This is an IELTS Reading Practice about astronomy Topic
HOW THE PAULI EXCLUSION PRINCIPLE REGULATES THE EVOLUTION OF STARS
All stars (like plants and animals) evolve, with each one following the same general pattern of evolution. Their journey along the evolutionary path, and ultimate fate at stellar death, is determined by their initial mass, which is measured in multiples of the solar mass of our own Sun. (11)
Perturbations of nebulous interstellar clouds, in space result in gravitational interaction, with the consequent contraction of gaseous matter to create protostars (1), which are much larger than the stars they will finally become. As the temperature increases, the gas become completely ionized to form plasma (2) and gravi-tational contraction of the cores then takes place. The onset of hydrogen- burning hap- pens at a core temperature of several mil-lion degrees, and converts hydrogen to helium (4) through nuclear fusion (3). The greater part of star’s evolutionary lifetime spent hydrogen-burning and, during this period, it is said to be on the Main Sequence (7). The end of hydrogen-burning is marked by the evolution of a star into the red giant (5), then it is sais to leave the Main Sequence. Burning ceases completely in the core, which under- goes gravitational contraction to maintain mechanical equilibrium (6).
Now, the Pauli Exclusion Principe states that “no two identical particles can occupy the same quantum state” (9) (Kaufmann, 1994): that is, loosely, they cannot have the same spatial location and momentum. This prin-ciple is important in determining the ulti-mate fate of stars. Consider low Main Sequence mass stars( this is, stars of less than three solar masses) which have passed through the hydrogen- burning phase to he-lium-burning. Such bodies require extreme compression of the core to raise their tem-perature sufficiently for the onset of he-lium-burning. Increasing desity of elec-trons occurs, so that they are squashed into close proximity with each other, until a limit is reached when they resist any further compression. This phenomenon is called de-generacy, and is a manifestation of the Pauli Exclusion Principle (8). Resistance to further compression results in degenerate- electron pressure which supports the core, prevent-ing its contraction. However, this pressure is independent of temperature continues to increase. Helium ignition takes place and the thermonuclear reaction proceeds at an increasing rate until a helium-flash occurs. The temperature is so great that degeneracy cannot be maintained: the core suddenly ex-pands with a corresponding decrease in temperature that abruptly ends the he-lium-flash. This cycle may be repeated until all the core helium is converted to carbon.
More massive stars do not undergo a be- lium-flash (10). Moreover, their cores are suffi-ciently massive for further element-burning to occur, until they, too, reach a limit im-pased by degeneracy. That is, as the pro-duct of each phase of element-burning is al-ways nuclei of greater mass, it requires even greater compression of the core remnant in order to raise the temperature sufficiently high enough to initiate the next phase. Such compression can only occur until the degen-erate condition is achieved.
Stellar death comes about when the core cannot carry out further element-burning, because of its degenerate nature. Stars of Main Sequence mass less than seven solar masses become white dwarfs (12). The stability of a white dwar5f is only maintained if its final(post- Main Sequence) mass does not exceed the Chandrasekhar Limit of 1.4 solar masses. Degenerate-electron pressure supports the core against collapse, thereby conforming to the Principle.
Neutron stars (13) are the stellar corpses of stars whose Main Sequence mass is between seven and twenty solar masses. Before death, these stars have undergone some fur-ther element-burning and the final core mass exceeds the Chandrasekhar Limit. This is too great for degeneracy equilib-rium is achieved once more. It is degener-ate-neutron pressure that halts the collapse, and, thereby, upholds the Principle.
The most massive stars have completed burn-ing to obtain an iron core, and have a Main Sequence mass exceeding twenty solar masses. This is so great that degener-ate-neutron pressure cannot support it, and rapid collapse ensues. Since density is in-versely proportional to volume and the mass is vast, then, as the volume dwindles, the density tends to infinity and a Black Hole is formed (14).
Black Hole are a violation of Pauli’s Ex-clusion Principe. If the Principle did not regulate the evolution of stars, nothing would prevent the inexorable collapse of an inter-stellar cloud from its initial disturbance into a massive Black Hole.
Questions 1-7: The flow chart below summarises Paragraph 2. You may use NO MORE THAN THREE WORDS to complete each space.
Shapless clouds in space become...........1.............With the rise in temperature, ......2............is formed from the ionised gas and the core then contracts. At extremely high temperatures, hydrogen-burning occurs through............3............. and hydrogen is converted to..........4.............. When the hydrogen-burning stops,........5............is formed. The core then stops burning altogether and contracts to........6............ This stage is called..........7................
3. Nuclear fusion
5. Red giant
6. Maintain mechanical equilibrium
7. Main sequence
Question 8-11: Choose the appropriate letters A-D and write them in Boxes 8-11 on your answer sheet.
8. What can be said about degeneracy?
A It violates the Pauli Exclusion Principle. stars, but not of smaller ones?
B It is not dependent upon temperature.
C It is the point where the core of a star withstands further compression.
D It happens to most, but not all stars.
9. According to the Pauli Exclusion Principle,...
A no two stars are the same.
B low mass stars do not degenerate.
C it is not possible for two identical particles to be in the same space at the same time.
D when a star is compressed, the temperature and the pressure rise.
10. Which of the following is true of the largest
A Compression of their core is halted by degeneracy.
B Their core becomes iron.
C Their fate is stellar death.
D They undergo a helium flash.
11. What affects the development of all stars?
A Their stellar death.
B Their evolutionary path.
C Their mass when they are first formed.
D Their size when compared to our own Sun.
Questions 12- 14: Use information from the passage to complete the table below. Use No More Than Two Words for each space.
Stars with main sequence mass of
Less than 7
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