# Behold! Milky Way's monster black hole imaged for the 1st time.

#### rod

"The new image shows that the size of Sagittarius A*'s event horizon is 51.8 microarcseconds on the sky."

My observation. Using 8kpc distance, this works out to be ~ 0.41 au diameter, stunning here Using the Schwarzschild radius for Sgr A* at 4.3 million solar masses, the diameter of the BH ~ 0.17 au. At 8 kpc, angular size ~ 2.122 x 10^-2 mas.

sam85geo

#### Unclear Engineer

Question: In lengths units of miles (yes millions of them) what is the calculated Event Horizon radius for a 4.3 million solar mass black hole, and what is the orbital velocity at that radius (in miles per second, please)? Yes, I see something like 0.41 au EH diameter (so 0.205 au radius = 19 million miles) from the angular dimension and estimated distance, compared to a number of 0.17 au = about 16 million miles estimated from the mass-based calculation. So, I am trying to get an idea of what things should look like near the event horizon. Knowing the calculated orbital velocity is my first step at understanding it. Then I will probably be asking questions about what General Relativity says things should look like at that velocity near that much mass. For people contemplating how to answer my question, remember that orbital velocity is less than escape velocity, which is why it is an orbit, not an escape trajectory. But, light from orbiting objects would still need to escape for us to be able to see it, so there should be a speed limit on what we can see. And, that would appear to be altered by the proximity to mass, according to General Relativity. Plus Special Relativity says we should see shortening of motions along the line of sight, but not across the line of sight at right angles. Trying to envision it all. Help would be appreciated.

rod

#### rod

Sgr A* has a number of very fast stars reported moving around it but not at the EH. This star is orbiting close to 24,000 km/s or faster, Fastest star ever seen is moving at 8% the speed of light, https://phys.org/news/2020-08-fastest-star.html

"In the center of our galaxy, hundreds of stars closely orbit a supermassive black hole. Most of these stars have large enough orbits that their motion is described by Newtonian gravity and Kepler's laws of motion. But a few orbit so closely that their orbits can only be accurately described by Einstein's theory of general relativity. The star with the smallest orbit is known as S62. Its closest approach to the black hole has it moving more than 8% of light speed...The most studied star orbiting Sgr A* is known as S2. It is a bright, blue giant star that orbits the black hole every 16 years..."

I used my spreadsheet for a Keplerian orbit with a = 0.25 au, e = 0, mass = 1 Jupiter mass (like an exoplanet) and star mass 4.3 million solar masses. The calculation comes back nearly 124,000 km/s for the average velocity. This is not GR math though and distance 0.25 au from Sgr A* center. Unclear Engineer said in post #3, "So, I am trying to get an idea of what things should look like near the event horizon."

Looking at S62 and my guess work , things would be moving very fast near the EH of Sgr A*, perhaps faster than 8% c to faster than 124,000 km/s or 40% c or faster.

This sites contains some interesting info on the BH observation. Astronomers Unveil Image of the Milky Way’s Central Black Hole - Sky & Telescope - Sky & Telescope (skyandtelescope.org)

"Gas whips around Sgr A* in only a few minutes, compared to the days that gas takes to circuit M87*, which is roughly 1,500 times larger. That means that Sgr A* doesn’t sit patiently for its portrait: Its image is constantly changing, the thin, turbulent gas flow burbling and gurgling, Özel said. Furthermore, we look at Sgr A* through the dusty plane of the Milky Way, which blurs the image as though we were looking through frosted glass. The team used some of the most sophisticated computer algorithms ever written to reconstruct the image. Even so, Sgr A* proved daunting. As they had with M87*, members split into multiple teams, each using its own methods to reconstruct an image from the data. Last time, all the teams fairly quickly had an image, and they all agreed remarkably well. This time, the teams were stumped — many images showed a ring, but not all. It was so unclear what was going on that people didn’t even want to show their images, said computer scientist Katie Bouman (Caltech), who co-led the imaging effort. “That’s really what we have spent years trying to figure out,” she explained. To solve the conundrum, scientists simulated different images and unleashed their algorithms on the mock data, to learn how their methods reacted to different situations. Finally, they were confident that indeed they do detect a ring, and always of the same size: about 50 microarcseconds wide, exactly as Einstein’s theory of gravity predicts."

I could interpret that some image cleanup took place here

Last edited: