请教大家补充题２２篇（２１，２２）

irence_928

这篇文章我觉得结构清晰，可就是觉得定位不对．

Passage 84 (21/22)

Astrophysicists wrestling with the study of a new kind of star, the flat, “two-dimensional” configurations known as accretion disks have recently gained new insights into the behavior of these stars. Accretion disks exist in a variety of situations where matters swirl around a compact star such as a white dwarf star or a neutron star. Accretion disks are also suspected of playing a part in more exotic situations, in which the central object is imagined to be a supermassive black hole, the ultimate form of collapsed matter, rather than a compact star. The modeling of accretion disks is still in its infancy, a situation analogous to the days when ordinary stars were modeled by using elementary scaling laws without benefit of knowledge of the nuclear processes that power the stars. Similarly, the basic physics of the power by which accretion disks radiate, thought to originate in a form of turbulent friction, is known only at the crudest level.

Accretion disks were first defined in the context of Cataclysmic variables. In these systems, matter from the outer layers of an ordinary star is attracted by the gravitational influence of a nearby orbiting white dwarf star, the matter lost from the ordinary star cannot strike the surface of the tiny white dwarf directly but settles into an orbit around the star. The viscosity in the disk thus formed causes heating, radiation, and a slow spiraling of disk matter onto the surface of the white dwarf.

The rapid advances made in x-ray astronomy in the past decade have identified a second type of system in which accretion disks occur. In such a system, an accretion disk whirls about a neutron star rather than a white dwarf. The inner reaches of the accretion disk extend deeply into the gravitational potential of the neutron star where very rapid motion is the rule. The energy released by friction and the actual raining of the material from the disk onto the surface of the neutron star is so great that radiation is given off in a powerful flood of x-rays. And in at least one case, x-ray astronomers believe that the object in the center of an accretion disk is a black hole, suggesting that a third system may exist.

It had been assumed that portions of accretion disks would be unstable and that, as a result, clumping of their matter into rings would occur. There is no evidence from observation, however, that accretion disks do, in fact, suffer from these instabilities. In recent work, Abramowicz has shown that added gravitational effects due to general relativity may alter the expected Newtonian gravitational relationships in such a way that the disk remains stable, indicating that it is possible that these predicted instabilities do not occur.

Further progress toward understanding accretion disks will involve defining and proposing solutions to restricted problems just as was done in this case and was done and continues to be done for ordinary stars. Abramowicz’ work is a valuable example of the care (a person or thing that is an object of attention, anxiety, or solicitude)
                that must be taken before reaching conclusions regarding accretion disks.

4.     The passage suggests which of the following about current scientific knowledge of the nuclear processes of ordinary stars?

(A) Its pattern of development has been analogous to that of developments in x-ray astronomy.

(B) Its role in the explanation of turbulent friction has been significant.

(C) It has contributed to a more accurate modeling of ordinary stars.

(D) It lags behind knowledge of scaling laws.

(E) It explains the behavior of accretion disks as well as that of ordinary stars.

它的答案是Ｃ哦．有哪位做过了又做对了的，能不能告诉我在哪里定位呢．

8.     According to the passage, some accretion disks originated in

(A) an increase in heat and radiation around an ordinary star

(B) a powerful flood of x-rays emitted by a neutron star

(C) a collision between two stars

(D) the turbulent friction on the surface of a compact star

(E) the accumulation of matter removed from an ordinary star

答案是Ｅ哦．我选了Ａ　．也请好心人告诉我定位．