Chapter 4: Space and Time Flashcards
Define ‘Asteroid’.
A sub-planetary object orbiting the Sun. The orbits of most asteroids lie between the orbits of Mars and Jupiter.
Define ‘Astronomical unit’.
Unit of measure used by astronomers; it is the average distance from Earth to the Sun; 149,600,000 kilometers.
Define ‘Comet’.
Small solar system body com- posed primarily of ice with some dust and rock particles, which orbits the Sun in a highly elliptical orbit.
Define ‘Galaxy’.
A cluster of a billion or more stars, plus gas and dust, that is held together by gravity.
Define ‘Geological column’.
A composite diagram combining in chronological order the succession of known strata, fitted together on the basis of their fossils or other evidence of relative or actual age.
Define ‘Half-life’.
The time needed for the number of parent atoms of a radioactive isotope to be reduced by one-half.
Define ‘Jovian planets’.
Giant planets in the outer regions of the solar system that are characterized by great masses, low densities, and thick atmospheres consisting primarily of hydrogen and helium.
Define ‘Main sequence’.
The principal series of stars in the Hertzsprung-Russell diagram, which includes stars that are converting hydrogen to helium.
Define ‘Meteorite’. vs meteoroid and meteor?
Piece of natural debris that falls to Earth.
Define ‘Moon’.
A natural object in a regular orbit around a planet.
Define ‘Nebular hypothesis’.
The proposition that the Sun and planets formed from a huge, swirling cloud of cosmic gas and dust. Elements heavier than hydrogen and helium came from the remains of an older star that exploded in a supernova.
Define ‘Numerical age’.
The time in years when a specific event happened or a specific material formed or was deposited.
Define ‘Planet’.
A natural body in orbit around a star that is massive enough to be spherical and to have cleared its orbital path of other objects.
Define ‘Planetary accretion’.
The process by which bits of condensed solid matter were gathered to form the planets.
Define ‘Planetary differentiation’.
The process of chemical segregation by which a planet separates into a core of dense matter surrounded by one or more layers of less dense rocky matter.
Define ‘Primary (primordial) atmosphere’.
The original envelopes of hydrogen and helium which surrounded the terrestrial planets early in the history of the solar system.
Define ‘Principle of uniformitarianism’.
The same external and internal processes we recognize in action today have been operating unchanged, though at different rates, throughout most of the Earth’s history.
Define ‘Radiometric dating’.
Determination of the time in years since the formation of a rock or other natural object using the contained radioactive isotopes.
Define ‘Relative age’.
The age of an object, material, or event, as determined by comparison to an older or younger object or event.
Define ‘Secondary atmosphere’.
The envelope of gaseous volatile elements that leaked from the interior of a terrestrial planet via volcanoes and was trapped by the planet’s gravity.
Define ‘Solar nebula’.
A flattened rotating disc of gas and dust surrounding the Sun.
Define ‘Solar system’.
The group of planets, moons, asteroids, comets, and other natural objects in orbit around the Sun.
Define ‘Terrestrial planets’.
The innermost planets of the solar system (Mercury, Venus, Earth, and Mars), which have high densities and rocky compositions.
The Sun is a star, the central body of our solar system. It is part of the Milky Way galaxy, one of about 100 billion galaxies, each of which contains between 200 and 400 billion stars. Though an ordinary star, the Sun overwhelmingly dominates our solar system, containing ___ percent of its mass—mostly hydrogen and helium, with small amounts of the heavier elements.
99.8%
