Artist concept of matter swirling around a black hole. (NASA/Dana Berry/SkyWorks Digital)
According to Einstein’s general theory of relativity, a black hole is a place where the gravity well is so great that a gravitational time dilation has occurred. This causes time to stop. The time dilation causes an event horizon into which objects can fall or be pulled, but will never reappear. They are referred to as black because they absorb anything that hits them, including all light. Analysis has shown that a black hole does posses a temperature and Hawking radiation. Although it is invisible, a black hole can be observed through its interaction with other matter. One can be inferred by tracking the movement of a group of stars that orbit a region in space which looks empty. Also, scientists can see gas falling into a relatively small black hole from a companion star. This gas spirals inward, heating up to very high temperatures and emitting large amounts of radiation that can be detected by Earth bound and Earth-orbiting telescopes.
There are two accepted types of black holes: those that have 4-15 solar masses(stellar mass) and those that have thousands of solar masses(supermassive black holes). What happens is that when an object in excess of 3 solar masses collapses there is nothing to support the weight. Neither neutrons nor anything else we are currently aware is able to support the weight, so the star continues to collapse, so a singularity forms that is within the Schwarzschild radius.
All galaxies, including ours, have supermassive black holes at their centers. Supermassive black holes are thought to have formed by the accretion of material by a stellar mass black hole until it hits the suppermassive stage. This type have properties which distinguish them from stellar mass black holes:
- The average density of a supermassive black hole can be much less than the density of water. This is because the Schwarzschild radius is directly proportionate to mass, while density is inversely proportional to the volume. Since the volume of a spherical object(the event horizon in this case) is directly proportional to the cube of the radius, average density decreases for larger black holes, being inversely proportional to the square of the mass.
- The tidal forces near the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, the forces are weaker at the edges of the horizon making it theoretically easier to escape from.
Like many other things in life, size matters and that is exactly how the types of black holes are defined. Current scientific understanding of the phenomenon is limited, so we may find additional classifying factors in the future.
We have written many articles about Black Holes for Universe Today. Here’s an article about black holes, and here are some facts about black holes.
If you’d like more info on Black Holes, check out NASA Science Page about Black Holes, and here’s more information from NASA from their educational pages.
We’ve also recorded an episode of Astronomy Cast all about Black Holes. Listen here, Episode 18: Black Holes Big and Small.
Sources: NASA, Hubblesite
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