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Cake day: August 3rd, 2023

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  • The definition for ‘galaxy’ is independent of ‘being in space’. A galaxy is “a system of large numbers of stars, dust, and other interstellar matter bound together by gravity”. In much the same way that our solar system is bound by a gravitational center inside the sun, a galaxy is a collection of stars bound to a gravitational center in its core.

    The stars in the interstellar stream are not bound by a single gravitational center. There might be multiple star systems inside the stream, but scale matters and a handful of stars do not constitute a galaxy. Note how this definition of ‘galaxy’ still does not include ‘moving through intergalactic space’, so that characteristic of the stars in this stream is ignored for this pedagogical exercise.