Global Telescope Seeks First Black Hole Photo

The supermassive black hole at the center of our galaxy, called Sagittarius A*  (pronounced “A star”), or Sgr A*, is estimated to contain about 4 million times the mass of our sun. This is one of the best current photos of it but the resolution isn’t good enough to reveal the black hole. That will change soon! Credit: X-ray: NASA/UMass/D.Wang et al.; IR: NASA/STScI

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 Atacama Large Millimeter / submillimeter array showing the many radio dishes used in concert to see the tiniest of details in distant star-forming clouds and a key instrument in the quest to image the Milky Way’s supermassive black hole. The array is located in the northern Chilean desert. Credit: Christoph Malin / ESO

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.

In this artist’s illustration, a supermassive black hole is surrounded by matter flowing onto the black hole from a whirling disk. This disk forms as dust, gas and stars fall onto the hole and are torn apart and heated before crossing the black hole’s “edge” — called the event horizon — and disppearing forever. Also shown is an outflowing jet of energetic particles believed to be powered by the black hole’s spin. Credit: NASA/JPL-Caltech

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.

A computer simulation of superheated plasma swirling around the black hole at the center of our galaxy. The dark shadow at center is what astronomers hope to finally see with the Event Horizon Telescope. Credit: Scott Noble/RIT

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.

Can’t wait!

2 Responses

  1. Richard West, Bristol, England

    VLBI has has been around for quite a while, but why is is never done at optical wavelengths? Surely optical VLBI would with its shorter wavelengths make it possible to e.g. image other solar systems.

    1. astrobob

      There are several visual VLBI programs underway notably CHARA, NPOI and the British COAST. Visual VLBI requires bright targets since many small apertures don’t increase the brightness of an object, but they do provide the highest resolutions. Stars and star spots are favorite targets.

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