Jun 28, 2017 | Updated: 09:10 PM EDT

Einstein Proven Both Right and Wrong As Hubble Weighs a Star

Jun 19, 2017 12:44 PM EDT

Hubble Space Telescope
Hubble Space Telescope aids in white dwarf star study.

Over seventy years ago, Albert Einstein said it was theoretically possible to measure the mass of a star -- but that we would never have the technical ability to do it.

He was right and wrong. Astrophysicists have now measured the mass of a white dwarf star for the first time using gravitational microlensing.

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Knowing the mass of star is important. Our understanding of a star's birth, death, size, and composition is based on a star's mass. Will a star become a supernova? Its mass gives us the answer. What course of evolution will a star take? We need to know its mass.

White dwarf stars are of special interest. Stripped of electrons, the atomic nuclei remain, and the star is astoundingly dense. To become a white dwarf is the fate of over 97% of all the stars in the universe, including our sun. Knowing their masses tells us a lot about how the universe is structured.

In a triumph of persistence and precision, a team of scientists from the Space Telescope Science Institute in Baltimore used the Hubble Space Telescope to calculate the mass of Stein 2051B by detecting how light bent around it.

The team knew, as did Einstein, that light coming from a distant star will be bent when it encounters a massive object. This is a consequence of how space is altered by mass. From previous estimates, astronomers predicted the bend would be small -- just 0.0000005555555556 degrees, or about 2 milliarcseconds. But if that tiny bend could be measured, it would directly reveal the mass of the white dwarf.

The team faced two huge challenges. First, they had to find two stars that were in perfect alignment with Earth. Second, they had to measure how much the light deviated.

The Baltimore team searched through a catalog of 10,000 stars until they found the two stars they needed. One was Stein 2051B, 17 light years from Earth. The second, in the far background, was 5000 light years away -- and the two were perfectly lined up. Because it is so much closer to us, the white dwarf seemed much brighter, another hurdle the team had to deal with.

When they were ready to take the measurements that were once technically impossible, they turned to Hubble.

Albert Einstein could not envision the power of the Hubble telescope. In 1936, he wrote in Science magazine, "Of course there is no hope of observing this phenomenon directly. First, we shall never approach close enough to a central line. Second, the angle (beta) will defy the resolving power of our instruments."

Publishing in Science on June 9, 2017, the Baltimore group proved him wrong. The star, Stein 2015B, has a mass 68% of our sun.

The next goal? Using gravitational microlensing to determine the mass of a wide range of stars. When the James Webb telescope is operational in 2018, its superior resolving power will allow us to measure more star masses with greater accuracy.

One guesses that Einstein would have been pleased to be wrong.

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