Gravitational microlensing

Gravitational microlensing is a temporary brightening of a background star caused by the gravity of a foreground object bending its light. In Astrophysics II, it is used to detect dark matter candidates and planets that would otherwise stay hidden.

What is gravitational microlensing?

Gravitational microlensing is a lensing event in Astrophysics II where a foreground object, like a star, brown dwarf, or planet, passes close to our line of sight to a distant background star and briefly magnifies that star’s light. You do not see the lensing object glow more. Instead, you see the background star brighten in a very specific time pattern because gravity bends the light around the foreground mass.

This works because mass curves spacetime, and light follows that curvature. When the alignment is close but not perfect, the background star can appear brighter for days, weeks, or even months. The exact shape of the light curve depends on the mass of the lens, the speed of the lens across the line of sight, and how closely the alignment happens. Smaller masses usually make shorter, subtler events, while larger masses can stretch the event over longer times.

Microlensing is called “micro” because the angular separation of the multiple images is far too small to resolve directly with a normal telescope. You usually do not see two separate star images. What you measure is the combined brightness change, which rises and falls as the lens moves. That means the main data product is a light curve, not an image.

A classic feature is that the light curve is often smooth and symmetric for a simple single-lens event. If the lens has a planet, the curve can show short extra bumps or dips when the planet adds its own gravitational signature. That is why microlensing is such a good planet-hunting method for planets far from their stars, including systems that are hard to study with transit or radial velocity methods.

Astrophysics II also uses microlensing as a probe of invisible mass in the Milky Way. In past searches, astronomers used it to look for MACHOs, compact objects that could contribute to dark matter. The method does not directly tell you the lens is dark matter, but it can count how much unseen compact mass is floating in the halo and rule out some candidate populations.

Why gravitational microlensing matters in Astrophysics II

Gravitational microlensing matters in Astrophysics II because it turns gravity into a detection tool for objects that do not emit much or any light. That makes it one of the few observational methods that can find dark compact objects, especially candidates discussed in the dark matter unit, such as MACHOs or other faint masses that slip past ordinary imaging.

It also connects directly to the class’s data-analysis side. You are not just naming a phenomenon, you are reading a brightness curve and asking what kind of lens could create it. That means matching event duration, symmetry, and anomalies to physical causes, which is a very astrophysics-style move.

Microlensing also widens how you think about exoplanet discovery. Some planets are too far from their stars or too dim for other methods, but they still leave a small fingerprint in the lensing signal. So the concept sits at the intersection of stellar populations, planetary systems, and the mass budget of the galaxy.

If you are studying galactic structure or dark matter, microlensing gives you an observational constraint, not just a theory. It tells you what kinds of unseen objects are present, how common they are, and how they are distributed along a line of sight.

How gravitational microlensing connects across the course

Einstein Ring

An Einstein ring is the idealized geometry behind microlensing, when the alignment is nearly perfect and the lensed image forms a ring. In real microlensing events, you usually do not resolve the ring, but the same lensing geometry is what produces the temporary brightening. It is the cleanest picture of how gravity bends light around a foreground mass.

Dark Matter

Microlensing has been used to search for compact dark matter candidates by counting lensing events that cannot be explained by visible stars alone. If many halo objects were causing these events, that would support a compact dark matter population. The results have helped narrow what dark matter can and cannot be, especially for MACHO-style ideas.

Lensing Effect

Gravitational microlensing is a small-scale version of the broader lensing effect. The general principle is the same, mass bends light and changes what we observe. What makes microlensing special is that the angular split is too tiny to see directly, so the signal shows up as a short change in brightness instead of a visible arc or multiple image.

Sterile Neutrinos

Sterile neutrinos are a dark matter candidate, but microlensing usually does not detect them directly because they would not act like compact lenses in the same way a star or black hole does. That contrast matters in Astrophysics II, because it shows how some dark matter ideas are tested with particle physics while others are tested with gravitational observations.

Is gravitational microlensing on the Astrophysics II exam?

A quiz question or problem set usually asks you to identify microlensing from a light curve, explain why the star brightened without any intrinsic change, or connect the event to a compact foreground mass. You might also be asked to compare it with transit photometry or radial velocity, especially in exoplanet questions. If the graph has a smooth brightening with a brief extra bump, that is a strong clue that a planet or other small mass perturbed the lensing event. In a dark matter prompt, the skill is tracing how repeated microlensing surveys can limit the number of invisible compact objects in the galaxy’s halo.

Gravitational microlensing vs Transit Photometry

Transit photometry measures a dip in brightness when a planet passes in front of its own star. Microlensing does the opposite, it causes a temporary increase in brightness because a foreground mass bends and focuses light from a background star. Transit events repeat on regular orbits, while microlensing is usually a one-time alignment event.

Key things to remember about gravitational microlensing

• Gravitational microlensing is a temporary brightening of a background star caused by a foreground mass bending its light.

• You usually do not resolve separate images in microlensing, so the main clue is the shape of the light curve.

• The event length depends on lens mass, relative speed, and how close the alignment is.

• Microlensing can reveal dark or faint objects, including some dark matter candidates and distant exoplanets.

• In Astrophysics II, it is a data-driven way to infer mass from light, not from direct visibility.

Frequently asked questions about gravitational microlensing

What is gravitational microlensing in Astrophysics II?

It is the temporary magnification of a distant star’s light when a foreground object passes close to the line of sight and bends the light with gravity. In Astrophysics II, you use it to study hidden mass, especially compact dark objects and exoplanets. The main evidence is a characteristic light curve, not a visible image.

How is gravitational microlensing different from an Einstein ring?

An Einstein ring is the ideal geometric result of almost perfect alignment, usually described as a ring-like image. Microlensing is the observational effect you get when the images are too close together to resolve, so you detect a change in brightness instead. They are related, but one is the ideal image geometry and the other is the measurable light variation.

Why can microlensing find exoplanets?

A planet orbiting the lens star can create a short extra disturbance in the light curve. That disturbance happens because the planet adds its own gravitational influence to the lensing pattern. This makes microlensing useful for finding planets that are hard to detect with transits or radial velocity.

Can microlensing prove dark matter exists?

Not by itself. Microlensing can show that some compact, unseen objects are present and help rule out how much of dark matter could be made of those objects. It is a constraint tool, so it tests certain dark matter ideas rather than directly identifying the full dark matter particle or object.