# The Milky Way

1. Sep 10, 2015

### Passionate Eng

How did we know that our galaxy is spiral?!

2. Sep 10, 2015

### marcus

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.

Last edited: Sep 10, 2015
3. Sep 10, 2015

### Chalnoth

Huh. Apparently the first suggestion that the Milky Way is a spiral came all the way back in 1852: