New study reveals light-bending celestial phenomenon

The first observation of displacement of a star due to bending of its light by another celestial body other than the Sun is revealed today (Wednesday 7 June 2017) in a new study from the University of St Andrews.

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 100 years later, similar observations have been made for another celestial body.

As will be reported in the renowned journal Science this week (Friday 9 June 2017), astronomers were able to precisely determine the mass of the nearby white dwarf star Stein 2051B 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.

Dr Martin Dominik, of the School of Physics and Astronomy at the University of St Andrews, who co-authored the new study said: “While Eddington measured an already incredibly small angle corresponding to the diameter of a human hair seen from 10m distance, we measured displacements that were 1000 times smaller, corresponding to the angle subtended by a virus at the same distance.”

The study’s lead researcher Dr Kailash Sahu from the Space Telescope Science Institute (STScI) in Baltimore said: “It’s like placing the star on a scale: the deflection is analogous to the movement of the needle on the scale.”

Twenty years ago Dr Dominik and Dr Sahu laid the theoretical foundations for the study.

The challenging observations became possible with the resolution provided by the Hubble Space Telescope (HST), a joint venture between the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA), with data being acquired during eight 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 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 5000 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 is a white dwarf, the burnt-out remnant of a normal star, which forms a binary star system with the brighter Stein 2051A, a red dwarf star. The closest encounter between Stein 2051B and the background star was predicted to occur during March 2014 at 0.1 arcseconds, with the relative motion being about 2.5 arcseconds per year. Stein 2051A was missed, given that it is more than 10 arcseconds apart from Stein 2051B. 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 (with 8 per cent uncertainty), in perfect agreement with the theoretical mass-radius relation for white dwarfs found in 1935 by Subrahmanyan Chandrasekhar.

The power of astrometric microlensing for measuring masses of celestial bodies will soon be unleashed. By the end of its mission in 2019, ESA’s Gaia satellite will have collected data that will provide reliable mass measurements via astrometric microlensing signatures for more than a thousand nearby bodies, including the evolutionary remnants of stars, namely white dwarfs, neutron stars and black holes, as well as brown dwarfs (“failed” stars not massive enough to sustain nuclear fusion of hydrogen), thereby surveying objects that otherwise cannot be studied due to being faint or invisible and contributing a cornerstone to our understanding of stellar evolution.

Dr Dominik, who coined the term “astrometric microlensing”, said: “While astrometric microlensing currently seems to be mostly a curious effect, it will very soon turn into a most useful tool to survey our dark neighbours.”

Image caption

How gravitation can bend starlight: This illustration reveals how the gravitation of a white dwarf star warps space and bends the light of a distant star behind it.

White dwarfs are the burned-out remnants of normal stars. The Hubble Space Telescope captured images of the dead star, called Stein 2051B, as it passed in front of a background star. During the close alignment, Stein 2051B deflected the starlight, which appeared offset by about 2 milli-arcseconds from its actual position. This angle is 1000 times smaller than that subtended by the width of a human hair seen from 10m distance, and about equal to that of a virus at the same distance. From this measurement, astronomers calculated that the white dwarf’s mass is roughly 68 per cent of the Sun’s mass.

Stein 2051B resides 17 light-years from Earth. The background star is about 5000 light-years away (about 20 per cent of the distance between Earth and the centre of the Milky Way). The white dwarf is named for its discoverer, Dutch astronomer Johan Stein SJ (1871 to 1951), former Director of the Vatican Observatory.

Credits: NASA, ESA and A Feild (STScI)

Background information

Stein 2051B is the fainter component of the binary star system Stein 2051. While the brighter component, Stein 2051A, is a (main-sequence) M-dwarf star, Stein 2051B is the sixth-nearest white dwarf known. The two components are separated by about 10.1 arcseconds, putting them at least 5 billion miles apart.

The Albert Einstein quotation is taken from A Einstein, ‘Lens-like action of a star by the deviation of light in the gravitational field’, Science 84, 506 (1936).

‘Relativistic deflection of background starlight measures the mass of a nearby white dwarf star’ by Jay Anderson, Stefano Casertano, Howard E Bond, Pierre Bergeron, Edmund P Nelan, Laurent Pueyo, Thomas M Brown, Andrea Bellini, Zoltan G Levay, Joshua Sokol, Annalisa Calamida, Noé Kains, Mario LivioKailash C Sahu, Space Telescope Science Institute (STScI), Baltimore; and Dr Martin Dominik, is published by Science. DOI: 10.1126/science.aal2879

Issued by the University of St Andrews Communications Office, contactable on 01334 467310 or [email protected].