Stars Can Now Be “Weighed” With Gravity


In a paper published in Science in 1936, Einstein claimed that there was no hope of ever observing the mass of a star directly, but that it could be done indirectly. Now, slightly more than 100 years after Einstein developed his theory of general relativity, researchers have done exactly that by using the iconic scientist’s laws.

The scientists have observed the bending of distant starlight by gravity and this has enabled them to calculate the mass of a white dwarf star. Up to now, this has only been possible in theory. The results of the study show a way in which the masses of objects that scientists can’t easily measure by other means, can be determined.

In general relativity as postulated by Einstein, one of the key predictions is that the bending of space close to a massive body, such as a star, causes a ray of light that passes close to it to be deflected by twice the amount that would be expected based on classical laws of gravity. Based on this, Einstein predicted that when a star in the foreground passes exactly between us and a background star, a perfectly circular ring of light would be formed – the so-called “Einstein ring.” This phenomenon is called gravitational microlensing.

An eclipse in 1919 provided the first evidence of the bending of light in this form, proving for one of the first times Einstein’s general theory of relativity.

This illustration shows how the gravity of a white dwarf star warps space and bends the light of a distant star behind it. (CREDIT: NASA, ESA, and A. Feild (STScI)
This illustration shows how the gravity of a white dwarf star warps space and bends the light of a distant star behind it. (CREDIT:
NASA, ESA, and A. Feild (STScI)

In spite of 100 years of advances in technology, observing a slightly different scenario – two stars just out of alignment, has not been possible for stars outside our solar system. This scenario should result in an asymmetrical Einstein ring. Einstein noted that such an asymmetrical ring is notable because it would cause the background star to appear off-center in such a way that it could be used to determine the mass of the foreground star directly.

Kailash Chandra Sahu and colleagues used the superior angular resolution of the Hubble Space Telescope to proactively search more than 5,000 stars to find such an asymmetric alignment. They calculated that the white dwarf Stein 2051 B was set to be in such a position in March 2014. The team used the Hubble Space Telescope to witness the phenomenon and measured minute shifts in the apparent position of a background star behind it.

Based on the data, the researchers estimated the white dwarf star’s mass is roughly 68% of that of our sun. The measuring of Stein 2051 B’s mass is important to understand the evolution of white dwarf stars, as most of the stars that have ever formed in the galaxy, including our sun, are already white dwarfs, or will become so in the future.

The study was published in the journal Science.