Gravitational lensing

How mass bends light into rings, crosses and arcs, and lets us weigh what we cannot see

Step by step

Mass bends light

For the Sun the bend is tiny. For a whole galaxy, or a cluster of galaxies, it is large enough to show: a mass acts like an uneven lens, and light from something far behind it can reach us from more than one direction. Here the dark dot is the lens (drawn), and the galaxy behind it is seen twice.

Lining up

As the galaxy behind slides into line with the lens, its two images swing round the lens, stretch and brighten, and join into a ring.

The lens equation says where light seems to come from. When the source is a little off line, there are two images, one on each side of the lens, a long arc on the far side and a short one close in. The closer the line-up, the more the images stretch, and the brighter they get.

At perfect alignment the two images become a full circle, the Einstein ring. Its radius, the Einstein radius, grows with the square root of the lens's mass. You can try it: dragging the picture pushes the galaxy behind across the lens, and the images follow.

The Einstein Cross

Each image brightens and fades on its own, because single stars in the lens galaxy drift in front of it and lens it again. Astronomers use that flicker to study the quasar's disk, a region only light-days across that no telescope could see directly.

Weighing dark matter

Clowe's team studied it in 2006. The pink glow is the hot gas, seen in X-rays, which holds most of the ordinary matter; the blue is the mass that lensing finds. Both are drawn here where they were measured: the mass is not where the gas is.

Gas is slowed by the collision, as two clouds of smoke would be. The mass kept moving with the galaxies, as if it were something that passes through itself. This is among the best evidence that most matter is dark and hardly interacts, and it is hard to explain by changing gravity alone.

A star seen across time

In 2022 Welch's team found Earendel in an arc, the Sunrise Arc, that a cluster stretches from a galaxy at redshift 6.2. Right on a line where the lens is strongest, the magnification reaches hundreds or thousands, enough to show one star at a distance where none could otherwise be seen.

Lensing is nature's own telescope, and the James Webb Space Telescope uses clusters this way for its deepest pictures. Whether Earendel is one star or a few is still being studied. What the picture shows is a point of light, drawn on the arc where it was found.