Brown+Dwarfs



BROWN DWARFS 1. A brown dwarf is a celestial body that has never quite become a star. A typical brown dwarf has a mass that is 8 percent or less than that of the Sun. The mass of a brown dwarf is too small to generate the internal temperatures capable of igniting the nuclear burning of hydrogen to release energy and light.

2. A brown dwarf contracts at a steady rate, and after it has contracted as much as possible, a process that takes about 1 million years, it begins to cool off. Its emission of light diminishes with the decrease in its internal temperature, and after a period of 2 to 3 billion years, its emission of light is so weak that it can be difficult to observe from Earth.

3. Because of these characteristics of a brown dwarf, it can be easily distinguished from stars in different stages of formation. A brown dwarf is quite distinctive because its surface temperature is relatively cool and because its internal composition—approximately 75 percent hydrogen—has remained essentially the same as it was when first formed. A white dwarf, in contrast, has gone through a long period when it burns hydrogen, followed by another long period in which it burns the helium created by the burning of hydrogen and ends up with a core that consists mostly of oxygen and carbon with a thin layer of hydrogen surrounding the core.

4. It is not always as easy, however, to distinguish brown dwarfs from large planets. Though planets are not formed in the same way as brown dwarfs, they may in their current state have some of the same characteristics as a brown dwarf. The planet Jupiter, for example, is the largest planet in our solar system with a mass 317 times that of our planet and resembles a brown dwarf in that it radiates energy based on its internal energy. It is the mechanism by which they were formed that distinguishes a high-mass planet such as Jupiter from a low-mass brown dwarf.

16. It is stated in the passage that the mass of an average brown dwarf

A. is smaller than the mass of the Sun

B. generates an extremely high internal temperature

C. is capable of igniting nuclear burning

D. causes the release of considerable energy and light

17. According to paragraph 2, a brown dwarf cools off

A. within the first million years of its existence

B. after its contraction is complete

C. at the same time that it contracts

D. in order to begin contracting core

18. What is stated in paragraph 2 about a brown dwarf that has cooled off for 2 to 3 billion years?

A. Its weak light makes it difficult to see from Earth.

B. It no longer emits light.

C. Its weak light has begun the process of re- strengthening.

D. Scientists are unable to study it.

19. It is indicated in paragraph 3 that

A. the amount of hydrogen in a brown dwarf has increased dramatically

B. a brown dwarf had far more hydrogen when it first formed

C. three-quarters of the core of a brown dwarf is hydrogen

D. the internal composition of a brown dwarf is always changing

Answer key: 16, 17, 18, 19, 20, 21, 22 ABACDCD