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?(1 point)
A. 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.
B. Dark matter emits the light that astronomers need to see 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.
The correct answer is C. Dark matter provides the gravity that allows the stars to rotate at the same speed.
To understand why this is evidence for the existence of dark matter, let's break it down:
In a galaxy, the gravitational force between stars and other objects determines how they move. According to Newton's laws of motion, as the distance from the center of a rotating object increases, the orbital speed should decrease. This principle is known as Kepler's laws of planetary motion.
However, observations have shown that the stars at the edges of galaxies are rotating around the galactic center at roughly the same speed as the stars in the middle of the galaxy. This is unexpected because if the only matter in the galaxy were visible matter (stars, planets, gas, and dust), the outer stars should be moving slower due to the weaker gravity at their distances from the center.
The fact that the stars at different distances from the center have the same rotational speeds points towards the existence of additional mass in the galaxy, one that does not emit light or interact with electromagnetic radiation—this is dark matter.
Dark matter, being invisible and not emitting light, does not directly affect the observed orbital speeds of stars. However, since it has mass and interacts gravitationally, its presence provides the necessary gravitational pull to keep the stars at the edges of galaxies moving at similar speeds as the stars in the middle. In other words, dark matter provides the additional gravity needed to explain the observations.
Therefore, option C, "Dark matter provides the gravity that allows the stars to rotate at the same speed," is the correct answer.