I understand the fundamentals of how collapsed stars form, but in trying to grasp the mass=energy relationship, I am stuck wondering that since pulsars are emitting radiation (heaps of x-rays I gather) then where is this energy coming from?
If it’s from rotational energy then the pulsar must slow down, thus the frequency of the emission would also be slowing. Since I haven’t read this anywhere then I can only assume it must be losing mass.
If so, then as the mass reduces then so must the gravitational effect - so what are the implications of this? I assume that as gravitational effect is reduced then the effect (x-rays) is also reduced? If this is so then I guess it would all end at some point in equilibrium where there is no longer any x-ray emission?
I myself wonder why at all this is important, but it is really annoying me! My only defense argument is that I like to know the real facts, so therefore Newtons gravitational theory just isn’t good enough, and when you read any layman’s books on Einstein this sort of astro-physics stuff always arises to supposedly make things easier to understand!
Yes, the mass will be decreasing and it will slow down. But c[sup]2[/sup] is big, so this will tend to be a small effect over time scales we can appreciate.
Star quakes also affect the rotation rate - the star “settles” and the radius changes. Conservation of angular momentum requires that the rotation speed change.
Firstly, a pulsar is a neutron star which emits strongly at radio frequencies. An X-ray binary pulsar is a neutron star which emits brightly in the X-ray.
A neutron star is a very dense, very small collapsed stellar core. Since the star originally had a magnetic field, it is sensible to assume that the dense core will have a stronger magnetic field, since you will have the same flux density as in the original star. Also, a pulsar rotates extremely rapidly (this is conservation of angular momentum - a neutron star tends to be the diameter of a small town!).
Now, the exact process by which energy is radiated isn’t clear. Our leading model is that the star’s axis of rotation and magnetic field axis aren’t the same. The intense magnetic field and rapid rotation acts like an electric generator, and part of the magnetic field energy goes into creating electrons and positrons - a process called pair production. The electromagnetic fields in the pulsar push the electrons and positrons into the star’s magnetic field. As the particles spiral through the field, they are accelerated and emit radio waves. The magnetic field also confines the radiation along the star’s magnetic axis. We therefore see two collimated beams of intense radio radiation.
So, that’s how the radiation is produced. As you point out, it is the pulsar’s own energy that is radiated into space (well, into the nebula surrounding the pulsar), and so the pulsar does indeed slow down. We observe this effect in a decrease in the frequency of radio waves emitted. The pulsar doesn’t lose mass, simply rotational energy.
Now, for X-ray emitting pulsars and neutron stars. These are pulsars which are in a binary system. The gravitational field of a neutron star is so strong that any companion star is disrupted, and expands to fill its Roche lobe. Matter from the companion star is accreted onto the pulsar via an accretion disc. The infalling gas is essentially confined and funelled down onto the neutron star’s magnetic poles, and hits the star with enough energy to create two X-ray hotspots. As the neutron star spins, we get two collimated beams of X-rays. These are called X-ray binary pulsars.
Another class of energetic neutron star seems to be the Magnetar,
which is responsible for fantastically powerful gamma ray emissions, apparently caused by star quakes as mentioned by swansont.
The crust atoms of these stars are distorted into needles hundreds of times as long as they are wide; http://solomon.as.utexas.edu/~duncan/magnetar.html
Mmm, helps a lot! So what happens when the rotational energy has reduced to the point where the neutron star is no longer emitting EM radiation. Can we identify them?
I guess it is a question of whether these “dead” neutron stars are part of the missing mass of the universe or have these already been accounted for?
Pulsars rotate at anthything from a rate of the order of milliseconds to tens of seconds. The slowdown rate tends to be of the order of billionths of seconds a day, so we tend not to see totally slowed down pulsars.
However, what can happen to a pulsar if it has a companion star which is of a lower mass, is that as the low mass star evolves, whilst the neutron star is happily spinning down (as we call this slow down), the low mass star begins to expand into a red giant phase. Once the red giant fills its Roche lobe, the neutron star starts accreting matter from the red giant, as in the scenario I’ve described above for X-ray emitting pulsars. This extra energy and angular momentum is impacted onto the star. This causes the pulsar to spin up again. Essentially the pulsar is re-energised.
Also, a lone neutron star could capture a stray individual star, and make a binary pair that way. Which would lead to the same process.
Certainly, they’re a part of the dark matter, but not a very significant part. To explain: Some of the lighter elements, such as hydrogen, helium, and lithium, were produced in the Big Bang. By comparing the abundances of different isotopes of these elements, we can estimate how many baryons (protons and neutrons) were made in the Big Bang (this is referred to as Big Bang Nucleosynthesis). And based on that, we can put an upper bound on how much of the Universe’s mass could possibly be made up of baryons or of things which were once baryons. All of the various types of stellar remnants fall into this category, and we know that all of the baryonic matter in the Universe isn’t enough to give us the mass densities that gravitational evidence tells us is there. Since neutron stars are only a small portion of the baryonic matter in the Universe, they can only be an even smaller portion of the dark matter.
Cool. Thanks to all who have posted, really interesting and informative and answered my question exactly! I always loved astronomy as a child, in another life I think I will study astronomy/astro-physics.