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 that must be taken before reaching conclusions regarding accretion disks.
1. The author of the passage is primarily concerned with
(A) comparing Abramowicz’ work to the work of earlier astrophysicists
(B) providing information about accretion disks and discussing significant new work
(C) defining the conditions under which accretion disks can be observed
(D) exploring the question of whether a black hole can ever be the central object of an accretion disk
(E) describing the phenomenon of accretion disks and reviewing several conflicting theories of their origins
2. It can be inferred from the passage that predictions of the instability of accretion disks were based on which of the following?
(A) A calculation of the probable effects of standard Newtonian gravitational relationships
(B) A calculation of the probable relationship between general relativity and standard Newtonian gravitational relationships
(C) A calculation of the energy released by friction within a compact star
(D) Observation of the x-rays radiated by compact stars
(E) Observation of the clumping of accretion disk matter into rings around compact stars
3. The author’s attitude toward Abramowicz’ work can best be described as one of
(A) uncertain approval
(B) unqualified respect
(C) mild interest
(D) careful dismissal
(E) hostile skepticism
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.
5. The passage suggests that Abramowicz’ work was motivated by which of the following assumptions?
(A) The quantity of energy released by accretion disks can be as large as it is only if the disks are stable.
(B) Improved techniques in x-ray astronomy would reveal any instabilities occurring in accretion disks.
(C) The lack of observational evidence of instabilities in accretion disks suggests that predictions of their occurrence might be wrong.
(D) Known methods of observing accretion disk surrounding compact stars and black holes do not permit the observation of the matter in accretion disks.
(E) The gravitational potential of compact stars does not vary from star to star.
6. The passage implies which of the following about the progress of knowledge in astrophysics?
(A) Adherence to outdated theories has, in the past, limited the activities of astrophysicists and restricted progress.
(B) Progress has, in the past, occurred only as a result of significant breakthroughs in basic physics and chemistry.
(C) Progress has, in the past, occurred as a result of a process of defining and solving restricted problems.
(D) Given the recent acquisition of knowledge about the nuclear processes of stars, further progress is likely to be limited to the refinement of what is already known.
(E) Conclusions in astrophysics have, in the past, been seriously flawed, thus limiting progress, although there have recently been signs of change.
7. The passage suggests that, compared to the study of ordinary stars, the study of accretion disks is
(B) more sophisticated
(C) less clearly focused
(D) at an earlier stage of development
(E) more dependent on technological advances
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
9. It can be inferred from the passage that the significance of Abramowicz’ work is that it
(A) provides a means of measuring the gravitational potential of neutron stars
(B) opens a new area for exploration in the field of x-ray astronomy
(C) proves that scaling laws cannot be applied to accretion disks
(D) proposes a new system of classification of stars
(E) suggests a resolution of a discrepancy between a theoretical prediction and actual observation
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