Stein 2051B Shows How Gravity Can Bend Starlight

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The NASA/ESA Hubble Space Telescope observed the nearby white dwarf star 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 milliarcseconds from its actual position. This deviation is so small that it is equivalent to observing an ant crawl across the surface of a quarter from 1,500 miles (2,300 km) away.

The white dwarf star Stein 2051B (center) is 17 light-years away and the background star is 5,000 light-years away. Image credit: NASA / ESA / K. Sahu, STScI.

The white dwarf star Stein 2051B (center) is 17 light-years away and the background star is 5,000 light-years away. Image credit: NASA / ESA / K. Sahu, STScI.

A century ago, Albert Einstein published his famous theory of relativity. He proposed that all objects physically warp the fabric of space, with larger masses producing a more pronounced effect, and very massive objects causing light to travel along curved paths through space.

Such an effect was first observed during the 1919 solar eclipse by English astronomer Arthur Eddington.

Scientists had to wait a century, however, to get a telescope powerful enough to detect this gravitational microlensing caused by a star outside the Solar System.

Even around objects with very large masses, such as stars, this effect is very slight, making such detections extremely challenging for ground-based telescopes. It is, however, within the capabilities of Hubble.

This Hubble image shows the binary star system Stein 2051 on October 1, 2013, consisting of the brighter, redder Stein 2051A component at lower right and the fainter, bluer Stein 2051B component near the center. Because these stars are only 17 light-years away they appear to move in the sky relative to the much more distant background stars in several months of observations with Hubble. The wavy blue line traces this motion, due to their true motion relative to the Sun combined with the parallax due to the motion of Earth around the Sun. Stein 2051B appeared to pass close enough to one of these background stars, labeled ‘source’ that the light from the background star was bent due to the mass of the white dwarf. This color image was made by combining images taken in two filters with Hubble’s Wide Field Camera 3. Image credit: NASA / ESA / K. Sahu, STScI.

This Hubble image shows the binary star system Stein 2051 on October 1, 2013, consisting of the brighter, redder Stein 2051A component at lower right and the fainter, bluer Stein 2051B component near the center. Because these stars are only 17 light-years away they appear to move in the sky relative to the much more distant background stars in several months of observations with Hubble. The wavy blue line traces this motion, due to their true motion relative to the Sun combined with the parallax due to the motion of Earth around the Sun. Stein 2051B appeared to pass close enough to one of these background stars, labeled ‘source’ that the light from the background star was bent due to the mass of the white dwarf. This color image was made by combining images taken in two filters with Hubble’s Wide Field Camera 3. Image credit: NASA / ESA / K. Sahu, STScI.

Stein 2051B resides 17 light-years from Earth and forms a binary system with the red dwarf star Stein 2051A.

The background star is approximately 5,000 light-years away.

Space Telescope Science Institute researcher Dr. Kailash Sahu and co-authors observed Stein 2051B eight times within two years while the white dwarf traveled in front of the background star.

During the close alignment, the white dwarf’s gravity bent the light from the distant star, making it appear offset by about 2 milliarcseconds from its actual position.

From this measurement, the astronomers calculated that the white dwarf’s mass is 68% of the Sun’s mass.