# Milky Way; How do we know it's a spiral?

## Main Question or Discussion Point

... Obviously we can't fly out and look down on it, so what observations are made to deduce the structure of the galaxy?

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Count the number of stars seen in each direction.
Then do essentially the same maths as a CAT scan to get the 3D distribution

You can also use the velocity of the stars (from doppler shift) to confirm the dynamics

phyzguy
Actually, counting the stars in each direction led early astronomers to the conclusion that the galaxy was spherical. This is because we are in the galactic disk, which has a large amount of light absorbing dust. So the number of stars we see in each direction is limited by how far we can see through the dust, not by the structure of the galaxy itself. The first indications that the Milky Way was a spiral were from observations of the density of neutral hydrogen gas made from observations of the 21cm hydrogen line with radio telescopes. This allowed us to see through the dust and make a valid 3D map of the galaxy, which clearly showed the spiral arms. These days, there are many different observations at differernt wavelengths which show the spiral structure.

Staff Emeritus
2019 Award
It's not very simple - as you say, we're on the inside, and also much of it is obscured by dust. Also, it's difficult to measure the distance to many stars, which makes a 3-D map difficult.

Until very recently, there were just two strong lines of argument. One was by making a velocity map of so-called H(I) regions - neutral hydrogen clouds. They emit radio waves of a specific frequency, and by measuring the Doppler shift one can make a velocity map of the galaxy and see that it rotates. Like a spiral.

The other is looking for star-forming regions, which give bright, blue stars. The fact that such stars are in the sky at all makes it likely we are in a spiral, but the fact that these regions are concentrated in "arms" confirms it.

DaveC426913
Gold Member
The fact that such stars are in the sky at all makes it likely we are in a spiral...
Can you elaborate briefly?

Star formation seems to occur mostly in the spiral arms of galaxies, we only see nearby stars so seeing nearby star formation means we are in the arms.

turbo
Gold Member
Star formation seems to occur mostly in the spiral arms of galaxies, we only see nearby stars so seeing nearby star formation means we are in the arms.
Please! Someone use a couple of brain-cells and shut off this idiocy.

Staff Emeritus
2019 Award
Elliptical galaxies have little to no star formation (most of the star formation is during mergers, and it averages less than 1% that of spirals), so their stars are old. If we lived in an elliptical galaxy we wouldn't see many young stars. But we have a sky full of them - Rigel, the Pleiades, Spica, Algol, etc.

Please! Someone use a couple of brain-cells and shut off this idiocy.
It's not exactly my area but I think star formation being concentrated in the arms is still the orthodox model?
A quick arvix search suggests lots of recent papers and an IAU conference on it.

cepheid
Staff Emeritus