Main sequence stars: stars whose temperature and luminosity place them on the main sequence of the H-R diagram. Main-sequence stars release energy by fusing hydrogen into helium in their cores.
Planetary nebula: the glowing cloud of gas ejected from a low-mass star at the end of its life.
Supernova: the explosion of a star.
White dwarf: the hot, compact corpse of a low-mass star, typically with a mass similar to that of the Sun compressed to a volume the size of the Earth.
Brown dwarf: an object that forms much like a star but too low in mass to sustain nuclear fusion in its core; brown dwarfs have masses much greater than Jupiter but always less than 0.08MSun.
AU (astronomical unit): the average distance (semimajor axis) of Earth from the Sun, which is about 150 million km.
Light years: the distance that light can travel in 1 year, which is 9.46 trillion km.
Parsecs: 1 parsec (pc) = 3.09 x 1013 km = 3.26 light years
Typical distance between neighboring stars
Wavelength: the distance between adjacent peaks (or troughs) of a wave.
Frequency: describes the rate at which peaks of a wave pass by a point; measured in units of 1/s, which are often called cycles per second or hertz.
Speed of light: the speed at which light travels, which is about 300,000 km/s
Doppler effect: the effect that shifts the wavelengths of spectral features in objects that are moving toward or away from the observer.
Gravitational lensing: the magnification or distortion (into arcs, rings, or multiple images) of an image caused by light bending through a gravitational field, as predicted by Einstein’s general theory of relativity.
Transit method: a transit is an event in which a planet passes in front of a star (or the Sun) as seen from Earth. Only Mercury and Venus can be seen in transit from our Sun. The search for transits of extrasolar planets is an important planet detection strategy.
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