For First Time, Einstein's Theory of Relativity Used to Weigh a Star
Binary stars are the only stars scientists can directly gauge, because their orbits around each other reveal their masses. Now, astronomers from the Space Telescope Science Institute in Baltimore have succeeded in measuring the mass of an isolated star using a technique first suggested by Albert Einstein's general theory of relativity 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. Einstein's theory was proven in 1919 when scientists measured the curving of starlight around the sun during a total solar eclipse.
Transcript
00:00:01 A new observation of the bending of distant starlight by gravity has allowed researchers to weigh a white dwarf star, virtually impossible before now for an isolated star. One of the key predictions of Einstein’s general theory of relativity is that the curvature of space near a massive body, such as a star, causes a ray of light passing near it to be deflected by twice the amount
00:00:24 that would be expected. So when a star passes between us and a more distant star, we should see the distant star shift position or -- in the case of perfect alignment -- become distorted into a perfectly circular ring of light – a so-called “Einstein ring.” One of the first convincing proofs of general relativity
00:00:43 came in 1919 during an eclipse of the sun when astronomers saw its gravity shift the position of stars close to its edge. In recent decades, astronomers have seen the gravity of galaxies produce Einstein rings of more distant galaxies, and stars briefly brighten the image of other stars in a process known as gravitational microlensing, but detecting a position shift caused by a star
00:01:07 other than the sun has been too small to discern. Seeing such a shift would be very useful to astronomers because it would provide a way to to directly measure the mass of the intervening star, something that is very hard to do unless a star is in a binary pair when their orbits reveal their mass. Recently, researchers realized that the keen eyesight of the Hubble Space Telescope might
00:01:30 be able to spot such movement so went looking for stars that might soon be in alignment. They found that the nearby white dwarf Stein 2051 B was set to pass nearly in front of another star in March of 2014. They then used Hubble to measure tiny shifts in the apparent position of the background star as Stein 2051 B passed by. The researchers’ measurements
00:01:54 put the white dwarf star’s mass to be about 0.675 solar masses. As well as demonstrating the power of this new technique, the work also solved an enigma surrounding Stein 2051 B. An earlier mass estimate had made it substantially lighter, too light for a white dwarf of that type, casting doubt on the theory of how such burnt-out stellar remnants form.
00:02:16 But the new measurement puts Stein 2051 B’s mass exactly where it should be. With this one measurement, the researchers have confirmed Einstein’s prediction, demonstrated a valuable new technique, and reinforced the theory of stellar evolution. That's a good day’s work.