The Attraction Of A Star

In White Dwarfs by Brian Koberlein1 Comment

In 1919 Arthur Eddington traveled to the island of Principe off the coast of West Africa to photograph a total eclipse. Mission was to observe the apparent shift of nearby stars by the Sun’s gravity. His experiment was a success, and it verified Einstein’s gravitational theory of general relativity. Since then, we have observed the gravitational deflection of starlight by the Sun numerous times. We have also seen the deflection of the light from distant galaxies, but we haven’t seen the deflection of distant starlight by another star. The gravitational effect of a single star is extraordinarily small. But recently a team has observed the deflection of starlight by a single white dwarf star.

The white dwarf is known as Stein 2051 B, and it’s been in the middle of a controversy for nearly a century. White dwarfs are formed with a star such as our Sun runs out of the hydrogen necessary to generate heat and pressure through nuclear fusion. The star collapses under its own weight until it reaches a point where the pressure of electrons keeps it from collapsing any further. White dwarfs have the mass of a star, but are about the size of Earth. This small size makes them difficult to study.

In 1930, a 19-year old named Subrahmanyan Chandrasekhar calculated the theoretical structure of white dwarfs. Similar models were developed by Wilhelm Anderson and E. C. Stoner, but Chandrasekhar’s was more accurate, and included the calculation of an upper limit to the mass of white dwarf, now known as the Chandrasekhar limit. His model was highly controversial, and Eddington was one of the biggest opponents of the model. As astronomers found more examples of white dwarfs, it became clear that Chandrasekhar’s model worked. But Stein 2051 B seemed to be an exception. It has a distant companion star that allowed us to get a rough idea of its mass and it seemed that Stein 2051 B had a mass that is much smaller than most white dwarfs. This would imply it has some strange structure such a s a large iron core.

A schematic of the gravitational lensing effect. Credit: Wikipedia

But then in 2014 the white dwarf happened to pass in front of a more distant star, as seen from Earth. This allowed astronomers to observe the effects of gravitational lensing by the white dwarf. Using data from the Hubble Space Telescope, the team analyzed the gravitational deflection of the distant star. Using general relativity, they then calculated the size and mass of the white dwarf. They found it has a mass of 0.675 solar masses, which is larger than previously thought and well within the typical range of a white dwarf. So Stein 2051 B doesn’t have an exotic composition after all.

Arthur Eddington was an extremely prominent astronomer, and his opposition to the young upstart’s model meant it gained little traction early on. But the good thing about science is that in the end the data rules. It is perhaps poetic justice that Chandrasekhar’s model has been vindicated by the very experiment Eddington so famously first used.

Paper: Kailash C. Sahu, et al. Relativistic deflection of background starlight measures the mass of a nearby white dwarf star. Science
DOI: 10.1126/science.aal2879 (2017)


  1. I’m not sure if this is the first detection of lensing by a distant star. I guess it depends on the definition of ‘distant’. Or, more likely, by me misunderstanding the article.
    Gravitational microlensing has been used for some years as a means of exoplanet detection. In this, the distant star is lensed by the closer lens star. If the lens star has a planetary companion, then there is a second (much smaller) spike, caused by the planet. Collaborations such as MOA and OGLE have been doing this for some years, with a fair bit of success. Purely random, and therefore our best indication of the true distribution of planets re size and separation within the galaxy.

Leave a Reply