The first observation of displacement of a star due to bending of its light by another celestial body other than our Sun is revealed in new research.
This illustration reveals how the gravitation of the white dwarf Stein 2051B warps space and bends the light of a distant star behind it. The NASA/ESA Hubble Space Telescope captured images of Stein 2051B as it passed in front of the background star. During the close alignment, Stein 2051B deflected the starlight, which appeared offset by about 2 milli-arcseconds from its actual position. From this measurement, astronomers calculated that the white dwarf’s mass is roughly 68% of the Sun’s mass. Image credit: NASA / ESA / A. Field, STScI.
In 1919, Arthur Eddington’s observations during a solar eclipse of the displacement of stars caused by the bending of their light by the Sun marked the breakthrough of Albert Einstein’s theory of general relativity.
Now almost a century later, similar observations have been made for another celestial body.
Dr. Kailash Sahu of the Space Telescope Science Institute and co-authors were able to precisely determine the mass of the white dwarf star Stein 2051B, the fainter component of the nearby binary system Stein 2051, by repeatedly observing the changing position of another closely aligned star passing in the background over two years.
The bending of light by gravitation is a most curious effect, resulting directly from the warping of space-time by massive bodies according to Einstein’s theory.
As a consequence, light rays take an apparent turn due to the curved space despite the fact that light itself does not have a mass which could account for an attraction as given by Isaac Newton’s law of universal gravitation.
Like invisible glass lenses affecting light, the gravitational field of stars displaces and distorts the images of background stars passing in angular proximity on the sky, thereby providing a direct measurement of the mass of the foreground star.
“While Eddington measured an already incredibly small angle corresponding to the diameter of a human hair seen from 10 m distance, we measured displacements that were 1,000 times smaller, corresponding to the angle subtended by a virus at the same distance,” said co-author Dr. Martin Dominik, a Royal Society University Research Fellow at the School of Physics & Astronomy at the University of St Andrews, UK.
“It’s like placing the star on a scale: the deflection is analogous to the movement of the needle on the scale,” Dr. Sahu added.
The new observations became possible with the resolution provided by the NASA/ESA Hubble Space Telescope, with data being acquired during 8 epochs from October 2013 to October 2015.
While the distortion of the images of background stars, resulting in an apparent brightening (known as photometric microlensing), has been observed more than 10,000 times since 1992, the positional shift of the images (astrometric microlensing) was observed for the very first time.
In either case, one relies on a very rare close angular alignment of two stars.
Being unable to predict the technological advances over the coming decades, Albert Einstein himself concluded in 1936: “Of course, there is no hope of observing this phenomenon directly.”
Photometric microlensing can be well observed for distant stars, but the observability of astrometric microlensing requires nearby foreground stars, which puts a strong further restriction on the number of such events.
The team was able to identify Stein 2051B and its background star after combing through data of more than 5,000 stars in a catalogue of nearby stars that appear to move quickly across the sky, given that stars with a higher apparent motion across the sky have a greater chance of passing in front of a distant background star.
Stein 2051B, the sixth-nearest white dwarf known, resides 17 light-years from Earth.
It is estimated to be about 2.7 billion years old and forms a binary system with the brighter Stein 2051A, a red dwarf star.
The two components are separated by about 10.1 arcseconds, putting them at least 5 billion miles apart.
The Stein 2051 system is named for its discoverer, Dutch astronomer Johan Stein S.J., former Director of the Vatican Observatory.
The closest encounter between Stein 2051B and the background star, which is about 5,000 light-years away, was predicted to occur during March 2014 at 0.1 arcseconds, with the relative motion being about 2.5 arcseconds/year.
During the close alignment with Stein 2051B, the background star was observed to be offset by up to 2 milli-arcseconds from its actual position.
The deflection yielded the mass of Stein 2051B as 0.68 solar masses, in perfect agreement with the theoretical mass-radius relation for white dwarfs found in 1935 by Subrahmanyan Chandrasekhar.
The research will be published this week in the journal Science.
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Jay Anderson et al. Relativistic deflection of background starlight measures the mass of a nearby white dwarf star. Science, published online June 7, 2017;
This article is based on text provided by the University of St Andrews.
