This list is ranked by mean, with 1=highly essential, 5=not at all essential.

Concept	Mean	SD
mass	1.06	0.23
Hertzsprung-Russell diagram	1.11	0.31
red shift	1.17	1.05
radiation	1.22	0.42
stellar evolution	1.22	0.82
atom	1.28	0.73
distances	1.28	0.56
Doppler shift	1.28	0.56
electromagnetic spectrum	1.28	0.65
main sequence	1.28	0.56
terrestrial planets	1.28	1.20
Hubble's law	1.33	0.67
Jovian planets	1.33	0.58
Kepler's laws	1.33	0.58
spectra	1.33	1.38
wavelength	1.33	1.15
blackbody radiation	1.39	0.76
Milky Way Galaxy	1.39	0.49
Big Bang model	1.44	0.30
inverse-square law for light	1.44	0.60
law of gravitation (Newton)	1.44	0.83
luminosity	1.44	0.60
orbits	1.44	0.60
spectral line	1.44	1.27
energy	1.50	0.76
galaxies	1.50	0.76
heliocentric model	1.50	0.76
observations	1.50	0.76
radial velocity	1.50	0.60
stellar lifetimes	1.50	1.11
age	1.56	0.76
astronomical unit	1.56	0.69
nucleosynthesis	1.56	0.90
spectroscopy	1.56	1.13
speed	1.56	1.00
stellar colors	1.56	0.60
Sun (model)	1.56	0.97
velocity	1.56	0.90
blue shift	1.61	0.89
cosmic microwave blackbody radiation	1.61	0.68
density	1.61	0.76
elements	1.61	1.06
supernova	1.61	0.96
temperature	1.61	0.85
universality of physical laws	1.61	0.78
white dwarf	1.61	0.42
black hole	1.67	0.67
laws of motion (Newton)	1.67	1.05
nuclear fusion	1.67	1.00
photon	1.67	1.11
radius	1.67	1.17
scientific model	1.67	1.20
spiral galaxy	1.67	1.24
angular diameter	1.72	0.80
ellipse	1.72	0.73
energy levels	1.72	0.80
globular clusters	1.72	0.73
pulsar	1.72	0.87
stellar spectral classes	1.72	1.41
binary stars	1.78	0.79
eclipse (lunar, solar)	1.78	0.92
ecliptic	1.78	1.03
electrons	1.78	1.18
supernova remnant	1.78	0.94
cosmology	1.83	0.90
force	1.83	0.90
Kirchhoff's rules	1.83	1.01
protons	1.83	1.26
cratering	1.89	0.74
nucleus (atom)	1.89	1.24
quasar	1.89	1.15
radiative energy	1.89	0.81
spiral arm	1.89	0.90
asteroid	1.94	1.03
colors	1.94	0.97
comets	1.94	0.91
galactic (open) clusters	1.94	0.91
neutron star	1.94	0.91
pressure	1.94	0.85
solstice	1.94	1.34
conservation of energy	2.00	1.05
disk (galaxy)	2.00	0.75
meteorite	2.00	1.20
period-luminosity relationship	2.00	0.82
planetary systems	2.00	1.11
starbirth	2.00	0.94
comparative planetology	2.06	1.18
equinoxes	2.06	0.91
heat	2.06	0.97
neutrons	2.06	1.27
Planck curve	2.06	0.97
angular separation	2.11	0.88
chemical composition	2.11	0.81
distances	2.11	1.05
frequency	2.11	1.15
planetary nebula	2.11	0.99
planets (models)	2.11	0.94
escape speed	2.17	0.90
geocentric model	2.17	1.30
mass-luminosity relation (stars)	2.17	1.12
nova	2.17	1.17
solar day	2.17	1.33
stellar heliocentric parallax	2.17	1.24
tidal forces	2.17	0.96
excitation	2.22	0.92
inertia	2.22	1.08
perihelion	2.22	1.08
superclusters (galaxies)	2.22	0.99
supergiant (star)	2.22	1.06
thermal energy	2.22	1.26
Balmer series	2.28	0.87
critical density (cosmos)	2.28	0.99
dark matter	2.28	0.93
heliocentric parallax	2.28	1.28
physical universe	2.28	1.24
physics	2.28	1.41
retrograde motion	2.28	0.56
semimajor axis	2.28	0.49
star model	2.28	0.76
weight	2.28	0.60
focus (ellipse)	2.33	0.82
kinetic energy	2.33	1.16
potential energy	2.33	1.25
proton-proton chain	2.33	1.20
Schwarzschild radius	2.33	1.12
stellar corpses	2.33	1.08
differentiated (planets, stars)	2.39	1.01
halo (galaxy)	2.39	1.06
nucleus (galaxy)	2.39	0.89
pre-main-sequence star	2.39	1.01
states of matter	2.39	1.01
clusters (galaxies)	2.44	0.96
conservation of angular momentum	2.44	1.12
cosmos	2.44	1.38
irregular galaxies	2.44	0.90
opposition	2.44	1.01
radioactive dating	2.44	1.12
acceleration	2.50	1.26
aphelion	2.50	1.12
center of mass	2.50	1.07
elliptical galaxy	2.50	0.83
interstellar medium	2.50	1.01
constellations	2.56	1.42
magnetic fields	2.56	1.17
molecular cloud	2.56	1.07
planetesimals	2.56	0.96
zodiac	2.56	1.12
noon	2.61	1.38
angular momentum	2.67	1.05
luminosity classes (stars)	2.67	1.16
conjunction	2.72	0.92
solar nebula	2.72	1.32
tectonics	2.72	1.01
horizon	2.78	1.08
isotropic	2.78	1.03
nebular models	2.78	1.31
nuclear fission	2.78	1.13
spectroscopic distances	2.78	1.17
centripetal force	2.83	1.01
flux	2.83	1.34
neutrino	2.83	1.34
protostars	2.83	0.90
degenerate gas	2.89	1.15
centripetal acceleration	2.94	0.91
elongations	2.94	1.03
homogeneous	2.94	1.18
protoplanets	2.94	0.78
singularity	2.94	1.07
synchrotron radiation	2.94	1.25
active galaxies	3.00	1.11
condensation sequence	3.00	1.05
geometry (flat, open, closed)	3.00	1.16
mass-luminosity ratio (galaxies)	3.00	1.16
opacity	3.00	0.94
ring systems	3.00	0.23
spiral tracers	3.00	0.89
volcanism	3.00	1.34
brown dwarf	3.06	0.71
convection	3.06	0.91
rotation curve (galaxy)	3.06	1.16
spacetime	3.06	0.91
voids	3.06	0.87
young stellar objects	3.06	0.81
angular speed	3.11	1.29
freefall	3.11	0.99
general relativity	3.11	1.29
aesthetics	3.17	1.26
conduction	3.22	0.92
dynamo model (magnetic field)	3.22	1.13
magnetosphere	3.33	1.05
natural motion	3.33	1.33
inflationary universe model	3.50	0.96
intergalactic medium	3.50	0.76
protogalaxies	3.50	0.96
quantum physics	3.56	1.34
black dwarf	3.67	0.94
grand unified theories	3.78	1.03
antimatter	4.00	0.75