ASTROPHYSICS definitions

universe

large collection of billions of galaxies

galaxy

large collection of billions of stars

planets

large objects that orbit a star

moons

orbit planets with almost circular orbits. They form a natural satellite

asteroids

lumps of rock and metal that orbit the sun, usually found in an asteroid belt

comets

lumps of ice and dust that orbit the sun in highly elliptical orbits

weight

force on an object due to gravity, depends on the strength of the gravitational field it is in

absolute brightness / magnitude

brightness of each star at a standard distance

apparent brightness

brightness of the star as seen by the observer on earth

luminosity

measures how much light is emitted from the start surface every second

doppler effect

an apparent change in wavelength and frequency due to the relative speed of the observer with respect to the source of the wave