Download Astronomy Vocabulary File

Chapter 18—Observing the Sky
Astronomy—the study of all physical objects beyond Earth
Calendar—a system for organizing time; most calendars organize time within a
single unit called a year
Year—the time required for the Earth to orbit the sun once
Month—roughly the amount of time required for the moon to orbit the Earth once
Day—the time required for the Earth to rotate once on its axis
Leap year—a year in which an extra day is added to the calendar
Constellation—a section of the sky that contains a recognizable star pattern
Altitude—the angle between an object in the sky and the horizon
Right ascension—a measure of how far east an object is from the point at which
the sun appears on the first day of spring
Declination—a measure of how far north or south an object is from the celestial
equator
Celestial equator—imaginary circle created by extending Earth’s equator into
space
Ecliptic—the apparent path the sun takes across the celestial sphere each year
Light-year—a unit of length equal to the distance that light travels through space in
1 year
Telescope—an instrument that collects electromagnetic radiation from the sky and
concentrates it for better observation
Refracting telescope—a telescope that uses a set of lenses to gather and focus light
Chapter 18—Continued
Reflecting telescope—a telescope that uses curved mirrors to gather and focus light
Electromagnetic spectrum—all the wavelengths of electromagnetic radiation
Chapter 19—Formation of the Solar System
Solar system—the system composed of the sun (a star) and the planets and other
bodies that travel around the sun
Nebula—a large cloud of dust and gas in interstellar space; the location of star
formation
Solar nebula—the nebula that formed into the solar system
Planetesimal—the tiny building block of the planets that formed as dust particles
stuck together and grew in size
Rotation—the spinning motion of a body on its axis
Orbit—the elliptical path a body takes as it travels around another body in space;
the motion itself
Revolution—the elliptical motion of a body as it orbits another body in space
Period of revolution—the time it takes for one body to make one complete orbit; or
revolution, around another body in space
Ellipse—a closed curve in which the sum of the distances from the edge of the
curve to two points inside the ellipse is always the same
Astronomical unit (AU)—the average distance between the Earth and the sun, or
approximately 150,000,000 km
Corona—the sun’s outer atmosphere, which can extend outward a distance equal to
10-12 times the diameter of the sun
Chromosphere—a thin region of the sun’s atmosphere between the corona and the
photosphere; too faint to see unless there is a total solar eclipse
Photosphere—the layer of the sun at which point the gases get thick enough to see;
the surface of the sun
Chapter 19—Continued
Convective zone—a region of the sun where gases circulate in convection currents,
bringing the sun’s energy to the surface
Radiative zone—a very dense region of the sun in which the atoms are so closely
packed that light can take millions of years to pass through
Core—the center of the sun where the sun’s energy is produced
Nuclear fusion—the process by which two or more nuclei with small masses join
together, or fuse, to form a larger, more massive nucleus, along with the production
of energy
Sunspot—an area on the photosphere of the sun that is cooler than surrounding
areas, showing up as a dark spot
Crust—the thin, outermost layer of the Earth, or the uppermost part of the
lithosphere
Mantle—the layer of the Earth between the crust and the core
Core—the central, spherical part the Earth below the mantle
Chapter 20—A Family of Planets
Astronomical unit (AU)—the average age distance between the Earth and the sun,
or approximately 150,000,000 km
Terrestrial planets—the small, dense, rocky planets of the inner solar system
Prograde rotation—the counter-clockwise spin of a planet or moon as seen from
above the planet’s North Pole
Retrograde rotation—the clockwise spin of a planet or moon as seen from above
the planet’s North Pole
Gas giants—the large, gaseous planets of the outer solar system
Satellite—a natural or artificial body that revolves around a planet
Phases—the different appearances of the moon due to varying amounts of sunlight
on the side of the moon that faces the Earth; results from the changing relative
positions of the moon, Earth, and the sun
Eclipse—an event in which the shadow of one celestial body falls on another
Comet—a small body of ice, rock, and cosmic dust loosely packed together that
gives off gas and dust in the form of a tail as it passes close to the sun
Asteroid—a small, rock body that revolves around the sun
Asteroid belt—the region of the solar system most asteroids occupy; roughly
between the orbits of Mars and Jupiter
Meteoroid—a very small, rocky body that revolves around the sun
Meteorite—a meteoroid that reaches the Earth’s surface without burning up
completely
Meteor—a streak of light caused when a meteoroid or comet dust burns up in the
Earth’s atmosphere before it reaches the ground
Chapter 21—The Universe Beyond
Spectrum—the rainbow of colors produced when white light passes through a
prism or spectrograph
Apparent magnitude—how bright a light appears to an observer
Absolute magnitude—the actual brightness of a star
Light-year—a unit of length equal to the distance that light travels through space in
1 year
Parallax—an apparent shift in the position of an object when viewed from different
locations
H-R diagram—Hertzsprung-Russell diagram; a graph that shows the relationship
between a star’s surface temperature and its absolute magnitude
Main sequence—a diagonal pattern of stars on the H-R diagram
White dwarf—a small, hot star near the end of its life; the leftover center of an old
star
Red giant—a star that expands and cools once it runs out of hydrogen fuel
Supernova—the death of a large star by explosion
Neutron star—a star in which all the particles have become neutrons; the collapsed
remains of a supernova
Pulsar—a spinning neutron star that emits rapid pulses of light
Black hole—an object with more than three solar masses squeezed into a ball only
10 km across whose gravity is so strong that not even light can escape
Galaxy—a large grouping of stars in space
Spiral galaxy—a galaxy with a bulge in the center and very distinctive spiral arms
Chapter 21—Continued
Elliptical galaxy—a spherical or elongated galaxy with a bright center and very
little dust and gas
Irregular galaxy—a galaxy that does not fit into any other category; one with an
irregular shape
Nebula—a large cloud of dust and gas in interstellar space; the location of star
formation
Open cluster—a group of stars that are usually located along the spiral disk of a
galaxy
Globular cluster—a group of older stars that looks like a ball of stars
Quasar—“quasi-stellar” object; a star-like source of light that is extremely far
away; one of the most powerful sources of energy in the universe
Cosmology—the study of the origin and future of the universe
Big bang theory—the theory that states the universe began with a tremendous
explosion
Cosmic background radiation—radiation left over from the big bang that fills all of
space
Chapter 22—Exploring Space
Rocket—a machine that uses escaping gas to move
NASA—National Aeronautics and Space Administration; founded to combine all
of the separate rocket-development teams in the United States
Thrust—the force that accelerates a rocket
Orbital velocity—the speed and direction a rocket must have in order to orbit the
Earth
Escape velocity—the speed and direction a rocket must travel in order to
completely break away from a planet’s gravitational pull
Artificial satellite—any human-made object placed in orbit around a body in space
Low Earth orbit—an orbit located a few hundred kilometers above the Earth’s
surface
Geosynchronous orbit—an orbit in which a satellite travels at a speed that matches
the rotational speed of the Earth exactly, keeping the satellite positioned above the
same spot on Earth at all times
Space probe—a vehicle that carries scientific instruments to planets or other bodies
in space
Space shuttle—a reusable vehicle that takes off like a rocket and lands like an
airplane
Space station—a long-term orbiting platform from which other vehicles can be
launched or scientific research can be carried out