Gravity is a force of attraction between two objects. All objects with mass (weight) have gravity. Gravity acts like a magnet, pulling objects together. What causes gravity is not really known. The Earth has gravity, which holds everything close to the planet so that it does not float into space: trees, water, animals, buildings, and the air we breathe are all held here by gravity. All of the planets, the stars and the moons of the Universe also have gravity. Even our own bodies exert gravitational forces on other objects. The Earth’s gravity is far stronger than our own, so the gravity our bodies possess is not noticeable.
Gravity is affected by the size and proximity (closeness) of objects. The Earth and the Moon have a stronger pull on each other than the Earth and Jupiter because the Earth and Moon are closer to one another. Earth has a stronger pull than the Moon because it is larger, so there is more pull on our bodies here on the Earth than the pull on the bodies of astronauts visiting the Moon. That’s why astronauts can jump higher on the moon than on Earth. We don’t actually ‘feel’ gravity. We only feel the effects of trying to overcome it by jumping, or when we fall. Actually, the man who thought of the ‘Universal Law of Gravitation‘ was inspired by an apple falling on his head as he sat thinking in the garden. The apple was being pulled to Earth by gravity!
A black hole is a region in space where gravity is so strong that nothing that enters it can escape, not even light! Black holes form when a massive star runs out of fuel and becomes unable to support its heavy outer layers of gas. If the star is large enough—approximately 25 solar masses—then gravity pulls on the gas and causes the star to grow smaller and smaller until its density reaches infinity at a single point. This is called a ‘singularity’.
After the black hole forms, it can continue to grow by absorbing mass from its surroundings, such as other stars and other black holes. If a black hole absorbs enough material, growing to over one million solar masses, it becomes a ‘supermassive black hole’. It is believed that supermassive black holes exist in the centres of most galaxies, including the Milky Way.
A black hole is made up of three parts: the singularity (the collapsed star), the ‘inner event horizon’ (the region around the singularity where nothing, not even light, can escape), and the ‘outer event horizon’ (where objects will still feel the gravity of the black hole but do not become trapped).
Astronomers usually observe objects in space by looking at the light. However, since black holes don’t emit any light, they can’t be observed in the usual way. Instead, astronomers have to observe the interaction between the black hole and other objects. For example, as black holes pull material in, like water being sucked down a plughole, the material forms a disc around the black hole. As the disc spins faster and faster, it heats up to extreme temperatures, causing enormous amounts of light and material to be emitted into space as dazzling jets. If they’re pointed towards us, these jets are extremely bright and can easily be picked up by our telescopes on Earth. For black holes that are not ‘feeding‘, one way to detect them is to observe the motions of stars around the black hole, since their orbits will be altered by its presence.
Space is made up of three dimensions (up-down, left-right and forward-backward). If you add the fourth dimension, time, then you have what’s called the space-time continuum. This might sound strange, but imagine you are meeting somebody; you need to know which location (place in space) to meet them at, but you also need to know which time!
Albert Einstein was the first person to propose the idea of the ‘fabric of space’ (space-time), in his ‘General Theory of Relativity‘. Before Einstein’s theories, it was believed that gravity was a force, as explained by Isaac Newton. But Einstein’s general theory of relativity explains gravity as the ‘curvature of space-time’. This concept can be pictured by imagining space-time as a rubber sheet. The balls on the rubber sheet bend the sheet around them, similar to the way matter bends space-time.