It has long been assumed that our galaxy is a classic example of many of the galaxies that pepper the cosmos. That’s important, because astronomers have intensely studied the Milky Way and, with this assumption, can use what they find to inform their view of the wider universe.
But what if our neck of the woods isn’t that normal after all? It would be a big deal for cosmologists and astronomers. That question is coming to the fore because yet more observations suggest that our galaxy is an “outlier” in important ways. And if it isn’t a representative galaxy, a fair chunk of astronomical thinking could be out of kilter.
This all goes back to astronomers’ love of putting things into boxes. Objects seen through telescopes are separated into groups, such as planets, stars and galaxies, and then subdivided into smaller and smaller boxes.
Galaxies get lumped into broad categories based upon their gross features – spiral, elliptical or irregular – and are then more finely divided based upon smaller variations, such as just how elliptical their shape is, or how tightly wound their spiral arms are.
Such classification can be powerful as it is used to reveal the underlying physical processes that shaped a galaxy, such as the flow of dark matter and gas over cosmic history. With galaxies grouped this way, we are able to directly test our ideas for galactic formation and evolution.
The Milky Way remains the prime example of such cosmic populations, with astronomers able to map, in detail, the stars, gas and dust that circle the supermassive black hole at its heart. This remains central to ideas of galaxy evolution. But while many of its observed properties agree with expectations, a number do not. Prominent on this list is the population of dwarf galaxies that accompany the Milky Way in space.
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