Academic Reading — Test 119

Reading passage

For centuries, astronomers assumed that a point of light in the night sky represented a single, solitary star. It is now understood that this assumption is frequently mistaken. A great many stars are not alone at all but exist in pairs or larger groupings, bound together by gravity and orbiting a shared centre of mass. In many such systems, only one star is bright enough to be seen directly from the Earth; its partner remains hidden, either because it is intrinsically faint or because the glare of the brighter star overwhelms it. The challenge for astronomers, then, is to detect something that cannot be observed at all. The most powerful technique developed for this purpose does not rely on seeing the companion. Instead, it relies on measuring the subtle motion that the unseen object imposes upon the star we can see. The principle rests on a simple consequence of gravity. When two bodies orbit one another, neither stays still while the other moves around it. Both bodies circle a common balance point, much as two children of unequal weight must sit at different distances to balance a seesaw. A massive companion therefore causes the visible star to trace a small orbit of its own. As the star advances along this path, it periodically moves a little towards the Earth and then a little away from it. This rhythmic approach and retreat is known as a wobble. The wobble is far too small to be seen as a change in the star's position in the sky, but it can be detected because motion towards or away from an observer alters the light that reaches us. The key to measuring this motion lies in the spectrum of the star. When starlight is spread out into its component colours, it is crossed by dark lines at particular wavelengths, produced as specific chemical elements in the star's atmosphere absorb light. These absorption lines act as fixed markers. When the star moves towards the Earth, its light waves are compressed and the lines shift towards the blue end of the spectrum; when the star recedes, the waves are stretched and the lines shift towards the red. This phenomenon, called the Doppler effect, is the same one that makes the siren of a passing ambulance change in pitch. By recording how far the lines shift, astronomers can calculate the speed at which the star is moving along the line of sight. This particular component of motion is what gives the method its name: the radial velocity technique. A single measurement reveals little, because it captures the star at only one instant of its orbit. The strength of the method comes from repetition. By observing the same star over many nights, weeks or even years, astronomers build up a record of how its radial velocity changes through time. If the star is genuinely accompanied by an unseen partner, this record traces out a smooth, repeating curve that rises and falls in a regular cycle. The time taken to complete one full cycle reveals the orbital period of the system. The size of the swing in velocity, meanwhile, depends on the mass of the hidden companion and on how close it lies to the visible star. A heavier or nearer companion tugs the star more forcefully and produces a larger wobble; a lighter or more distant one produces only a faint signal that may be difficult to distinguish from noise. From these measurements, a surprising amount can be deduced about an object that has never been seen. The shape of the velocity curve indicates whether the orbit is nearly circular or markedly elongated. The period and the amplitude together allow astronomers to estimate a lower limit for the companion's mass. This last point carries an important caution. The radial velocity technique measures only the portion of motion directed towards or away from the Earth, so the result depends on the angle at which the orbit is tilted relative to our line of sight. If the orbit is viewed nearly face-on, much of the true motion is sideways and therefore invisible to the method, causing the companion's mass to be underestimated. For this reason, the figure obtained is described as a minimum rather than an exact value. The same approach has applications well beyond the study of paired stars. When the unseen companion is not a faint star but a planet, the wobble it produces is correspondingly tiny, since a planet is far less massive. The detection of such minuscule shifts demanded instruments of extraordinary precision, and their development transformed the search for worlds beyond our own solar system. Many of the first planets found orbiting distant stars were identified not by direct imaging but by the faint gravitational tug they exerted, betrayed by the rhythmic reddening and blueing of their parent star's light. In this way, a method first conceived to weigh hidden stars became a principal tool for discovering hidden planets.