Astronomers capture first image of black hole at center of Milky Way

Driving the news: Astronomers imaged Sagittarius A* (Sgr A*) using the Event Horizon Telescope (EHT), scientists announced at a press conference Thursday.

• Most galaxies are thought to have a supermassive black hole at their center.
• The false color orange-yellow in the image is the silhouette of the black hole created by matter teetering on its edge, or event horizon.
• Light can't escape a black hole but hot plasma swirling around it emits short radio waves that radio telescopes can pick up. In the image, the gas silhouettes the black hole itself.

What they're saying: “For decades, astronomers have wondered what lies at the heart of our galaxy, pulling stars into tight orbits through its immense gravity," Michael Johnson, an astrophysicist at the Center for Astrophysics, said in a press release.

• “With the EHT image, we have zoomed in a thousand times closer than these orbits, where the gravity grows a million times stronger. At this close range, the black hole accelerates matter to close to the speed of light and bends the paths of photons in the warped spacetime," Johnson added.

The very big picture: Black holes can tear apart and devour nearby stars, generate massive bursts of gamma-ray energy that shape the galaxies around them and, recent evidence suggests, ignite the formation of new stars.

• Einstein's theory of general relativity predicts the existence of black holes — and his century-old rule consistently passes cosmic tests near and far.
• But scientists hope these massive, dense objects may also reveal instances where general relativity doesn't hold. These important limits could point them to conditions that require a new physics to describe.
• Any new physical laws, together with general relativity and Newtonian physics, could give us a more complete view of the physics of the universe.

Details: Sgr A* is less than 26,000 light years from Earth — far closer than the supermassive black hole at the center of galaxy Messier 87, the first imaged by EHT. At about 4 million times the mass of the Sun, it is also far smaller than the 6.5 billion solar masses of M87* suggested by the EHT measurements.

• M87* is one of the biggest black holes in the universe, while Sgr A* is of a more common size, Johnson said.

The "gentle giant in the center of our galaxy" is consuming very little matter compared to others but its temperature and magnetic fields are quite high, astronomer Feryal Özel of the University of Arizona said.

• The diameter of the shadow was determined to within 10% and is consistent to indirect observations and that predicted by general relativity, Johnson said.
• The results were published today in a series of papers in the Astrophysical Journal Letters.

How it works: The EHT is an international collaboration that uses data from ground-based telescopes in Hawai'i, the Spanish Sierra Nevada, the South Pole, the Atacama Desert in Chile and other locations.

• Working in pairs, eight observatories formed an Earth-sized telescope capable of measuring the interference of millimeter radio waves from hot plasma near the edge of black holes — a technique called very-long-base interferometry (VLBI). The data was collected over five nights in April 2017, alongside data from M87.*

• But Sgr A* is fast-changing — on the order of minutes rather than the changes seen in M87* over weeks — and needed to be imaged quickly. The international team of more than 300 scientists tackled that challenge by creating more than five million images in total and averaging them together to reveal the features of the ring.
• Comparing the data to computer simulations of the black hole suggests it is rotating, Johnson said at the press conference.

The intrigue: The team didn't release all of the data they collected. On one night, Sgr A* was active and erupting, sending sudden bursts of energy seen in infrared and X-ray wavelengths, theoretical astrophysicist Ramesh Narayan of Harvard University told Axios.

• By analyzing eruption data, they may get clues about how the plasma near the black hole is being heated — possibly by flares.

General relativity didn't crack in the results reported Thursday.

• If it is going to break down, it would likely be seen in data from the LIGO experiments, says Matthew Strassler, a theoretical physicist at Harvard who isn't involved in EHT. Those experiments detect gravitational wave signals from colliding black holes and neutron stars.
• "That said, anytime you open up a new window into some type of complex physical phenomena, you can not predict what surprises might lie in store," he said. As the black hole imaging measurements become sharper, there may be opportunities to ask new questions, he added.

What's next: Astronomers hope to create videos of a black hole by the end of the decade and to incorporate data from space-based telescopes that don't have to contend with interference from Earth's atmosphere.

Editor's note: This story has been updated with additional details throughout.