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These two galaxies are so tight, the stellar formation encompassing them both actually has a name of its own.

Say hello to Arp 91, a pair of spiral galaxies that are situated so close together (in relative terms, space is big) we can actually see their outer arms reaching out and colliding with one another. BFFs on an intergalactic scale.

Say hello to the two galaxies comprising Arp 91.Credit: ESA/Hubble & NASA, J. DalcantonA

Like a good marriage, these galaxies may share a name but they are their own individuals as well. In the center of the frame is NGC 5953. Just above it and slightly to the right is NGC 5954. They're both spiral galaxies, but their shapes differ slightly as a result of perspective. Earth is situated more than 100 million light-years away, so it's not like Hubble can zoom around to catch them at a different angle.

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These interacting galaxies are drawn together by the immense inward pull of gravitational force that each one produces. Spiral galaxies are so-named and shaped because of their unique swirling shape around a bright center, which is typically a supermassive black hole.

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The description accompanying this particular Hubble image notes that interacting galaxies like Arp 91 are just a glimpse at one stage of galactic evolution. Researchers believe that on a long enough timeline, colliding spiral galaxies eventually merge completely into one elliptical galaxy, populated primarily by older stars and with little of the gas and dust that characterizes the hazy areas of the two galaxies making up Arp 91.

Time and distance are the two major complicating factors when it comes to studying these interactions and determining the role they play (or not) in the ever-evolving landscape that we call outer space. Every elliptical galaxy we've observed could have at one point been multiple galaxies in close proximity, just like Arp 91. But it's all so far away and happening so slowly (in relative terms) that the history of human space study is barely a blip within the big picture.

(责任编辑：綜合)