# The Existence of Black Holes: Finding the Evidence

From the Lecture Series: Great Heroes and Discoveries of Astronomy

## Black holes represent the ultimate extreme of what we know about physics, gravity, and space-time. Sometimes, after the collapse of a very massive star’s core, even quantum physics can’t battle the immense inward press of gravity. In a case like this, the core will keep collapsing, forming a tiny and dense object that severely warps space-time. Gravity, in this tiny corner of space-time, becomes so strong that nothing can escape it, not even light.

### Achieving Escape Velocity

Let’s imagine a rocket being launched into space from the surface of the Earth. In order to break free of Earth’s gravitational force and reach outer space, the rocket needs to travel at what we call escape velocity.

This means that the rocket must be traveling at about 25,000 miles per hour, or about seven miles per second, to leave the Earth. Objects with stronger gravity have a higher escape velocity; for that same rocket to escape from the gravitational pull of our Sun, it would need to be traveling nearly four times faster, covering more than 380 miles in a single second.

The gravity of a black hole is so strong that even light itself, traveling at 186,000 miles per second, isn’t moving fast enough to achieve escape velocity. Instead, that light—and everything else—becomes irrevocably trapped inside the warped region of spacetime around the black hole. It would be like trying to launch yourself to Mars by jumping into the air; one simply can’t jump at a speed that will let us escape Earth’s gravity. Inside a black hole, even jumping at the speed of light wouldn’t be enough to escape.

### Predicting a Black Hole

Astronomers had toyed with the idea of objects like this for decades. The earliest prediction of a black hole can be traced back to the 18th century. In the early 20th century, Albert Einstein’s general theory of relativity sparked a new wave of interest in black holes from theoretical physicists, who began studying what they could do to spacetime and light and how we might eventually observe them.

Still, it was Jocelyn Bell’s discovery that paved the way for astronomers hoping to actually observe and prove the existence of black holes.

When not even light can escape from an object, it’s easy to see why astronomers might have a hard time studying them! Fortunately, Einstein’s theories gave us the tools for predicting what black holes might do to their surroundings. And this ultimately gave us our first glimpse of a black hole.

This article comes directly from content in the video series Great Heroes and Discoveries of AstronomyWatch it now, on Wondrium.

### Cygnus X-1

In 1964, a team of astronomers launched a sounding rocket from the White Sands Missile Range in New Mexico. The rocket was carrying a special instrument designed to detect x-rays coming from deep space. The suborbital rocket spent only a few minutes above the Earth’s atmosphere, but in that short time it surveyed the sky and found the first evidence of a strange object in the constellation Cygnus: something in that part of the sky was emitting x-rays, and those x-rays seemed to be varying with time.

Later observations with satellites and other telescopes revealed that the x-rays appeared to be coming from a bright blue massive star, named Cygnus X-1. This surprised and confused astronomers at first, since stars don’t typically emit many x-rays by themselves.

### A Binary Companion

It wasn’t until 1971 that three astronomers—Louise Webster and Paul Murdin, and Charles Bolton working separately—discovered that this blue star had a close neighbor—what’s called a binary companion. What’s more, the binary companion seemed to be invisible and very massive, 15 times as massive as our Sun.

They proposed that the companion must be a black hole—it was far too massive to be a neutron star—and that the changing x-rays were coming from matter being dragged from the blue star into the black hole.

### Searching for Evidence

For years, astronomers continued to study Cygnus X-1 and other objects like it; we’ve since discovered many more x-ray-emitting binaries with bright stars and neutron star or black hole companions. Every detail of these companions matches the predicted properties of black holes. Still, most astronomers wanted incontrovertible evidence that black holes truly existed.

Another exciting discovery came in 1995, when astronomers began tracking the motion of stars orbiting what was believed to be a supermassive black hole at the center of our own Milky Way.

### Warped Space-time

Today, we believe that most galaxies have supermassive black holes near their centers. We’re still not sure how black holes like this are made, but most astronomers believe that they form by merging with other black holes or trapping huge amounts of matter in the warped space-time surrounding their masses, growing over time. This leads to black holes containing huge amounts of mass: the black hole believed to be at the center of our Milky Way would be as massive as 2.6 million suns.

The stars near this black hole are careening through space in tight and high-speed orbits. From watching them move astronomers could see clear evidence that a huge amount of invisible mass was concentrated at the center of our galaxy. It was another strong piece of evidence for the existence of black holes.

### Common Questions about the Existence of Black Holes

Q: What is escape velocity?

In order to break free of Earth’s gravitational force and reach outer space, a rocket needs to travel at what we call escape velocity. This means that the rocket must be traveling at about 25,000 miles per hour, or about seven miles per second, to leave the Earth.

Q: How are black holes made?

We’re still not sure how black holes are made, but most astronomers believe that they form by merging with other black holes or trapping huge amounts of matter in the warped space-time surrounding their masses, growing over time.

Q: How big is the black hole in the center of our Milky Way believed to be?

Black holes contain huge amounts of mass. The black hole believed to be at the center of our Milky Way would be as massive as 2.6 million suns.