According to Einstein's general theory of relativity, the gravity from huge objects like stars and galaxies can curve the fabric of spacetime, to the point that passing light will bend from its usual straight path. This can alter how we see distant stars through a phenomenon called gravitational lensing, and now astronomers have seen a rare form of the process in action, directly observing a star bend the light of another, more distant star.
Gravitational lensing has been used in the past to help us find objects hiding behind closer and brighter objects, but for the light-bending effects to make an observable impact, the "lens" object is usually gigantic, like a galaxy or even a cluster of galaxies.
Gravitational microlensing uses a much smaller object as a lens, like a single star or galaxy. From our perspective here on Earth, when a "nearby" star passes in front of a more distant one, the background star will appear to change its position in the sky, as the closer object's gravity causes the light to bend. Albert Einstein called this effect astrometric lensing, and it's been seen in action in the past using our own Sun as a lens.
But there's a glaring problem: the Sun is so bright that it washes out the light from the stars, so the effect can only be seen during a solar eclipse. In a new study, a team of astronomers has used the Hubble Space Telescope to measure how this interaction takes place between two other stars, using a relatively-nearby white dwarf, Stein 2051 B, as a lens to magnify a background star.
The researchers took measurements of the background star's light on eight different days over a two-year period, and noticed that the further star seems to wobble around as Stein 2051 B passes by in front of it (as seen in the video above). This not only marks the first time this kind of gravitational microlensing has been observed in stars other than the Sun, but using the data collected the astronomers managed to work backwards to solve some mysteries about the closer star itself.
Stein 2051 B is the sixth-closest white dwarf star to us, but its composition and mass were still largely unknown. By measuring the extent of the lensing effect, the team was able to determine that the star has a mass about two-thirds that of the Sun. In future, this method could help scientists determine the mass and makeup of other stars, and shed light on white dwarves in general.
"(The) team nicely confirms astrophysicist Subrahmanyan Chandrasekhar's 1930 Nobel Prize-winning theory about the relationship between the mass and radius of white dwarf stars," says Terry Oswalt, a professor at Embry-Riddle Aeronautical University who wrote a perspective piece on the study. "We now know that Stein 2051 B is perfectly normal; it's not a massive white dwarf with an exotic composition, as has been believed for nearly a century."