Over the past decade we've discovered hundreds of celestial objects we call brown dwarfs. Actually are more reddish than brown. Theories about them have been around for decades but it's only recently that we've been able to find and observe them. Brown dwarfs are challenging for astronomers because they're tough to classify. They have masses too large for a planet but too small for a star, and they share some characteristics with planets and others with stars. For example, they seem more like planets in that many of them orbit around stars, but they apparently form not like planets but in much the same way stars do at least initially. Remember stars originate in huge clouds of dust and gas thousands of light years across, ur molecular clouds each with enough material to make dozens of stars. Young stars forming in the denser regions of molecular clouds known as cores, which eventually collapse due to their own gravity . Now within any given molecular cloud there can be several cores. And when they collapse, the inner portions break up into humps, which are stellar embryos, stars in the process of forming. So a collapsing core can contain several stellar embryos, several of which can become stars. 。。。。。余下托福听力真题原文省略！
Question 1 of 6
What is the main purpose of the lecture?
O A. To explain methods astronomers use to classify stars
O B. To explain the formation of molecular clouds in the universe
O C. To discuss how some stellar embryos fail to become stars
O D. To discuss similarities between brown dwarfs and planets
Question 2 of 6
According to the professor, why is the study of brown dwarfs particularly challenging?
O A. They cannot be detected directly.
O B. They combine characteristics of very distinct celestial objects.
O C. They appear in colors ranging from brown to red.
O D. They are always near very bright stars.