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Gravitational collapse
    Try to jump so high that you fly right off of the Earth into outer space. What happens? Why don't you get very far? The gravitational force pulls you back down again very quickly. You could jump much higher on Mars, still higher on the moon, because they're both less massive than the Earth. The strength of gravity at the surface of the moon is only 1/6 the strength of gravity at the surface of the Earth.
A black hole surrounded by a dust cloud
A gigantic black hole surrounded by a dust cloud in the center of a distant galaxy, as seen by the Hubble Space Telescope
    You are essentially trapped on Earth, unless you can find a rocket that can travel at escape velocity away from the Earth. This is how our space program works. If you shoot something fast enough, it can escape gravity and make it to outer space.
    But hold the phone -- there's supposedly a maximum speed in the Universe, the speed of light. What happens if the escape velocity of a planet were greater than the speed of light? In other words, what if gravity were strong enough to trap light itself?
    Then you'd have yourself a black hole. A black hole is a gravitating object whose gravitational field is so strong that light cannot escape. The event horizon is where light loses the ability to escape from the black hole. Nothing that goes inside the event horizon can ever get back out again, not even light.
    Black holes can be created by the gravitational collapse of large stars that are at least twice as massive as our Sun. Normally, stars balance the gravitational force with the pressure from the nuclear fusion reactions inside. When a star gets old and burns up all of its hydrogen into helium and then turns the helium into heavier elements like iron and nickel, it can have three fates. The first two fates occur for stars less than about twice the mass of our Sun (and one of them will be our Sun's eventual fate). These two fates both depend on the fermionic repulsion pressure described by quantum mechanics -- two fermions cannot be in the same quantum state at the same time. This means that the two stable destinies for a collapsing star will be:

1. a white dwarf supported by the fermionic repulsion pressure of the electrons in the heavy atoms in the core
2. a neutron star supported by the fermionic repulsion pressure of the neutrons in the nuclei of the heavy atoms in the core


    If the mass of the collapsing star is too large, bigger than twice the mass of our Sun, the fermionic repulsion pressure of either the electrons or the neutrons is not strong enough to prevent the ultimate gravitational collapse into a black hole.
    The estimated age of the Universe is several times the lifespan of an average star. This means there must have been a lot of stars bigger than twice the mass of our Sun that have burned their hydrogen and collapsed since the Universe began. Our Universe ought to contain many black holes, if the model that astrophysicists use to describe their formation is correct. Black holes created by the collapse of individual stars should only be about 2 to 100 times as massive as our Sun.
    Another way that black holes can be created is the gravitational collapse of the center of a large cluster of stars. These types of black holes can be very much more massive than our Sun. There may be one of them in the center of every galaxy, including our galaxy, the Milky Way. The black hole shown above sits in the middle of the galaxy called NGC 7052, surrounded by a bright cloud of dust 3,700 light-years in diameter. The mass of this black hole is 300 million times the mass of our Sun.
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