# ASTR 120 Midterm 2

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1. Angular distance
The distance between two "lines" drawn from the eye to connect two close stars
2. Angular diameter
measurement to describe the apparent size of a celestial object. (also called angular size)
3. Astronomical unit (AU)
average distance between earth and the sun (also semi-major axis of earth's orbit around the sun)
4. Light-year (ly)
the distance light travels in one year
5. Parsec (pc)
the distance at which 1 AU subtends an angle of 1 arcsec
6. geocentric model
model of universe where the earth is stationary while the sun, moon, stars and planets revolve around it
7. direct motion
the eastward motion of planets relative to the stars
the westward motion of planets relative to the stars
9. Ptolemaic system
most enduring model to explain retrograde motion within the geocentric model using epicycles and deferents
10. epicycle
small circle each planet was assumed to move around, centered on the deferent
11. deferent
larger circle planet's epicycle moved around, centered on the earth
12. heliocentric model
model of solar system in which all planets, including earth, orbit the Sun
13. ecliptic
the projection onto the celestial sphere of the plane of earth's orbit
14. period
time it takes a planet to complete one orbit
15. synodic period
time that elapses between two successive identical configurations as seen from earth (time it takes for a planet to appear in the same spot of the sky compared to the stars)
16. sidereal period
time it takes a planet to complete one full orbit of the sun relative to the stars
17. parallax
the apparent change in position of an object due to the observers movement
18. Nicolaus Copernicus
devised the first comprehensive heliocentric model
19. Tycho Brahe's
astronomical observations disproved ancient ideas about the heavens
20. Johannes Kelper
proposed elliptical paths for the planets movement around the Sun
21. major axis

semi-major axis
longest diameter of an ellipse

half the distance of the major axis (also mean distance between planet and star)
22. eccentricity
describes the shape of an ellipse which depends on its elongation
23. perihelion
point on orbit closest to the Sun causing the planet to move most rapidly
24. aphelion
point on orbit farthest from Sun causing planet to move most slowly
25. Kepler's first law
the orbit of a planet about the Sun is an ellispe with the sun at one focus
26. Kepler's second law
a line joining a planet and the sun sweeps out equal areas in equal intervals of time (lso called law of equal areas)
27. Kepler's third law
the square of the sidereal period of a planet is directly proportional to the cube of the semi-major axis of the orbit
28. Galileo
discoveries with a telescope strongly supported a heliocentric model
29. Newton
formulated laws of motion and gravity that describe fundamental properties of physical reality
30. Newton's first law of motion
an object remains at rest, or moves in a straight line at a constant speed, unless acted on by a net outside force
31. Newton's second law of motion
in order to give an object acceleration a net outside force must act upon it
32. weight
the force of gravity that acts on an object
33. Newton's third law
whenever one object exerts a force on a second object, the second object exerts an oppositely dircted force of equal strength on the first
34. law of universal gravitation
two objects attract each other with a force that is directly proportional to the mass of each object and inversely proportional to the square of the distance between them
35. orbital energy
trajectories with both kinetic and gravitational potential energy
36. escape speed
energy required to break out of bound orbits and never return to the original source of gravity
37. tidal forces
difference in gravitational pull at different points in an object
38. blackbody
ideal type of dense object, reflects no light but instead absorbs all radiation falling on it
light emitted by a blackbody as a result of temperature
40. energy flux
"rate of flow"; the amount of energy emitted from each square meter of an object's sureface in a second
41. Wien's law
relates temperature to wavelength of maximum emission
42. Stefan-Boltzmann law
relates flux to temperature
43. photon
particle of light, a distinct packet of electromagnetic energy
44. spectral lines
fine dark lines in solar spectrum caused when light at a specific wavelength is partially absorbed (also called absorption lines)
45. Kirehhoff's law: 1
a hot, opaque body, such as a perfect blackbody or a hot dense gas produces a continuous spectrum (a complete rainbow of colors without any spectral lines)
46. Kirehhoff's law:2
a hot, transparent gas produces an emission line spectrum
47. emission line spectrum
a series of bright spectral lines against a dark background
48. Kirehhoff's law: 3
a cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum
49. absorption line spectrum
a series of dark spectral lines amoung the solors of the continuous spectrum
50. Balmer lines
the spectral lines of hydrogen at visible wavelengths
51. Lyman series
series of spectral lines completely in the ultraviolet, n=1
52. Paschen series
series of spectral lines completely in the infrared, n=3
53. blueshift
as light source moves closer to observer, all lines in spectrum are shifted toward the short-wavelength end of the spectrum
54. red shift
as light source moves away from observer, all lines in spectrum are shifted toward the long-wavelength end of the sepctrum
55. Doppler effect
the effect of relative motion on wavelength
56. light gathering power
the ability of a telescope to detect light. directly proportional to area of object lens which in turn is proportional to the square of the lens' diameter
57. chromatic aberration
optical defect caused by light bending differently in different lenses and resulting in fuzzy, rainbow colored halos
58. refracting telescope
telescope using two lenses to make an image formed on a focal plane appear larger
59. refecting telescope
telescope that uses a curved mirror to make an image of a distant object
60. angular resolution
quantity that gauges how well fine details can be seen
61. diffraction
tendency of light waves to spread out when they are confined to a small area like the lens or mirror of a telescope
62. seeing disk
a measure of the limit that atmospheric turbulence places on telescope's resolution
63. very long baseline interferometry
technique to obtain better angular resolution by combining the signal of radio telescopes at different observatories 1000s of km apart
64. luminosity
total amount of energy emitted by a star
65. hydrogen fusion
process of converting hydrogen into helum through a nuclear reaction
66. proton-proton chain
process of converting 4 hydrogen nuclei into one helium nucleus
67. hydrostatic equilibrium
the outer layers of stellar mater are held in place (neither exploding or collapsing nor significantly heating or cooling) because downward pressure of pressure of layers of material and weight of material are balanced by the upward pressure of hot gases beneath them
68. thermal equilibrium
while temperature in solar interior is different at different depthes the temperature at each depth remains constant in time
69. neutrinos
particles that have no electric charge
70. photosphere
single thin layer of gas that all of the sun's visible light emanates from creating what appears to be a sharp, well defined solar surface
71. chromosphere
glowing pinking layer of gas above the photosphere revealed during total solar eclipse
72. corona
outermost layer of sun's atmosphere
73. solar wind
coronal gas that escapes sun's atmosphere due to corona's high temperature
74. sunspots
irregularly shaped dark regions in the photosphere caused by differential roation making the sun's poles not aligned all the time
75. sunspot cycle
phenomenon where average number of sunspots is not constant but varies in a predictable pattern (11 years between two sunspot maximims or two sunspot minimums)
76. 22-year solar cycle
the time ti takes the Sun's magnetic pattern to repeat itself (occurs after 2 sunspot cycles
77. space weather
variations in the solar wind and magnetic field that can affect satellites and astronauts
78. magnetic reconnection
process in which, if two magnetic fields of two arches come into proximity, their magnetic field can rearrange
79. space velocity
describes how fast and in what direction a star is moving
80. tangential velocity
star's velocity perpendicular to our line of sight
81. proper motion
the number of arcsecs that the star appears to move per year on selestial plane
star's motion parallel to our line of sight
83. brightness
the amount of energy that passes each second through a square meter of the sphere's surface area
84. apparent magnitude
describe how bright an object appears to an earth-based observer
85. absolute magnitude
measure a star's true energy output by calculating the star's apparent magnitude if it were located 10 pc from earth
86. spectra classes
groups of similar appearing stellar spectra (Oh, Be A Fine Girl Kiss Me)
87. Hertsprung-Russell diagrams
graphs showing relationship/trneds between stars absolute magnitude (luminosity) and surface temperature
88. interstellar medium
combination of gas and dust that spans the space between stars
89. dark nebula
nebula so opaque it blocks any visible light coming from stars that lie behind it
90. interstellar reddning
effect where light from remote stars is reddened as it passes through the interstellar medium because the blue component of their starlight is scattered and absorbed by interstellar dust
91. Dok globules
other relatively small, nearly spherical dark nebulae
92. protostars
clumps formed within densest portions of dark nebulae that contract under their own mutual gravitational attractoin
93. clusters
groups of young stars

open: a loose collection of stars

stellar association: group of stars gravitationally unbound from the beginning
94. giant molecular clouds
huge clouds that contain enormous amounts of hydrogen
95. supernova remnants
nebulae that are the shredded funeral shrouds of dead stars
96. core hydrogen fusion
process of converting hydrogen to helium through thermonuclear fusion that takes place in the core of main sequence stars
total time a star spends fusing hydrogen into helium at its core
98. red dwarfs
stars of spectral class M having masses between 0.08 and 0.4 solar mass
99. shell hydrogen fusion
in stars with more than 0.4 solar mass, hydrogen fusion continues in materiel just outside the core after all hydrogen in the core has been consumed
100. red giants
as shell hydrogen fusion works it way outward, luminosity increases and radius increases causing outer layers to cool and surface temperature to drop.
101. core helium fusion
helium fusion that takes place in a red giants core after it reaches temperatures of 108 K
102. triple alpha process
process of fusing 3 helium nuclei to form a carbon nucleus
103. helium flash
in red giants with a mass less that about 2-3 solar mass, helium fusion beings explosively and suddenly
104. degenercy
phenomenon in which closely packed particles resist compression as a consequence of the Pauli exclusion principle
105. Pauli exclusion principle
states that two electrons cannot simultaneously occupy the same quantum state
106. turn off point
peak of the surviving portion of the main sequence stars on the cluster's H-R diagram
107. Population 1 stars
(metal rich) stars whose spectra contain many prominent spectral lines of heavy elements
108. Population 2 stars
(metal poor) stars whose spectra show weak lines of heavy elements
109. pulsating variable stars
stars that undergo substantial changes in size, their surface temperature varies and vary dramatically in brightness
110. Cepheid variable
characterized by rapid brightening followed by gradual dimming
111. red giant branch
as luminosity increases and surface temperature drops, post main sequence stars move up and to the right of H-R digram
112. core helium fusion
second post main sequence stage
113. horizontal branch
stars with helium fusing cores and hydrogen fusion shells
114. shell helium fusion
after all helium in red giant core has been burned the core contracts releasing heat and beginning helium fusion in thin shell around the core (second phase of red giants)
115. asymptotic giant branch stars
stars in second red giant phase
116. planetary nebula
low mass stars eventually become a fiercely hot, exposed core, surrounded by glowing shells of ejected gas
117. white dwarf
relic of star created when there's no nuclear reactions occuring so core cools down like dying ember
118. Chandrasekhar limit
upper limit to mass of a white dwarf
119. superjiant
star whose luminosity and radius are much larger than those of a giant caused by nuclear reactions simultaneously in several shells
120. supernova
when shock wave breaks out of the star, a portion of the explosive energy escapse in a torrent of light
121. core-collapse supernova
type of supernova that occurs when there is no more fuel to burn in core so the star falls in on it's self due to gravitational forces but bounces back because of degenerate neutron pressure, sending a shock wave through layers of the star resulting in supernova
122. progenitor star
star before it "goes supernova"
shock wave caused by positron that moves through a substance faster than the speed of light in that substance
124. Type 2 supernovae
supernovae with hydrogen emission lines (core-collapse supernova)
125. Type 1a supernovae
have spectra that include strong absorption line of ionized silicon but not hydrogen lines
126. Type 1b Supernovae
lack ionized silicon line as well as lack hydrogen line but have strong helium absorption line
127. Type 1c Supernovae
lack ionized silicon line, helium absorption line and hydrogen line
128. Thermonuclear supernova
Type 1a supernova caused by the thermonuclear explosion of a white dwarf
129. neutron star
an incredibly dense sphere composed of mostly of neutrons
130. pulsars
pulsating radio sources, neutron stars rapidly rotating
 Author: nikachan ID: 305632 Card Set: ASTR 120 Midterm 2 Updated: 2015-07-24 05:00:50 Tags: astronomy definitions vocab Folders: vocab,science,midterm Description: Definitions of astronomy terms Show Answers: