Question

question 13
1 pts
why do astronomers propose that the milky way galaxy contains a lot of dark matter?
the light from stars in the disk is redder than their spectral types indicate.
all of these
the light from stars in the disk is dimmed about 2 magnitudes per kiloparsec.
the galaxys rotation curve flattens out at great distances.
dark silhouettes of material are observed blocking the light from stars.

Explanation:

• The reddening of starlight (redder than spectral types indicate) can be due to dust, but dark matter doesn't directly cause this.
• Dimming of starlight (2 magnitudes per kiloparsec) is often due to interstellar dust, not dark matter.
• Dark silhouettes blocking starlight are caused by dust clouds, not dark matter.
• The key evidence for dark matter in the Milky Way is the rotation curve. In a galaxy, if most of the mass were in the visible stars (in the inner regions), the rotation speed should decrease with distance (like in the solar system). But the Milky Way's rotation curve flattens at great distances. This implies there is a large amount of unseen (dark) matter providing additional gravitational pull to keep the rotation speed from decreasing, which is consistent with the presence of dark matter.

Answer:

The Galaxy's rotation curve flattens out at great distances.