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Listen to an Academic Talk Test #005
Listen to an Academic Talk
1. What is the lecture mainly about?
A) The life cycle of a star
B) A type of matter that cannot be seen
C) Methods for observing distant galaxies
D) The formation of the solar system
2. According to the lecturer, how did scientists discover dark matter?
A) By detecting strange light emissions
B) By noticing the unexpected movement of stars
C) By analyzing the composition of gas clouds
D) By measuring the temperature of galaxies
3. What would likely happen to galaxies without dark matter?
A) They would become much brighter
B) They would not be able to hold their shape
C) They would spin much more slowly
D) They would contain fewer stars
4. Why does the lecturer mention the laws of physics?
A) To introduce a theory that was proven wrong
B) To explain how telescopes are built
C) To provide the basis for an expectation
D) To question the accuracy of observations
Lecturer: When we look at galaxies, we see stars, gas, and dust. But astronomers have found that this visible matter only accounts for a small fraction of a galaxy's total mass. The rest is made up of something we can't see, which we call dark matter. It doesn't emit or reflect any light.
So, how do we know it's there? The evidence comes from gravity. According to the laws of physics, stars farther from the center of a rotating galaxy should move more slowly than stars closer to the center. However, observations consistently show this is not the case. Stars on the outer edges of galaxies are moving just as fast, if not faster.
This unexpected speed tells us there must be a huge amount of extra, unseen mass providing the gravitational pull needed to keep these fast-moving stars in orbit. Without this dark matter, the galaxies would fly apart.
So, how do we know it's there? The evidence comes from gravity. According to the laws of physics, stars farther from the center of a rotating galaxy should move more slowly than stars closer to the center. However, observations consistently show this is not the case. Stars on the outer edges of galaxies are moving just as fast, if not faster.
This unexpected speed tells us there must be a huge amount of extra, unseen mass providing the gravitational pull needed to keep these fast-moving stars in orbit. Without this dark matter, the galaxies would fly apart.
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