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 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
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.