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Oort proposed in 1950 that long-period comets are derived from what we now call the Oort cloud, which surrounds the Sun out to about 50,000 AU (near the limit of the Sunâs gravitational sphere of influence) and contains between 1012and 1013comets. Comets also come from the Kuiper belt, a disk-shaped region beyond the... | https://openstax.org/books/astronomy-2e/pages/13-summary |
accretion : the gradual accumulation of mass, as by a planet forming from colliding particles in the solar nebula | https://openstax.org/books/astronomy-2e/pages/14-key-terms |
exoplanet : a planet orbiting a star other than our Sun | https://openstax.org/books/astronomy-2e/pages/14-key-terms |
iron meteorite : a meteorite composed primarily of iron and nickel | https://openstax.org/books/astronomy-2e/pages/14-key-terms |
meteor : a small piece of solid matter that enters Earthâs atmosphere and burns up, popularly called ashooting starbecause it is seen as a small flash of light | https://openstax.org/books/astronomy-2e/pages/14-key-terms |
meteor shower : many meteors appearing to radiate from one point in the sky; produced when Earth passes through a cometary dust stream | https://openstax.org/books/astronomy-2e/pages/14-key-terms |
meteorite : a portion of a meteor that survives passage through the atmosphere and strikes the ground | https://openstax.org/books/astronomy-2e/pages/14-key-terms |
stony meteorite : a meteorite composed mostly of stony material, either primitive or differentiated | https://openstax.org/books/astronomy-2e/pages/14-key-terms |
stony-iron meteorite : a type of differentiated meteorite that is a blend of nickel-iron and silicate materials | https://openstax.org/books/astronomy-2e/pages/14-key-terms |
When a fragment of interplanetary dust strikes Earthâs atmosphere, it burns up to create a meteor. Streams of dust particles traveling through space together produce meteor showers, in which we see meteors diverging from a spot in the sky called the radiant of the shower. Many meteor showers recur each year and are a... | https://openstax.org/books/astronomy-2e/pages/14-summary |
Meteorites are the debris from space (mostly asteroid fragments) that survive to reach the surface of Earth. Meteorites are calledfindsorfallsaccording to how they are discovered; the most productive source today is the Antarctic ice cap. Meteorites are classified as irons, stony-irons, or stones accordingly to their c... | https://openstax.org/books/astronomy-2e/pages/14-summary |
A viable theory of solar system formation must take into account motion constraints, chemical constraints, and age constraints. Meteorites, comets, and asteroids are survivors of the solar nebula out of which the solar system formed. This nebula was the result of the collapse of an interstellar cloud of gas and dust, w... | https://openstax.org/books/astronomy-2e/pages/14-summary |
The first planet circling a distant solar-type star was announced in 1995. Twenty years later, thousands of exoplanets have been identified, including planets with sizes and masses between Earthâs and Neptuneâs, which we donât have in our own solar system. A few percent of exoplanet systems have âhot Jupiters,â... | https://openstax.org/books/astronomy-2e/pages/14-summary |
After their common beginning, each of the planets evolved on its own path. Different possible outcomes are illustrated by comparison of the terrestrial planets (Earth, Venus, Mars, Mercury, and the Moon). All are rocky, differentiated objects. The level of geological activity is proportional to mass: greatest for Earth... | https://openstax.org/books/astronomy-2e/pages/14-summary |
active region : an area on the Sun where magnetic fields are concentrated; sunspots, prominences, flares, and CMEs all tend to occur in active regions | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
aurora : light radiated by atoms and ions in the ionosphere excited by charged particles from the Sun, mostly seen in the magnetic polar regions | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
chromosphere : the part of the solar atmosphere that lies immediately above the photospheric layers | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
corona : (of the Sun) the outer (hot) atmosphere of the Sun | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
coronal hole : a region in the Sunâs outer atmosphere that appears darker because there is less hot gas there | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
coronal mass ejection (CME) : a solar flare in which immense quantities of coronal materialâmainly protons and electronsâis ejected at high speeds (500â1000 kilometers per second) into interplanetary space | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
differential rotation : the phenomenon that occurs when different parts of a rotating object rotate at different rates at different latitudes | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
granulation : the rice-grain-like structure of the solar photosphere; granulation is produced by upwelling currents of gas that are slightly hotter, and therefore brighter, than the surrounding regions, which are flowing downward into the Sun | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
Maunder Minimum : a period during the seventeenth century when the number of sunspots seen throughout the solar cycle was unusually low | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
photosphere : the region of the solar (or stellar) atmosphere from which continuous radiation escapes into space | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
plage : a bright region of the solar surface observed in the light of some spectral line | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
plasma : a hot ionized gas | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
prominence : a large, bright, gaseous feature that appears above the surface of the Sun and extends into the corona | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
solar flare : a sudden and temporary outburst of electromagnetic radiation from an extended region of the Sunâs surface | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
solar wind : a flow of hot, charged particles leaving the Sun | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
sunspot : large, dark features seen on the surface of the Sun caused by increased magnetic activity | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
sunspot cycle : the semiregular 11-year period with which the frequency of sunspots fluctuates | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
transition region : the region in the Sunâs atmosphere where the temperature rises very rapidly from the relatively low temperatures that characterize the chromosphere to the high temperatures of the corona | https://openstax.org/books/astronomy-2e/pages/15-key-terms |
The Sun, our star, has several layers beneath the visible surface: the core, radiative zone, and convective zone. These, in turn, are surrounded by a number of layers that make up the solar atmosphere. In order of increasing distance from the center of the Sun, they are the photosphere, with a temperature that ranges f... | https://openstax.org/books/astronomy-2e/pages/15-summary |
Sunspots are dark regions where the temperature is up to 2000 K cooler than the surrounding photosphere. Their motion across the Sunâs disk allows us to calculate how fast the Sun turns on its axis. The Sun rotates more rapidly at its equator, where the rotation period is about 25 days, than near the poles, where the... | https://openstax.org/books/astronomy-2e/pages/15-summary |
Signs of more intense solar activity, an increase in the number of sunspots, as well as prominences, plages, solar flares, andcoronal mass ejections, all tend to occur in active regionsâthat is, in places on the Sun with the same latitude and longitude but at different heights in the atmosphere. Active regions vary w... | https://openstax.org/books/astronomy-2e/pages/15-summary |
Space weather is the effect of solar activity on our own planet, both in our magnetosphere and on Earthâs surface. Coronal holes allow more of the Sunâs material to flow out into space. Solar flares and coronal mass ejections can cause auroras, disrupt communications, damage satellites, and cause power outages on E... | https://openstax.org/books/astronomy-2e/pages/15-summary |
conduction : process by which heat is directly transmitted through a substance when there is a difference of temperature between adjoining regions caused by atomic or molecular collisions | https://openstax.org/books/astronomy-2e/pages/16-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/16-key-terms |
fission : breaking up of heavier atomic nuclei into lighter ones | https://openstax.org/books/astronomy-2e/pages/16-key-terms |
fusion : building up of heavier atomic nuclei from lighter ones | https://openstax.org/books/astronomy-2e/pages/16-key-terms |
helioseismology : study of pulsations or oscillations of the Sun in order to determine the characteristics of the solar interior | https://openstax.org/books/astronomy-2e/pages/16-key-terms |
hydrostatic equilibrium : balance between the weights of various layers, as in a star or Earthâs atmosphere, and the pressures that support them | https://openstax.org/books/astronomy-2e/pages/16-key-terms |
neutrino : fundamental particle that has no charge and a mass that is tiny relative to an electron; it rarely interacts with ordinary matter and comes in three different types | https://openstax.org/books/astronomy-2e/pages/16-key-terms |
positron : particle with the same mass as an electron, but positively charged | https://openstax.org/books/astronomy-2e/pages/16-key-terms |
proton-proton chain : series of thermonuclear reactions by which nuclei of hydrogen are built up into nuclei of helium | https://openstax.org/books/astronomy-2e/pages/16-key-terms |
radiation : emission of energy as electromagnetic waves or photons also the transmitted energy itself | https://openstax.org/books/astronomy-2e/pages/16-key-terms |
The Sun produces an enormous amount of energy every second. Since Earth and the solar system are roughly 4.5 billion years old, this means that the Sun has been producing vast amounts for energy for a very, very long time. Neither chemical burning nor gravitational contraction can account for the total amount of energy... | https://openstax.org/books/astronomy-2e/pages/16-summary |
Solar energy is produced by interactions of particlesâthat is, protons, neutrons, electrons, positrons, and neutrinos. Specifically, the source of the Sunâs energy is the fusion of hydrogen to form helium. The series of reactions required to convert hydrogen to helium is called the proton-proton chain. A helium ato... | https://openstax.org/books/astronomy-2e/pages/16-summary |
Even though we cannot see inside the Sun, it is possible to calculate what its interior must be like. As input for these calculations, we use what we know about the Sun. It is made entirely of hot gas. Apart from some very tiny changes, the Sun is neither expanding nor contracting (it is in hydrostatic equilibrium) and... | https://openstax.org/books/astronomy-2e/pages/16-summary |
Studies of solar oscillations (helioseismology) and neutrinos can provide observational data about the Sunâs interior. The technique of helioseismology has so far shown that the composition of the interior is much like that of the surface (except in the core, where some of the original hydrogen has been converted int... | https://openstax.org/books/astronomy-2e/pages/16-summary |
apparent brightness : a measure of the amount of light received by Earth from a star or other objectâthat is, how bright an object appears in the sky, as contrasted with its luminosity | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
brown dwarf : an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is about 0.075 the mass of the Sun; brown dwarfs are capable of deuterium fusion, but not hydrogen fusio... | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
color index : difference between the magnitudes of a star or other object measured in light of two different spectral regionsâfor example, blue minus visual (BâV) magnitudes | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
giant : a star of exaggerated size with a large, extended photosphere | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
luminosity : the rate at which a star or other object emits electromagnetic energy into space; the total power output of an object | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
magnitude : an older system of measuring the amount of light we receive from a star or other luminous object; the larger the magnitude, the less radiation we receive from the object | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
proper motion : the angular change per year in the direction of a star as seen from the Sun | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
radial velocity : motion toward or away from the observer; the component of relative velocity that lies in the line of sight | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
space velocity : the total (three-dimensional) speed and direction with which an object is moving through space relative to the Sun | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
spectral class : (or spectral type) the classification of stars according to their temperatures using the characteristics of their spectra; the types are O, B, A, F, G, K, and M with L, T, and Y added recently for cooler star-like objects that recent survey have revealed | https://openstax.org/books/astronomy-2e/pages/17-key-terms |
The total energy emitted per second by a star is called its luminosity. How bright a star looks from the perspective of Earth is its apparent brightness. The apparent brightness of a star depends on both its luminosity and its distance from Earth. Thus, the determination of apparent brightness and measurement of the di... | https://openstax.org/books/astronomy-2e/pages/17-summary |
Stars have different colors, which are indicators of temperature. The hottest stars tend to appear blue or blue-white, whereas the coolest stars are red. A color index of a star is the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of sta... | https://openstax.org/books/astronomy-2e/pages/17-summary |
The differences in the spectra of stars are principally due to differences in temperature, not composition. The spectra of stars are described in terms of spectral classes. In order of decreasing temperature, these spectral classes are O, B, A, F, G, K, M, L, T, and Y. These are further divided into subclasses numbered... | https://openstax.org/books/astronomy-2e/pages/17-summary |
Spectra of stars of the same temperature but different atmospheric pressures have subtle differences, so spectra can be used to determine whether a star has a large radius and low atmospheric pressure (a giant star) or a small radius and high atmospheric pressure. Stellar spectra can also be used to determine the chemi... | https://openstax.org/books/astronomy-2e/pages/17-summary |
binary stars : two stars that revolve about each other | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
brown dwarf : an object intermediate in size between a planet and a star; the approximate mass range is from about 1/100 of the mass of the Sun up to the lower mass limit for self-sustaining nuclear reactions, which is about 1/12 the mass of the Sun | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
eclipsing binary : a binary star in which the plane of revolution of the two stars is nearly edge-on to our line of sight, so that the light of one star is periodically diminished by the other passing in front of it | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
HâR diagram : (HertzsprungâRussell diagram) a plot of luminosity against surface temperature (or spectral type) for a group of stars | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
main sequence : a sequence of stars on the HertzsprungâRussell diagram, containing the majority of stars, that runs diagonally from the upper left to the lower right | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
mass-luminosity relation : the observed relation between the masses and luminosities of many (90% of all) stars | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
selection effect : the selection of sample data in a nonrandom way, causing the sample data to be unrepresentative of the entire data set | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
spectroscopic binary : a binary star in which the components are not resolved but whose binary nature is indicated by periodic variations in radial velocity, indicating orbital motion | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
visual binary : a binary star in which the two components are telescopically resolved | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
white dwarf : a low-mass star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size; such a star is near its final state of life | https://openstax.org/books/astronomy-2e/pages/18-key-terms |
To understand the properties of stars, we must make wide-ranging surveys. We find the stars that appear brightest to our eyes are bright primarily because they are intrinsically very luminous, not because they are the closest to us. Most of the nearest stars are intrinsically so faint that they can be seen only with th... | https://openstax.org/books/astronomy-2e/pages/18-summary |
The masses of stars can be determined by analysis of the orbit of binary starsâtwo stars that orbit a common center of mass. In visual binaries, the two stars can be seen separately in a telescope, whereas in a spectroscopic binary, only the spectrum reveals the presence of two stars. Stellar masses range from about ... | https://openstax.org/books/astronomy-2e/pages/18-summary |
The diameters of stars can be determined by measuring the time it takes an object (the Moon, a planet, or a companion star) to pass in front of it and block its light. Diameters of members of eclipsing binary systems (where the stars pass in front of each other) can be determined through analysis of their orbital motio... | https://openstax.org/books/astronomy-2e/pages/18-summary |
The HertzsprungâRussell diagram, or HâR diagram, is a plot of stellar luminosity against surface temperature. Most stars lie on the main sequence, which extends diagonally across the HâR diagram from high temperature and high luminosity to low temperature and low luminosity. The position of a star along the main ... | https://openstax.org/books/astronomy-2e/pages/18-summary |
cepheid : a star that belongs to a class of yellow supergiant pulsating stars; these stars vary periodically in brightness, and the relationship between their periods and luminosities is useful in deriving distances to them | https://openstax.org/books/astronomy-2e/pages/19-key-terms |
light curve : a graph that displays the time variation of the light from a variable or eclipsing binary star or, more generally, from any other object whose radiation output changes with time | https://openstax.org/books/astronomy-2e/pages/19-key-terms |
luminosity class : a classification of a star according to its luminosity within a given spectral class; our Sun, a G2V star, has luminosity class V, for example | https://openstax.org/books/astronomy-2e/pages/19-key-terms |
parallax : an apparent displacement of a nearby star that results from the motion of Earth around the Sun | https://openstax.org/books/astronomy-2e/pages/19-key-terms |
parsec : a unit of distance in astronomy, equal to 3.26 light-years; at a distance of 1 parsec, a star has a parallax of 1 arcsecond | https://openstax.org/books/astronomy-2e/pages/19-key-terms |
period-luminosity relation : an empirical relation between the periods and luminosities of certain variable stars | https://openstax.org/books/astronomy-2e/pages/19-key-terms |
pulsating variable star : a variable star that pulsates in size and luminosity | https://openstax.org/books/astronomy-2e/pages/19-key-terms |
RR Lyrae : one of a class of giant pulsating stars with periods shorter than 1 day, useful for finding distances | https://openstax.org/books/astronomy-2e/pages/19-key-terms |
Early measurements of length were based on human dimensions, but today, we use worldwide standards that specify lengths in units such as the meter. Distances within the solar system are now determined by timing how long it takes radar signals to travel from Earth to the surface of a planet or other body and then return... | https://openstax.org/books/astronomy-2e/pages/19-summary |
For stars that are relatively nearby, we can âtriangulateâ the distances from a baseline created by Earthâs annual motion around the Sun. Half the shift in a nearby starâs position relative to very distant background stars, as viewed from opposite sides of Earthâs orbit, is called the parallax of that star an... | https://openstax.org/books/astronomy-2e/pages/19-summary |
Cepheids and RR Lyrae stars are two types of pulsating variable stars. Light curves of these stars show that their luminosities vary with a regularly repeating period. RR Lyrae stars can be used as standard bulbs, and cepheid variables obey a period-luminosity relation, so measuring their periods can tell us their lumi... | https://openstax.org/books/astronomy-2e/pages/19-summary |
Stars with identical temperatures but different pressures (and diameters) have somewhat different spectra. Spectral classification can therefore be used to estimate the luminosity class of a star as well as its temperature. As a result, a spectrum can allow us to pinpoint where the star is located on an HâR diagram a... | https://openstax.org/books/astronomy-2e/pages/19-summary |
baryon cycle : the cycling of mass in and out of the interstellar medium, including accretion of gas from intergalactic space, loss of gas back into intergalactic space, and conversion of interstellar gas into stars | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
cosmic rays : atomic nuclei (mostly protons) and electrons that are observed to strike Earthâs atmosphere with exceedingly high energies. | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
H II region : the region of ionized hydrogen in interstellar space | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
interstellar dust : tiny solid grains in interstellar space thought to consist of a core of rocklike material (silicates) or graphite surrounded by a mantle of ices; water, methane, and ammonia are probably the most abundant ices | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
interstellar extinction : the attenuation or absorption of light by dust in the interstellar medium | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
interstellar medium (ISM) : (or interstellar matter) the gas and dust between the stars in a galaxy | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
Local Bubble : (or Local Hot Bubble) a region of low-density, million degree gas in which the Sun and solar system are currently located | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
Local Fluff : a slightly denser cloud inside the Local Bubble, inside which the Sun also lies | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
molecular cloud : a large, dense, cold interstellar cloud; because of its size and density, this type of cloud can keep ultraviolet radiation from reaching its interior, where molecules are able to form | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
nebula : a cloud of interstellar gas or dust; the term is most often used for clouds that are seen to glow with visible light or infrared | https://openstax.org/books/astronomy-2e/pages/20-key-terms |
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