According to the general relativity theory of Einstein, near a large mass, time happens slowly due to the gravitational action.
Einstein deduced (as we read in his book “The Meaning of Relativity”) the following formula
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It is possible to find the relationship between the mass and the radius of a spherical black hole keeping in mind that the maximum speed that could reach an object, according to the theory of relativity, it is the speed of light.
The scape velocity in the surface of a spherical star will be the maximum velocity that could reach an object in order to stay in orbit around the star. This will happen when the kinetic energy of one object is equal to the potential energy due to the star gravitational attraction. Continue reading
We are studying an object, that people may think from recent conception, but that it was conceived more than two centuries ago.
In their initial conception, a black hole was an object with a gravity force in their surface so big that nothing could escape from him; not even light if it were affected by gravity (this was not known 200 years ago). Before measuring the speed of light and before the theory of relativity, by means of which it was demonstrated that nothing could surpass the speed of light, people thought that a body could reach an infinite speed and therefore the black hole was a body in which the scape speed was also infinite. This could only happen in the case of a infinite mass star or infinite density star. This cases were illogical cases and the scientists did not give any importance to the matter, falling it in the oblivion.
As we have described a black hole, we never can observe one of them because they would not reflect or emit any radiation or particle. But there are certain effects that can be detected. One of these effects is the gravitational effect on a neighboring star.
Suppose a binary star system (two stars so close to turning the one around the other) in which one of the stars is visible and than we can calculate its distance from Earth, and its mass. The visible star will make a few oscillatory movements in space due to the gravitational attraction of the invisible star. From these movements we can calculate the mass of the invisible star .
If this invisible star exceeds a mass of about 2.5 times the mass of our Sun, we must assume that it’s a black hole.