Answers for The Life Cycle of a Star - IELTS Reading Test

A. It has been conservatively estimated that there are some 10,000 billion stars in the universe. It is difficult to know the exact age of a star (astronomers have identified stars as young as 25,000 years old and others are thought to be over 10 billion years old), but what astronomers do know is that there are many different kinds.
B. How each star is formed, and its mass, influences its type and longevity. A star is born in a nebula, which is a giant cloud of gas and dust. The larger the amount of matter that is into the nebula, the greater the mass of the star that is created. Inside these nebulae are dense areas of gas, which, due to their density, have a stronger gravitational pull than the rest of the nebula. Gradually, gravity drags the gas in the nebula together and it begins to spin and become increasingly hotter.
Once the temperature reaches 15,000,000°C, nuclear fusion occurs in the center of the cloud and it begins to glow brightly. It stabilizes at this temperature, contracts slightly, and becomes what is known as a main-sequence star (an example of this is our own Sun). It can remain in this stage for millions or billions of years.
C. As it glows, hydrogen in the center (through nuclear fusion) becomes helium. Eventually, the hydrogen supply in the core diminishes and the core of the star becomes unstable, contracting more. However, the outer parts of the star (which are still mainly hydrogen) expand and cool, and in doing so, the star starts to glow red.
It is at this stage that the star becomes a red giant. It is anticipated that it will take the Sun another 5 billion years to reach this stage. By then it will have grown large enough to engulf the three closest planets (Mercury, Venus, and Earth) and glow 2,000 times brighter than it currently does.
D. Exactly how a star will react in the red giant phase depends on its mass. Throughout the red giant phase, the hydrogen in the outer parts carries on burning, and the center gets hotter and hotter. On reaching 200,000,000°C, the helium atoms fuse forming carbon atoms. The remainder of the hydrogen explodes and forms a ring around the core called a planetary nebula.
E. With medium-sized stars, once the final helium atoms have fused into carbon atoms, the star starts to die. The gravitational pull leads to the last of the star's matter collapsing inwards and compacting to become extremely dense. A star like this is called a white dwarf. It will shine white- hot until the remaining energy (thermal energy trapped in its interior) has been exhausted after which it will no longer emit light. This can take in excess of several billion years. It is then termed a black dwarf (a cold, dark star, perhaps replete with diamonds) and remains in that stage forever.
F. When the larger red giants (massive stars) collapse, which happens in an instant, so much planetary nebula is created that this gas and dust can be used as a building material for planets in developing solar systems. In addition, with massive stars, as the temperature increases, the Carbon atoms get pulled together to form increasingly heavier elements like oxygen, nitrogen, and finally iron. Once this happens, fusion ceases and the iron atoms begin absorbing energy. At some point in the future, this energy is released in a huge explosion called a supernova. A supernova can have a core temperature of up to 1,000,000,000°C and the explosion can light up the sky for weeks, outshining an entire galaxy. Astronomers believe that Earth is made up of elements formed from the inside of stars, in particular, red giants that explode as supernovas. These massive stars have an average lifespan of one million years.
G. After becoming a supernova, the remaining core of a massive star that is 1.5 to 4 times as massive as the Sun becomes a neutron star. It starts to spin and often emits radio waves. If these waves occur in pulses, the neutron star is referred to as a pulsar. When a massive star has eight or more times the mass of the Sun, it will remain massive after the supernova. It has no nuclear fusion at the core and becomes engulfed by its own gravity. This results in a black hole, which sucks in any matter or energy that passes close to it. The gravitational field of a black hole is powerful enough to prevent the escape of light and is so dense that it cannot be measured. The phrase ‘black hole’ originated from the physicist John Archibald Wheeler; before this, black holes were known as ‘frozen stars.’ Wheeler came up with this name two years before the proof of the existence of the first black hole, X-ray binary star Cygnus X-1, in 1971. Astronomers think that there may be a black hole at the center of each galaxy.
H. The life cycle of a star is really that — the materials from an exploded star mix with the hydrogen of the universe. This mixture, in turn, will be the starting point of the next star. The Sun is a case in point, containing the debris from numerous other stars that exploded long before the Sun was born.

Questions 1-7

Complete the summary below. 

Write NO MORE THAN ONE WORD from the text for each answer. 

Vast clouds of gas and dust called 1._________ are where stars are created. The denser regions of these clouds generate stronger 2. ________, drawing gas together. When temperatures exceed 15,000,000°C, this process causes the nebula to rotate and heat up, which results in 3.________ in its core. A stabilised star will become a main sequence and stay that way for millions to billions of years. 

 Through nuclear combustion, the star gradually transforms hydrogen into 4. ________. While the outer layers expand and cool, the core becomes unstable and compresses as its hydrogen runs out. At this point, the star enters the crimson giant phase, when it glows red. In roughly 7.5 billion years, the Sun will turn big enough to swallow up Earth and its nearby planets, according to 5._________. By this point, the Sun's brightness is anticipated to have increased two thousand times above its current level. While smaller stars may stabilise as white 6. _______stars or eventually cool and become black, stars with more mass than the Sun will eventually undergo more transformations. A star's 7. _________is the primary determinant of its life cycle.

Questions 8-13

The Reading Passage has sections, A-H.

Which section contains the following information?

Write the correct letter, A-H, in boxes 10-13 on your answer sheet.