Question

How do we know that there is much more mass in the halo of our galaxy than in the disk?

A) There are so many globular clusters in the halo that their total mass is greater than the mass of stars in the disk.
B) Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk.
C) Although the question of mass in the halo was long mysterious, we now know it exists because we see so many brown dwarfs in the halo.
D) The recent discovery of photinos, combined with theoretical predictions, tells us that there must be a huge mass of photinos in the halo.
E) We don't know that there is more mass in the halo; it is only a guess based on theory

Answer 1

The much greater mass in the Milky Way's halo compared to its disk is inferred from the high orbital speeds of stars on its outskirts, which suggest a significant amount of dark matter providing the needed gravitational pull to keep these stars in their rapid orbits. The correct answer is option B.

Step-by-step explanation:

We know there is much more mass in the halo of our galaxy than in the disk because of the gravitational effects of this unseen matter. The correct answer to the question is B) Stars in the outskirts of the Milky Way orbit the galaxy at much higher speeds than we would expect if all the mass were concentrated in the disk. This unexpected orbital speed is explained by the presence of a large amount of dark matter in the halo exerting additional gravitational force, keeping those fast-moving stars bound to the galaxy.

These outcomes have been observed not just in the Milky Way but in other spiral galaxies as well. Furthermore, studies have shown that the total mass of the Galaxy is at least 2 × 10¹² Msun (solar masses), which is about twenty times greater than the amount of luminous matter. This vast dark matter halo extends to at least 200,000 light-years from the center of the Galaxy, well beyond the luminous stars, and impacts the orbits of distant star clusters and dwarf galaxies associated with the Milky Way.