Academic Reading — Test 69

Reading passage

For most of human history, the stars were little more than points of light, admired but fundamentally unknowable. In the early nineteenth century, the French philosopher Auguste Comte famously declared that the chemical composition of the stars was a question science could never hope to answer, because no instrument could ever travel to them and bring back a sample. Within a few decades, however, his confident prediction had been overturned. The key did not lie in reaching the stars at all, but in carefully studying the light they send across the vast emptiness of space. The technique that made this possible is called spectroscopy, and it remains one of the most powerful tools available to astronomers today. When sunlight passes through a glass prism, it spreads out into a band of colour known as a spectrum, ranging from red through to violet. In 1814, the German optician Joseph von Fraunhofer examined the solar spectrum more closely and noticed something curious: the smooth ribbon of colour was interrupted by hundreds of narrow dark lines. He could not explain why these gaps appeared, but he measured their positions with great precision and catalogued them. These features, now called Fraunhofer lines, turned out to hold the secret to the composition of the Sun and, eventually, of countless other stars. The explanation emerged later in the century through the work of the physicist Gustav Kirchhoff and the chemist Robert Bunsen. They demonstrated that each chemical element, when heated until it glows, emits light at a unique set of wavelengths. Sodium, for example, produces a characteristic pair of bright yellow lines, while hydrogen and iron each leave their own distinctive signatures. Crucially, the same element, when present as a cooler gas, will absorb light at exactly those wavelengths, producing dark lines in precisely the positions Fraunhofer had recorded. The pattern of lines therefore acts like a fingerprint, allowing scientists to identify which elements are present without ever touching the source. The dark lines in the solar spectrum revealed that the Sun contains hydrogen, calcium, iron, sodium and many other familiar substances. This insight transformed astronomy from a science of position into a science of substance. By spreading out the faint light of a distant star and recording the resulting pattern of lines, astronomers can determine not only which elements the star contains but also their relative abundances. The strength, or darkness, of a particular line gives a measure of how much of that element is present, although interpreting these strengths correctly requires careful allowance for the temperature and pressure of the star's outer layers. A hotter star and a cooler one may contain identical elements yet display strikingly different spectra, because temperature governs how the atoms absorb and emit light. For this reason, astronomers must model the physical conditions of a star alongside reading its chemical signature. Spectroscopy also yields information that goes well beyond composition. Because a star's spectrum shifts slightly towards the red end when the object is moving away from Earth, and towards the blue end when it is approaching, the positions of the lines reveal motion. This so-called Doppler effect enabled astronomers to measure how fast stars and galaxies are travelling, and it provided some of the earliest evidence that the universe is expanding. The same shifts can betray the presence of an unseen planet tugging on its parent star, or the rotation of a star spinning on its axis. A single spectrum, patiently analysed, can thus disclose a remarkable amount about an object that no telescope will ever visit. One of the most celebrated triumphs of the method came in 1868, when astronomers studying the Sun detected a set of lines that matched no element then known on Earth. The new substance was named helium, after the Greek word for the Sun, helios. It was not isolated in a laboratory on Earth until almost three decades later. The episode demonstrated, more vividly than any argument, that the light of a distant body could reveal genuinely new knowledge about the material universe. Today, spectroscopy underpins almost every branch of astrophysics, from charting the chemical evolution of galaxies to the search for the faint chemical traces of life in the atmospheres of planets orbiting other stars. What Comte deemed forever beyond reach has become routine, all because scientists learned to read the messages encoded in starlight.