Astronomers have been busy this week using the Event Horizon Telescope (EHT) to make the first-ever pictures of the supermassive black hole at the center of the Milky Way galaxy. The project began on April 5 and will finish on the 14th. Rather than a single telescope, the EVT will link eight radio telescopes around the world into one through a technique called Very Long Baseline Interferometry or VLBI. Signals from the black hole and its immediate vicinity are being collected from each location and combined, effectively creating a instrument equal in size to the maximum separation of the telescopes — essentially the diameter of the Earth.
That’s a big scope! But it’s what you need if you want to see darkness at the core of our galaxy 26,000 light years away.
The telescopes include the Atacama Large Millimeter/submillimeter Array in Chile (ALMA), the Caltech Submillimeter Observatory in Hawaii, the Large Millimeter Telescope Alfonso Serrano in Mexico, the South Pole Telescope in Antarctica, and others in France and Spain. None of these telescopes is a traditional one that observes in visible light. That won’t get you very far when it come to peering into the center of the Milky Way because of all the interstellar dust shed by aging stars and supernovas. Dust absorbs light. Instead, these telescopes observe in a slice light between infrared (which we sense as heat) and radio waves called the millimeter and submillimeter light, referring to the light’s wavelength.
These longer waves of light aren’t absorbed by the dust; they make it through to the big dishes and allow astronomers to create a detailed picture of the black hole. Kind of like knowing there’s a heat source behind a wall even if you can’t see it by touching the wall with your hand. Based on how fast stars close to black hole are orbiting around it, we know that it contains about 4 million times the mass of the sun and measures some 27 million miles (44 million km) across.
The hole itself is darkness, blackness. Astronomers will be imaging the hole’s shadow cast on the bright light given off as matter goes whirling down its maw. As matter goes down the hole, friction causes it to heat up to billions of degrees and radiate light across the entire spectrum. To our best knowledge, a black hole will look something like all those illustrations you see of them: a bright disk with an empty, black center or a giant and very lethal doughnut.
So much data will be collected during the two observation periods that it’s faster to fly them to a processing facility called the Haystack Observatory in Westford, Mass. than to transmit them electronically. Petabytes (1 petabyte equals a million gigabytes) of data will be flown from telescopes around the world to Haystack for correlation and processing before images of the black hole can be created. After months of preparation, the images should be ready to view by early 2018.