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A region of space having a gravitational field so intense that no matter or radiation can escape.

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Black holes have helped us test Einstein's theory of general relativity, which describes how mass, space, and time are related to one another.

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A star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.

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Supernovae are considered one of the original sources of the elements heavier than iron in the Universe.

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A star showing a sudden large increase in brightness and then slowly returning to its original state over a few months. 

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A nova reaches maximum luminosity within hours after its outburst and may shine intensely for several days or occasionally for a few weeks

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Any star having a relatively large radius for its mass and temperature.

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Massive stars easily rank as the most luminous in the cosmos, the brightest outshine the Sun by a million times.

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A very large star that is even brighter than a giant, often despite being relatively cool.

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Supergiants can contain 8 - 200 times the mass of the Sun!

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A white dwarf is what stars like the Sun become after they have exhausted their nuclear fuel.

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Near the end of its nuclear burning stage, this type of star expels most of its outer material, creating a planetary nebula. Only the hot core of the star remains.

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A series of star types to which most stars belong, represented on a Hertzsprung–Russell diagram as a continuous band extending from the upper left to the lower right.

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Main sequence stars make up around 90% of the universe's stellar population.

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The Hertzsprung-Russell diagram shows the relationship between a star's temperature and its luminosity.

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It is a very useful graph because it can be used to chart the life cycle of a star.

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Any of the numerous luminous or dark formations or irregularly distributed dust and gas seen within the Milky Way galaxy and in other spiral galaxies.

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Nebulae are always in motion.

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A supernova remnant is the remains of a supernova explosion.

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They are a major source of heating of interstellar gas using the magnetic turbulence and violent shocks that they produce.

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A planetary nebula is a region of cosmic gas and dust formed from the cast-off outer layers of a dying star.

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They are formed from low-mass stars and come in a variety of shapes and sizes. When a low-mass star runs out of both hydrogen and helium fuel, it collapses and ejects the hot gas surrounding the core.

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The dust and gases in a nebula are very spread out, but gravity can slowly begin to pull together clumps of dust and gas. As these clumps get bigger and bigger, their gravity gets stronger and stronger.