Measuring the mass of stars isn’t an easy feat – you can’t exactly pop them on a pair of scales. But thanks to one of Einstein’s key predictions of general relativity, astrophysicists have directly measured the mass of a white dwarf star for the first time.
Einstein’s prediction, called gravitational lensing, says that the curvature of space near a massive body, such as a star or black hole, causes light passing that body to bend instead of travel in a straight line as it normally would. But in a 1936 paper he claimed that it would be impossible to directly observe the phenomenon since telescopes wouldn’t be able to see this level of detail.
It seems he underestimated the power of later generations of telescopes, however, as this new observation was made from the Hubble Space Telescope.
Kailash Sahu at the Space Telescope Science Institute in Baltimore, Maryland, and his colleagues measured the deflection of light by white dwarf Stein 2051 B to measure its mass. This star is 18 light years from Earth.
“The mass of the star essentially decides everything about the star,” says Sahu. Knowing a star’s mass can help astrophysicists work out how old a star is and what it will become after it dies.
Bending background light
Stein 2015 B, it turns out, is about 68 per cent the mass of the sun. That’s not too far from previous estimates of the star’s mass, but this most recent calculation is important because, unlike previous estimates, it doesn’t rely on assumptions about the star’s composition or its orbit around other stars. For example, early mass estimates for this white dwarf assumed it had an iron core, but this direct measurement suggests that is incorrect.
Martin Barstow at the University of Leicester in the UK, who wasn’t involved in this study, is impressed with the results. “Measuring the mass of white dwarfs without using models is really challenging,” he says.
Sahu and his colleagues observed the white dwarf as it moved in front of another star over a two-year period. By measuring how the light from that background star changed course as the white dwarf moved in front of it, Sahu was able to calculate its mass, as the greater a star’s mass, the greater the deflection.
The team is now trying to measure the mass of Proxima Centauri, a red dwarf that’s just 4.25 light years from Earth.
Gravitational lensing is a handy way to independently measure the mass of a star, but finding stars that are passing in front of another star with the right alignment to take these measurements can be tricky, says Barstow. Sahu and his colleagues started with a survey of more than 5000 stars before they settled on Stein 2051 B.
Nonetheless, Barstow says that gravitational lensing is the only way to accurately measure the mass of stars that don’t come in pairs. In binary systems, where two stars orbit around a common centre, astrophysicists can estimate stellar masses by studying their orbits.
The mass measurements for Stein 2051 B may be off by up to 7.5 per cent due to a lack of perfect resolution, says Sahu, but it is still our best calculation of the white dwarf’s mass. Future space telescopes like NASA’s James Webb Space Telescope could help make more accurate measurements, he says.
Weighing a star is hard. In fact, binary stars are the only ones scientists can directly gauge, because their orbits around each other reveal their masses. Now, a team of astronomers has succeeded in measuring the mass of an isolated star using a technique first suggested by Albert Einstein in 1936. The method exploits the fact that a large mass, like a star, can bend the path of light. Although the effect is tiny, measuring the deflection can reveal the mass of the light-bending star.
“This is a really elegant piece of work they’ve done,” says astronomer Martin Barstow of the University of Leicester in the United Kingdom, “and a nice echo of a century of general relativity.”
Astronomers have seen many examples of gravity-bending light, including galaxies distorting images of even more distant ones, sometimes stretching them out into circular “Einstein rings.” In our own galaxy, when one star passes in front of another, astronomers see a brief brightening of the more distant one as the nearer star acts as a lens, bending more of its passing rays toward Earth. This effect, known as gravitational microlensing, has been used to detect exoplanets and search for dark matter, black holes, and brown dwarfs.
But Einstein also predicted that if the source of the light and light-bending star are not in exact alignment, the bending will cause the source star to appear to move when viewed from Earth. The size of that shift tells scientists the light-bending star’s mass. The effect is so tiny and the likelihood of such a near-alignment so rare, Einstein thought it could never be done.
But a team of astronomers from the United States, the United Kingdom, and Canada had a hunch that the keen-eyed view of the Hubble Space Telescope might be able to detect such a shift. They started by looking for stars that might be coming into alignment, and found that Stein 2051 B—a white dwarf just 18 light-years from Earth—was due to pass almost directly in front of another star in March 2014. When it did, the team captured the slightest shifts in position of the background star. That shift let the team calculate that Stein 2051 B’s mass is about two-thirds the mass of the sun, 0.675 solar masses, they report today in Science.
But this was more than just a skillful demonstration of a new technique. Stein 2051 B is something of an enigma for specialists in white dwarfs, the husks left behind when stars have burned up all their fuel. (This fate awaits 97% of stars, including the sun.) From observations of its size, temperature, and the light it emits, researchers had estimated that Stein 2051 B was a particular variety of white dwarf that should weigh about 0.67 solar masses. But, using the method to measure mass through binary stars, other researchers had paired it with another nearby star, Stein 2051 A, and had calculated a weight of just 0.5 solar masses. The latest calculation puts Stein 2051 B’s mass exactly where it should be, and it also casts doubt on the idea that A and B are actually a binary pair. “It’s a nice addition to our understanding of white dwarf composition,” Barstow says.
Astronomer Markus Hundertmark of the University of Heidelberg in Germany says that simply the detection of such a shift would have deserved publication in its own right. “Measuring the mass of a nearby white dwarf seems to make the result even better, and at first glance it is a surprising discovery.”
The technique’s application seems limited at the moment, because the near-alignment of stars is so rare, but that will change next year when the second star catalog from the European Space Agency’s Gaia satellite is released, giving exact positions and motions of many thousands of stars. “It’s likely we’ll find many more examples to pursue,” Barstow says. The new result “seems to be a most curious effect,” says team member Martin Dominik of the University of St. Andrews in the United Kingdom, but it will “turn into a quite useful astrophysical technique sooner rather than later.”