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Starts With A Bang

Einstein’s Ultimate Test: Star S0–2 To Encounter Milky Way’s Supermassive Black Hole

After a 16 year wait, the star SO-2 will speed past our galaxy’s supermassive black hole at 2.5% the speed of light. It will be the first-of-its-kind test of Einstein’s theory.

This year, for the first time, astronomers will observationally measure a gravitational redshift around a supermassive black hole.

The spacetime curvature around any massive object is determined by the combination of mass and distance from the center-of-mass. At our galaxy’s center, a black hole 4 million times the mass of the Sun will allow us to measure gravitational redshift there, testing Einstein’s General Relativity in a new way. (T. Pyle/Caltech/MIT/LIGO Lab)

When an object gets close to a massive source, its light must “climb out” of the deformed spacetime predicted by Einstein.

When a quantum of radiation leaves a gravitational field, its frequency must be redshifted to conserve energy; when it falls in, it must be blueshifted. Only if gravitation itself is linked to not only mass but energy, too, does this make sense. (Vlad2i and mapos / English Wikipedia)

The gravitational potential well can cause the light to lose enough energy that it becomes observably redshifted.

The supermassive black hole at the center of our galaxy, Sagittarius A*, flares brightly in X-rays whenever matter is devoured. In other wavelengths of light, from infrared to radio, we can see the individual stars in this innermost portion of the galaxy. (X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI)

The largest, closest single mass to Earth is Sagittarius A*, our Milky Way’s supermassive black hole, weighing in at 4,000,000 solar masses.

The star S0–2 makes the closest known approach to this black hole, reaching a minimum distance of just 18 billion kilometers.

Double Lasers from KECK I and KECK II create an artificial laser guide star to better help the telescope focus on a particular location and account for the atmosphere’s properties, taking advantage of some of the most advanced adaptive optics systems and techinques in the world. (Ethan Tweedy Photography —

That’s only three times the Sun-Pluto distance, or a meager 17 light-hours.

This 2-panel shows observations of the Galactic Center with and without Adaptive Optics, illustrating the resolution gain. Adaptive optics corrects for the blurring effects of the Earth’s atmosphere. Using a bright star, we measure how a wavefont of light is distorted by the atmosphere and quickly adjust the shape of a deformable mirror to remove these distortions. This enables individual stars to be resolved and tracked over time, in the infrared, from the ground. (CLA Galactic Center Group — W.M. Keck Observatory Laser Team)

Later in 2018, S0–2 will reach a maximum speed of 2.5% the speed of light as it zips past Sagittarius A*.

This event will test Einstein’s predictions for gravitational redshift as never before.

The orbit of S0–2 (light blue) located near the Milky Way’s supermassive black hole will be used to test Einstein’s Theory of General Relativity. Other stars, like S0–102 and S0–38, make close approaches to Sagittarius A*, but S0–2 is the closest. If there are any departures from Einstein’s predictions observed, these results will lead the way towards a new, more fundamental and accurate theory of gravity. (S. Sakai / A. Ghez / W.M. Keck Observatory / UCLA Galactic Center Group)

Einstein also predicts a slight kick to the orbit as the star makes its closest approach, an effect astronomers hope to measure.

In Newton’s theory of gravity, orbits make perfect ellipses when they occur around single, large masses. However, in General Relativity, there is an additional precession effect due to the curvature of spacetime, and this causes the orbit to shift over time, in a fashion that may be measurable with current equipment. This 3D visualization illustrates stellar motion in the galactic center at a particular instant in time. (NCSA, UCLA / Keck, A. Ghez group; Visualization: S. Levy and R. Patterson / UIUC)

With no evidence for a binary companion, conditions are ideal for this test.

This multiwavelength view of the Milky Way’s galactic center goes from the X-ray through the optical and into the infrared, showcasing Sagittarius A* and the intragalactic medium located some 25,000 light years away. We understand how stars form in this environment only very poorly. (X-ray: NASA/CXC/UMass/D. Wang et al.; Optical: NASA/ESA/STScI/D.Wang et al.; IR: NASA/JPL-Caltech/SSC/S.Stolovy)

Someday, we hope to learn how stars like S0–2 can form in such an extreme environment.

Mostly Mute Monday tells the scientific story of an astronomical object, event, or phenomenon in images, visuals, and no more than 200 words.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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