celestial sphere
An imaginary sphere of infinite extent with the earth at its center on which the stars, planets, and other heavenly bodies appear to be located.
celestial poles
Either of two diametrically opposite points at which the extensions of the earth's axis intersect the celestial sphere.
celestial equator
A great circle on the celestial sphere in the same plane as the earth's equator.
ecliptic
The intersection plane of the earth's orbit with the celestial sphere, along which the sun appears to move as viewed from the earth.
solar day
A mean solar day.
sidereal day
The time required for a complete rotation of the earth in reference to any star or to the vernal equinox at the meridian, equal to 23 hours, 56 minutes, 4.09 seconds in units of mean solar time.
horizon
The apparent intersection of the earth and sky as seen by an observer.
zenith
The point on the celestial sphere that is directly above the observer.
meridian
An imaginary great circle on the earth's surface passing through the North and South geographic poles. All points on the same meridian have the same longitude.
Earth's tilt
The reason for the seasons and has a tilt of 23.45 degrees and it perpendicular to the plane of the ecliptic
Kepler's law 1
The orbit is an ellipse with 2 centers called foci, and points all have the same sum of distances from foci
Kepler's law 2
Planet on elliptic orbit moves faster when near the sun, slower when away
Kepler's law 3
Planets on orbits farther from the sun take longer to complete one orbit
Newton's law of gravity -- how force depends on masses & distances
F=ma and every piece of mass in the universe exerts a gravitational force on every other piece
Kirchhoff's law 1
hot, opaque source (solid or dense) = continuous spectrum
Kirchhoff's law 2
hot, low density gas = emission lines
Kirchhoff's law 3
hot, opaque source and light travels through cold low-density gas = continuous + absorption lines
emission lines
bright object gives off light
absorption lines
lights hits something then gets "eaten
blackbody radiation
The electromagnetic radiation that a perfect blackbody would give off at a given temperature. A warm blackbody would emit radiation with a higher average frequency than a cooler one.
Stefan-Boltzmann and Wein laws
the hotter the star the brighter the star
energy levels
The energy characteristic of a stationary state of a physical system, especially a quantum mechanical system.
Doppler shift
A change in the observed frequency of a wave, as of sound or light, occurring when the source and observer are in motion relative to each other, with the frequency increasing when the source and observer approach each other and decreasing when they move a
Luminosity
The condition or quality of being luminous. Emitting light, especially emitting self-generated light.
Flux
imagine dividing surface of object into tiles of unit size
absolute magnitude
The intrinsic magnitude of a celestial body computed as if viewed from a distance of 10 parsecs, or 32.6 light-years.
apparent magnitude
a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere.
parallax
An apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight.
H-R diagram
A graph of the absolute magnitude of stars plotted against their surface temperature or color, used in the study of stellar evolution.
main sequence
A major grouping of stars that forms a relatively narrow band from the upper left to the lower right when plotted according to luminosity and surface temperature on the Hertzsprung-Russell diagram.
star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered g
main sequence turn-off point
The turnoff point for a star refers to the point on the Hertzsprung-Russell diagram where it leaves the main sequence after the exhaustion of its main fuel.
hydrostatic equilibrium
the outward force due to the pressure gradient within the star is exactly balanced by the inward force due to gravity.
convection
The transfer of heat or other atmospheric properties by massive motion within the atmosphere
neutrino
an elementary particle with zero charge and zero mass
nuclear fusion
a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy
molecular cloud
a cool dense interstellar region composed of a wide variety of molecules, mainly hydrogen, plus some dust, in which stars are forming
protostar
a cloud of interstellar gas and dust that gradually collapses, forming a hot dense core, and evolves into a star once nuclear fusion can occur in the core
accretion disk
A disk of interstellar material surrounding a celestial object with an intense gravitational field, such as a black hole.
jet
long thin feature extending from an active galaxy and usually observed at radio wavelengths
red giant
A star of great size and brightness that has a relatively low surface temperature.
white dwarf
The remnant of a star that has collapsed, having an extremely dense state with no empty space between its atoms, but not reaching the extremely dense state of a neutron star or black hole.
supernova
A rare celestial phenomenon involving the explosion of most of the material in a star, resulting in an extremely bright, short-lived object that emits vast amounts of energy.
neutron star
A celestial body consisting of the superdense remains of a massive star that has collapsed with sufficient force to push all of its electrons into the nuclei that they orbit, thus leaving only neutrons, and having a powerful gravitational attraction from
black hole
An area of space-time with a gravitational field so intense that its escape velocity is equal to or exceeds the speed of light.
event horizon
The region, usually described as spherical, marking the outer boundary of a black hole, inside which the gravitational force is strong enough to prevent matter or radiation from escaping.
Galaxy
Any of numerous large-scale aggregates of stars, gas, and dust that constitute the universe
Active galaxy
A galaxy emitting unusually high quantities of radiation from an active galactic nucleus at its center. Some active galaxies emit more energy in radio wavelengths than they do visible light
spiral
A curve on a plane that winds around a fixed center point at a continuously increasing or decreasing distance from the point.
elliptical
Of, relating to, or having the shape of an ellipse.
dark matter
Matter that emits little or no detectable radiation of its own, postulated to account for gravitational forces observed on astronomical objects and to be part of the missing mass.