What happens inside an atom that causes it to give off light?

The Photon is the "release" of energy in the form of light. Energy absorbed by the atom causes a change in energy of the electrons of a stable atom. This change forces an electron to move from one valance ring to another. The energy is released in the form of light called a photon. Depending on the amount of energy released and the valance the electron jumps to is what determines the light (photon) wave length. If an atom has too much light than it may disappear/explode.

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Explanation:

A quasar (/ˈkweɪzɑːr/; also known as a quasi-stellar object, abbreviated QSO) is an extremely luminous active galactic nucleus (AGN), in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk. As gas in the disk falls towards the black hole, energy is released in the form of electromagnetic radiation, which can be observed across the electromagnetic spectrum. The power radiated by quasars is enormous; the most powerful quasars have luminosities thousands of times greater than a galaxy such as the Milky Way. Usually, quasars are categorized as a subclass of the more general category of AGN. The redshifts of quasars are of cosmological origin.

a massive and extremely remote celestial object, emitting exceptionally large amounts of energy and typically having a starlike image in a telescope. It has been suggested that quasars contain massive black holes and may represent a stage in the evolution of some galaxies.