Astronomy Questions pertaining to Galaxies.?

What use are 21 cm radio waves to galactic astronomers?





A) They cut through the dusty cocoons to let us watch star birth.


B) We can reflect them off the core of the Galaxy.


C) Their Doppler shifts let us map the motions and locations of spiral arms.


D) They bounce off stars like our Sun to let us precisely measure their distances.


E) They pick up the cool, dark matter much better than can optical telescopes.





and





16. The visible color of the direction to the Galactic Center is





A) blue from the OB associations in the Sagittarius arm.


B) yellow from the old Population II stars in the Galactic Bulge.


C) red from the emission nebulae.


D) black from the dust clouds.


E) brown from the dwarfs in the dark matter.

Astronomy Questions pertaining to Galaxies.?
Lets see, for the first question:


A - might be true


B - would require us to send 21 cm waves from earth to the core of the galaxy to be reflected - that is many many thousands of light years, so this can't be right.


C - Doppler shift is not affected by wavelength - all wave lengths are doppler shifted the same amount, so this can't be right.


D - bouncing 21 cm waves off of any star BUT the sun would take years because they are so far away, so this can't be right.


E - the reason that cool, dark matter is "dark" is that it doesn't emit ANY radiation - and that includes 21 cm waves, so this can't be right.





Only A is left. And it is correct. 21 cm waves emitted by stars penetrate dust clouds well so we can peek inside the dust cocoons.








For the next question, I have to admit that I just don't know, though I suspect it is D.
Reply:First problem: B and D are clearly wrong. The 21 cm radiation is from hydrogen, and penetrates dust clouds better than optical radiation. It should be possible to measure Doppler shift, although I haven't heard of any work along those lines.





Second problem: D.