L4：
讲座
1. What is the main purpose of the lecture?
To explain methods astronomers use to classify stars
To explain the formation of molecular clouds in the universe
To discuss how some stellar embryos fail to become stars
To discuss similarities between brown dwarfs and planets
2. According to the professor, why is the study of brown dwarfs particularly challenging?
They cannot be detected directly.
They combine characteristics of very distinct celestial objects.
They appear in colors ranging from brown to red.
They are always near very bright stars.
3. Why does the professor discuss how stars originate?
To explain how brown dwarfs begin to form
To suggest that brown dwarfs do not originate in molecular clouds
To explain why brown dwarfs emit light billions of years
To show that stellar embryos cause turbulence within molecular clouds
4. According to the ejection theory, why do some stellar embryos stop growing before they become stars?
The motion of dust and gas inhibits their growth.
The cores in which they form are not dense enough.
They start forming in the area of a molecular cloud with the least amount of material. They are moved by gravitational forces to areas outside cores.
5. Why does the professor mention that newborn stars are surrounded by disks of dust and gas?
To describe a method for testing two theories about brown dwarfs
To clarify how brown dwarfs are drawn into star systems
To emphasize that brown dwarfs move at low velocities
To introduce planet formation as the topic of the next lecture
6. What is the professor’s attitude toward the two theories?
He is convinced that neither of them can explain why brown dwarfs have stellar disks.
He hopes both theories will be confirmed by computer simulations.
He thinks evidence supports the turbulence theory even if he cannot rule out the ejection theory.
He finds the ejection theory more attractive than the turbulence theory.
