Gravity is a force that makes objects move, bringing things together. Everything with mass has gravity. We perceive gravity when we jump up and get pulled back down to the ground. Planets, stars, moons and other objects in the Universe have gravity. That’s why they orbit around each other, like for example the Earth orbits the Sun or the moon orbits around the Earth, instead of flying randomly in space. That’s why we see the moon and the Sun every day.
The more mass something has, the stronger the gravity it produces. The Earth’s gravity is stronger than the Moon’s because it is more massive. So our bodies are pulled down on Earth more than if we were on the Moon. That’s why astronauts can jump higher and more easily on the moon than on Earth. Our bodies also exert gravitational forces on other objects, but because our mass is so small, the gravity from our bodies does not affect objects any way we can see. The strength of gravity also changes with the distance to an object. The pull between the Earth and the moon is stronger than that between the Earth and Jupiter. This is because the Earth is closer to the Moon than to Jupiter.
Gravity was first described by Newton as a force. Described more than 300 years ago, Newton’s theory of gravity is still applied today and it was used when scientists plotted the course to land man on the moon and is still used to build bridges across rivers. Although Newton’s theory describes the strength of gravity fairly accurately, he didn’t know what caused gravity or how it worked. These concepts remained unknown for nearly 250 years, until Albert Einstein described gravity as the curvature of space. Space has 3 dimensions: up-down, left-right and forward-backward; and it can be visualized as a fabric, like a stretchy sheet. Any object with mass deforms space, just like a marble creating a dimple on the surface of the stretchy sheet. This curvature of space causes objects to interact with each other, often by moving towards each other, which is seen as gravity, a natural consequence of a mass’s influence on space. The more mass something has, the more the space is curved, and, therefore, the more gravity there is.
A black hole is a region in space where the pull of gravity is so strong that nothing, not even light, can escape. A black hole can be very very small in size but can contain a lot of mass. Some black holes are a result of dying giant stars, several to hundreds times bigger than our Sun. At the end of its life, the star burns out and collapses, causing some of the material from the dead star to be compressed into a tiny space. Such compression of enormous mass bends space so much it acts like a deep well. The result is the formation of what we call black holes, characterized by extreme gravity. Because its mass is so concentrated, another object can get very close to the black hole, to where the space is greatly curved, and gets trapped forever in this gravitational well.
Black holes do not necessarily have more mass than other objects in the Universe but the compression of its mass creates its extreme gravity near to it. After its formation, black holes can continue to grow more massive as they accumulate more matter from its surroundings, such as stars, gas, dust and other black holes. Black holes can be as massive as three Suns to over a million Suns. Things that enter into the black holeare lost forever once they pass the border of no escape - the event horizon. As we get closer to a black hole, space is bent more and more, and gravity becomes more extreme very rapidly. The event horizon is the boundary region of black hole where the pull of gravity becomes so extreme that nothing, not even light, can escape. It is thought that there are around 100 million stellar-mass black holes orbiting within our own galaxy.
Black hole observation
Because of their extreme gravity, black holes are perfect space traps. Light travels extremely fast (nothing can travel faster), only taking 1.3 second to travel between the moon and the Earth. If we moved at the speed of light, we could travel around the Earth 7.5 times just in one second. Therefore, the pull of gravity in a black hole must be extremely high if light cannot escape from it. As light enters a black hole there is no path out and remains inside. This is why a black hole is invisible and very difficult for astronomers to detect. However, there are still some ways for black holes to be detected.
When a black hole is feeding on matter within its vicinity, that material can become very bright, allowing its detection, in a phenomenon called an accretion disk. A black hole’s extreme gravity causes nearby material to be pulled into it; this material can include small amounts of gas from a nearby star, or the dust and other stellar debris from a complete star being torn apart. As this material spirals closer into the black hole, it orbits faster and rubs and collides against other material spiraling into the hole. The friction created causes the material to be heated to very high temperatures. Wherever there is heat, there is emission of thermal electromagnetic radiation, which is light. An example is our body, which also emits thermal radiation and this can be seen when looking through thermal imaging goggles. Another example is when a piece of metalwork is heated to very high temperature that we can see the heated part glow.
When the temperature of the material around the black hole increases, the energy emitted in the form of radiation also increases. Because the materials spiraling in black hole get extremely heated, they glow and form a bright flattened ring of spinning matter, called an accretion disk, around the event horizon. Although black holes are themselves invisible, this accretion disk can be seen by astronomers because the spiraling materials have yet to pass the event horizon to be lost forever. The light from the accretion disk radiates out into the universe and allows us to see where a black hole is and what it is doing to its surroundings. As the material spirals in, when it is very close to the black hole but before it goes beyond the event horizon, some of it can be shot out in the form of super-fast, focused beams of material, called ‘jets’.
For black holes that are not feeding on surrounding material and cannot be observed via their accretion disks, the presence of a black hole can be revealed by the effect of its gravity on normal stars that apparently orbit around nothing. This is like planets keeping their orbit around the Sun. But instead of something visible like our Sun, the stars just orbit around an empty space. Thus astronomers can assume that there is a black hole there. This happens at the centre of our own galaxy, where we see stars, such as star S2, orbiting fast around an invisible point.
Additionally, the existence of a black hole can be evident through an optical effect, called gravitational lensing, seen as the result of light being bent by the gravity generated by black hole. Light normally travels straight in space. But due to a black hole’s extreme gravity, which is the curvature of space, light passing nearby a black hole will travel along this curvature, instead of going straight through, i.e. light is bent. Therefore, a black hole can act as a lens, bending any light passing behind it. When viewing distant galaxies with telescopes, astronomers have observed strange rings and arcs of light, despite no observation of any visible mass. This indicates the presence of invisible black hole that is creating curvature due to gravity and focusing the light towards us like a lens.