A Brief Introduction to Black Holes: What Are They and Where Did They Come From?
What is a Black Hole?
A black hole is super dense region of spacetime that exhibits such strong gravitational effects that nothing—not even light—can escape. A black hole’s gravitational force is so strong because black holes are so dense. Black holes come in various sizes and many have masses similar to that of ordinary stars. Most black holes are formed from the collapsed remnants of a star that was at least thirty times the mass of the sun. To form a black hole, the collapsed star will shrink down to an infinitely dense point called a singularity.
Around the singularity, there is an imaginary line called the event horizon. Beyond the event horizon all light is turned black by the force of its own gravity. Einstein proved that not only can gravity capture light, it can also distort time and space. Inside the event horizon the entire concept of time and space completely breaks down.
Black holes are known for sucking in everything around them because of their massive gravitational force. Black holes can tear apart and devour entire stars. They only “eat” a small amount (about 1%) of the stars that they suck in, however. The rest of the matter from the star gets flung back out into space.
Types of Black Holes
There are several different types of black holes, which come in various sizes. Many have masses similar to that of an ordinary star.
Micro Black Holes
Some scientists theorize that there may be micro black holes that were formed shortly after the Big Bang. These scientists believe that these micro black holes are still scattered around our galaxy today. None of these micro black holes have been observed yet, however, so their existence remains purely theoretical. These black holes are theorized to be as small as an atom, but with a mass rivaling that of a large mountain.
Stellar Black Holes
Stellar black holes are formed by the gravitational collapse of a massive star. They have a mass up to 20 times more than the mass of the sun. There are likely many stellar mass black holes in the Milky Way (the galaxy where Earth is located).
Intermediate-Mass Black Holes
Intermediate-mass black holes are significantly more massive than stellar black holes, but less massive than supermassive black holes. Unlike stellar black holes, intermediate-mass black holes are too massive to be formed by the collapse of a single star. Scientists have developed three theories for how intermediate-mass black holes may have formed.
- Intermediate-mass black holes may form due to the merging of stellar black holes and other compact objects by means of accretion.
- They may form in a runaway collision of massive stars in dense stellar clusters. The collapse of this collision product may form an intermediate-mass black hole.
- Intermediate-mass black holes may also be primordial black holes formed in the Big Bang, similar to micro black holes.
Supermassive Black Holes
The largest type of black holes are supermassive black holes. These black holes are more massive than 1 million of our suns. Every large galaxy has a supermassive black hole at its center. The supermassive black hole at the center of our galaxy is called Sagittarius A.
Supermassive black holes can continue to become more massive over time. A supermassive black hole at the center of a galaxy can grow by accretion of matter as well as by merging with other black holes.
How Do Black Holes Form?
Black holes are thought to form in several different ways. Black holes require immense amounts of energy to form. They may form from gravitational collapse (such as when stars collapse) or from high-energy collisions.
Many black holes are thought to form due to gravitational collapse. This may occur when an object's internal pressure is insufficient to resist the its own gravity. For stars this typically happens because a star runs out of fuel and is no longer able to maintain its temperature. The gravitational collapse of heavy stars may result in the formation of stellar black holes.
Scientists believe that some black holes were formed at the same time as the universe. These black holes are referred to as primordial black holes. Micro black holes are thought to be primordial black holes. Gravitational collapse requires enormous density. In the current universe, these high densities are only found in stars. However, densities were much greater in the early universe shortly after the Big Bang, which may have allowed for the creation of these primordial black holes.
In addition to gravitational collapse, some black holes may form as a result of high-energy collisions that achieve sufficient density.
What Would Happen if You Fell into a Black Hole?
You could never see a person fall into a black hole. As they approached the event horizon time would slow down to the point where it would take infinitely long to reach the event horizon. The gravitational pull on light would also make the person falling into the black hole appear to fade away.
If you fell feet-first into a black hole, the difference in the gravitational pull on your head and your feet would be so great that you would be stretched out like spaghetti. As you near the singularity at the center, you would feel yourself being torn apart atom by atom. If you could look back as you fell into the black hole, you would see the future history of the universe flash before your eyes.
How did the Idea of Black Holes Originate?
The idea of black holes originated in 1783 when Rev. John Michell used Newton’s theory of gravity to predict the possibility of “dark stars.” In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does, in fact, influence the motion of light. Only a few months later, Karl Schwarzschild found a solution to the Einstein field equations, which describes the gravitational field of a point mass and a spherical mass. These super dense points in space became known as “Schwarzschild singularities.” In 1967, the new name "black hole" was suggested for these points by a student at a lecture by John Wheeler. The name stuck.
How Do Scientists Know Black Holes Are There?
Black holes cannot be seen because their gravitational pull is so great that even light cannot escape. This makes them appear to be invisible. Black holes are not completely black, however. English physicist Stephen Hawking showed that black holes give off faint radiation, which implies that their mass must eventually evaporate into a haze of subatomic particles. Black holes can be located with space telescopes equipped with special tools. These special tools allow scientists to observe how stars that are very close to black holes behave differently from other stars. This allows scientists to detect the locations of black holes.
Are Black Holes a Threat to Earth?
Black holes are of no danger to Earth. This is because only objects within a black hole’s event horizon can actually be pulled into it, and Earth is nowhere close enough to a black hole. The closest stellar black hole to Earth is V616 Monocerotis, and it is 3,000 light-years away. Sagittarius A, the supermassive black hole at the center of the Milky Way galaxy, is 26,000 light-years away from Earth.
Black Holes and the Future of Science
There are certainly many more black holes out there in the universe, just waiting to be discovered. Who knows what amazing new discoveries scientists will discover about black holes in the future, and how many more black holes they will find? Perhaps scientists will make even more bizarre discoveries about black holes than what we already know.
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© 2018 Jennifer Wilber