the stars at the edges of a galaxy are rotating around the galaxy's center at the same speed as the stars in the middle of the galaxy. how does this provide evidence of dark matter?
a. dark matter emits the light that astronomers need to see the orbital speeds of the stars
b. dark matter absorbs light from outside the galaxy, allowing the light of the stars in the galaxy to show the orbital speeds of the stars
c. dark matter provides the gravity that allows the stars to rotate at the same speed
d. dark matter subtracts gravity, which allows the stars to rotate at the same
speed
c. dark matter provides the gravity that allows the stars to rotate at the same speed
color-speed.
The correct answer is c. Dark matter provides the gravity that allows the stars to rotate at the same speed.
This is known as the "galaxy rotation problem." According to Newtonian gravity, stars farther from the center of a galaxy should have slower orbital speeds compared to stars closer to the center. However, observations have shown that the stars at the edges of galaxies are rotating at the same speed as stars in the middle.
This indicates that there must be more mass in the galaxy than can be accounted for by the visible matter (stars, gas, and dust). Dark matter, which does not emit or absorb light, provides the additional gravitational force needed to explain the observed rotational velocities of stars. Therefore, the presence of dark matter is inferred by its gravitational effects on the rotation of galaxies.
The correct answer is c. dark matter provides the gravity that allows the stars to rotate at the same speed.
To understand why this provides evidence for dark matter, we need to consider the behavior of visible matter such as stars. In a galaxy, stars closer to the center should experience stronger gravitational forces and thus move faster than stars at the outer edges. This is similar to how planets closer to the Sun move faster in their orbits compared to those farther away.
Yet, observations have shown that stars at the edges of galaxies rotate around the galactic center at approximately the same speed as stars in the middle. This is unexpected because according to the known laws of gravity, the outer stars should experience weaker gravitational forces and should rotate at slower speeds.
To explain this observation, scientists propose the presence of dark matter. Dark matter is a hypothetical form of matter that does not emit, absorb, or reflect light, making it invisible to telescopes. However, it interacts with matter through gravity.
The idea is that the gravitational pull from dark matter provides the additional gravity needed to keep the outer stars in the galaxy from flying off into space. This additional gravitational force allows stars at the outer edges to rotate at roughly the same speed as stars in the middle.
Therefore, the fact that stars at the edges of galaxies rotate at similar speeds to central stars provides strong evidence for the existence of dark matter, as this behavior can only be explained by the presence of additional unseen mass to provide the necessary gravitational forces.