Contents
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
- Absolute Bolometeric Magnitude
- Absolute Visual Magnitude
- Absolute Zero
- Absorption Line
- Absorption Spectrum
- Acceleration
- Acceleration of Gravity
- Accretion
- Accretion Disk
- Achromatic Lens
- Active Galaxy
- Active Galaxy Nucleus (AGN)
- Active Optics
- Adaptive Optics
- Alt-Azimuth Mounting
- Angstrom
- Angular Momentum
- Annual Eclipse
- Anorthosite
- Aphelion
- Apogee
- Apparent Relative Orbit
- Apparent Visual Magnitude (mv)
- Arachnoids
- Archaeoastronomy
- Association
- Asterism
- Asteroids
- Astrometric Binary
- Astronomical Unit (AU)
- Astronomy
- Atmospheric Window
- Aurora
- Autumnal Equinox
- B Canis Majoris Variable
- Barred Spiral Galaxy
- Basalt
- Belts
- Big Bang Theory
- Binary Stars
- Binding Energy
- Bipolar Flow
- BL Lac Object
- Black Body Radiation
- Black Dwarf
- Black Hole
- Blue and Red Shifts
- Bok Globules
- Bow Shock
- Breccia
- Brown Dwarf
- Burster
- Capture Hypothesis
- Carbon Detonation
- Cassegrain Telescope
- Celestial Equator
- Celestial Sphere
- Center of Mass
- Cepheid Variable Star
- Chandrasekhar Limit
- Charge-Coupled Device CCD
- Chromatic Aberration
- Chromosphere
- Circular Velocity
- Circumpolar Constellation
- Closed Orbit
- Closed Universe
- Cluster Method
- CNO Cycle
- Co-Accretion Hypothesis
- Cocoon
- Collisional Broadening
- Color Index
- Comet
- Compact Object
- Comparative Planetology
- Condensation Sequence
- Constellations
- Continuity of Energy Law
- Continuity of Mass Law
- Continuous Spectrum
- Corona
- Coronae
- Coronagraph
- Coronal Hole
- Cosmic Ray
- Cosmological Principle
- Cosmology
- Coude Focus
- Critical Density
- Critical Point
- Dark Nebula
- Decameter Radiation
- Decimeter Radiation
- Deferent
- Degenerate Matter
- Density Wave Theory
- Diamond Ring Effect
- Differential Rotation
- Differentiation
- Diffraction Fringe
- Dirty Snowball Theory
- Disk Component
- Distance Indicator
- Distance Modulus
- Diurnal Motion
- Doppler Broadening
- Doppler Effect
- Double Galaxy Method
- Double Lobed Radio Source
- Double Stars
- Double-Exhaust Model
- Double-Line Spectroscopic Binary
- Dwarf Nova
- Dynamo Effect
- Earthlike Planets
- Eccentrics
- Eclipse Season
- Eclipse Year
- Eclipsing Binary
- Ecliptic
- Ejecta
- Electroglow
- Electromagnetic Radiation
- Electron
- Ellipse
- Elliptical Galaxy
- Emission Line
- Emission Nebula
- Emission Spectrum
- Energy Level
- Energy Machine
- Epicycle
- Equant
- Equatorial Mounting
- Ergosphere
- Escape Velocity
- Event Horizon
- Excited Atom
- Extinction
- Eyepiece
- False Color
- Field
- Filar Micrometer
- Filtergrams
- Fission Hypothesis
- Flare
- Flash Spectrum
- Flat Universe
- Flatness Problem
- Flocculent
- Focal Length
- Focus
- Forward Scattering
- Frequency
- Galactic Cannibalism
- Galactic Corona
- Galilean Satellites
- Gauss
- General Relativity
- Geocentric Universe
- Giant Molecular Cloud
- Giant Stars
- Glacial Period
- Glitch
- Globular Star Cluster
- Graben Rille
- Granulation
- Grating
- Gravitational Lens Effect
- Gravitational Red Shift
- Gravitational Wave
- Greenhouse Effect
- Grooved Terrain
- Ground State
- H II Region
- Half-Life
- Halo
- Head-Tail Radio Galaxy
- Heat
- Heat of Formation
- Heliocentric Universe
- Helioseismology
- Helium Flash
- Herbig-Haro Object
- Heterogeneous Accretion
- High-Velocity Star
- Homogeneity
- Homogeneous Accretion
- Horizontal Branch
- Horoscope
- Hot Spot
- H-R Diagram
- Hubble Constant (H)
- Hubble’s Law
- Hydrostatic Equilibrium
- Infrared Cirrus
- Infrared Outburst
- Infrared Radiation
- Instability Strip
- Intercrater Plain
- Interglacial Period
- Interstellar Absorption Lines
- Interstellar Medium
- Interstellar Reddening
- Inverse Square Law
- Io Flux Tube
- Ion
- Ionization
- Irregular Galaxy
- Isotope
- Isotropy
- Joule (J)
- Jovian Planets
- Julian Day
- Jumbled Terrain
- Kelvin Temperature Scale
- Keplerian Motion
- Kerr Black Hole
- Kiloparsec (kpc)
- Lagrangian Point
- Large-Impact Hypothesis
- Light Curve
- Light Gathering Power
- Lighthouse Theory
- Light-Year
- Limb
- Limb Darkening
- Line of Nodes
- Line Profile
- Lobate Scarp
- Local Hypothesis
- Long Period Variable
- Luminosity
- Luminosity Class
- Lyman, Balmer and Paschen Series
- Magellanic Cloud
- Magnetosphere
- Magnifying Power
- Magnitude Scale
- Main Sequence
- Mantle
- Mare
- Mass
- Mass Function
- Maunder Butterfly Diagram
- Maunder Minimum
- Megaparsec (Mpc)
- Metal Hydrogen
- Metals
- Meteor
- Meteorite
- Meteoroid
- Midocean Rift
- Midocean Rise
- Minute of Arc
- Missing Mass
- Model
- Molecule
- Momentum
- Morning and Evening Stars
- Nanometer
- Natural Motion
- Nebula
- Neutrino
- Neutron
- Neutron Star
- Newtonian Focus
- Node
- North and South Celestial Poles
- Nova
- Nuclear Bulge
- Nucleosynthesis
- Nucleus
- Objective Lens
- Objective Mirror
- Oblate Spheroid
- Oblateness
- Occultation
- Olber’s Paradox
- Opacity
- Open (Escape) Orbit
- Open Star Cluster
- Open Universe
- Optical Binary
- Oscillating Universe Theory
- Outgassing
- Ozone Layer
- Parallax
- Parsec
- Partial Eclipse (lunar or Solar)
- Path of Totality
- Penumbra
- Penumbra Eclipse
- Perigee
- Perihelion
- Period-Luminosity Diagram
- Photometer
- Photon
- Photosphere
- Planet Motion
- Planetary Nebula
- Planetisimal
- Plastic
- Plate Tectonics
- Polar Axis
- Poor Galaxy Cluster
- Population I
- Population II
- Position Angle
- Precession
- Pressure (P) Waves
- Primary Minimum
- Prime Focus
- Primeval Atmosphere
- Primordial Background Radiation
- Prolate Spheroid
- Prominence
- Proper Motion
- Proton
- Proton-Proton Chain
- Protoplanet
- Protostar
- Pulsar
- Quantum Mechanics
- Quasar
- Quasi-Periodic Object (QPO)
- Radial Velocity
- Radial Velocity Curve
- Radiation Pressure
- Radio Galaxy
- Radio Interferometer
- Ray
- Recombination
- Recurrent Nova
- Red Dwarf
- Red Shifts and Blue
- Reflecting Telescope
- Reflection Nebula
- Refracting Telescope
- Regolith
- Relative Age
- Relativistic Jet Model
- Relativistic Red Shift
- Resolving Power
- Resonance
- Retrograde loop
- Rich Galaxy Cluster
- Rift Valley
- Ring Galaxy
- Roche Limit
- Rolling Plains
- Rotation Curve
- RR Lyrae Variable
- Sagittarius A
- Saros Cycle
- Schmidt Camera
- Schmidt Cassegrain Telescope
- Schwartzchild Radius
- Scientific Notation
- Seasons
- Second of Arc
- Secondary Atmosphere
- Secondary Minimum
- Secondary Mirror
- Seeing
- Seismic Waves
- Seismograph
- Self-Sustaining Star Formation
- Semimajor Axis
- Seyfert Galaxy
- Shear (S) Waves
- Shepherd Satellite
- Shield Volcanoes
- Shock Wave
- Sidereal Drive
- Sidereal Period
- Single-Line Spectroscopic Binary
- Singularity
- Sinuous Rille
- Smooth Plain
- Solar Constant
- Solar Nebula Theory
- Solar Wind
- Special Relativity
- Spectral Class
- Spectral Sequence
- Spectrograph
- Spectroscopic Binary
- Spectroscopic Parallax
- Speed of Light
- Spherical Aberration
- Spherical Component
- Spicules
- Spiral Arm
- Spiral Galaxy
- Spiral Tracers
- Spoke
- Spring and Neap Tides
- Starburst Galaxy
- Steady State Theory
- Stellar Density Function
- Stellar Model
- Stellar Parallax
- Subsolar Point
- Summer Solstice
- Sunspot
- Super Giants
- SuperCluster
- Superconductor
- Supergranule
- Superluminal Expansion
- Supernova Remnant
- Supernova Type I
- Supernova Type II
- Synchrotron Radiation
- Synodic Period/(Synodic Month)
- T Tauri Stars
- Temperature
- Terminator
- Terrestrial Planets (Earthlike Planets)
- Tidal Coupling
- Tidal Heating
- Time Dilation
- Titius-Bode Rule
- Total Eclipse (lunar or Solar)
- Transition
- Transverse Velocity
- Triaxial Ellipsoid
- Triple Alpha Process
- True Relative Orbit
- Tuning Fork Diagram
- Turnoff Point
- Ultraviolet Radiation
- Umbra
- Uncompressed Density
- Uniform circular motion
- Universality
- Van Allen Belts
- Variable Star
- Velocity
- Velocity Dispersion Method
- Vernal Equinox
- Very Long Baseline
- Vesicular Basalt
- Violent Motion
- Visual Binary
- Wavelength
- Wavelength of Maximum
- White Dwarf Stars
- Winter Solstice
- Zeeman Effect
- Zenith
- Zero-Age Main Sequence (ZAMS)
- Zodiac
- Zone of Avoidance
- Zones
1. Absolute Bolometeric Magnitude
The Absolute Magnitude we would observe if we could detect all wavelengths[ Top of Page ]
2. Absolute Visual Magnitude
Intrinsic brightness of a star; the apparent visual magnitude the star would have if it were 10 Parsecs away from earth.[ Top of Page ]
3. Absolute Zero
The lowest possible temperature. The temperature at which particles in a material, atoms or molecules, contain no energy of motion that can be extracted from a body.[ Top of Page ]
4. Absorption Line
A dark line in a spectrum. Produced by the absence of photons absorbed by atoms or molecules.[ Top of Page ]
5. Absorption Spectrum
A Spectrum that contains absorption lines.[ Top of Page ]
6. Acceleration
A change in velocity.[ Top of Page ]
7. Acceleration of Gravity
Falling objects fall with an increasing acceleration of 9.8 meters per second per second.[ Top of Page ]
8. Accretion
The sticking together of solid particles to form a larger particle.[ Top of Page ]
9. Accretion Disk
The whirling disk of gas that forms around a compact object such as a white dwarf, neutron star or black hole as matter is drawn in.[ Top of Page ]
10. Achromatic Lens
A telescope lens composed of two lenses ground from different types of glass and designed to bring two selected colors to the same focus and correct for chromatic aberration.[ Top of Page ]
11. Active Galaxy
A galaxy that is a source of excess radiation, usually radio waves, X rays, gamma rays or some combination.[ Top of Page ]
12. Active Galaxy Nucleus (AGN)
The central energy source of an active galaxy.[ Top of Page ]
13. Active Optics
Optical elements whose position or shape is continuously controlled by computers.[ Top of Page ]
14. Adaptive Optics
Computer controlled telescope mirrors that can adjust for some changes in seeing conditions.[ Top of Page ]
15. Alt-Azimuth Mounting
A telescope mounting capable of motion parallel to and perpendicular to the horizon.[ Top of Page ]
16. Angstrom
1 x 10 -10 meters (used to measure wavelengths).[ Top of Page ]
17. Angular Momentum
Measure of the rotation of the body around some point.[ Top of Page ]
18. Annual Eclipse
A type of total eclipse in which the moon is too far from the earth to totally cover the suns surface. A ring of the photosphere surrounds the moon during mideclipse. The corona is not visible and neither are the prominence because of the glare from the photosphere.[ Top of Page ]
19. Anorthosite
Rock of aluminum and calcium silicates found in the lunar highlands.[ Top of Page ]
20. Aphelion
Point in the earth’s orbit where the earth is farthest away from the Sun (summer for us).[ Top of Page ]
21. Apogee
Orbital point of greatest distance from the earth. The Moon’s apogee is approximately 252,710 miles. See also perigee.[ Top of Page ]
22. Apparent Relative Orbit
The Orbit of one star in a visual binary with respect to the other star as seen from earth.[ Top of Page ]
23. Apparent Visual Magnitude (mv)
Is the magnitude of the stars without compensating for their distance from Earth.[ Top of Page ]
24. Arachnoids
On Venus, one of a number of round networks of fractures in the crust, resembles spider webs.[ Top of Page ]
25. Archaeoastronomy
The study of astronomy by ancient peoples.[ Top of Page ]
26. Association
H=Groups of widely scattered stars (10 – 1000) moving together through space; not gravitationally bound to clusters.[ Top of Page ]
27. Asterism
Named group for stars not identified as constellations.[ Top of Page ]
28. Asteroids
Small Rocky worlds, most of which lie between Mars and Jupiter in the asteroid belt.[ Top of Page ]
29. Astrometric Binary
A binary star identified by its irregular proper motion.[ Top of Page ]
30. Astronomical Unit (AU)
Is the average distance from the Earth to the Sun. 1 AU = 93 million miles or 1.5 X 1011 meters or 1.5 X 109 km.[ Top of Page ]
31. Astronomy
Is the study of the universe.[ Top of Page ]
32. Atmospheric Window
H=Wavelength regions in which our atmosphere is transparent at visual wavelengths, infrared and radio.[ Top of Page ]
33. Aurora
The glowing light display that results when a planet’s magnetic field guides charged particles toward the north and south magnetic poles, where they strike the upper atmosphere and excite atoms to emit photons.[ Top of Page ]
34. Autumnal Equinox
The place where the Sun crosses the celestial equator heading south.
[ Top of Page ]
35. B Canis Majoris Variable
Short Period variables stars that do not lie in the instability strip.[ Top of Page ]
36. Barred Spiral Galaxy
A spiral galaxy with an elongated nuclei resembling a bar from which the arm originate.[ Top of Page ]
37. Basalt
Dark, igneous rock characteristic of solidified lava.[ Top of Page ]
38. Belts
Dark bands of clouds that circle Jupiter parallel to its equator; generally red, brown, or blue-green; believed to be regions of descending gas.[ Top of Page ]
39. Big Bang Theory
The theory that the universe began with a violent explosion from which the expanding universe of galaxies formed.[ Top of Page ]
40. Binary Stars
Pairs of stars that orbit around their common center of mass.[ Top of Page ]
41. Binding Energy
The energy needed to pull an electron away from its atom.[ Top of Page ]
42. Bipolar Flow
Opposite directed jets of gas ejected by some protostellar objects.[ Top of Page ]
43. BL Lac Object
Objects resemble quasars; thought to highly luminous cores of distant galaxies.[ Top of Page ]
44. Black Body Radiation
Radiation emitted by a hypothetical perfect radiator. The spectrum is continuous, and the wavelength of maximum emission depends only on the body’s temperature.[ Top of Page ]
45. Black Dwarf
The end state of a white dwarf that has cooled to a low temperature.[ Top of Page ]
46. Black Hole
A mass that has collapsed to such a small volume that its gravity prevents the escape of all radiation; also, the volume of space from which the radiation may not escape.[ Top of Page ]
47. Blue and Red Shifts
Blue shifts: Shorting of wavelengths of observed light when an object in moving toward the observer. Red Shifts: Lengthening of wavelengths of light as the object moves away from the observer.[ Top of Page ]
48. Bok Globules
Small, dark clouds only about 1 light year in diameter that contain 10-1000 solar masses of gas and dust. Believed to be related to star formation.[ Top of Page ]
49. Bow Shock
The boundary between the undisturbed solar wind and the region being deflected around the planet or comet.[ Top of Page ]
50. Breccia
A rock composed of fragments of earlier rocks bonded together.[ Top of Page ]
51. Brown Dwarf
A very cool, low luminosity star whose mass is not sufficient to ignite nuclear fusion.[ Top of Page ]
52. Burster
A source of bursts of X-rays or in some cases, gamma rays; believed to be associated with neutron stars.
[ Top of Page ]
53. Capture Hypothesis
The theory that the moon formed elsewhere in the solar system and was later captured by the earth.[ Top of Page ]
54. Carbon Detonation
The explosive ignition of carbon burning in some giant stars. A possible cause of some supernova explosions.[ Top of Page ]
55. Cassegrain Telescope
A reflecting telescope in which the secondary mirror reflects light back down the tube through a hole in the center of the primary mirror .[ Top of Page ]
56. Celestial Equator
An imaginary line around the sky which is directly above the earth’s equator.[ Top of Page ]
57. Celestial Sphere
Imaginary sphere surrounding the Earth to which the stars, planets, Sun and moon seem to be attached.[ Top of Page ]
58. Center of Mass
Two bodies revolve around a common center, called the balance point of the system.[ Top of Page ]
59. Cepheid Variable Star
Variable stars with a period of 1-60 days. Their period is related to luminosity.[ Top of Page ]
60. Chandrasekhar Limit
The maximum mass of a white dwarf, about 1.4 solar masses. A white dwarf of greater mass can not support itself and will collapse.[ Top of Page ]
61. Charge-Coupled Device CCD
A CCD is an electronic device consisting of an array of photosensitive elements, used to record images.[ Top of Page ]
62. Chromatic Aberration
A distortion found in refracting telescopes because lenses focus different colors at slightly different distances. Images are consequently surrounded by color fringes.[ Top of Page ]
63. Chromosphere
A line layer of gas just above the photosphere of the sun, it is often marked by solar eruptions called solar flares and prominence.[ Top of Page ]
64. Circular Velocity
The lateral velocity an object must have to remain in orbit.[ Top of Page ]
65. Circumpolar Constellation
Constellations which appear around the celestial poles, which never seem to rise or set.[ Top of Page ]
66. Closed Orbit
A circular orbit, one which returns back on itself.[ Top of Page ]
67. Closed Universe
A model universe in which the average density is great enough to stop the expansion and make the universe contract.[ Top of Page ]
68. Cluster Method
The method of determining the masses of galaxies in a cluster.[ Top of Page ]
69. CNO Cycle
A series of nuclear reactions that use carbon as a catalyst to combine four hydrogen atoms to make one helium atom plus energy; effective in stars more massive than the sun.[ Top of Page ]
70. Co-Accretion Hypothesis
The theory that the moon and the earth formed together.[ Top of Page ]
71. Cocoon
The cloud of gas and dust around a contracting protostar that conceals it at visible wavelengths.[ Top of Page ]
72. Collisional Broadening
The smearing out of a spectrum line because of collisions among the atoms of the gas.[ Top of Page ]
73. Color Index
A numerical measure of the color of a star.[ Top of Page ]
74. Comet
One of the small, icy bodies that orbit the sun and produce tails of gas and dust when they near the sun.[ Top of Page ]
75. Compact Object
A star that has collapsed to form a white dwarf, neutron star or black hole.[ Top of Page ]
76. Comparative Planetology
The study of planets by comparing the characteristics of different examples.[ Top of Page ]
77. Condensation Sequence
The sequence in which different materials condense from the solar nebula as we move outward from the sun.[ Top of Page ]
78. Constellations
Apparent arrangement of stars, usually named after ancient gods, heroes, animals or mythological beings.[ Top of Page ]
79. Continuity of Energy Law
One of the basic laws of stellar structure, The amount of energy flowing out of the top of a shell must equal the amount coming in at the bottom plus whatever energy is generated within the shell.[ Top of Page ]
80. Continuity of Mass Law
One of the basic laws of stellar structure. The total mass of the star must equal the sum of the masses of the shells, and the mass must be distributed smoothly throughout the star.[ Top of Page ]
81. Continuous Spectrum
A spectrum in which there are no absorption or emission lines.[ Top of Page ]
82. Corona
The faint outer atmosphere of the Sun that is exposed during a total solar eclipse.[ Top of Page ]
83. Coronae
On Venus, circular features, not caused by impacts, they are domed plains caused by the rising plumes of molten rock from below.[ Top of Page ]
84. Coronagraph
A telescope designed to photograph the inner corona of the sun.[ Top of Page ]
85. Coronal Hole
An Area of the solar surface that is dark at X-ray wavelengths; thought to be associated with divergent magnetic fields and the source of the solar wind.[ Top of Page ]
86. Cosmic Ray
Atomic nuclei that enter earth’s atmosphere at nearly the speed of light. Some originate in solar flares, and some may come from supernova explosions, but their true nature is not well understood.[ Top of Page ]
87. Cosmological Principle
The assumption that any observer in any galaxy sees the same general features of the universe.[ Top of Page ]
88. Cosmology
The study of the nature, origin and evolution of the universe.[ Top of Page ]
89. Coude Focus
The focal arrangement of a reflecting telescope in which mirrors direct the light o a fixed focus beyond the bounds of the telescope’s movement. typically in a separate room, used primarily for spectroscopy.[ Top of Page ]
90. Critical Density
The average density of the universe needed to make its curvature flat.[ Top of Page ]
91. Critical Point
The temperature and pressure at which vapor and liquid phases of a material have the same density.
[ Top of Page ]
92. Dark Nebula
A nebula consisting of dust and gas blocking our view of more distant stars.[ Top of Page ]
93. Decameter Radiation
Radio signals from Jupiter with wavelengths about 10m.[ Top of Page ]
94. Decimeter Radiation
Radio signals from Jupiter with wavelengths about 0.1m.[ Top of Page ]
95. Deferent
The larger circle of which an epicycle revolves (the earth in this case).[ Top of Page ]
96. Degenerate Matter
Extremely high density matter in which pressure no longer depends on temperature due to the quantum mechanical effects.[ Top of Page ]
97. Density Wave Theory
Theory proposed to account for spiral arms as compressions of the interstellar medium in the disk of the galaxy.[ Top of Page ]
98. Diamond Ring Effect
Just as totality begins during a solar eclipse a small portion of the Suns photosphere can peak out from behind the moon through a valley at the edge of the lunar disk. It is not visible during every solar eclipse.[ Top of Page ]
99. Differential Rotation
The rotation of a body in which different parts of the body have different periods of rotation. This is true of the sun, Jovian planets, and the disk of the galaxy.[ Top of Page ]
100. Differentiation
The separation of planetary material according to density.[ Top of Page ]
101. Diffraction Fringe
Blurred fringe surrounding and image caused by wave properties of light. because of this no image detail smaller than the fringe can be seen.[ Top of Page ]
102. Dirty Snowball Theory
The hypothesis that comets are kilometer-size balls of ices with embedded impurities.[ Top of Page ]
103. Disk Component
All material confined to the plane of the galaxy.[ Top of Page ]
104. Distance Indicator
Objects whose luminosity’s or diameters are known; used to find the distance to a star cluster or galaxy.[ Top of Page ]
105. Distance Modulus
The difference between the apparent and absolute magnitude of a star. A measure of how far away the star is.[ Top of Page ]
106. Diurnal Motion
The apparent daily rotation of the sky.[ Top of Page ]
107. Doppler Broadening
The smearing of spectral lines because of the motion of atoms in the gas.[ Top of Page ]
108. Doppler Effect
A change in the wavelength of radiation due to relative radial motion of the source and the observer.[ Top of Page ]
109. Double Galaxy Method
A method of finding the masses of galaxies from orbiting pairs of galaxies.[ Top of Page ]
110. Double Lobed Radio Source
A galaxy that emits radio energy from two regions (lobes) located on opposite sides of the galaxy.[ Top of Page ]
111. Double Stars
A pair of stars close together in the sky. Not all double stars are necessarily in orbit around each other.[ Top of Page ]
112. Double-Exhaust Model
The theory that double radio lobes are produced by pairs of jets emitted in opposite directions from the centers of active galaxies.[ Top of Page ]
113. Double-Line Spectroscopic Binary
A spectroscopic binary star in which spectral lines from both stars are visible in the spectrum.[ Top of Page ]
114. Dwarf Nova
A star that undergoes novalike explosions every few days or weeks; believed to be associated with mass transfer onto a white dwarf in a binary system.[ Top of Page ]
115. Dynamo Effect
The theory that the earth’s magnetic field is generated in the conducting material of its molten core.
[ Top of Page ]
116. Earthlike Planets
See Terrestrial Planets.[ Top of Page ]
117. Eccentrics
An offcenter circular path.[ Top of Page ]
118. Eclipse Season
Is the season when the Sun is close enough to a node for an eclipse to occur, An eclipse season is 32 days. Any new moon during this period will cause a solar eclipse. For Lunar eclipses the period is shorter only about 22 days. A full moon occurring during this time will cause a lunar eclipse.[ Top of Page ]
119. Eclipse Year
The 346.62 days it takes the sun to return to a node.[ Top of Page ]
120. Eclipsing Binary
A binary star system in which the stars eclipse each other.[ Top of Page ]
121. Ecliptic
The apparent path of the Sun around the Sky.[ Top of Page ]
122. Ejecta
Pulverized rock scattered by meteorite impacts on a planetary surface.[ Top of Page ]
123. Electroglow
The UV radiation produced in the upper atmosphere of Jupiter, Saturn, Uranus by high-energy particles in the planet’s magnetosphere.[ Top of Page ]
124. Electromagnetic Radiation
H=Changing electric and magnetic fields that travel through space and transfer energy from one place to another light, radio waves and magnetism.[ Top of Page ]
125. Electron
A low mass atomic particles carrying a negative charge.[ Top of Page ]
126. Ellipse
A closed curve enclosing two points (foci) such that the total distance from one focus to any other point on the curve back to the other focus equals a constant.[ Top of Page ]
127. Elliptical Galaxy
A galaxy that is round or elliptical in outline. It contains little gas and dust, no disk or spiral arms, and a few hot, bright stars.[ Top of Page ]
128. Emission Line
A bright line in a spectrum caused by the emission of photons from atoms.[ Top of Page ]
129. Emission Nebula
A cloud of gas excited by UV wavelengths of hot stars.[ Top of Page ]
130. Emission Spectrum
A spectrum containing emission lines.[ Top of Page ]
131. Energy Level
One of a number of states an electron may occupy in an atom, depending on its binding energy.[ Top of Page ]
132. Energy Machine
An object that releases energy. Commonly used to refer to the source of energy in active galactic nuclei.[ Top of Page ]
133. Epicycle
An attempt to explain the retrograde loop in the earth centered universe, by attaching the planets to epicycles and having them revolve around that, and in turn around the earth.[ Top of Page ]
134. Equant
Ptolemy placed the earth off center and the opposite point from the deferent on this off center circle is called the equant.[ Top of Page ]
135. Equatorial Mounting
A telescope mounting that allows motion parallel to and perpendicular to the celestial equator.[ Top of Page ]
136. Ergosphere
The region surrounding a rotating black hole within one could not resist being dragged around the black hole. It is possible for a particle to escape from the erosphere and extract energy from the black hole.[ Top of Page ]
137. Escape Velocity
The velocity needed to escape from the surface of a body.[ Top of Page ]
138. Event Horizon
The boundary of the region of a black hole from which no radiation may escape. No event that occurs with the event horizon is visible to a distant observer.[ Top of Page ]
139. Excited Atom
An atom in which an electron has moved from a lower to higher orbit.[ Top of Page ]
140. Extinction
The dimming of light by intervening material; commonly, dimming by the interstellar medium.[ Top of Page ]
141. Eyepiece
A short focal length lens used to enlarge the image in a telescope; the lens nearest the eye.
[ Top of Page ]
142. False Color
A graphical representation of data in which an images is colored to reveal additional detail[ Top of Page ]
143. Field
A way of explaining action at a distance.[ Top of Page ]
144. Filar Micrometer
An instrument that permits precise measurements at the telescope of the position of visual binary stars and similar objects.[ Top of Page ]
145. Filtergrams
H=A photograph (usually of the sun) taken in the light of a specific region of the spectrum e.g., an H-alpha filtergram.[ Top of Page ]
146. Fission Hypothesis
The theory that the moon formed by breaking away from the earth.[ Top of Page ]
147. Flare
A violent eruption on the sun’s surface.[ Top of Page ]
148. Flash Spectrum
The emission spectrum of the chromosphere that is visible for the few seconds during a total solar eclipse when the moon has covered the photosphere but has not covered the chromosphere.[ Top of Page ]
149. Flat Universe
A model of the universe in which space-time is not curved.[ Top of Page ]
150. Flatness Problem
In cosmology the circumstance that the early universe must have contained almost exactly the right amount of matter to close the space-time (to make space-time flat).[ Top of Page ]
151. Flocculent
Woolly, fluffy; used to refer to certain galaxies that have a woolly appearance.[ Top of Page ]
152. Focal Length
A distance from a lens to a point where it focuses parallel rays of light.[ Top of Page ]
153. Focus
The points around which an ellipse is drawn.[ Top of Page ]
154. Forward Scattering
The optical property of finely divided particles to preferentially direct light in the original direction of the light’s travel.[ Top of Page ]
155. Frequency
The number of times a given event occurs in a given time; for a wave the number of cycles that pass the observer in one second.
[ Top of Page ]
156. Galactic Cannibalism
The theory that large galaxies absorb smaller galaxies.[ Top of Page ]
157. Galactic Corona
The low-density extensions of the halo of a galaxy; now suspected to extend many times the visible diameter of the galaxy.[ Top of Page ]
158. Galilean Satellites
The four largest moons of Jupiter, named after their discoverer Galileo.[ Top of Page ]
159. Gauss
A unit used to measure the strength of a magnetic field.[ Top of Page ]
160. General Relativity
Observers can not distinguish locally between inertial forces due to acceleration and uniform gravitational forces due to the presence of a massive body. Mass tells space-time how to curve, and the curvature of space-time (gravity) tells mass how to accelerate.[ Top of Page ]
161. Geocentric Universe
Aristotle believed the universe was divided into two parts, the earth corrupt and the heavens perfect and immutable. The geocentric universe described the universe with the earth at the center.[ Top of Page ]
162. Giant Molecular Cloud
Very large, cool clouds of dense gas in which stars form.[ Top of Page ]
163. Giant Stars
Large, cool, highly luminous stars in the upper right of the H-R diagram. Typically 10-100 times the diameter of the sun.[ Top of Page ]
164. Glacial Period
Time when glaciers advance and engulf huge sheets of land.[ Top of Page ]
165. Glitch
A sudden change in the period of a pulsar.[ Top of Page ]
166. Globular Star Cluster
A star cluster containing 50,000 to 1 million stars in a sphere about 75 light years in diameter; generally old metal poor and found in the spherical component of the galaxy.[ Top of Page ]
167. Graben Rille
A linear feature on a planetary surface caused by the faulting and sinking of portions of the crust.[ Top of Page ]
168. Granulation
The fine structure visible on the solar surface caused by rising currents of hot gas and sinking currents of cool gas below the surface.[ Top of Page ]
169. Grating
Material onto which microscopic parallel lines are inscribed. used to create a spectrum of colors from light.[ Top of Page ]
170. Gravitational Lens Effect
The focusing of light from a distant galaxy or quasar by an intervening galaxy to produce multiple images of the distant body.[ Top of Page ]
171. Gravitational Red Shift
A lengthening of the wavelength of a photon due to its escape from a gravitational field.[ Top of Page ]
172. Gravitational Wave
A transport of energy by the motion of waves in a gravitational field; predicted by general relativity.[ Top of Page ]
173. Greenhouse Effect
The process by which a carbon dioxide atmosphere traps heat and raises the temperature of a planetary surface.[ Top of Page ]
174. Grooved Terrain
Regions of the surface of Ganymede consisting of parallel grooves; believed to have formed by repeated fracture and re-freezing of the icy crust.[ Top of Page ]
175. Ground State
The lowest permitted orbit of an electron in an atom.
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176. H II Region
A region of ionized hydrogen around a hot star.[ Top of Page ]
177. Half-Life
The time required for half of the atoms in a radioactive sample to decay.[ Top of Page ]
178. Halo
The spherical region of a spiral galaxy containing a thin scattering of stars, star clusters, and small amounts of gas.[ Top of Page ]
179. Head-Tail Radio Galaxy
A radio galaxy with a contour consisting of a head and a tail; believed caused by the motion of an active galaxy through the intergalactic medium.[ Top of Page ]
180. Heat
Thermal energy present in a body as agitation (motion) among its particles (atoms or molecules).[ Top of Page ]
181. Heat of Formation
In planetology, the heat released by the infall of matter during the formation of a planetary body.[ Top of Page ]
182. Heliocentric Universe
The Sun is at the center of the universe and everything revolves around that. Was first proposed by Copernicus.[ Top of Page ]
183. Helioseismology
The study of the interior of the sun by the analysis of its modes of vibration.[ Top of Page ]
184. Helium Flash
The explosive ignition of helium burning that takes place in some giant stars.[ Top of Page ]
185. Herbig-Haro Object
Small nebula that vary irregularly in brightness; believed associated with star formation.[ Top of Page ]
186. Heterogeneous Accretion
H=The formation of a planet by the accumulation of planetisimals of different composition, e.g., first iron particles, then silicates.[ Top of Page ]
187. High-Velocity Star
A star with a large space velocity. Such stars are halo stars passing through the disk of the galaxy at steep angles.[ Top of Page ]
188. Homogeneity
The assumption that, on the large-scale, matter is uniformly spread throughout the universe.[ Top of Page ]
189. Homogeneous Accretion
The formation of a planet by the accumulation of planetisimals of the same composition.[ Top of Page ]
190. Horizontal Branch
In the H-R diagram of a globular cluster, the sequence of stars extending from the red giants toward the blue side of the diagram; includes RR Lyrae stars.[ Top of Page ]
191. Horoscope
A chart showing the location of heavenly bodies among the zodiacal signs and with respect to the horizon at the persons birth.[ Top of Page ]
192. Hot Spot
In radio astronomy, a bright spot in a radio lobe.[ Top of Page ]
193. H-R Diagram
A plot of the intrinsic brightness versus the surface temperature of the stars. It separates the effects of temperature and surface area versus spectral type, but also luminosity versus surface temperature or color.[ Top of Page ]
194. Hubble Constant (H)
A measure of the rate of expansion of the universe; the average value of velocity of recession divided by distance.[ Top of Page ]
195. Hubble’s Law
The linear relation between the distance to a galaxy and its radial velocity.[ Top of Page ]
196. Hydrostatic Equilibrium
The balance between weight of the material pressing downward on a layer in a star and the pressure in that layer.
[ Top of Page ]
197. Infrared Cirrus
A fine network of filaments covering the sky detected in the far infrared by the IRAS satellite; believed associated with dust in the interstellar medium.[ Top of Page ]
198. Infrared Outburst
A sudden brightening of an object at infrared wavelengths.[ Top of Page ]
199. Infrared Radiation
Electromagnetic Radiation with wavelengths intermediate between visible light and radio waves.[ Top of Page ]
200. Instability Strip
The region of the H-R diagram in which stars are unstable to pulsation. A star passing through this strip becomes a variable star.[ Top of Page ]
201. Intercrater Plain
The relatively smooth terrain on Mercury.[ Top of Page ]
202. Interglacial Period
Time when glaciers melt back (cycles are roughly 40,000 years).[ Top of Page ]
203. Interstellar Absorption Lines
Dark lines in some stellar spectra that are formed by the interstellar gas.[ Top of Page ]
204. Interstellar Medium
The gas and dust distributed between the stars.[ Top of Page ]
205. Interstellar Reddening
The process in which dust scatters blue light out of starlight and makes the stars look redder than they actually are.[ Top of Page ]
206. Inverse Square Law
Force of gravity decreases as the square of the distance increases.[ Top of Page ]
207. Io Flux Tube
A tube of magnetic lines and electric currents connecting Io and Jupiter.[ Top of Page ]
208. Ion
An atom that has lost or gained one or more electrons.[ Top of Page ]
209. Ionization
The process in which atoms lose or gain electrons.[ Top of Page ]
210. Irregular Galaxy
A galaxy with a chaotic appearance, large clouds of gas and dust, and both population I and population II stars, but without spiral arms.[ Top of Page ]
211. Isotope
Atoms that have the same number of protons but a different number of neutrons.[ Top of Page ]
212. Isotropy
The assumption that in its general properties the universe looks the same in every direction.
[ Top of Page ]
213. Joule (J)
A unit of energy roughly equivalent to the energy given up when an apple falls on the floor. Equivalent to the force of 1 Newton acting over a distance of 1 meter; 1 joule per second = 1 watt of power.[ Top of Page ]
214. Jovian Planets
Jupiterlike planets with large diameters and low densities.[ Top of Page ]
215. Julian Day
The Julian Day is the number of days since the year -4712. The Julian Day begins at 12:00 Noon Greenwich mean time.[ Top of Page ]
216. Jumbled Terrain
Strangely disturbed regions of the moon opposite the locations of the Imbrium basin and Mare Oriental.
[ Top of Page ]
217. Kelvin Temperature Scale
The temperature, in Celsius (Centigrade) degrees, measured above absolute zero.[ Top of Page ]
218. Keplerian Motion
Orbital motion in accord with Kepler’s laws of planetary motion.[ Top of Page ]
219. Kerr Black Hole
A solution to the equations of general relativity that describes the properties of a rotating black hole.[ Top of Page ]
220. Kiloparsec (kpc)
A unit of distance equal to 1000 pc or 3260 ly.
Kirchoff’s Laws -A set of laws that describes the Absorption and emission of light by matter.
[ Top of Page ]
221. Lagrangian Point
Points of stability in the orbital plane of a binary system, planet, or moon. One is located 60 degrees ahead and one 60 degrees behind the orbiting bodies. Another is located between the orbiting bodies.[ Top of Page ]
222. Large-Impact Hypothesis
The theory that the moon formed from debris ejected during a collision between the earth and a large planetisimal.[ Top of Page ]
223. Light Curve
A graph of brightness versus time commonly used in analyzing variable stars and eclipsing binaries.[ Top of Page ]
224. Light Gathering Power
The ability of a telescope to collect light. Proportional to the are of the telescope objective lens or mirror.[ Top of Page ]
225. Lighthouse Theory
A theory that a neutron star produces pulses of radiation by sweeping radio beams around the sky as it rotates.[ Top of Page ]
226. Light-Year
Is the distance that light travels in one year. Abbreviation is ly.[ Top of Page ]
227. Limb
The edge of the apparent disk of a body, as in the “limb of the moon”.[ Top of Page ]
228. Limb Darkening
The decrease in the brightness of the sun or other body from its center to its limb.[ Top of Page ]
229. Line of Nodes
The nodes of the moons orbit are the points where it passes through the plane of the earth’s orbit. An eclipse season occurs whenever the line connecting these nodes (line of nodes) points toward the sun.[ Top of Page ]
230. Line Profile
A graph of light intensity verses wavelength showing the shape of an absorption line Liquid.[ Top of Page ]
231. Lobate Scarp
A curved cliff such as those found on Mercury.[ Top of Page ]
232. Local Hypothesis
The theory that quasars are not at great distances but relatively nearby.[ Top of Page ]
233. Long Period Variable
A variable star with a period ranging from 100 days to over 400 days.
Look Back Time The amount by which we look into the past when we look at a distant galaxy; a time equal to the distance to the galaxy in light-years.
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234. Luminosity
The total amount of energy a star radiates in one second.[ Top of Page ]
235. Luminosity Class
A category of stars of similar luminosity; determined by the widths of lines in their spectra.[ Top of Page ]
236. Lyman, Balmer and Paschen Series
Spectral lines in the UV spectrum of hydrogen produced by transitions whose lowest orbit of electrons is at ground state (Lyman). Spectral lines in the visible and near UV of hydrogen produced by transitions whose lowest orbit is second (Balmer). Spectral lines in the infrared spectrum of hydrogen produced by transitions whose lowest orbit is the third. (Paschen).
[ Top of Page ]
237. Magellanic Cloud
Small irregular galaxies that are companions to the Milky Way; visible in the southern sky.[ Top of Page ]
238. Magnetosphere
The volume of space around a planet within which the motion of charged particles is dominated by the planetary magnetic field rather than the solar wind.[ Top of Page ]
239. Magnifying Power
The ability of a telescope to make an image larger.[ Top of Page ]
240. Magnitude Scale
Method developed by Hipparchus who divided the stars into 6 classes. The brightest stars are first class and those slightly fainter are second class. The sixth class is the faintest stars that he cold see with the unaided eye. The magnitude scale is logarithmic like the eye.[ Top of Page ]
241. Main Sequence
The region of the H-R diagram running from upper left to lower right, which includes roughly 90 percent of all stars.[ Top of Page ]
242. Mantle
The layer of dense rock and metal oxides that lies between the molten core and the surface of the earth; also, similar layers in other planets.[ Top of Page ]
243. Mare
One of the lunar lowlands filled by successive flows of dark lava.[ Top of Page ]
244. Mass
A measure of the amount of matter in an object.[ Top of Page ]
245. Mass Function
A measure of the ratio of the masses in a single-line spectroscopic binary. Also includes the inclination, which is unknown for some systems.[ Top of Page ]
246. Maunder Butterfly Diagram
A graph showing the latitude of sunspots versus time, first plotted by W.W. Maunder in 1904.[ Top of Page ]
247. Maunder Minimum
A period of less numerous sunspots and other solar activity from 1645-1715.[ Top of Page ]
248. Megaparsec (Mpc)
A unit of distance equal to 1 million pc.[ Top of Page ]
249. Metal Hydrogen
A form of hydrogen under high pressure that is a good electrical conductor.[ Top of Page ]
250. Metals
In astronomical usage, all atoms heavier than helium.[ Top of Page ]
251. Meteor
A small bit of matter heated by friction to incandescent vapor as it falls into the atmosphere.[ Top of Page ]
252. Meteorite
A meteor that has survived its passage through the atmosphere and strikes the ground.[ Top of Page ]
253. Meteoroid
A meteor in space before it enters the earth’s atmosphere.[ Top of Page ]
254. Midocean Rift
Chasms that split the midocean rises where the crustal plates move apart.[ Top of Page ]
255. Midocean Rise
One of the undersea mountain ranges that push up from the seafloor in the center of the oceans.[ Top of Page ]
256. Minute of Arc
A measurement of the sky which includes degrees, minutes and seconds.. There are 60 minutes of arc in one degree.[ Top of Page ]
257. Missing Mass
Unobserved mass in clusters of galaxies believed to provide sufficient gravity to bind the cluster together.[ Top of Page ]
258. Model
An intellectual concept of how nature works.[ Top of Page ]
259. Molecule
Two or more atoms bonded together.[ Top of Page ]
260. Momentum
The measurement of the amount of motion. the product of mass and velocity.[ Top of Page ]
261. Morning and Evening Stars
A planet visible in the Morning shortly before sunrise is a morning star, a planet visible just before sunset is the Evening Star.
[ Top of Page ]
262. Nanometer
(nm) 1 x 10-9 meters.[ Top of Page ]
263. Natural Motion
Objects move toward their proper place. earth and water downward, fire and air upward.[ Top of Page ]
264. Nebula
A cloud of gas and dust in space.[ Top of Page ]
265. Neutrino
A neutral massless atomic particle that travels at the speed of light.[ Top of Page ]
266. Neutron
An atomic particles with no charge and about the same mass as a proton.[ Top of Page ]
267. Neutron Star
A small highly dense star composed almost entirely of tightly packed neutrons; radius about 10 km.[ Top of Page ]
268. Newtonian Focus
A focal arrangement of a reflecting telescope in which a diagonal mirror reflects light out the side of the telescope for easier access.[ Top of Page ]
269. Node
The point twice a month where the moon crosses the ecliptic. Once a month the moon crosses heading north and two week later crosses again heading south.[ Top of Page ]
270. North and South Celestial Poles
The north ands south pivots points around which the sky appears to rotate.[ Top of Page ]
271. Nova
From Latin meaning “new”; a sudden brightening of a star, making it appear as a new star in the sky; believed associated with eruptions on white dwarfs in binary systems.[ Top of Page ]
272. Nuclear Bulge
The spherical cloud of stars that lies at the center of spiral galaxies.[ Top of Page ]
273. Nucleosynthesis
The production of elements heavier than helium by the fusion of atomic nuclei in stars and during supernova explosions.[ Top of Page ]
274. Nucleus
The central core of an atom, containing protons and neutrons; carries a net positive charge.
[ Top of Page ]
275. Objective Lens
In a refracting telescope, the long focal length lens that forms an image of the object viewed; the lens closest to the object.[ Top of Page ]
276. Objective Mirror
In a reflecting telescope the principle mirror (reflecting surface) that forms an image of the object viewed.[ Top of Page ]
277. Oblate Spheroid
A sphere flattened such that its polar diameter is smaller than its equatorial diameter.[ Top of Page ]
278. Oblateness
The flattening of a spherical body; usually caused by rotation.[ Top of Page ]
279. Occultation
The passage of a larger body in front of a smaller body.[ Top of Page ]
280. Olber’s Paradox
The conflict between observation and theory as to why the night sky should or should not be dark.[ Top of Page ]
281. Opacity
The resistance of a gas to the passage of radiation.[ Top of Page ]
282. Open (Escape) Orbit
An orbit which leads away from the central body, never to return.[ Top of Page ]
283. Open Star Cluster
A cluster of 10 to 10,000 stars with an open, transparent appearance. The stars are not tightly grouped. Usually relatively young and located in the disk of the galaxy.[ Top of Page ]
284. Open Universe
A model universe in which the average density is less than the critical density needed to halt the expansion of the universe.[ Top of Page ]
285. Optical Binary
A binary star in which the stars are only apparently associated. One star is nearby and one is more distant.[ Top of Page ]
286. Oscillating Universe Theory
The theory that the universe begins with a big bang, expands, slows by its own gravity, collapses to create another big bang.[ Top of Page ]
287. Outgassing
The release of gas from a planets interior.[ Top of Page ]
288. Ozone Layer
In earth’s atmosphere, the layer of oxygen ions (O3) lying 15 to 30 km high that protects the surface by absorbing ultraviolet rays.
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289. Parallax
The apparent change in the position of an object due to a change in the location of the observer. It was because ancient astronomers did not observe parallax that they though the earth was the center of the universe and everything in the heavens revolved around it.[ Top of Page ]
290. Parsec
A hypothetical distance to a star whose parallax is one arcsecond; 1pc = 206,265 A.U. = 3.26 ly.[ Top of Page ]
291. Partial Eclipse (lunar or Solar)
Partial eclipses are caused when the moon passes through only part of the umbra shadow of the earth. Or when the earth passes through the only part of the umbra shadow of the moon.[ Top of Page ]
292. Path of Totality
The path of a total eclipse that is swept out by the umbra shadow of the moon on the earth.[ Top of Page ]
293. Penumbra
A Partial blocking of the Sun by the Earth creates an Penumbra Shadow. The Sunlight is dimmed but not extinguished.[ Top of Page ]
294. Penumbra Eclipse
When the moon passes only through the penumbra shadow of the earth or when the earth passes through only the penumbra shadow of the moon.[ Top of Page ]
295. Perigee
Point of closet approach to the earth. The Moon’s perigee is approximately 221,463 miles. See also apogee.[ Top of Page ]
296. Perihelion
Point in earth’s orbit around the sun where it is closest to the Sun (winter for us).[ Top of Page ]
297. Period-Luminosity Diagram
A graph showing the relation-between period of pulsation and intrinsic brightness among Cepheid variable stars.[ Top of Page ]
298. Photometer
A device used to measure the intensity and color of light.[ Top of Page ]
299. Photon
A quantum of electromagnetic energy Carries an amount of energy that depends inversely on its wavelength.[ Top of Page ]
300. Photosphere
The bright disk of the sun that is covered completely by a total Solar eclipse.[ Top of Page ]
301. Planet Motion
Always found near the ecliptic (except Pluto).[ Top of Page ]
302. Planetary Nebula
An expanding shell of gas ejected by a star in the latter stages of its evolution.[ Top of Page ]
303. Planetisimal
One of the small bodies that formed from the solar nebula and eventually grew into protoplanets.[ Top of Page ]
304. Plastic
A material with properties of a solid but capable of flowing under pressure.[ Top of Page ]
305. Plate Tectonics
The constant destruction and renewal of the earth’s surface by the motions of sections of the crust.[ Top of Page ]
306. Polar Axis
The axis of a telescope around which the celestial sphere rotates.[ Top of Page ]
307. Poor Galaxy Cluster
An irregularly shaped cluster that contains fewer than 1000 galaxies, many spiral, and no giant ellipticals.[ Top of Page ]
308. Population I
Stars rich in atoms heavier than helium; nearly always relatively young stars found in the disk of the galaxy.[ Top of Page ]
309. Population II
Stars poor in atoms heavier than helium; nearly always relatively old stars found in the halo, globular clusters, or the nuclear bulge.[ Top of Page ]
310. Position Angle
The angular direction of one body with respect to another; measured from north toward the east; typically used in the study of visual binaries.[ Top of Page ]
311. Precession
A wobbling of the Earth’s Axis. It takes 26,000 years for the earth to complete one wobble.[ Top of Page ]
312. Pressure (P) Waves
In geophysics, mechanical waves of compression and rarefaction that travel through the earth’s interior.[ Top of Page ]
313. Primary Minimum
In the light curve of an eclipsing binary, the deeper eclipse.[ Top of Page ]
314. Prime Focus
The point at which an object mirror forms an image in a reflecting telescope.[ Top of Page ]
315. Primeval Atmosphere
Earth’s first air, composed of gases from the solar nebula.[ Top of Page ]
316. Primordial Background Radiation
Radiation from the hot clouds of the big-bang explosion. Because of its large red shift it appears to come from a body whose temperature is only 2.7K.[ Top of Page ]
317. Prolate Spheroid
A sphere stretched along its polar axis so its polar diameter is greater than its equatorial diameter.[ Top of Page ]
318. Prominence
Eruptions of the solar surface. Visible during total solar eclipses.[ Top of Page ]
319. Proper Motion
The rate at which a star moves across the sky. Measured in arc seconds per year.[ Top of Page ]
320. Proton
A positively charged atomic particle contained in the nucleus of an atom. The nucleus of hydrogen atom.[ Top of Page ]
321. Proton-Proton Chain
A series of three nuclear reactions that build a helium atom by adding together protons. The main energy source in our sun.[ Top of Page ]
322. Protoplanet
Massive object resulting from the coalescence of planetisimals in the solar nebula and destined to become a planet.[ Top of Page ]
323. Protostar
A collapsing cloud of gas and dust destined to become a star.[ Top of Page ]
324. Pulsar
A source of short, precisely times radio bursts believed to be spinning neutron stars.
[ Top of Page ]
325. Quantum Mechanics
The study of behavior of atoms and atomic particles.[ Top of Page ]
326. Quasar
Small powerful source of energy believed to be the active core of very distant galaxies.[ Top of Page ]
327. Quasi-Periodic Object (QPO)
Certain X-rays sources that “flicker” rapidly for short intervals.
[ Top of Page ]
328. Radial Velocity
(Vr) That component of an object’s velocity directed away from or toward the earth.[ Top of Page ]
329. Radial Velocity Curve
A graph of the velocity of recession or approach of the stars in a spectroscopic binary.[ Top of Page ]
330. Radiation Pressure
The force exerted on the surface of a body by its absorption of light. Small particles floating in the solar system can be blown outward by the pressure of sunlight.[ Top of Page ]
331. Radio Galaxy
A galaxy that is strong source of radio signals.[ Top of Page ]
332. Radio Interferometer
Two or more radio telescopes that combine their signals to achieve the resolving power of a larger telescope.[ Top of Page ]
333. Ray
Ejecta from meteorite impacts forming white streamers radiating from some lunar craters.[ Top of Page ]
334. Recombination
The stage within 1 million years of the big bang when the gas became transparent to radiation.[ Top of Page ]
335. Recurrent Nova
Stars that erupt as nova every few dozen years.[ Top of Page ]
336. Red Dwarf
Cool, low mass stars on the lower main sequence.[ Top of Page ]
337. Red Shifts and Blue
Red Shifts: Lengthening of wavelengths of light as the object moves away from the observer. Blue shifts: Shorting of wavelengths of observed light when an object in moving toward the observer.[ Top of Page ]
338. Reflecting Telescope
A telescope which uses a concave mirror to focus light into an image.[ Top of Page ]
339. Reflection Nebula
A nebula produced by starlight reflecting off dust particles in the interstellar medium.[ Top of Page ]
340. Refracting Telescope
A telescope that forms images by bending light through an objective lens.[ Top of Page ]
341. Regolith
A soil made up of crushed rock fragments.[ Top of Page ]
342. Relative Age
The age of a geological feature referred to other features. For example, relative ages tell us the lunar maria and younger than the highlands.[ Top of Page ]
343. Relativistic Jet Model
An explanation of superluminal expansion based on a high velocity jet from a quasar directed approximately toward the earth.[ Top of Page ]
344. Relativistic Red Shift
The red shift due to Doppler effect for objects traveling near the speed of light.[ Top of Page ]
345. Resolving Power
The ability of a telescope to reveal fine detail. depends on the diameter of the telescope objective.[ Top of Page ]
346. Resonance
The coincidental agreement between two periodic phenomena; commonly applied to agreements between orbital periods, which can make orbits more or less stable.[ Top of Page ]
347. Retrograde loop
The planets sometimes speed up in their movements, slow down, stop and even reverse direction. The motion traces out a retrograde loop.[ Top of Page ]
348. Rich Galaxy Cluster
A cluster containing over 1000 galaxies, mostly elliptical, scattered over a volume about 3 Mpc in diameter.[ Top of Page ]
349. Rift Valley
A long, straight, deep valley produced By the separation of crustal plates.[ Top of Page ]
350. Ring Galaxy
A galaxy that resembles a ring around a bright nucleus; believed to be the result of a head-on collision of two galaxies.[ Top of Page ]
351. Roche Limit
The minimum distance between a planet and a satellite that holds itself together by its own gravity. If a satellites orbit brings it inside the Roche limit, tidal forces will break the satellite up.[ Top of Page ]
352. Rolling Plains
The most common type of terrain on Venus.[ Top of Page ]
353. Rotation Curve
A graph of orbital velocity versus radius in the disk of a galaxy.[ Top of Page ]
354. RR Lyrae Variable
Variable stars with periods of 12-24 hours, common in some globular clusters.
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355. Sagittarius A
The powerful radio source located at the core of the Milky Way.[ Top of Page ]
356. Saros Cycle
After 18 years and 11 1/3 days the eclipse pattern repeats The saros cycle does not mean the eclipse will occur in the same place. Sine the cycle take one third of a day. The earth will have rotated 8 hours westward from the original location. It takes 3 saros cycles for the eclipse to repeat in the exact same place or 54 years and 1 month.[ Top of Page ]
357. Schmidt Camera
A photographic telescope that take wide angle photographs.[ Top of Page ]
358. Schmidt Cassegrain Telescope
A Cassegrain telescope that uses a thin correcting lens as in a Schmidt camera.[ Top of Page ]
359. Schwartzchild Radius
The radius of the event horizon around a black hole.[ Top of Page ]
360. Scientific Notation
A method of writing large numbers in a simple way. For example 380,000 is written 3.8 x106. In scientific notation a number is always written as a x 10 h Where a >= 1 and a <10.[ Top of Page ]
361. Seasons
Caused by the tilt of the earth’s Axis 23.5 degrees.[ Top of Page ]
362. Second of Arc
See Minute of Arc. 60 seconds of arc make up one minute.[ Top of Page ]
363. Secondary Atmosphere
The gases outgassed from a planet’s interior; rich in carbon dioxide.[ Top of Page ]
364. Secondary Minimum
In the light curve of an eclipsing binary, the shallower eclipse.[ Top of Page ]
365. Secondary Mirror
In a reflecting telescope, the mirror that reflects the image to a point for easy observation.[ Top of Page ]
366. Seeing
Atmospheric conditions on a given night When the atmosphere is unsteady, producing blurred images the seeing is considered poor.[ Top of Page ]
367. Seismic Waves
A mechanical vibration that travels through the earth. Usually caused by an earthquake.[ Top of Page ]
368. Seismograph
An instrument that records seismic waves.[ Top of Page ]
369. Self-Sustaining Star Formation
The process by which the birth of stars compress the surrounding gas clouds and triggers the formation of more stars, proposed to explain spiral arms.[ Top of Page ]
370. Semimajor Axis
Half the long diameter of an ellipse.[ Top of Page ]
371. Seyfert Galaxy
An otherwise normal spiral galaxy with an unusually bright, small core that fluctuates in brightness; believed to indicate the core is erupting.[ Top of Page ]
372. Shear (S) Waves
Mechanical waves that travel through earth’s interior by the vibration of particles perpendicular to the direction of the wave travel.[ Top of Page ]
373. Shepherd Satellite
A satellite that, by its gravitational field, confines particles to a planetary ring.[ Top of Page ]
374. Shield Volcanoes
Wide, low profile volcanic cones produced by highly liquid lava.[ Top of Page ]
375. Shock Wave
A sudden change in pressure that travels as an intense sound wave.[ Top of Page ]
376. Sidereal Drive
The motion and gears on a telescope that turn westward to keep it pointing at a star.[ Top of Page ]
377. Sidereal Period/Siderial Month
27.5 Days. Actual Lunar orbit around the earth. 1 revolution with respect to the stars. Moon drifts eastward by 13 degrees per day.[ Top of Page ]
378. Single-Line Spectroscopic Binary
A spectroscopic binary in which lines of one star are visible in the spectrum.[ Top of Page ]
379. Singularity
The object of zero radius into which the matter of a black hole is believed to fall.[ Top of Page ]
380. Sinuous Rille
A narrow, winding valley on the moon caused by ancient lava flows along narrow channels.[ Top of Page ]
381. Smooth Plain
Apparently young plains on Mercury formed by lava flows at or soon after the formation of the Caloris Basin.[ Top of Page ]
382. Solar Constant
A measure of the energy output of the sun. The total solar energy striking 1 sq. meter just above the earth’s atmosphere in 1 second.[ Top of Page ]
383. Solar Nebula Theory
The theory that the planets formed from the same cloud of gas and dust that formed the sun.[ Top of Page ]
384. Solar Wind
Rapidly moving atoms and ions that escape from the solar corona and blow outward through the solar system.[ Top of Page ]
385. Special Relativity
Observers can never detect their uniform motion except relative to other objects. The velocity of light is constant for all observers.[ Top of Page ]
386. Spectral Class
A star’s position in the temperature classification system O,B,A,F,G,K, and M. Based on the appearance of the star’s spectrum.[ Top of Page ]
387. Spectral Sequence
The arrangement of spectral classes (O,B,A,F,G,K, and M) ranging from hot to cool stars.[ Top of Page ]
388. Spectrograph
Device that separates light by wavelengths to produce a spectrum.[ Top of Page ]
389. Spectroscopic Binary
A star system in which the stars are too close together to be visible separately. We see in a single point of light, and only by taking the spectrum can we determine that there are two stars.[ Top of Page ]
390. Spectroscopic Parallax
A method of determining a star’s distance by comparing its apparent magnitude with its absolute magnitude as estimated from its spectrum.[ Top of Page ]
391. Speed of Light
186,000 miles per second or 300,000 km per second or 3 X 10E8 meters per second.[ Top of Page ]
392. Spherical Aberration
An image distortion caused by a telescope mirror failing to be exactly parabolic.[ Top of Page ]
393. Spherical Component
The part of the galaxy including all matter in a spherical distribution around the center (halo and nuclear bulge).[ Top of Page ]
394. Spicules
Small, flame like projections in the chromosphere of the sun.[ Top of Page ]
395. Spiral Arm
Long spiral patterns of bright stars, star clusters, gas and dust that extend from the center to the edge of the disk of spiral galaxies.[ Top of Page ]
396. Spiral Galaxy
A galaxy with an obvious disk component containing gas; dust; hot, bright stars; and spiral arms.[ Top of Page ]
397. Spiral Tracers
Objects used to map the spiral arms (e.g. O and B associations, open clusters, clouds of ionized hydrogen, and some types of variable stars).[ Top of Page ]
398. Spoke
A radial feature in the rings of Saturn.[ Top of Page ]
399. Spring and Neap Tides
Spring Tides occur at new and full moon, when the combined gravitation effect of the Sun and Moon act to make high tides higher and low tides lower. Neap tides occur at first and 3rd qtr moon when the moons gravitation pulls helps to cancel out the Suns. The high tides are lower and the low tides are higher than Spring tides.[ Top of Page ]
400. Starburst Galaxy
A bright blue galaxy in which many new stars are forming. Believed caused by collisions between galaxies.[ Top of Page ]
401. Steady State Theory
The theory (now generally abandoned) the universe does not evolve.[ Top of Page ]
402. Stellar Density Function
A description of the abundance of stars of different types in space.[ Top of Page ]
403. Stellar Model
A table of numbers representing the conditions in various layers within a star.[ Top of Page ]
404. Stellar Parallax
A measure of the stellar distance.[ Top of Page ]
405. Subsolar Point
The point on a planet which is directly below the sun.[ Top of Page ]
406. Summer Solstice
The point on the ecliptic where the Sun reaches it most northern track.[ Top of Page ]
407. Sunspot
Relatively dark spots on the sun that contain intense magnetic fields.[ Top of Page ]
408. Super Giants
Exceptionally luminous stars 10 to 1000 times the sun’s diameter.[ Top of Page ]
409. SuperCluster
A cluster of galaxy clusters.[ Top of Page ]
410. Superconductor
A material that can conduct electricity with essentially zero resistance.[ Top of Page ]
411. Supergranule
A large granule on the sun’s surface including many smaller granules.[ Top of Page ]
412. Superluminal Expansion
The apparent expansion of parts of a quasar at speeds greater than the speed of light.[ Top of Page ]
413. Supernova Remnant
The expanding shell of gas marking the site of a supernova explosion.[ Top of Page ]
414. Supernova Type I
The explosion of a star believed to be caused by mass transfer to a white dwarf.[ Top of Page ]
415. Supernova Type II
The explosion of a star believed to be caused by the collapse of a massive star.[ Top of Page ]
416. Synchrotron Radiation
Radiation emitted when high speed electrons move through a magnetic field.[ Top of Page ]
417. Synodic Period/(Synodic Month)
29.5 days. One revolution with respect to the Sun. This is the time frame that determines lunar phases. Used as the basis for the first Roman Calendar.
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418. T Tauri Stars
Young stars surrounding by gas and dust. Believed to be contracting toward the main sequence.[ Top of Page ]
419. Temperature
A measure of the velocity of random motions among atoms or molecules in a material.[ Top of Page ]
420. Terminator
The dividing line between daylight and darkness on a planet or moon.[ Top of Page ]
421. Terrestrial Planets (Earthlike Planets)
Small, dense, rocky.[ Top of Page ]
422. Tidal Coupling
The locking of the rotation of a body to its revolution around another body.[ Top of Page ]
423. Tidal Heating
The heating of a planet or satellite because of friction caused by tides.[ Top of Page ]
424. Time Dilation
The slowing of time in curved space time, believed to occur as one approaches the speed of light or crosses the even horizon of a black hole.[ Top of Page ]
425. Titius-Bode Rule
A simple series of steps that produces numbers approximately matching the sizes of the planetary orbits.[ Top of Page ]
426. Total Eclipse (lunar or Solar)
When the umbra part of the Earth’s Shadow (lunar eclipse) totally blocks the light being received by the moon, these can only occur during a Full Moon. A Total Solar Eclipse is the earth passing through the umbra shadow of the moon and can only occur during a New moon.[ Top of Page ]
427. Transition
The movement of an electron from one atomic orbit to another.[ Top of Page ]
428. Transverse Velocity
The velocity of a star perpendicular to the line of sight.[ Top of Page ]
429. Triaxial Ellipsoid
A geometrical solid whose three axes are equal.[ Top of Page ]
430. Triple Alpha Process
The nuclear fusion process that combines three helium nuclei (alpha particles) to make one carbon nucleus.[ Top of Page ]
431. True Relative Orbit
The orbit of one star in a visual binary with respect to the other star after correction for orbital inclination.[ Top of Page ]
432. Tuning Fork Diagram
A system of classification for elliptical, spiral and irregular galaxies.[ Top of Page ]
433. Turnoff Point
The point on the H-R diagram where a cluster’s stars turn off the main sequence and move toward the red giant region revealing the approximate age of the cluster.
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434. Ultraviolet Radiation
Electromagnetic Radiation with wavelengths shorter than visible light but longer than X-rays.[ Top of Page ]
435. Umbra
Is the Earth shadow that is total. No part of the Sun can be seen when in the umbra shadow.[ Top of Page ]
436. Uncompressed Density
The density a planet would have if its gravity did not compress it.[ Top of Page ]
437. Uniform circular motion
Plato argued that the most perfect form should be a circle and therefore motions of the heavens should be made of up combinations of circular motion. The most perfect motion would be uniform circular motion, so astronomers of ancient times tried to describe the motions of the heavens in these terms.[ Top of Page ]
438. Universality
The assumption that the physical laws observed on earth apply throughout the universe.
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439. Van Allen Belts
Radiation belts of high-energy particles trapped in the earth’s magnetosphere.[ Top of Page ]
440. Variable Star
A star whose brightness changes periodically.[ Top of Page ]
441. Velocity
A directed rate of motion.[ Top of Page ]
442. Velocity Dispersion Method
A method of finding a galaxy’s mass by observing the range of velocities within the galaxy.[ Top of Page ]
443. Vernal Equinox
The place where the Sun crosses the celestial equator heading North.[ Top of Page ]
444. Very Long Baseline
The use of radio telescopes located thousands of miles apart to resolve detail in radio sources.[ Top of Page ]
445. Vesicular Basalt
A porous rock formed by solidified lava with trapped bubbles.[ Top of Page ]
446. Violent Motion
Motion other than natural motion.[ Top of Page ]
447. Visual Binary
A binary star system in which two stars are separately visible in a telescope.
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448. Wavelength
The distance between successive peaks or troughs of a wave.[ Top of Page ]
449. Wavelength of Maximum
The wavelength at which a perfect radiator emits the maximum amount of energy; depends only on the object’s temperature.[ Top of Page ]
450. White Dwarf Stars
Dying stars that have collapsed to the size of the earth and are slowly cooling off; at the lower left of the H-R diagram.[ Top of Page ]
451. Winter Solstice
The point on the ecliptic where the Sun reaches it most southern point.
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452. Zeeman Effect
The splitting of spectral lines into multiple components when the atoms are in a magnetic field.[ Top of Page ]
453. Zenith
Point in the Sky that is directly overhead.[ Top of Page ]
454. Zero-Age Main Sequence (ZAMS)
The locus in the H-R- diagram where stars first reach stability as hydrogen burning stars.[ Top of Page ]
455. Zodiac
The 12 constellations near the ecliptic through which the Sun passes.[ Top of Page ]
456. Zone of Avoidance
A region around the Milky Way where almost no galaxies are visible because our view is blocked by dust in our galaxy.[ Top of Page ]
457. Zones
Yellow-white regions that circle Jupiter parallel to its equator; believed to be areas of rising gas.
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