An international group of researchers has achieved something that Einstein thought would be incredibly unlikely to happen. They used a rare stellar alignment to assess the mass of a white dwarf and solve a long-standing mystery about this type of star.
In a paper published in Science, researchers have reported the first example of a particular type of gravitational microlensing by a star other than the Sun. The microlensing star is a white dwarf known as Stein 2051 B and, according to the analysis, this white dwarf is perfectly normal. For almost a century, people believed that this object was more massive and made of more exotic matter than it truly is.
Massive objects in the foreground can bend the light of distant background stars when they become aligned. The difficulty is actually finding two stars that are so aligned. In 1919, this effect was seen using the Sun and considered the first strong evidence that Einstein’s theory of general relativity was correct. But it was considered impossible for it to be seen in distant stars. Einstein himself wrote in Science in 1936 that "there is no hope of observing this phenomenon directly."
"This is the first time we are able to use the gravitational deflection of light by a star to measure its mass," co-author Dr Stefano Casertano from the Space Telescope Science Institute told IFLScience. "Eddington's experiment in 1919 used the gravitational deflection due to the Sun – a thousand times larger than the effect we measured – to test the (then new) theory of general relativity; nearly 100 years later, we are instead using the deflection of light in general relativity to measure the mass of a known star."
Better instruments mean deeper observations, which ultimately led to this discovery. Between 2013 and 2015, Stein 2051 B deflected the light of a star, although they weren’t perfectly aligned. It produced an effect called astrometric lensing, which was enough for the researchers to estimate the mass of the white dwarf. The team took eight observations of the alignment using the Hubble Space Telescope.
The failed star has a mass roughly 68 percent of our Sun. This measurement is an important addition to the small group of white dwarfs that have a well-determined radius and mass. This information helps scientists not only understand the astrophysics of white dwarfs, but to use it as a window on the past. Based on its mass and temperature, researchers estimate that the progenitor star that formed this white dwarf was about 2.6 times the mass of the Sun. The team is now using this technique on other objects.
"We are observing several other systems, including Proxima Centauri, the star closest to the Sun, to measure their mass in the same way," Dr Casertano continued. "We are also working to find isolated black holes in the Galaxy using the same basic technique, but applied to millions of background stars – finding the one that moves because an invisible black hole passes in front of it."
White dwarfs are the final stage of life for stars that are not big enough to explode in a supernova. They are the leftover cores made of degenerate matter, where helium and carbon ions move in a sea of electrons. White dwarfs are luminous not because they are producing more energy, but because they have trapped so much energy in their center that it’s slowly radiating away