Astrophysics is the study of the objects, traditionally referred to as 'heavenly bodies', which can be observed in the Earth's night sky. It uses physics and chemistry to attempt to discern the composition and structure of stars, nebulae, galaxies, etc., whereas Astronomy derives its information primarily from observations of movements of heavenly bodies, and Cosmology is concerned with the big picture, the history and evolution of the universe as a whole.
Astrophysicists are currently engaged in studying the Sun, stars, exoplanets, our Milky Way galaxy, other galaxies, as well interstellar medium, black holes, and the cosmic microwave background. They use many types of instruments, such as optical telescopes and radio telescopes, and measure such things as the electromagnetic spectrum of stars, which reveals which elements are present in them through distinctive absorption lines.
The discipline is very broad, and also includes highly theoretical work, such as the existence of wormholes, general relativity, gravity waves, and even the possibility of time travel and extraterrestrial life. The expansion and possible future contraction of the universe, inflation, dark energy and dark matter, are hotly debated issues of modern astrophysics, running ingenious experiments from the Earth and space satellites.
In the early 1800s, Pierre-Simon de Laplace applied Newton's law of gravitation to a hypothetical case of the extreme compression of a large amount of matter to a very small volume. He realised that the gravitational force near the mass would be so great nothing could escape, not even light. This was possibly the first concept of black hole with a threshold of 'no return'.
Albert Einstein took the idea further with his General Relativity theory, published in November 1915. Karl Schwarzschild, a German astrophysicist very quickly worked out the mathematics of the equations describing the behaviour of matter, light and energy in space, known as the Field Equations. He sent them to Einstein in January 1916 while he was serving on the eastern front during World War One, who promptly recognised their importance and published them.
Schwarzschild's radius is the sphere of equidistant points from the centre of the black hole where light would not escape because the gravitational field is so intense that space-time is bent back on itself. This is a confirmation of Einstein's explanation of gravity, as a distortion of space-time, since photons, the quanta of light, have no mass, so would not be affected by Newtonian's gravitational force. This is known as the Event Horizon, a point where all information concerning matter entering a black hole is lost forever.
Although the mechanism was theoretically described by Einstein and Schwarzschild, the term 'Black Hole' was not popularised until 1967, through the work of John Wheeler (1911 - 2008). And their actual existence was not detected until the pulsar phenomenon was discovered by radio astronomers, primarily Jocelyn Bell. Many of the laws and theoretical descriptions of black holes and their radiation was worked out by Stephen Hawking and others. Now it is believed that there are many millions of black holes in the galaxy, and every galaxy probably has a giant black hole at its centre.
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