IELTS Sample Exam

The life cycle of a star

It has been conservatively estimated that there are some 10,000 billion, 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. 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 in 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 I5,000,000 degree celsius, nuclear fusion occurs in the centre 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.

As it glows, hydrogen in the centre (through the 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 warmly 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.

Exactly how a star will react in the red giant phase depends on its rnass. Throughout the red giant phase, the hydrogen in the outer parts carrier on burimig, and the centre gets hotter and hotter. On reaching 200,000,000 degree celsius, the helium atoms fuse forming carbon atoms. The remainder of the hydrogen explodes and forms a ring sound the core called a planetary nebula.

With medium-sized stars, once tote 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.

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When the larger red giants (massive stars) collapse, which happens in an instant, so much planetary nebulae is created that this gas and dust can be used as 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 header 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 hugo explosion called a supernova. A supernova can have a core temperature of up to 1,000,000,000 degree celcius 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 exploded as supernovas. These massive stars have an average life span of one million years.

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 supporting 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 this existence of the first black hole, X-ray binary star Cygnus X-1 in 1971. Astronomers think that there may be a blackhole at the center of each galaxy.

The life cycle of a star is really that - the materials aon 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 1-13 are based on Reading Passage 1
Questions 1 - 6
Different Stages and types of stars are mentioned in Reading Passage 1. Choose ONE of the types or stages (A-H) from the list below which best matches the description (Question 1-6). Enter your answers in boxes 1-6 below.
NOTE: you may use any answer more than once.
A - nebula
B - main sequence star
C - red giant
D - white dwarf
E - black dwarf
F - supernova
G - neutron star
H - black hole
Example:
Question: Hottest, brightest point of star
Answer: F

1. the Sun
2. birthplace of a star
3. a dying star
4. sometimes has pulsating waves
5. its size is immeasurable
6. its supply of energy has run out

Questions 7 - 13
Complete the sentences using NO MORE THAN THREE WORDS for each answer. Enter your answers in the boxes 7 - 13 below

7. Hydrogen will turn to helium after
occurs.
8. The colour of the red giant is formed as the
becomes smaller and the outer areas cool.
9. At 200,000,000 degree celsius the star's helium atoms fuse into carbon atoms, and then the star
10. Unlike small and medium-sized stars, largs stars
quickly.
11. A black hole's
stops light from being emitted.
12. Astronomers knew about
before they were able to confirm their existence.
13. Planets and stars are likely to consist of
from explodes celedtial bodies.