# What is a Black Hole?

From far away, stars are tiny points of light. But up close, stars are massive, seething, fiery balls of burning gas. This fierce display does not last forever. Eventually, the nuclear fusion which powers the star will burn allits fuel.

Gravity then collapses the remaining matter together. For very large stars, what happens next is a display of extremes. First, the star explodes in a supernova, scattering much of its matter throughout the universe.

For a brief moment, the dying star out shine sits entire galaxy. But once the light fades and darkness returns, the remaining matter forms an object so dense that anything that gets too close will completely disappear from view.

THIS is a black hole… The idea of a black hole originated hundreds of years ago. In 1687, Isaac Newton published his landmark work known as The Principia. Here he detailed his laws of motion and the universal law of gravitation. Using a thought experiment involving a cannon placed on a very tall mountain, Newton derived the notion of escape velocity.

This is the launch speed required to break free from the pull of gravity. In 1783, the English clergyman John Michell found that a star 500 times larger than our sun would have an escape velocity greater than the speed of light. He called these giant objects “dark stars” because they could not emit starlight. This idea lay dormant for more than a century.

Then, in the early 20th century, Albert Einstein developed two theories of relativity that changed our view of space and time: the special theory and the general theory. The special theory is famous for the equation E=mc2.

The general theory painted a new and different picture of gravity. According to the general theory of relativity, matter and energy bend space and time. Because of this, objects which travel near a large mass will appear to move along a curved path because of the bending in spacetime. We call this effect gravity.

One consequence of this idea is that light is also affected by gravity. After all, if spacetime is curved, then everything must follow along a curved path, including light. Einstein published his general theory of relativity in 1915. And while Newton’s theory of gravity could be expressed using a simple formula, Einstein’s theory required a set of complex equations known as the “field equations.

”Only a few months after Einstein’s publication, the German scientist Karl Schwarzschild found surprising solution. According to the field equations, an extremely dense ball of matter creates a spherical region in space where nothing can escape, not even light. A curious result, but did such things actually exist? At first, the idea of a black sphere in space from which nothing could escape was considered purely a mathematical result, but one which would not really happen.

However, as the decades passed, our understanding of the lifecycle of stars grew. It was observed that some dying stars became pulsars, another exotic object predicted by theory. This suggested that dark stars could actually be real as well. These strange spheres were named “black holes,” and scientists began the hard work of finding them, describing them and understanding how they are created.

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But how do you find an object in space that is completely black? Luckily, because black holes have a large mass, they also have allergen gravitational field. So while we may not be able to SEE a black hole, we can observe its gravity pulling on its neighbors. With this in mind, astronomers looked for a place where a visible star and a black hole were in close proximity to one another.

One such place is binary stars. A binary star is a system of two stars orbitingone another. We can spot them in several ways. You can look for stars that change position back and forth ever-so-slightly. Alternatively, if you observe a binary star from the side, the brightness will change when one star passes behind the other.

So it’s possible that somewhere in space, there’s a binary star consisting of a black hole and a visible strain fact, such binary systems have been observed! Astronomers have found stars orbiting an invisible companion. From the size of the visible star and its orbit, astronomers calculated themes of its invisible neighbor. It fit the profile of a black hole.

Since we can’t see a black hole, isothere a way to find its size? From Einstein’s field equations, we know that given the massif a black hole, we can determine the size of the sphere that separates the region on escape from the rest of space. The radius of this sphere is called the Schwarzschild radius in honor of Karl Schwarzschild.

The surface of the sphere is called the event horizon. If anything crosses the event horizon, it’s gone forever — hidden from the rest of the universe. This means, once you know the MASS of a blackhole, you can compute its SIZE using a simple formula. And it’s actually quite easy to measure the mass of a black hole.

Just take a standard issue space probe and shoot it into orbit around the black hole. Just like any other system of orbiting bodies — like the Earth orbiting the Sun, or the Moon orbiting the Earth — the size and period of the orbit will tell you the mass of the black hole.

If you don’t have a space probe handy, then compute the mass and orbit of a companion star and use that to find the Schwarzschild radius. Black holes come in many sizes. If it wastage from a dying star, then we call it a “stellar mass” black hole, because isomass is in the same range as stars. But we can go bigger – much bigger. And to do so, we are going to visit the center of a galaxy.

Galaxies can contain billions and billions of stars, all orbiting a central point. Scientists now believe that in the center of most galaxies lives a black hole which we call a “supermassive black hole,” because of its tremendous mass.The size can vary from hundreds of thousands to even billions of solar masses. For example, at the center of our own Milky Way galaxy is a supermassive black hole with a mass 4million times that of our sun.

Black holes have another property we can measure- their spin. Just like the planets, stars rotate. And different stars spin at different speeds. Imagine we can adjust the size of this star but keep the mass constant. If we increase the radius, the spinning slows down… If we decrease the size, the spinning speedup. But while the rotational speed can vary, the angular momentum never changes – it remains constant.

Even if the star were to collapse into a black hole, it would still have angular momentum. We could measure this by firing two probes into opposite orbits close to tieback hole. Because of their angular momentum, black holes create a spinning current in spacetime.

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The probe orbiting along with the current will travel faster than the one fighting it, and by measuring the difference in their orbital periods we can compute the black hole’s angular momentum. This spacetime current is so extreme it creates region called the ergo sphere where nothing, including light, can overcome it. Inside thermosphere, nothing can stand still.

Everything inside this region is dragged along by the spinning spacetime. The event horizon fits inside the ergo sphere, and they touch at the poles. So in one sense, black holes are like whirlpools of spacetime. Once inside the ergo sphere, you are caught by the current. And after you cross the event horizon, you disappear.

One final property of black holes we can measures electric charge. While most of the matter we encounter in our day-to-day lives is uncharged, a black hole may have a net positive or negative charge. This can easily be measured by seeing how hard the black hole pulls on a magnet.

But charged black holes are not expected to exist in nature. This is because the universe is teeming with charged particles, so a chargeback hole would simply attract oppositely charged particles until the overall charges neutralized. There are 3 fundamental properties of a blackhole we can measure – mass, angular momentum, and electric charge. It is believed that once you know these three values, you can completely describe the black hole.

This result is humorously known as the “no hair theorem,” since other than these 3 properties, black holes have no distinguishing characteristics. It’s not a blonde, brunette, or a redhead. We now have a good idea of a black hole from the outside, but

what does it look like on the inside? Unfortunately we can’t send probe inside to take a look.

Once any instrument crosses the event horizon, it’s gone. But! Don’t forget we have Einstein’s field equations. If these correctly describe spacetime outside the black hole, then we can use them to predict what’s going on inside as well. To solve the field equations, scientists consideredtwo separate cases: rotating black holes, and non-rotating black holes.

Non-rotating lack holes are simpler and were the first to be understood. In this case, all the matter inside the black hole collapses to a single point in the center, called a singularity. At this point, spacetime is infinitely warped. Rotating black holes have a different interior. IN this case, the mass inside a black hole will continue to collapse, but because of the rotation it will coalesce into a circle, not a point.

This circle has no thickness and is called a ring singularity. Black hole research continues to this day. Scientists are actively investigating the possibility that black holes appeared rightafter the big bang, and the idea that black holes can create bridges called wormholes connecting distant points of our universe.