Andromeda's Light Lets Us See a 2.5-Million-Year Moment

Seeing Andromeda with the naked eye means viewing light that began its journey about 2.5 million years ago—the era when early Homo in Africa were first making stone tools; Andromeda (M31) is the Local Group’s largest galaxy and the farthest naked-eye object at 2.5 million light-years, approaching the Milky Way at ~110 km/s with a predicted merger in ~4.5 billion years; the visible glow is the bright core, while deep images reveal hundreds of millions of stars, illustrating astronomy as time travel into the past.