The distances between stars are so vast that even our nearest stellar neighbors appear as unresolved points of light. This hasn’t stopped astronomers figuring out an array of inventive techniques to find out more about them, but it remains a challenge to study in detail all but the closest objects. Yet thanks to the curious effects of gravity on light itself, there exists a kind of natural telescope which can magnify distant stars. This phenomenon can teach us a lot, not just about those stars but about the ‘telescope’ as well.

Gravitational lensing occurs when the gravity of a massive object deflects light. It is a consequence of Einstein’s Theory of General Relativity, which predicts that massive objects cause a curvature in spacetime. Lensing can happen for objects of all masses, even up to clusters of galaxies. When it is caused by a star, it is termed ‘microlensing’. A microlensing event occurs whenever a foreground massive object (called the ‘lens’) passes directly between the observer and a luminous background source (known as the ‘source’).

Animation of gravitational microlensing by NASA's Goddard Space Flight Center Conceptual Image Lab
Animator: Walt Feimer (HTSI), producer: Scott Wiessinger (USRA), scientist: Neil Gehrels (NASA/GSFC)

We can observe microlensing because the source, lens and observer are moving. This means that the alignment between the three changes as a function of time, and so does the observed magnification of the source star. By plotting the source star’s brightness as a function of time we can detect a gradual brightening and fading as the bodies move into and out of alignment.

Notice that what we’re measuring here is the brightness of the background source star rather than the lens. This phenomenon is a very powerful way to detect objects that are faint or even invisible, since we don’t need to detect any light directly from the lens. It has the extraordinary property that it can tell us about objects that we would otherwise never know existed.

Microlensing events are even sensitive to companions orbiting the lensing star. Companion objects such as stars or planets also cause deflection in the light from the source, resulting in ‘anomalies’ or additional peaks in the light curve of the source. This makes microlensing a very important way to detect planets.

In the rest of this section, we explore the concepts and theory describing this fascinating phenomenon. A few exercises are included to help illustrate the core concepts.