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A quasar is a celestial object extremely far away that gives off a blinding brightness.
Literal
Quasar [as-for-とは], extremely far-away [in-に] exist, dazzling radiance [object-を] emit celestial-object is.
A definition-style sentence. ~とは is the definitional topic marker, used to introduce a term and then define it — 'a quasar is...' The structure [term]とは[definition]だ/である is standard for encyclopedic or technical definitions. Inside the definition are two relative clauses modifying 天体: 非常に遠方にある ('existing in the extremely far distance') and まぶしい輝きを放つ ('emitting a dazzling radiance'). The second uses 放つ ('to emit, give off'), a slightly literary verb for radiance, light, or scent — more evocative than 出す.