How did we know that our galaxy is spiral?!
Neutral hydrogen 21 cm line
the spiral arms are copied by clouds of hydrogen. the clouds of hydrogen are moving, their doppler describes rotating shapes. they can be seen and mapped farther away than the galaxy disk stars can because the stars are obscured by dust. But the hydrogen radio wavelength is 21 cm and it is not blocked by dust and it is very precise so the doppler can be measured exactly.
My memory of this is vague. I suppose the spiral arms were being mapped by radio astronomers before 1970 perhaps already before 1960. Wikipedia might have some sources.
Huh. Apparently the first suggestion that the Milky Way is a spiral came all the way back in 1852:
The somewhat old language is a bit difficult for me to parse, but it looks like he was inferring the shape from the directions to the stars in the sky. I'm not completely certain, but I *think* this paper was part of an ongoing debate as to whether the Milky Way was the whole of the universe, or whether it was just one of many "nebulae" (at the time, any extended object was called a nebula), the debate that wasn't settled until Hubble definitively showed that what we now call other galaxies are quite far away.
Of course, accurate maps weren't made for quite some time, and they're still refining them. Apparently the best current maps make use of a combination of gas clouds and bright stars. Here's one example:
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