13.8 Key Terms

Active galactic nuclei (AGN): galaxies that are almost as luminous as quasars and share many of their properties, although to a less spectacular degree; abnormal amounts of energy are produced in their centres. 13.5

Galactic cannibalism: a process by which a larger galaxy strips material from or completely swallows a smaller one. 13.3

Galaxy evolution: changes in individual galaxies over cosmic time, inferred by observing snapshots of many different galaxies at different times in their lives. 13.2

Merger: a collision between galaxies (of roughly comparable size) that combine to form a single new structure. 13.3

Quasar: an object of very high redshift that looks like a star but is extragalactic and highly luminous; also called a quasi-stellar object, or QSO. 13.5

Redshift: how much the lines in a galaxy’s spectrum are shifted to the red because of the expansion of the universe. 13.2

Starburst: a galaxy or merger of multiple galaxies that turns gas into stars much faster than usual. 13.3

Supermassive black hole: the object in the centre of most large galaxies that is so massive and compact that light cannot escape from it; the Milky Way’s supermassive black hole contains 4.6 millions of Suns’ worth of mass. 13.4