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Overlapping galaxies

8 Jan 2019, 14:00 UTC
Overlapping galaxies ESO

As I (and many before me) have said many times, space is big. That's why we call it space.

The sky is so big that by eye it's relatively rare that two objects are really close together. That's why we get excited when two planets pass near each other, or the Moon gets near one (also: it's very pretty when that happens). The handful of stars that are really close to each other by eye get a lot of attention too, like Albireo in Cygnus, and Mizar and Alcor in the handle of the Big Dipper.

The odds of two stars actually overlapping are incredibly small; stars appear so small in our sky that only a handful can be seen as anything other than unresolved dots.

But some objects are bigger. A lot bigger: Like, galaxies. These are enormous collections of billions or trillions of stars, and even though many are very far away indeed, their spatial extent is so big they can appear to overlap each other. This is actually rather common in nearby galaxies — Andromeda, for example, the nearest big spiral, has a satellite companion called M32 that is superposed against its disk.

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