What are neutron stars?
Neutron stars are the collapsed remnants of massive stars that have used up all of their nuclear fuel and can no longer withstand the crushing inward pull of gravity. All the matter collapses into a tiny region and nearly forms a black hole. But, because of a quantum mechanical effect, the Pauli exclusion principle, some particles are prevented from coming too close to each other, which creates a physical pressure that allows neutrons to withstand any further collapse. It is this object that hovers on the brink of total collapse into a black hole, but uses quantum mechanics to survive.
Where does the name “neutron star“ come from?
The name comes from the fact that stars at very high densities, namely of roughly 1.4 times the mass of the Sun, are crushed. At those very high densities, it is favourable for particles to exist mainly as neutrons and thus the name “neutron star”.
In your talk, you mentioned pulsars. What is the difference?
Pulsars are neutron stars that rotate rapidly and are highly magnetized so they produce pulses of lights mostly in the radio band. We call an object pulsar because it produces pulses of light each time it rotates.
There are neutron stars that we don’t observe as pulsars because they spin too slowly to have the pulsar effect. However, we don’t really know how fast a star has to rotate at to produce pulses, although the slowest pulsar to be observed rotates every 8.5 seconds.
Why can some pulsars not be observed?
Some pulsars might have just died and stopped sending pulses. In fact, there is a sharp cut-off in the distribution of these objects in space: We know of about 2000 pulsars in the galaxy and you can plot a histogram of the number of objects as a function of period and you see that it trails off around a few seconds. Since we know that every pulsar is slowing down, this suggests that there are many unseen pulsars.
Do we know why they are spinning and emitting radio waves?
The original spin they are born with is thought to be a relic of the progenitor star that collapsed.
All stars that we know of rotate, including our Sun. The progenitors of neutron stars are more massive than the Sun and those stars rotate as well. The same way that figure skaters start to spin on the ice by giving themselves a little push and they have their arms extended and as they spin they pull them inwards to decrease their moment of inertia. Because angular momentum has to be conserved, the velocity increases; that way, they can spin very fast.
The same principle applies with a star: Even if it is only the core of the star that collapses and most of the star is not involved in the gravitational collapse, any initial rotation that was there is amplified.
How do we observe the explosion of massive stars?
We know that massive stars rotate and that neutron stars do too. But we have never seen one turn into the other because it is a very rare event. It happens between once and twice per century in our galaxy. Even so, most of these events are too far away to be observed due to the intervening dust that obscures light. However, there have been historical observations of nearby supernovae, which are explosions where stars collapse.
One such observation of supernovae was by Keppler in 1604, but there was no neutron star formed in that case, nor have we seen one since then, which is puzzling. That said, there was one star that went off in 1054 A.D., which today we know produced a neutron star in the Crab Nebula.
The most recent opportunity for this was in 1987, when there was a large supernovae explosion in a nearby satellite galaxy called the Large Magellanic Cloud, which is easily visible from the Southern Hemisphere with the naked eye because it looks like a cloud.
So if you were in Australia or South Africa, you could see a galaxy up in the sky that looked like a cloud! In that galaxy, there was a supernovae called 1987A that was discovered by a Canadian student. In that case, he saw the actual supernova minutes after it happens. The story goes that he was in the telescope control room when he saw a bright object in the sky and went out to look!
For more information about Prof. Kaspi and her research, visit: www.physics.mcgill.ca/~vkaspi
