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SYFY WIRE Bad Astronomy

Einstein was right (again)! Astronomers watch as a star’s gravity bends light from another star

By Phil Plait

For only the second time in history, astronomers watched as the gravity of a nearby star bent the light from a more distant one, changing its apparent position in the sky. This is pretty dang remarkable*, especially since the first time it was done was in 1919, and the star doing the gravitational bending that time was the Sun.

That event, nearly a century ago, was during a total solar eclipse (the Sun, you may note, is pretty bright, making it hard to see nearby stars unless it’s eclipsed by the Moon) and was correctly hailed as the first experimental confirmation of Einstein’s Theory of Relativity.

This more recent observation also capitalized on relativity, and did even more than that: It solved a long-standing issue about the physical properties of dead stars.

First, why does gravity bend starlight? In my episode of Crash Course Astronomy: Dark Matter I covered this very topic. Here’s what I said:

One of Albert Einstein’s big ideas was that space wasn’t just emptiness between stars. In a sense it was an actual thing, with all of matter and energy is embedded in it.

Although you have to be careful not to take the analogy too literally, in many ways it acts like a fabric with everything stuck to it. This was more than just a theoretical construct: It has real implications.

For one, what we perceive as gravity – the force pulling two objects together – was actually just a bending of this fabric of space, a warp. It’s like a bowling ball sitting on a soft mattress; the surface of the mattress bends, and if you roll a marble past it, the path of the marble will curve.

This is true for light, too! It’s like having a bend in the road; cars follow the bend as they move, and trucks do, too. Everything does. With light, it doesn’t bend nearly as much as matter does, but it does curve if it moves through space distorted by gravity. The more massive an object is, the more gravity it has, the more it warps space, and the more it can warp the path of a light beam.

This bending of light — called gravitational lensing — is a measurable effect, and why astronomers were so eager to view the 1919 eclipse. They could see the Sun’s gravity shifting the apparent position of a background star, pretty much right on with what Einstein’s equations predicted.

light bending

The new study does the same thing. Not too far from the Sun, about 18 light-years from us, there is a binary star (two stars orbiting each other) called Stein 2051. One star (called Stein 2051 A) is a red dwarf, a star cooler, smaller and fainter than the Sun, but otherwise normal. The other star (Stein 2051 B), though, is a white dwarf: the burned-out core of a star that was once much like the Sun but came to the end of its life (surprise! I have more about those in my episode Crash Course Astronomy: Low Mass Stars).

White dwarfs are extremely small — about the size of the Earth — but have a large fraction of the Sun’s mass. That makes them extremely dense. And while they’re very hot, they are so small that they’re pretty dim. Even though it’s much hotter than its red dwarf companion, it’s a lot fainter.

As it happens, this binary star is close enough to us that they move appreciably over time in the sky as they orbit the center of the galaxy. That “proper motion” is small, but if you measure their position over a few years, you can watch them move. This is the key to whole thing!

path of Stein 2051 B

Why? Because the astronomers doing this new research discovered that, as it moved over time and as seen from Earth, Stein 2051 B was going to pass extremely close to a much more distant background star. At their closest, they’d be separated by a mere .203 arcseconds. An arcsecond is an angle measuring the size of an object in the sky; there are 60 arcseconds to an arcminute, and 60 arcminutes to a degree (and 360° going all the way around the sky). For comparison, the Moon is 0.5 degrees, or 1800 arcseconds, in size. So yeah, 0.2 arseconds is close. Close enough that the white dwarf’s fierce gravity would bend the light from the background star. And because the white dwarf is so dim, it wouldn’t be too bright to swamp the light from the background star. So this confluence was a perfect observational setup.

Using Hubble, a team of astronomers observed Stein 2051 B and the background star many times from October 2013 to October 2015, as the two approached and passed. They very carefully measured the stars’ positions (in itself a very difficult task to the accuracy needed!), and what they found was — within their measurement accuracy — the background star moved exactly the amount predicted by Einstein’s relativity equations!

Woohoo! Einstein, you may have heard, was pretty smart.

lensing of background star

The theory of General Relativity has been tested a zillion times, and has always come through. It’s one of the best-tested ideas in all of history. If this observation were just done to test it again, it would’ve been interesting, but not critical.

But there’s more! It’s pretty hard to measure the mass of a star when it’s sitting all alone in space. If it happens to be orbiting another star, the mass can be determined because it falls out of the equations of how gravity affects orbits (the orbital period depends on the stars’ masses). Only a few white dwarfs are in binaries where we can measure their masses, and in general the accuracy isn’t fantastic.

We want to know the masses of white dwarfs as accurately as we can, because a lot depends on it. How we determine their age, how luminous they are, what they’re composed of … all of this in one way or another depends on their mass.

And this is where these new observations are so cool: They give an independent measurement of the star’s mass! The amount the light from the background star was bent depended on how close it appeared to get to Stein 2051 B, but also the latter’s mass. Plugging their observational measurements into the equations, the astronomers found that Stein 2051 B has a mass 0.675 ± 0.051 times the Sun’s mass.

Previous measurements had the mass of the white dwarf at more like half the Sun’s mass, and that was causing all sorts of problems with theories of how these critters form. If the mass was that low, it would have had to have some weird core of iron to be as small as calculations indicated it being. However, the new, higher mass means everything is in order, and the theories governing white dwarf structure are right on the money as well.

I must say, I’m very impressed with this. I worked on Hubble data for a long time, including trying to nail down the positions of stars in the images, and it’s pretty hard to do, especially at the ridiculous accuracy needed for this task. And the idea of even finding a white dwarf that would pass that close to another star, not be too bright or too dim, and do so in a reasonable time … it’s amazing this event happened at all, let alone was observable.

But it did, and it was. And not only did we get yet another test for relativity, but a weird mystery at the root of stellar astrophysics was solved at the same time. Not bad for a bunch of hairless apes who are pretty new to this whole science thing.

*As a word pedant, I have to point out that when someone uses the phrase “that’s remarkable” it’s a self-fulfilling statement, since they’re remarking on it.