| No. | Term | Definition |
|---|---|---|
| 1. | Ablation | Erosion by atmospheric heating. |
| 2. | Absolute magnitude | Intrinsic brightness at 10 parsecs. |
| 3. | Acceleration | Rate of change of velocity. |
| 4. | Aperture | Telescope opening for collecting light. |
| 5. | Aphelion | Farthest point from the Sun. |
| 6. | Asteroid | Small rocky body orbiting the Sun. |
| 7. | Astrometry | Measuring positions and motions of objects. |
| 8. | Astronaut | Person trained for space travel. |
| 9. | Astronomy | Study of celestial objects and space. |
| 10. | Atmosphere | Layer of gases around a planet. |
| 11. | Backscatter | Light scattered back toward the source. |
| 12. | Big Bang | Origin model of the expanding universe. |
| 13. | Binary star | Two stars orbiting each other. |
| 14. | Black hole | Region where gravity traps light. |
| 15. | Blueshift | Wavelengths shortened by approaching motion. |
| 16. | Bolide | Very bright meteor fireball. |
| 17. | Brightness | Perceived intensity of light. |
| 18. | Brown dwarf | Substellar object, not fully fused. |
| 19. | Buckyball | Carbon molecule found in space. |
| 20. | Celestial equator | Earth’s equator projected onto sky. |
| 21. | Celestial sphere | Imaginary sphere of the sky. |
| 22. | Chromosphere | Solar layer above the photosphere. |
| 23. | Coma | Comet’s diffuse gas-and-dust cloud. |
| 24. | Comet | Icy body forming tail near Sun. |
| 25. | Constellation | Recognized pattern of stars. |
| 26. | Corona | Sun’s hot outer atmosphere. |
| 27. | Cosmic rays | High-energy particles from space. |
| 28. | Cosmology | Study of universe’s origin and evolution. |
| 29. | Crater | Bowl-shaped impact depression. |
| 30. | Dark matter | Unseen mass inferred from gravity. |
| 31. | Dark nebula | Dust cloud blocking background light. |
| 32. | Declination | Sky coordinate like latitude. |
| 33. | Density | Mass per unit volume. |
| 34. | Doppler effect | Frequency shift from relative motion. |
| 35. | Dwarf planet | Round body not clearing its orbit. |
| 36. | Eccentricity | Measure of orbit’s ovalness. |
| 37. | Eclipse | One body blocks another’s light. |
| 38. | Ecliptic | Sun’s apparent yearly path. |
| 39. | Electromagnetic spectrum | All wavelengths of electromagnetic radiation. |
| 40. | Emission nebula | Glowing gas cloud emitting light. |
| 41. | Exoplanet | Planet orbiting another star. |
| 42. | Fast radio burst | Brief, intense radio pulse. |
| 43. | Filament | Threadlike solar or nebular structure. |
| 44. | Flare | Sudden burst of solar energy. |
| 45. | Flux | Amount of energy per area. |
| 46. | Focal length | Distance to focus in optics. |
| 47. | Galaxy | Vast system of stars and gas. |
| 48. | Gamma rays | Highest-energy electromagnetic radiation. |
| 49. | Geocentric | Centered on Earth viewpoint. |
| 50. | Geostationary orbit | Orbit matching Earth’s rotation period. |
| 51. | Gibbous | More than half illuminated phase. |
| 52. | Globular cluster | Dense, spherical group of old stars. |
| 53. | Gravitational lensing | Gravity bends light, magnifying sources. |
| 54. | Gravity | Attractive force between masses. |
| 55. | Heliocentric | Centered on the Sun viewpoint. |
| 56. | Heliosphere | Bubble of solar wind influence. |
| 57. | Hertzsprung-Russell diagram | Star brightness versus temperature plot. |
| 58. | Hubble constant | Universe expansion rate parameter. |
| 59. | Hydrogen | Most abundant element in stars. |
| 60. | Ice giant | Large planet rich in volatiles. |
| 61. | Inclination | Tilt of orbit relative to reference. |
| 62. | Infrared | Light beyond red, heat radiation. |
| 63. | Interstellar medium | Gas and dust between stars. |
| 64. | Ionization | Removing electrons from atoms. |
| 65. | Jet | Narrow, fast outflow of matter. |
| 66. | Kepler’s laws | Rules describing planetary orbits. |
| 67. | Kuiper Belt | Icy region beyond Neptune. |
| 68. | Lagrange point | Stable gravitational balance location. |
| 69. | Light-year | Distance light travels in one year. |
| 70. | Limb darkening | Sun appears dimmer near edge. |
| 71. | Luminosity | Total energy output of an object. |
| 72. | Magnetosphere | Region controlled by magnetic field. |
| 73. | Magnitude | Logarithmic measure of brightness. |
| 74. | Main sequence | Stars fusing hydrogen in cores. |
| 75. | Mass | Amount of matter in an object. |
| 76. | Meteor | Streak from burning space debris. |
| 77. | Meteorite | Space rock that reaches ground. |
| 78. | Milky Way | Our home galaxy. |
| 79. | Molecular cloud | Cold gas where stars form. |
| 80. | Moon | Natural satellite of a planet. |
| 81. | Neutrino | Tiny particle, weakly interacting. |
| 82. | Neutron star | Collapsed stellar core, extremely dense. |
| 83. | Nova | Sudden brightening from binary system. |
| 84. | Nucleus (comet) | Solid core of a comet. |
| 85. | Occultation | One object hides another from view. |
| 86. | Oort Cloud | Distant reservoir of long-period comets. |
| 87. | Opposition | Object opposite the Sun in sky. |
| 88. | Orbit | Path of one body around another. |
| 89. | Parallax | Apparent shift from viewing position. |
| 90. | Parsec | Distance unit, about 3.26 light-years. |
| 91. | Penumbra | Partial shadow region. |
| 92. | Perihelion | Closest point to the Sun. |
| 93. | Photon | Quantum particle of light. |
| 94. | Photosphere | Visible surface layer of the Sun. |
| 95. | Planet | Large body orbiting a star. |
| 96. | Pulsar | Rapidly spinning neutron star beacon. |
| 97. | Quasar | Extremely bright active galactic nucleus. |
| 98. | Radiation | Energy emitted as waves or particles. |
| 99. | Radio telescope | Instrument observing radio waves from space. |
| 100. | Red giant | Expanded, cool, late-stage star. |