## For the first time in human history, astronomers have photographed a black hole, as reported by BBC News. An array of eight telescopes spread across five continents spent 10 days taking the picture. The physics affiliated with black holes is as incredible as are the pictures.

Before this photograph was taken, black holes were still theoretical objects. Humanity had no proof of their existence. However, thanks to 200 astronomers operating a group of eight telescopes called the Event Horizon Telescope Array, the first-ever image of a black hole has been captured. The landmark picture shows a perfectly circular area of darkness surrounded by an uneven ring of vibrant orange and yellow. According to the BBC article, “The bright halo is caused by superheated gas falling into the hole. The light is brighter than all the billions of other stars in the galaxy combined—which is why it can be seen at such distance from Earth.” Here’s all you need to know about this groundbreaking phenomenon.

## Black Hole Physics

The photographed black hole, which is in a distant galaxy, is believed to be nearly 25 billion miles (40 billion km) wide. For perspective, when Pluto is at its farthest point away from Earth, the distance between the two celestial bodies is 4.67 billion miles (7.5 billion km). This means the full width of the black hole is over five times that distance. But what is a black hole?

“Newton’s Law of Gravity gives the magnitude of the force between two masses,” Dr. Joshua N. Winn, Professor of Astrophysical Sciences at Princeton University said. “Strictly speaking, it’s the force between two point masses—two idealized mathematical points with nonzero mass, but zero size.” According to Dr. Winn, a black hole is a real-life point mass. It has no surface area, but it still has a mass, and all objects with mass have a gravitational pull to some extent. “The mass really is concentrated into a single point in space, so as you approach, the gravitational force gets stronger, all the way down to *r* equals zero [with *r* standing for radius], where it becomes infinite,” Dr. Winn said.

This explanation means that a black hole is a point in space with such a strong gravitational pull that nothing within a certain distance from it—not even light—can escape it. This point in space known as the Schwarzchild radius, which also explains the bright halo of gas surrounding the pitch black center in the photograph of the black hole.

## The Schwarzchild Radius

The further an object is from the black hole, the less the black hole draws that object to itself, because black holes do not have infinite reach in their gravitational pulls. Conversely, a black hole’s gravitational pull becomes stronger the closer the object is to the point mass—or center of the black hole. This correlation means that the closer the object is to the black hole, the more energy that object must exert and the quicker it must travel to escape the pull of the black hole. For example, a spaceship would need to burn more fuel to reach an adequate speed to break the pull of the black hole. “And at some point, the escape velocity exceeds the speed of light—the fastest speed it’s possible for anything to attain—and in that case, no amount of fuel would have been enough,” Dr. Winn said. That point is a radius surrounding the black hole. This is the reason the first picture of a black hole shows the bright ring surrounding the inky darkness: that ring’s innermost point is where light is pulled into the incredible mass of the black hole.

“It’s the radius of no return,” Dr. Winn said. “It’s called the Schwarzchild radius, after Karl Schwarzchild, the first person to solve Einstein’s equations of general relativity exactly, for the case of a point mass.” The Schwarzchild radius is also called an event horizon. “From beyond that horizon, no light can reach you, so you can’t see any events that might be going on there,” Dr. Winn said.

Black holes have been an astronomical phenomenon for decades, intriguing science enthusiasts the world over. Their existence begs questions about space-time relativity and the origins of the universe. Now, with a little help from Newton, Einstein, Schwarzchild, and the 200 members of the Event Horizon Telescope Array team, humanity is one step closer to understanding them.

Dr. Joshua N. Winn contributed to this article. Dr. Winn is the Professor of Astrophysical Sciences at Princeton University. After earning his Ph.D. in Physics from MIT, he held fellowships from the National Science Foundation and NASA at the Harvard-Smithsonian Center for Astrophysics.