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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.

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. >> IELTS TUTOR lưu ý: Phân tích&Bài Sửa HS đạt 7.0"The diagram below shows how to recycle organic waste to produce fertiliser (compost)" IELTS WRITING TASK 1

Once the temperature reaches 15,000,000°C, 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 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. >> IELTS TUTOR lưu ý: PHÂN TÍCH ĐỀ THI 30/5/2020"Some education systems make students focus on certain subjects at the age of 15, while others require students to study a wide range of subjects until they leave school. What are the benefits of each system? Which is better?"IELTS WRITING TASK 2 (kèm bài sửa HS đạt 6.5)

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 centre 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.

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.

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 exploded as supernovas. These massive stars have an average lifespan 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 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 centre of each galaxy. >> IELTS TUTOR lưu ý: Phân tích & Sửa đề"Traffic and accommodation problems are increasing and the government should encourage some businesses to move from cities to rural areas. Does the advantages outweigh the disadvantages?" (ngày21/11/2020)

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 – 6: Different stages and types of stars are mentioned in Reading Passage 1. Choose ONE of the types or stages (A–H) from the box below which best matches the descriptions. NOTE: you may use any answer more than once.

A nebula E black dwarf
B main sequence star F supernova
C red giant G neutron star
D white dwarf H black hole

Example: hottest, brightest point of a 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. Write your boxes 7 – 13 on your Answer Sheet.

7. Hydrogen will turn into 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°C the star’s helium atoms fuse into carbon atoms, and then the star____.
10. Unlike small and medium-sized stars, large 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 exploded celestial bodies. >> IELTS TUTOR lưu ý: Phân tích +kèm sửa bài"The table below shows the number of visitors in the UK and their average spending from 2003 to 2008" IELTS WRITING TASK 1 (table)

Đáp án:

1. B

2. A

3. D

4. G

5. H

6. E








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