Astronomy 1-5

• Star


  Large, bright ball of gas that generates heat and light through nuclear fusion in its core
• Planet

  Moderately large object that orbits a star and shines by reflected light. It is made of rock, ice, or gas
• Moon (or satellite)


  An object that orbits a planet
• Asteroids


  Relatively small/rocky object that orbits a star
• Comet


  Relatively small/ice-rich object that orbits a star
• Solar (star) system


  A star and all orbiting material; includes its planets and their moons
• Nebula


  A(n interstellar) cloud of gas and/or dust
• Galaxy


  An "island in space" held together by gravity: stars and their corpses, gas, dust/planets, and dark matter
• Universe (or cosmos)


  Everything: all space, matter, and energy that is "accessible"
• Astronomical Unit (AU)


  Average distance from earth to sun
• Cluster (group of galaxies)

  Collection of galaxies bound together by gravity
• Milky way galaxy


  Our solar system belongs to this; disk-shaped collection of stars
• Local Group


  Galaxies that are grouped together
• Galaxy Clusters


  Group of galaxies with more than a few dozen members
• Super cluster


  Gigantic space where may groups of clusters are packed more closely
• Light years (ly)


  Distance light can travel in 1 year
• Observable Universe


  Section of entire universe that can be seen from Earth
• Expanse in universe


  Increase in average distance between galaxies as time progresses
• Big bang


  Birth of the universe
• Ecliptic Plane


  Plane of Earth's orbit around the sun; 23.5 degrees perpendicular
• Axis Tilt


  Amount by which a planet's axis is tilted with respect to a line perpendicular to the ecliptic plane; oriented so axis points almost directly at Polaris or the north star
• Rotation


  The spinning of an object about its axis
• Orbit (Revolution)


  The orbital motion of one object around another due to gravity
• Nuclear Fusion


  Process in which lightweight atomic nuclei smash together and stick (fuse) to make heavier nuclei
• Horizon


  Boundary that divides what we can see from what we cannot see, boundary between the earth and the sky
• Altitude


  (Above horizon) the angular distance between the horizon and an object in the sky
• Constellation


  A region of the sky; 88 total over celestial sphere
• Zenith


  Point directly overhead, altitude of 90 degree
• Celestial sphere


  Imaginary sphere which objects in the sky reside when seen from Earth:4 points/circles
• North celestial pole


  (Part of celestial sphere) point directly over Earth's North Pole
• South celestial pole


  (Part of celestial sphere) point directly over Earth's South Pole
• Celestial Equator


  Projection of Earth's equator into space, makes complete circle around celestial sphere
• Ecliptic


  Path the sun follows as it circles around the celestial sphere once each year. It crosses the celestial equator at a 23.5 degree angle, because that is the tilt of the Earth's axis
• Local sky


  Sky seen from wherever you're standing
• Meridian


  Imaginary half-circle stretching from the horizon due south, through the zenith, to the horizon due north
• Direction


  Position of object in the local sky along the horizon. Referred to as azimuth, which is degrees clockwise from due north
• Angular Size


  Angle it appears to span in your field of view. Doesn't tell us true size of object, depends on distance.
• Angular distance


  Angle that appears to separate 2 objects in the sky
• Arcminutes


  (Symbol = ') subdivide degrees into this (60 arcminutes).
• Arcseconds


  (Symbol = '') subdivide a arcminute into this (60 arcseconds)