The Beacons of the Universe
Keep piling on more weight, and at some point the
electrons and protons in a white dwarf are crushed
together and neutralize each other. The star turns into
one big soup of neutrons. The gravity keeps pulling down,
as before, pushing the neutrons closer together than they
could ever get with the electrons and protons
around.
This is where Nature's trickiness comes in. The neutrons also belong to the special category of particles called fermions . Like white dwarfs,
neutron stars weigh about as much as our Sun, but they are
packed into spheres that are just a few miles
across. Imagine all the matter in the Sun—about
500,000 times as much as is in the Earth—being
packed into a ball that could fit comfortably inside the
Grand Canyon! This is 100,000,000,000,000 (one hundred
million million) times as dense as lead.
With all this density, the gravity around a neutron star is much more intense than anything we on Earth will ever experience—just like in the case of the white dwarf, but worse. If you were separated at birth from a twin who went to live on the surface of a neutron star, by the time you reach 75 years old, your twin would only be 60 years old. (Of course, life on a neutron star is far worse than on a white dwarf, even.) In fact, gravity is so strong that not only the warping of time, but also the warping of space is noticeable. To visualize this warping, we take another disc slicing through the center of our star. But circles on the disc don't obey our high-school geometry rules. This disc that we cut straight through the star acts as though it is a hill, just like the one shown below.
The dark ring shown in the picture represents the surface
of the star. Take a rope reaching from the center of the
star to its surface. If high-school geometry worked in
this case, the circle would be about 6.2832 times as long
as the rope. It turns out, however, that the circle is
much smaller than that. That is the weirdness of warped
space.
The picture above shows a perfectly round neutron star—it is the same, no matter which angle you choose to look at it. But this is not necessarily how stars found in the Universe actually are. Neutron stars are very complicated objects. For example, they are believed to have solid crusts—like the Earth's solid rock floating over the liquid magma inside. Just as there are mountains on the Earth, we can expect to find mountains on a neutron star. In that case, the star won't look the same from any angle. If we look at our disc now, it will act like a big hill, with a little lump on it.
Suppose that our bumpy neutron star is also spinning. We know that the Earth spins—that's why the Sun seems to move across the sky. Astronomers have seen that the other planets rotate similarly. Even the Sun itself rotates. We should expect that distant stars also rotate. As these stars die, their rotation doesn't just stop. Instead, like an ice skater bringing her arms in as she twirls, the collapsing star will spin faster and faster. Once the star is squeezed down to the size of a neutron star, it will spin incredibly quickly.
Now we add one last ingredient to the mix. We know that
the Earth has a magnetic field—which is why a
compass works. Similarly astronomers have discovered
magnetic fields around other planets, and even the
Sun. Again, we would expect that most stars have magnetic
fields. When the star contracts to the size of a neutron
star, the magnetic field is squeezed and becomes much more
intense. A typical neutron star might have a magnetic
field 1,000,000,000,000 (one million million) times more
intense than the Earth's. Such a strong magnetic field can
shoot particles from the star out into interstellar
space.
In 1967, the astronomer Jocelyn Bell was looking at data she had been taking at a radio telescope when she noticed an unusual, steady pattern. Dr. Bell had discovered the first known pulsar . A pulsar is just a
neutron star that shoots out a beam of particles and
light. When the star rotates, its beam rotates with it,
like a tremendous lighthouse. It may happen that the beam
from this lighthouse sweeps across the Earth, where humans
see the signal as a flash from outer space. Some pulsars
flash hundreds of times per second. That means that the
entire star is spinning around hundreds of times per
second.
Now, if such a pulsar happens to also have a mountain on it, spacetime will get stirred up. Like a paddle spinning in water, that pulsar will give off gravitational waves. The spinning should be pretty constant, so we would expect the sound of its gravitational waves to be a very steady tone—one single, constant note carried off through space and time.
This is where Nature's trickiness comes in. The neutrons also belong to the special category of particles called fermions
A type of particle with "odd half-integral angular momentum"—a spin of 1/2, 3/2, etc. Spin refers to an intrinsic quality of all particles. Examples of fermions are electrons, neutrons, and protons. The other type of particle is the boson.
With all this density, the gravity around a neutron star is much more intense than anything we on Earth will ever experience—just like in the case of the white dwarf, but worse. If you were separated at birth from a twin who went to live on the surface of a neutron star, by the time you reach 75 years old, your twin would only be 60 years old. (Of course, life on a neutron star is far worse than on a white dwarf, even.) In fact, gravity is so strong that not only the warping of time, but also the warping of space is noticeable. To visualize this warping, we take another disc slicing through the center of our star. But circles on the disc don't obey our high-school geometry rules. This disc that we cut straight through the star acts as though it is a hill, just like the one shown below.
The geometry in and around a neutron star
one-and-a-half times as massive as the Sun. The
black ring represents the surface of the star. We
can see that space is warped, because a slice
straight through the center of the star has the same
geometry as a hill. Time's rate of flow in and
around the star is shown on the color scale. Time
slows significantly at the
center.
What does this mean?
What does this mean?
The picture above shows a perfectly round neutron star—it is the same, no matter which angle you choose to look at it. But this is not necessarily how stars found in the Universe actually are. Neutron stars are very complicated objects. For example, they are believed to have solid crusts—like the Earth's solid rock floating over the liquid magma inside. Just as there are mountains on the Earth, we can expect to find mountains on a neutron star. In that case, the star won't look the same from any angle. If we look at our disc now, it will act like a big hill, with a little lump on it.
Suppose that our bumpy neutron star is also spinning. We know that the Earth spins—that's why the Sun seems to move across the sky. Astronomers have seen that the other planets rotate similarly. Even the Sun itself rotates. We should expect that distant stars also rotate. As these stars die, their rotation doesn't just stop. Instead, like an ice skater bringing her arms in as she twirls, the collapsing star will spin faster and faster. Once the star is squeezed down to the size of a neutron star, it will spin incredibly quickly.
The Crab Nebula contains the remnants of
the supernova seen around
the world in 1054 A.D. The remaining core in its
center is a pulsar.
In 1967, the astronomer Jocelyn Bell was looking at data she had been taking at a radio telescope when she noticed an unusual, steady pattern. Dr. Bell had discovered the first known pulsar
A neutron star with a very high rate of spin, and very intense magnetic fields. The pulsar gives off beams of radiation along its magnetic poles. If these poles are not aligned with the spin poles, the beam will sweep around like the beam of a lighthouse.
Now, if such a pulsar happens to also have a mountain on it, spacetime will get stirred up. Like a paddle spinning in water, that pulsar will give off gravitational waves. The spinning should be pretty constant, so we would expect the sound of its gravitational waves to be a very steady tone—one single, constant note carried off through space and time.
