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Definition
| Human-made light that hinders astronomical observations. |
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Term
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Definition
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Term
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Definition
| A technique in which telescope mirrors flex rapidly to compensate for the bending of starlight caused by atmospheric turbulence. |
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Term
| grazing incidence mirrors |
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Definition
| Reflections in which light grazes a mirror surface and is deflected at a small angle; commonly used to focus high-energy ultraviolet light and X-rays. |
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