It’s one thing to discover a new star, quite another to find one that hurtles through space as if it’s in a hurry to get somewhere. But try this for a scientific trifecta: finding a speedy new star capable of putting Albert Einstein’s general theory of relativity to the test.
That’s exactly what a team of astronomers probing Sagittarius A, the super-massive black hole at the center of our Milky Way galaxy, has done. The faint star they’ve discovered, dubbed S0-102, is a celestial roadrunner that orbits closer to the black hole’s event horizon — the threshold at which nothing, not even light, can escape the tremendous gravity — than any other known star. But what makes S0-102 truly captivating is that while it’s the first star to get precisely this close to the event horizon, one other — dubbed S0-2 — has already been found that comes a close second. By comparing measurements of the two stars’ orbits, astrophysicists could, in effect, transform the center of our galaxy into an unprecedented, cosmic-scale laboratory for illustrating how black holes warp space and time as Einstein predicted they should.
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S0-102’s discovery, reported in the journal Science, is the latest milestone from a 17-year effort to study the star-rich galactic core using adaptive optics, a technique that enables telescopes to compensate for distortions created when light passes through Earth’s atmosphere. Using the Keck Observatory atop Hawaii’s dormant Mauna Kea volcano, a team of researchers led by led by UCLA astrophysicist Andrea Ghez, began their observing sessions by firing a laser into the Earth’s atmosphere, causing sodium atoms about 56 miles (90 km) above the surface to glow. The telescope collected the reflected light from this “virtual star” and divided it into multiple beams that a computer analyzed to determine how a reflective surface could best align the fragments and sharpen the focus. That’s where some mechanical magic came into play: the telescope’s flexible, “deformable” mirror is able to reshape itself, compensating for the atmosphere’s blurring effects. Suddenly, rather than seeing a jumble of blurred stars, astronomers could discern discrete points. Thus calibrated, the telescope could turn its attention to a one light month-wide region in Sagittarius A and better see what was to be found there.
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That’s when they found S0-102. The new star is located about 26,000 light-years from Earth and is 16 times fainter than S0-2 — but it was worth the effort to find it. S0-102 orbits the black hole at 3,107 miles (5,000 km) per second, a speed so tremendous that it completes its full, 409 billion-mile (658 billion km) orbit around the hole in just 11.5 years. S0-2, by comparison, takes 16 years, and most of the stars Ghez has studied in the region take an average of 60 years. It’s S0-102’s proximity to the event horizon — about 9.3 billion mi. (15 billion km), or roughly the size of our solar system — that accounts for its speed. And yes, when it comes to something as fearsome as an event horizon, the size of our solar system is just a whisker’s distance.
It didn’t take long for Ghez and her team to realize the significance of suddenly having two stars in reliable, elliptical orbits: a first-ever chance to map the geometry of space and time near a black hole by comparing orbital deviations. Einstein famously forecast that objects in space have both an apparent position and a true position, because anything with mass warps both space and time, bending the path of light as it travels over long distances. The concept was demonstrated to scientific acclaim during a 1919 solar eclipse, when astronomers found incoming starlight near the mostly blacked out sun was bending just as Einstein predicted.
Determining whether space-time warps light near Sagittarius A will theoretically be easy to do visually given the immense gravitational pull that S0-102 feels from the black hole. A warped orbit also has what’s known as a precession — a slow change in the rotational axis of the orbiting body — that can be charted. But there’s a complication: orbiting stars aren’t the only bodies out there. Other objects are also exerting gravitational influence, pulling an orbit outward even as the black hole draws it closer. That’s why two stars are required: because astronomers need to compare deviations in the two orbits to verify and then filter out the gravitational effects of all the extraneous bodies.
The curvature of space-time will yank the orbits of S0-102 and S0-2 most severely when they reach their closest orbital approach to the black hole — 2018 for S0-2, three years later for its new counterpart. That’s when astronomers plan on measuring how much, if at all, the gravitational pull bends the light departing from the stars. They also hope that three telescopes set to begin operating in the coming decade, each with mirrors three times the diameter of today’s most powerful telescopes, will enable them to measure the amount of precession.
The results will either add to Einstein’s legacy or place the fundamental laws of physics up for revision. That’s quite a burden for such a faint —and newly important — star.