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Quasar activity can be re-triggered by a collision between two galaxies, which provides a new source of fuel to feed the black hole. | https://openstax.org/books/astronomy-2e/pages/27-summary |
cold dark matter : slow-moving massive particles, not yet identified, that donât absorb, emit, or reflect light or other electromagnetic radiation | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
cosmological principle : the assumption that, on the large scale, the universe at any given time is the same everywhereâisotropic and homogeneous | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
dark energy : an energy that is causing the expansion of the universe to accelerate; the source of this energy is not yet understood | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
evolution (of galaxies) : changes in individual galaxies over cosmic time, inferred by observing snapshots of many different galaxies at different times in their lives | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
galactic cannibalism : a process by which a larger galaxy strips material from or completely swallows a smaller one | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
homogeneous : having a consistent and even distribution of matter that is the same everywhere | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
hot dark matter : massive particles, not yet identified, that donât absorb, emit, or reflect light or other electromagnetic radiation; hot dark matter is faster-moving material than cold dark matter | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
isotropic : the same in all directions | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
Local Group : a small cluster of galaxies to which our Galaxy belongs | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
merger : a collision between galaxies (of roughly comparable size) that combine to form a single new structure | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
starburst : a galaxy or merger of multiple galaxies that turns gas into stars much faster than usual | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
supercluster : a large region of space (more than 100 million light-years across) where groups and clusters of galaxies are more concentrated; a cluster of clusters of galaxies | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
void : a region between clusters and superclusters of galaxies that appears relatively empty of galaxies | https://openstax.org/books/astronomy-2e/pages/28-key-terms |
When we look at distant galaxies, we are looking back in time. We have now seen galaxies as they were when the universe was about 300 million years oldâonly about two percent as old as it is now. The universe now is 13.8 billion years old. The color of a galaxy is an indicator of the age of the stars that populate it... | https://openstax.org/books/astronomy-2e/pages/28-summary |
When galaxies of comparable size collide and coalesce we call it a merger, but when a small galaxy is swallowed by a much larger one, we use the term galactic cannibalism. Collisions play an important role in the evolution of galaxies. If the collision involves at least one galaxy rich in interstellar matter, the resul... | https://openstax.org/books/astronomy-2e/pages/28-summary |
Counts of galaxies in various directions establish that the universe on the large scale is homogeneous and isotropic (the same everywhere and the same in all directions, apart from evolutionary changes with time). The sameness of the universe everywhere is referred to as the cosmological principle. Galaxies are grouped... | https://openstax.org/books/astronomy-2e/pages/28-summary |
Stars move much faster in their orbits around the centers of galaxies, and galaxies around centers of galaxy clusters, than they should according to the gravity of all the luminous matter (stars, gas, and dust) astronomers can detect. This discrepancy implies that galaxies and galaxy clusters are dominated by dark matt... | https://openstax.org/books/astronomy-2e/pages/28-summary |
Initially, luminous and dark matter in the universe was distributed almostâbut not quiteâuniformly. The challenge for galaxy formation theories is to show how this ânot quiteâ smooth distribution of matter developed the structuresâgalaxies and galaxy clustersâthat we see today. It is likely that the filamen... | https://openstax.org/books/astronomy-2e/pages/28-summary |
anthropic principle : idea that physical laws must be the way they are because otherwise we could not be here to measure them | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
Big Bang : the theory of cosmology in which the expansion of the universe began with a primeval explosion (of space, time, matter, and energy) | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
closed universe : a model in which the universe expands from a Big Bang, stops, and then contracts to a big crunch | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
cosmic microwave background (CMB) : microwave radiation coming from all directions that is the redshifted afterglow of the Big Bang | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
cosmological constant : the term in the equations of general relativity that represents a repulsive force in the universe | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
cosmology : the study of the organization and evolution of the universe | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
critical density : in cosmology, the density that is just sufficient to bring the expansion of the universe to a stop after infinite time | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
dark energy : the energy that is causing the expansion of the universe to accelerate; its existence is inferred from observations of distant supernovae | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
dark matter : nonluminous material, whose nature we donât yet understand, but whose presence can be inferred because of its gravitational influence on luminous matter | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
deuterium : a form of hydrogen in which the nucleus of each atom consists of one proton and one neutron | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
flat universe : a model of the universe that has a critical density and in which the geometry of the universe is flat, like a sheet of paper | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
fusion : the building of heavier atomic nuclei from lighter ones | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
grand unified theories : (GUTs) physical theories that attempt to describe the four forces of nature as different manifestations of a single force | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
inflationary universe : a theory of cosmology in which the universe is assumed to have undergone a phase of very rapid expansion when the universe was about 10â35second old; after this period of rapid expansion, the standard Big Bang and inflationary models are identical | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
lithium : the third element in the periodic table; lithium nuclei with three protons and four neutrons were manufactured during the first few minutes of the expansion of the universe | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
multiverse : the speculative idea that our universe is just one of many universes, each with its own set of physical laws | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
open universe : a model in which the density of the universe is not high enough to bring the expansion of the universe to a halt | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
photon decoupling time : when radiation began to stream freely through the universe without interacting with matter | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
weakly interacting massive particles : (WIMPs) weakly interacting massive particles are one of the candidates for the composition of dark matter | https://openstax.org/books/astronomy-2e/pages/29-key-terms |
Cosmology is the study of the organization and evolution of the universe. The universe is expanding, and this is one of the key observational starting points for modern cosmological theories. Modern observations show that the rate of expansion has not been constant throughout the life of the universe. Initially, when g... | https://openstax.org/books/astronomy-2e/pages/29-summary |
For describing the large-scale properties of the universe, a model that is isotropic and homogeneous (same everywhere) is a pretty good approximation of reality. The universe is expanding, which means that the universe undergoes a change in scale with time; space stretches and distances grow larger by the same factor e... | https://openstax.org/books/astronomy-2e/pages/29-summary |
Lemaître, Alpher, and Gamow first worked out the ideas that are today called the Big Bang theory. The universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided with one another, they ... | https://openstax.org/books/astronomy-2e/pages/29-summary |
When the universe became cool enough to form neutral hydrogen atoms, the universe became transparent to radiation. Scientists have detected the cosmic microwave background (CMB) radiation from this time during the hot, early universe. Measurements with the COBE satellite show that the CMB acts like a blackbody with a t... | https://openstax.org/books/astronomy-2e/pages/29-summary |
Twenty-seven percent of the critical density of the universe is composed of dark matter. To explain so much dark matter, some physics theories predict that additional types of particles should exist. One type has been given the name of WIMPs (weakly interacting massive particles), and scientists are now conducting expe... | https://openstax.org/books/astronomy-2e/pages/29-summary |
The Big Bang model does not explain why the CMB has the same temperature in all directions. Neither does it explain why the density of the universe is so close to critical density. These observations can be explained if the universe experienced a period of rapid expansion, which scientists call inflation, about 10â35... | https://openstax.org/books/astronomy-2e/pages/29-summary |
Recently, many cosmologists have noted that the existence of humans depends on the fact that many properties of the universeâthe size of density fluctuations in the early universe, the strength of gravity, the structure of atomsâwere just right. The idea that physical laws must be the way they are because otherwise... | https://openstax.org/books/astronomy-2e/pages/29-summary |
amino acids : organic compounds that are the molecular building blocks of proteins | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
astrobiology : the multidisciplinary study of life in the universe: its origin, evolution, distribution, and fate; similar terms areexobiologyandbioastronomy | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
biomarker : evidence of the presence of life, especially a global indication of life on a planet that could be detected remotely (such as an unusual atmospheric composition) | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
DNA (deoxyribonucleic acid) : a molecule that stores information about how to replicate a cell and its chemical and structural components | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
Drake equation : a formula for estimating the number of intelligent, technological civilizations in our Galaxy, first suggested by Frank Drake | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
extremophile : an organism (usually a microbe) that tolerates or even thrives under conditions that most of the life around us would consider hostile, such as very high or low temperature or acidity | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
gene : the basic functional unit that carries the genetic (hereditary) material contained in a cell | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
habitable environment : an environment capable of hosting life | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
habitable zone : the region around a star in which liquid water could exist on the surface of terrestrial-sized planets, hence the most probable place to look for life in a starâs planetary system | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
organic compound : a compound containing carbon, especially a complex carbon compound; not necessarily produced by life | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
organic molecule : a combination of carbon and other atomsâprimarily hydrogen, oxygen, nitrogen, phosphorus, and sulfurâsome of which serve as the basis for our biochemistry | https://openstax.org/books/astronomy-2e/pages/30-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/30-key-terms |
protein : a key biological molecule that provides the structure and function of the bodyâs tissues and organs, and essentially carries out the chemical work of the cell | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
RNA (ribonucleic acid) : a molecule that aids in the flow of genetic information from DNA to proteins | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
SETI : the search for extraterrestrial intelligence; usually applied to searches for radio signals from other civilizations | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
stromatolites : solid, layered rock formations that are thought to be the fossils of oxygen-producing photosynthetic bacteria in rocks that are 3.5 billion years old | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
technosignature : any observable evidence of technology from an extraterrestrial civilization | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
thermophile : an organism that can tolerate high temperatures | https://openstax.org/books/astronomy-2e/pages/30-key-terms |
Life on Earth is based on the presence of a key unit known as an organic molecule, a molecule that contains carbon, especially complex hydrocarbons. Our solar system formed about 5 billion years ago from a cloud of gas and dust enriched by several generations of heavier element production in stars. Life is made up of c... | https://openstax.org/books/astronomy-2e/pages/30-summary |
The study of life in the universe, including its origin on Earth, is called astrobiology. Life as we know it requires water, certain elemental raw materials (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur), energy, and an environment in which the complex chemistry of life is stable. Carbon-based (or organic... | https://openstax.org/books/astronomy-2e/pages/30-summary |
The search for life beyond Earth offers several intriguing targets. Mars appears to have been more similar to Earth during its early history than it is now, with evidence for liquid water on its ancient surface and perhaps even now below ground. The accessibility of the martian surface to our spacecraft offers the exci... | https://openstax.org/books/astronomy-2e/pages/30-summary |
Some astronomers are engaged in the search for extraterrestrial intelligent life (SETI). Because other planetary systems are so far away, traveling to the stars is either very slow or extremely expensive (in terms of energy required). Despite many UFO reports and tremendous media publicity, there is no evidence that an... | https://openstax.org/books/astronomy-2e/pages/30-summary |
density = mass volume density = mass volume | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-equations |
T °C = 5 9 à ( T °F â 32 ) T °C = 5 9 à ( T °F â 32 ) | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-equations |
T °F = ( 9 5 à T °C ) + 32 T °F = ( 9 5 à T °C ) + 32 | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-equations |
T K = °C + 273.15 T K = °C + 273.15 | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-equations |
T °C = K â 273.15 T °C = K â 273.15 | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-equations |
accuracy : how closely a measurement aligns with a correct value | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
atom : smallest particle of an element that can enter into a chemical combination | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
Celsius (°C) : unit of temperature; water freezes at 0 °C and boils at 100 °C on this scale | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
chemical change : change producing a different kind of matter from the original kind of matter | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
chemical property : behavior that is related to the change of one kind of matter into another kind of matter | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
chemistry : study of the composition, properties, and interactions of matter | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
compound : pure substance that can be decomposed into two or more elements | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
cubic centimeter (cm3or cc) : volume of a cube with an edge length of exactly 1 cm | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
cubic meter (m3) : SI unit of volume | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
density : ratio of mass to volume for a substance or object | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
dimensional analysis : (also, factor-label method) versatile mathematical approach that can be applied to computations ranging from simple unit conversions to more complex, multi-step calculations involving several different quantities | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
element : substance that is composed of a single type of atom; a substance that cannot be decomposed by a chemical change | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
exact number : number derived by counting or by definition | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
extensive property : property of a substance that depends on the amount of the substance | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
Fahrenheit : unit of temperature; water freezes at 32 °F and boils at 212 °F on this scale | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
gas : state in which matter has neither definite volume nor shape | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
heterogeneous mixture : combination of substances with a composition that varies from point to point | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
homogeneous mixture : (also, solution) combination of substances with a composition that is uniform throughout | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
hypothesis : tentative explanation of observations that acts as a guide for gathering and checking information | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
intensive property : property of a substance that is independent of the amount of the substance | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
kelvin (K) : SI unit of temperature; 273.15 K = 0 ºC | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
kilogram (kg) : standard SI unit of mass | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
law : statement that summarizes a vast number of experimental observations, and describes or predicts some aspect of the natural world | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
law of conservation of matter : when matter converts from one type to another or changes form, there is no detectable change in the total amount of matter present | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
length : measure of one dimension of an object | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
liquid : state of matter that has a definite volume but indefinite shape | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
liter (L) : (also, cubic decimeter) unit of volume; 1 L = 1,000 cm3 | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
macroscopic domain : realm of everyday things that are large enough to sense directly by human sight and touch | https://openstax.org/books/chemistry-atoms-first-2e/pages/1-key-terms |
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