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interference : process in which waves mix together such that their crests and troughs can alternately reinforce and cancel one another | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
interferometer : instrument that combines electromagnetic radiation from one or more telescopes to obtain a resolution equivalent to what would be obtained with a single telescope with a diameter equal to the baseline separating the individual separate telescopes | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
interferometer array : combination of multiple radio dishes to, in effect, work like a large number of two-dish interferometers | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
prime focus : point in a telescope where the objective lens or primary mirror focuses the light | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
radar : technique of transmitting radio waves to an object and then detecting the radiation that the object reflects back to the transmitter; used to measure the distance to, and motion of, a target object or to form images of it | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
reflecting telescope : telescope in which the principal light collector is a concave mirror | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
refracting telescope : telescope in which the principal light collector is a lens or system of lenses | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
resolution : detail in an image; specifically, the smallest angular (or linear) features that can be distinguished | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
seeing : unsteadiness of Earthâs atmosphere, which blurs telescopic images; good seeing means the atmosphere is steady | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
telescope : instrument for collecting visible-light or other electromagnetic radiation | https://openstax.org/books/astronomy-2e/pages/6-key-terms |
A telescope collects the faint light from astronomical sources and brings it to a focus, where an instrument can sort the light according to wavelength. Light is then directed to a detector, where a permanent record is made. The light-gathering power of a telescope is determined by the diameter of its aperture, or open... | https://openstax.org/books/astronomy-2e/pages/6-summary |
New technologies for creating and supporting lightweight mirrors have led to the construction of a number of large telescopes since 1990. The site for an astronomical observatory must be carefully chosen for clear weather, dark skies, low water vapor, and excellent atmospheric seeing (low atmospheric turbulence). The r... | https://openstax.org/books/astronomy-2e/pages/6-summary |
Visible-light detectors include the human eye, photographic film, and charge-coupled devices (CCDs). Detectors that are sensitive to infrared radiation must be cooled to very low temperatures since everything in and near the telescope gives off infrared waves. A spectrometer disperses the light into a spectrum to be re... | https://openstax.org/books/astronomy-2e/pages/6-summary |
In the 1930s, radio astronomy was pioneered by Karl G. Jansky and Grote Reber. A radio telescope is basically a radio antenna (often a large, curved dish) connected to a receiver. Significantly enhanced resolution can be obtained with interferometers, including interferometer arrays like the 27-element VLA and the 66-e... | https://openstax.org/books/astronomy-2e/pages/6-summary |
Infrared observations are made with telescopes aboard aircraft and in space, as well as from ground-based facilities on dry mountain peaks. Ultraviolet, X-ray, and gamma-ray observations must be made from above the atmosphere. Many orbiting observatories have been flown to observe in these bands of the spectrum in the ... | https://openstax.org/books/astronomy-2e/pages/6-summary |
The largest space telescope yet, the 6.5-meterJames Webb Space Telescope, was launched in December 2021 and is sending back a flood of useful information about the infrared universe. New and even larger telescopes are on the drawing boards. Gamma-ray astronomers are planning to build the CTA to measure very energetic g... | https://openstax.org/books/astronomy-2e/pages/6-summary |
asteroid : a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
comet : a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
differentiation : gravitational separation of materials of different density into layers in the interior of a planet or moon | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
giant planet : any of the planets Jupiter, Saturn, Uranus, and Neptune in our solar system, or planets of roughly that mass and composition in other planetary systems | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
half-life : time required for half of the radioactive atoms in a sample to disintegrate | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
meteor : a small piece of solid matter that enters Earthâs atmosphere and burns up, popularly called ashootingstarbecause it is seen as a small flash of light | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
meteorite : a portion of a meteor that survives passage through an atmosphere and strikes the ground | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
planetesimals : objects, from tens to hundreds of kilometers in diameter, that formed in the solar nebula as an intermediate step between tiny grains and the larger planetary objects we see today; the comets and some asteroids may be leftover planetesimals | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
radioactivity : process by which certain kinds of atomic nuclei decay naturally, with the spontaneous emission of subatomic particles and gamma rays | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
solar nebula : the cloud of gas and dust from which the solar system formed | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
terrestrial planet : any of the planets Mercury, Venus, Earth, or Mars; sometimes the Moon is included in the list | https://openstax.org/books/astronomy-2e/pages/7-key-terms |
Our solar system currently consists of the Sun, eight planets, five dwarf planets, more than 280 known moons, and a host of smaller objects. The planets can be divided into two groups: the inner terrestrial planets and the outer giant planets. Pluto, Eris, Haumea, and Makemake do not fit into either category; as icy dw... | https://openstax.org/books/astronomy-2e/pages/7-summary |
The giant planets have dense cores roughly 10 times the mass of Earth, surrounded by layers of hydrogen and helium. The terrestrial planets consist mostly of rocks and metals. They were once molten, which allowed their structures to differentiate (that is, their denser materials sank to the center). The Moon resembles ... | https://openstax.org/books/astronomy-2e/pages/7-summary |
The ages of the surfaces of objects in the solar system can be estimated by counting craters: on a given world, a more heavily cratered region will generally be older than one that is less cratered. We can also use samples of rocks with radioactive elements in them to obtain the time since the layer in which the rock f... | https://openstax.org/books/astronomy-2e/pages/7-summary |
Regularities among the planets have led astronomers to hypothesize that the Sun and the planets formed together in a giant, spinning cloud of gas and dust called the solar nebula. Astronomical observations show tantalizingly similar circumstellar disks around other stars. Within the solar nebula, material first coalesc... | https://openstax.org/books/astronomy-2e/pages/7-summary |
bar : a force of 100,000 Newtons acting on a surface area of 1 square meter; the average pressure of Earthâs atmosphere at sea level is 1.013 bars | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
basalt : igneous rock produced by the cooling of lava; makes up most of Earthâs oceanic crust and is found on other planets that have experienced extensive volcanic activity | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
convection : movement caused within a gas or liquid by the tendency of hotter, and therefore less dense material, to rise and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
core : the central part of the planet; consists of higher density material | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
crust : the outer layer of a terrestrial planet | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
fault : in geology, a crack or break in the crust of a planet along which slippage or movement can take place, accompanied by seismic activity | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
granite : a type of igneous silicate rock that makes up most of Earthâs continental crust | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
greenhouse effect : the blanketing (absorption) of infrared radiation near the surface of a planetâfor example, by CO2in its atmosphere | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
greenhouse gas : a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range; on Earth, these atmospheric gases primarily include carbon dioxide, methane, and water vapor | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
igneous rock : rock produced by cooling from a molten state | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
magnetosphere : the region around a planet in which its intrinsic magnetic field dominates the interplanetary field carried by the solar wind; hence, the region within which charged particles can be trapped by the planetary magnetic field | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
mantle : the largest part of Earthâs interior; lies between the crust and the core | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
mass extinction : the sudden disappearance in the fossil record of a large number of species of life, to be replaced by fossils of new species in subsequent layers; mass extinctions are indicators of catastrophic changes in the environment, such as might be produced by a large impact on Earth | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
metamorphic rock : rock produced by physical and chemical alteration (without melting) under high temperature and pressure | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
ozone : (O3) a heavy molecule of oxygen that contains three atoms rather than the more normal two | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
photosynthesis : a complex sequence of chemical reactions through which some living things can use sunlight to manufacture products that store energy (such as carbohydrates), releasing oxygen as one by-product | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
plate tectonics : the motion of segments or plates of the outer layer of a planet over the underlying mantle | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
primitive rock : rock that has not experienced great heat or pressure and therefore remains representative of the original condensed materials from the solar nebula | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
rift zone : in geology, a place where the crust is being torn apart by internal forces generally associated with the injection of new material from the mantle and with the slow separation of tectonic plates | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
sedimentary rock : rock formed by the deposition and cementing of fine grains of material, such as pieces of igneous rock or the shells of living things | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
seismic wave : a vibration that travels through the interior of Earth or any other object; on Earth, these are generally caused by earthquakes | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
stratosphere : the layer of Earthâs atmosphere above the troposphere and below the ionosphere | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
subduction : the sideways and downward movement of the edge of a plate of Earthâs crust into the mantle beneath another plate | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
troposphere : the lowest level of Earthâs atmosphere, where most weather takes place | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
volcano : a place where material from a planetâs mantle erupts on its surface | https://openstax.org/books/astronomy-2e/pages/8-key-terms |
Earth is the prototype terrestrial planet. Its interior composition and structure are probed using seismic waves. Such studies reveal that Earth has a metal core and a silicate mantle. The outer layer, or crust, consists primarily of oceanic basalt and continental granite. A global magnetic field, generated in the core... | https://openstax.org/books/astronomy-2e/pages/8-summary |
Terrestrial rocks can be classified as igneous, sedimentary, or metamorphic. A fourth type, primitive rock, is not found on Earth. Our planetâs geology is dominated by plate tectonics, in which crustal plates move slowly in response to mantle convection. The surface expression of plate tectonics includes continental ... | https://openstax.org/books/astronomy-2e/pages/8-summary |
The atmosphere has a surface pressure of 1 bar and is composed primarily of N2and O2, plus such important trace gases as H2O, CO2, and O3. Its structure consists of the troposphere, stratosphere, mesosphere, and ionosphere. Changing the composition of the atmosphere also influences the temperature. Atmospheric circulat... | https://openstax.org/books/astronomy-2e/pages/8-summary |
Life originated on Earth at a time when the atmosphere lacked O2and consisted mostly of CO2. Later, photosynthesis gave rise to free oxygen and ozone. Modern genomic analysis lets us see how the wide diversity of species on the planet are related to each other. CO2and methane in the atmosphere heat the surface through ... | https://openstax.org/books/astronomy-2e/pages/8-summary |
Earth, like the Moon and other planets, has been influenced by the impacts of cosmic debris, including such recent examples as Meteor Crater and the Tunguska explosion. Larger past impacts are implicated in some mass extinctions, including the large impact 65 million years ago at the end of the Cretaceous period that w... | https://openstax.org/books/astronomy-2e/pages/8-summary |
highlands : the lighter, heavily cratered regions of the Moon, which are generally several kilometers higher than the maria | https://openstax.org/books/astronomy-2e/pages/9-key-terms |
mare : (plural: maria) Latin for âsea;â the name applied to the dark, relatively smooth features that cover 17% of the Moonâs surface | https://openstax.org/books/astronomy-2e/pages/9-key-terms |
Most of what we know about the Moon derives from the Apollo program, including 400 kilograms of lunar samples still being intensively studied. The Moon has one-eightieth the mass of Earth and is severely depleted in both metals and volatile materials. It is made almost entirely of silicates like those in Earthâs mant... | https://openstax.org/books/astronomy-2e/pages/9-summary |
The Moon, like Earth, was formed about 4.5 billion years ago. The Moonâs heavily cratered highlands are made of rocks more than 4 billion years old. The darker volcanic plains of the maria were erupted primarily between 3.3 and 3.8 billion years ago. Generally, the surface is dominated by impacts, including continuin... | https://openstax.org/books/astronomy-2e/pages/9-summary |
A century ago, Grove Gilbert suggested that the lunar craters were caused by impacts, but the cratering process was not well understood until more recently. High-speed impacts produce explosions and excavate craters 10 to 15 times the size of the impactor with raised rims, ejecta blankets, and often central peaks. Crat... | https://openstax.org/books/astronomy-2e/pages/9-summary |
The three standard hypotheses for the origin of the Moon were the fission hypothesis, the sister hypothesis, and the capture hypothesis. All have problems, and they have been supplanted by the giant impact hypothesis, which ascribes the origin of the Moon to the impact of a Mars-sized projectile with Earth 4.5 billion ... | https://openstax.org/books/astronomy-2e/pages/9-summary |
Mercury is the nearest planet to the Sun and the fastest moving. Mercury is similar to the Moon in having a heavily cratered surface and no atmosphere, but it differs in having a very large metal core. Early in its evolution, it apparently lost part of its silicate mantle, probably due to one or more giant impacts. Lon... | https://openstax.org/books/astronomy-2e/pages/9-summary |
runaway greenhouse effect : the process by which the greenhouse effect, rather than remaining stable or being lessened through intervention, continues to grow at an increasing rate | https://openstax.org/books/astronomy-2e/pages/10-key-terms |
tectonic : geological features that result from stresses and pressures in the crust of a planet; tectonic forces can lead to earthquakes and motion of the crust | https://openstax.org/books/astronomy-2e/pages/10-key-terms |
Venus, the nearest planet, is a great disappointment through the telescope because of its impenetrable cloud cover. Mars is more tantalizing, with dark markings and polar caps. Early in the twentieth century, it was widely believed that the âcanalsâ of Mars indicated intelligent life there. Mars has only 11% the ma... | https://openstax.org/books/astronomy-2e/pages/10-summary |
Venus has been mapped by radar, especially with theMagellanspacecraft. Its crust consists of 75% lowland lava plains, numerous volcanic features, and many large coronae, which are the expression of subsurface volcanism. The planet has been modified by widespread tectonics driven by mantle convection, forming complex pa... | https://openstax.org/books/astronomy-2e/pages/10-summary |
The atmosphere of Venus is 96% CO2. Thick clouds at altitudes of 30 to 60 kilometers are made of sulfuric acid, and a CO2greenhouse effect maintains the high surface temperature. Venus presumably reached its current state from more earthlike initial conditions as a result of a runaway greenhouse effect, which included ... | https://openstax.org/books/astronomy-2e/pages/10-summary |
Most of what we know about Mars is derived from spacecraft: highly successful orbiters, landers, and rovers. We have also been able to study a few martian rocks that reached Earth as meteorites. Mars has heavily cratered highlands in its southern hemisphere, but younger, lower volcanic plains over much of its northern ... | https://openstax.org/books/astronomy-2e/pages/10-summary |
The martian atmosphere has a surface pressure of less than 0.01 bar and is 95% CO2. It has dust clouds, water clouds, and carbon dioxide (dry ice) clouds. Liquid water on the surface is not possible today, but there is subsurface permafrost at high latitudes. Seasonal polar caps are made of dry ice; the northern residu... | https://openstax.org/books/astronomy-2e/pages/10-summary |
Earth, Venus, and Mars have diverged in their evolution from what may have been similar beginnings. We need to understand why if we are to protect the environment of Earth. | https://openstax.org/books/astronomy-2e/pages/10-summary |
photochemistry : chemical changes caused by electromagnetic radiation | https://openstax.org/books/astronomy-2e/pages/11-key-terms |
synchrotron radiation : the radiation emitted by charged particles being accelerated in magnetic fields and moving at speeds near that of light | https://openstax.org/books/astronomy-2e/pages/11-key-terms |
The outer solar system contains the four giant planets: Jupiter, Saturn, Uranus, and Neptune. The gas giants Jupiter and Saturn have overall compositions similar to that of the Sun. These planets have been explored by the Pioneer, Voyager, Galileo, and Cassini spacecraft. Voyager 2, perhaps the most successful of all s... | https://openstax.org/books/astronomy-2e/pages/11-summary |
Jupiter is 318 times more massive than Earth. Saturn is about 25% as massive as Jupiter, and Uranus and Neptune are only 5% as massive. All four have deep atmospheres and opaque clouds, and all rotate quickly with periods from 10 to 17 hours. Jupiter and Saturn have extensive mantles of liquid hydrogen. Uranus and Nept... | https://openstax.org/books/astronomy-2e/pages/11-summary |
The four giant planets have generally similar atmospheres, composed mostly of hydrogen and helium. Their atmospheres contain small quantities of methane and ammonia gas, both of which also condense to form clouds. Deeper (invisible) cloud layers consist of water and possibly ammonium hydrosulfide (Jupiter and Saturn) a... | https://openstax.org/books/astronomy-2e/pages/11-summary |
resonance : an orbital condition in which one object is subject to periodic gravitational perturbations by another, most commonly arising when two objects orbiting a third have periods of revolution that are simple multiples or fractions of each other | https://openstax.org/books/astronomy-2e/pages/12-key-terms |
tidal heating : the heating of a planet or moonâs interior by variable tidal forces caused by changing gravitational pull from a nearby planet or moon | https://openstax.org/books/astronomy-2e/pages/12-key-terms |
The four jovian planets are accompanied by impressive systems of moons and rings. Over 200 moons have been discovered in the outer solar system. Of the four ring systems, Saturnâs is the largest and is composed primarily of water ice; in contrast, Uranus and Neptune have narrow rings of dark material, and Jupiter has... | https://openstax.org/books/astronomy-2e/pages/12-summary |
Jupiterâs largest moons are Ganymede and Callisto, both low-density objects that are composed of more than half water ice. Callisto has an ancient cratered surface, while Ganymede shows evidence of extensive tectonic and volcanic activity, persisting until perhaps a billion years ago. Io and Europa are denser and sma... | https://openstax.org/books/astronomy-2e/pages/12-summary |
Saturnâs moon Titan has an atmosphere that is thicker than that of Earth. There are lakes and rivers of liquid hydrocarbons, and evidence of a cycle of evaporation, condensation, and return to the surface that is similar to the water cycle on Earth (but with liquid methane and ethane). The Cassini-Huygens lander set ... | https://openstax.org/books/astronomy-2e/pages/12-summary |
Pluto and Charon have been revealed by the New Horizons spacecraft to be two of the most fascinating objects in the outer solar system. Pluto is small (a dwarf planet) but also surprisingly active, with contrasting areas of dark cratered terrain, light-colored basins of nitrogen ice, and mountains of frozen water that ... | https://openstax.org/books/astronomy-2e/pages/12-summary |
Rings are composed of vast numbers of individual particles orbiting so close to a planet that its gravitational forces could have broken larger pieces apart or kept small pieces from gathering together. Saturnâs rings are broad, flat, and nearly continuous, except for a handful of gaps. The particles are mostly water... | https://openstax.org/books/astronomy-2e/pages/12-summary |
asteroid : a stony or metallic object orbiting the Sun that is smaller than a planet but that shows no evidence of an atmosphere or of other types of activity associated with comets | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
asteroid belt : the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
comet : a small body of icy and dusty matter that revolves about the Sun; when a comet comes near the Sun, some of its material vaporizes, forming a large head of tenuous gas and often a tail | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
Kuiper belt : a region of space beyond Neptune that is dynamically stable (like the asteroid belt); the source region for most short-period comets | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
near-Earth asteroid (NEA) : an Earth-approaching asteroid, one whose orbit could bring it on a collision course with our planet | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
near-Earth object (NEO) : a comet or asteroid whose path intersects the orbit of Earth | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
nucleus (of a comet) : the solid chunk of ice and dust in the head of a comet | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
Oort cloud : the large spherical region around the Sun from which most ânewâ comets come; a reservoir of objects with aphelia at about 50,000 AU | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
tail : (of a comet) a tail consisting of two parts: the dust tail is made of dust loosened by the sublimation of ice in a comet that is then pushed by photons from the Sun into a curved stream; the ion tail is a stream of ionized particles evaporated from a comet and then swept away from the Sun by the solar wind | https://openstax.org/books/astronomy-2e/pages/13-key-terms |
The solar system includes many objects that are much smaller than the planets and their larger moons. The rocky ones are generally called asteroids. Ceres is the largest asteroid; about 15 are larger than 250 kilometers and about 100,000 are larger than 1 kilometer. Most are in the asteroid belt between Mars and Jupite... | https://openstax.org/books/astronomy-2e/pages/13-summary |
Near-Earth asteroids (NEAs), and near-Earth objects (NEOs) in general, are of interest in part because of their potential to hit Earth. They are on unstable orbits, and on timescales of 100 million years, they will either impact one of the terrestrial planets or the Sun, or be ejected. Most of them probably come from t... | https://openstax.org/books/astronomy-2e/pages/13-summary |
Halley first showed that some comets are on closed orbits and return periodically to swing around the Sun. The heart of a comet is its nucleus, a few kilometers in diameter and composed of volatiles (primarily frozen H2O) and solids (including both silicates and carbonaceous materials). Whipple first suggested this â... | https://openstax.org/books/astronomy-2e/pages/13-summary |
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