Physical theory implies that the existence of astronomical entities ab

Physical theory implies that the existence of astronomical entities above a certain mass is evidence for the existence of black holes. The Earth does not itself collapse upon itself under gravitational force because gravity is countered by the outward pressure generated by the electromagnetic repulsion between the atoms making up the planet. But if these forces are overpowered, gravity will always lead to the formation of a black hole. Assuming the validity of general relativity, we can calculate the upper bound for a star, the Tolman-Oppenheimer-Volkoff limit, to be 3.6 solar masses; any object heavier than this will be unable to resist collapse under its own mass and must be a black hole.

The search for entities more massive than the Tolman-Oppenheimer-Volkoff limit brings us to the examination of X-ray binary systems. In an X-ray binary, two bodies rotate around their center of mass, a point between them, while one component, usually a normal star, sheds matter to the other more massive component known as the accretor. The shedding matter is released as observable X-ray radiation. Since binary stars rotate around a common center of gravity, the mass of the accetor can be calculated from the orbit of the visible one. By 2004, about forty X-ray binaries that contained candidates for black holes had been discovered. The accretors in these binary systems did not appear visible, as is to be expected of black holes, but that fact alone does not distinguish them from very dense and hence less luminescent stars, such as neutron stars. More to the point is that these accretors were of mass far in excess of 3.6 solar masses. Famously, Cygnus X-1, an X-ray binary in the constellation Cygnus, has an accretor whose mass has been calculated to be 14 solar masses, plus or minus 4 solar masses. While does not rule out other phenomena without further interpretation, it provides strong proof that black holes exist.

The conclusion that black holes exist depends on the reliability of the general-relativistic calculations involved. If more generous assumptions are made, the Tolman-Oppenheimer-Volkoff limit can be calculated to be as high as 10 solar masses. The finding also establishes plausibility, if not direct evidence, for the existence of supermassive black holes hypothesized to exist at the center of some galaxies.

1. The passage is primarily concerned with

A. defending a controversial approach
B. criticizing an accepted view
C. summarizing research findings
D. contrasting competing theories
E. describing an innovative technique

2. The passage indicates that an accretor found in an X-ray binary and which has a mass greater than Tolman-Oppenheimer-Volkoff limit

A. is probably not a black hole
B. is probably a black hole and has a mass between 3.6 and 10 solar masses
C. is probably a black hole and has a mass greater than 10 solar masses
D. is probably a black hole and is in motion
E. is probably a black hole and is stationary

3. Which of the following statements, if true, most weakens the authors' suggestion that the record of X-ray binaries provides evidence for the existence of black holes?

A. No accreting mass in any of the X-ray binaries recorded is large enough to be a supermassive black hole.
B. The record considers only binary systems in which a star that sheds mass is less massive than the accretor.
C. In most of these binary systems, the X-ray emissions cannot conclusively be attributed to black holes.
D. The accretors in most of these systems have a mass near the lower end of the range estimate for the mass of Cygnus X-1.
E. The accretors in most X-ray binary systems are known to be neutron stars or white dwarves and not to be black holes.

4. It can be inferred from information presented in the passage that if the mass of a stellar object is less than 3.6 solar masses, which of the following must be true as a consequence?

A. The object cannot be a neutron star.
B. The object cannot be a black hole.
C. The object will not necessarily collapse under its own gravitational pull.
D. The object cannot shed X-ray emissions.
E. The object cannot be an accretor in an X-ray binary system.

5. The author mentions all of the following as weaknesses in the argument for black holes' existence EXCEPT

A. The argument assumes the validity of the theory of general relativity.
B. The argument does not exhaustively rule out that seemingly invisible stellar entities could be entities other than black holes.
C. The Tolman-Oppenheimer-Volkoff limit may be higher than previously calculated.
D. The masses of many accretors in X-ray binaries thought to be black holes are less than the more conservative calculation of the Tolman-Oppenheimer-Volkoff limit.
E. The evidence given supports the existence of only certain types of black holes hypothesized to exist.