Contrast the observed visible mass of the Galaxy with its ...

[serike]

Contrast the observed visible mass of the Galaxy with its suspected total. Explain the difference.
 
  What will be an ideal response?

Question 2

Compare the rotation of the stars and the spiral density wave pattern in our Galaxy. How is it observed?
 
  What will be an ideal response?

[jgranad15]

Answer to Question 1

From the Sun inward, we measure about 100 billion solar masses from the speeds and distances of objects around us in the disk. But when the huge quantities of dark matter out beyond us in the halo is considered from the Galaxy's rotation curve, the dark matter adds up to enough to push the total to 600 billion Suns.

Answer to Question 2

The density wave moves slower than the stars do, based on the Doppler shifts of the stars compared to the gas and dust that forms the spiral arms.

[serike]

Thanks for the timely response, appreciate it

[abro1885]

TYSM