“The Last Photon Orbit” –Milky Way’s Supermassive Black Hole ‘On Deck’ for the EHT

Supermassive Black Hole

 

When it’s completed, the picture of the Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*), is an image sure to equal the famous “Earthrise” photo taken by Apollo 8 astronaut Bill Anders in December 1968. The obvious target for the Event Horizon Telescope, the team hopes to get imagery of our supermassive black hole soon, said Shep Doeleman, Director, Event Horizon Telescope following last week’s release of the first ever image of Galaxy M87’s gargantuan black hole.

The researchers looked at M87 below, first, because it’s an enormous elliptical galaxy 55 million light-years away that harbors a mind-boggling supermassive black hole somewhere between 3.5 billion and 7.2 billion times the mass of the sun. It’s a bit easier to resolve than Sagittarius A* because it’s less variable over short timescales, Doeleman , explained.

 

Galaxy M87

 

M87, is an enormous elliptical galaxy 55 million light-years away that harbors a mind-boggling supermassive black hole somewhere between 3.5 billion and 7.2 billion times the mass of the sun. At the small end of that range, M87 would be an impossible target for EHT observes . At the high end, it is possibly suitable. So M87 became a secondary target in the pursuit of Sagittarius A*.

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What we’ll see when the EHT actually sees Sagittarius A* is an area slightly outside the event horizon itself — a region defined by the location closest to the black hole where a beam of light could orbit on a circle, known as the “last photon orbit.” Were you to float there, says astrophysicist Janna Levin, professor of physics and astronomy at Barnard College of Columbia University and author of Black Hole Blues, “you could see light reflected off the back of your head after completing a round trip. Or, if you turned around quickly enough, you might see your own face.) Closer than that, all the light falls in.”

 

Sagittarius A* Milky Way Center

 

The M87 EHT image is unmistakable — a dark shadow the size of our solar system, writes Levin, enveloped by a bright, beautiful blob.

 

EHT Image

 

“While the scientific implications will take time to unpack” she says, “some of the anthropological impact feels immediate. The light EHT collected from M87 headed our way 55 million years ago. Over those eons, we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs. Looking at M87, I am reminded that scientific discoveries transcend those differences. We are all under the same sky, all of us bound to this pale blue dot, floating in the sparse local territory of our solar system’s celestial bodies, under the warmth of our yellow sun, in a sparse sea of stars, in orbit around a supermassive black hole at the center of our luminous galaxy.”

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Chandra X-ray image at the top of the page shows the innermost 10 light years at the center of our galaxy. The image has been smoothed to bring out the X-ray emission from an extended cloud of hot gas surrounding the supermassive black-hole candidate Sagittarius A* (larger white dot at the very center of the image- a little to the left and above the smallest white dot). This gas glows in X-ray light because it has been heated to a temperature of millions of degrees by shock waves produced by supernova explosions and perhaps by colliding winds from young massive stars.

The Daily Galaxy via Quanta and New York Times

Image Credit top of page: YouTube.com

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