On June 8th, 2017 we learned about

Sizing up a star based on its neighbor’s bent light

For all the things we know about distant stars in the galaxy, we don’t have a concrete measure of how big they are. Just about everything we can measure in a star is based on the light it emits – the color and brightness can tell us about the how hot they’re burning, and what elements they’re made of. Fortunately, it turns out that light from stars can also tell us about their mass, thanks to an idea called gravitational microlensing.

Light warped through gravity’s lens

Gravitational microlensing may sound like filler dialog on a science fiction show, but it’s actually pretty straight forward once you accept that light is affected by objects’ gravity. Similarly to how light is bent in a glass lens, gravity can also bend and warp the direction light travels in, usually in a circular manner around a big object. This idea was first proposed by Albert Einstein in 1936, and the arcs of warped light around massive objects like stars and black holes we’ve since observed have appropriately been named “Einstein rings.” Einstein even went further, suggesting that we might be able to measure the mass of an object by seeing how much it’s gravity bent the light around it.

Einstein’s predictions about measuring mass with bent light weren’t all correct though, as he also assumed that this technique would never actually be of practical use. To measure the mass of one star with gravitational microlensing, you need a second star in exactly the right position behind it to provide the light that will get bent. Even with that star in perfect near-alignment, Einstein assumed that the light would be warped to such a small degree that we’d never be able to detect the difference. Fortunately, the Hubble Space Telescope has proved sensitive enough to do just that, and astronomers were able to successfully see light warping around a white dwarf star 18 light years away from Earth.

White dwarf weigh-in

So how big is this star? It turned out to be 0.675 solar masses, or around two-thirds the size of our Sun (or around 1.342×1030 kilograms). This is actually the second shot at measuring this particular star’s mass, as it was previously calculated by comparing its orbit to that of its closest neighbor. The two stars were thought to be part of a binary pair, which allows for an alternative way to measure the effects of gravity and thus, mass. However, the latest measurement is thought to be more accurate, and it has some astronomers doubting that the two stars are even a binary pair in the first place.

In the future, astronomers will likely further proving Einstein right and wrong about these measurements, as they start relying on them more frequently as technology allows for more and more precise observations of light.

Source: Astronomers measure the mass of a star—thanks to an old tip from Einstein by Daniel Clery, Science

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