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