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star 
A sphere of gas massive enough to generate light and heat through nuclear reactions that transform hydrogen into helium in its core.
brown dwarf
Star whose mass is not sufficient to generate a nuclear reaction.
main-sequence star
Star whose mass is sufficient to generate a nuclear reaction.
planetary nebula
Expanding gaseous envelope that corresponds to the external layer of a red giant that is gradually fading away.
black dwarf
Dead star, likely the residue of a dwarf that has totally exhausted its energy resources.
low-mass stars
Stars whose mass is less than 1.5 times that of the Sun.
red giant
An old star whose hydrogen reserve has been exhausted; its luminosity can be 100 times that of the Sun.
white dwarf
An old, extremely dense star of faint luminosity, formed by the nucleus of a red giant contracting until it reaches the size of Earth.
nova
A white dwarf that assimilates gaseous matter from a neighboring star, suddenly becoming extremely bright before it returns to its initial brightness.
massive stars
Stars whose mass is more than 1.5 times that of the Sun; can be up to 50 times the mass of the Sun.
supergiant
An old, extremely luminous star of considerable mass; its diameter can be as much as 100 times that of the Sun.
neutron star
Star formed of compressed neutrons, believed to be the residue of a supernova explosion.
supernova
A supergiant that collapses onto itself and explodes with such force that it releases more energy than millions of suns.
pulsar
A neutron star that rotates rapidly on itself, thereby emitting regular radio waves.
black hole
Results when the core of a massive star collapses; the gravitational force is so strong that not even light can escape.
