Academic Reading — Test 155

Reading passage

A star is, in essence, an immense sphere of gas that holds itself together against the relentless inward pull of its own gravity. For a star roughly the mass of our Sun, this balancing act lasts an astonishingly long time. The story of such a star is not one of constant drama but rather of a long, stable middle age followed by a comparatively brief and spectacular series of changes near the end. Understanding this sequence has been one of the central achievements of modern astronomy, and it rests on the simple idea that a star's fate is decided, above all, by the amount of material it contains. A Sun-like star is born inside a vast cloud of gas and dust. Gravity draws this material together until the centre becomes so dense and hot that the nuclei of hydrogen atoms begin to fuse into helium. This fusion releases enormous quantities of energy, and the outward pressure it generates pushes back against gravity. When these two forces settle into a steady balance, the star is said to have reached the main sequence, the phase in which it will spend the vast majority of its existence. Our own Sun has been on the main sequence for roughly four and a half billion years and is expected to remain there for a similar span before any major change occurs. Throughout this period the star shines with remarkable steadiness, slowly converting hydrogen in its core into helium. No supply of fuel lasts for ever, however. Once the hydrogen in the core has been largely exhausted, the delicate equilibrium is disturbed. The core, now composed mainly of helium, contracts under gravity and grows hotter, while the outer layers respond by expanding dramatically. As these outer layers swell, they cool and shift towards redder colours, and the star becomes what astronomers call a red giant. During this stage the star may grow to many times its original diameter. In the distant future, when the Sun enters this phase, its bloated surface is expected to reach beyond the orbit of the innermost planets, and possibly as far as the Earth itself, although the precise outcome remains uncertain. Inside the red giant, conditions eventually become extreme enough for a new round of fusion to begin. The compressed helium core reaches a temperature at which helium nuclei fuse to form carbon and oxygen. This fresh source of energy temporarily stabilises the star once more, but the reprieve is short by stellar standards. A Sun-like star does not possess sufficient mass to push its core to the still higher temperatures that would be needed to fuse carbon into heavier elements. As a result, fusion cannot continue indefinitely, and the star approaches the final chapter of its life with a core of inert carbon and oxygen that it can no longer burn. What follows is comparatively gentle. The outer layers of the dying star, only loosely bound by gravity, are gradually puffed away into surrounding space. These expelled gases form a glowing shell known, somewhat misleadingly, as a planetary nebula, a name that has nothing to do with planets and was coined long ago because such objects looked round and planet-like through early telescopes. The nebula drifts outward and slowly fades, enriching the surrounding region with the elements the star has manufactured. Left behind at the centre is the exposed core: a small, extraordinarily dense object called a white dwarf. A white dwarf is a remarkable thing. Although it may contain a substantial fraction of the original star's mass, it is compressed into a volume no larger than that of a planet such as the Earth. It produces no new energy through fusion; instead, it shines only with the residual heat left over from earlier stages. Over an immense stretch of time, far longer than the present age of the universe, it will radiate this heat away and grow ever dimmer and cooler. This unhurried fading marks the true conclusion of the story. The eventual fate of a Sun-like star, then, is not a violent explosion but a quiet retirement, cooling gradually in the dark for billions upon billions of years.