NEUTRON STARS, PULSARS AND BLACK HOLES
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When the atoms in a star get compressed to very high densities when stars come to the end of their lifes, the only thing holding the atoms apart is their electrons. The lowest electron energy levels are filled, and the electrons are forced into higher and higher energy levels, filling the lowest unoccupied energy levels. The full electron shells provide enough internal pressure within the star to to prevent gravitational collapse providing the mass of the white dwarf does not become larger than 1.44Msun, this is called Electron degeneracy and is found in all white dwarf stars. These are the cores of normal AGB stars (Asymptotic Giant Branch) which have used up all their fuel and blown off their outer layer to become Planetary Nebulae. But what happens if the mass of the star is so great that even electron degeneracy cannot hold back the force of gravity?
You've guessed it, they become Neutron Stars.
The idea of the neutron star was thought up by two guys by the name of Robert Oppenheimer and George Volkoff in 1939. They suggested that a star could exist that was very similar to a White Dwarf, but instead of degenerate electrons, it would be much denser and be made up of degenerate neutrons. Even with a mass of 70% that of the Sun, it would measure only 10km across. They were thought to be impossible to find, because even though they would have a surface temperature of around 50,000K, the surface area of the star would be extremely small and therefore dim. They were also new, and only a theory, so nobody even bothered looking for them for almost 30 years.
Although they have since been discovered, the structure of neutron stars is still primarily unknown. What is known is quite interesting.
Like white dwarf stars, their size decreases with an increase in mass. This is known as the mass-radius relationship. It occurs because as mass is added to a degenerate star (either white dwarf or neutron star) it increases the internal pressure of the star, but also the gravitational potential energy. As the pressure is already at a maximum, the gravitational energy wins, and compresses the star. Roughly speaking, if the mass is doubled the radius is halved. Just as for white dwarf stars, neutron stars also have a Chandrasekhar limit. The lower and upper limit for a neutron star seems to be about 1.5 - 5M sun.
A newly formed neutron star may have an internal temperature of almost 1 trillion degrees K, but due to its degeneracy, its pressure remains almost constant. It will however cool down significantly to around 1 billion degrees within a day or so, and eventually settle down to sustain a surface temperature of a few hundred thousand degrees.
When neutron stars are newly formed they are known to rotate at various rates. Some neutron stars rotate at 1 revolution every 4 seconds, whereas some neutron stars rotate at unbelievable speeds up to 1000 revolutions per second. The spin rate of a neutron star is so high because angular momentum must be conserved. This means that the rotational velocity of the star will increase as the radius decreases, and will decrease as the radius increases. Due to the mass of neutron stars and their very small size, it makes sense that their rotational velocity will be very high.
Angular momentum, L = M*V*R
M = mass, V = Velocity, R = Radius.....so, as L is conserved (never changes), if either M, V, or R increase or decrease, one or more of the other variables must increase or decrease to maintain L.
The magnetic field around a neutron star is also enormous. As with the angular momentum of the star, the magnetic field of a star also increases as the diameter decreases. The strength of a magnetic field is measured in TESLA'S. The average neutron star has a magnetic field strength of around 108 Tesla's (100 million), whereas the average star has a magnetic field strength of around 1 Tesla.
A pulsar is a rapidly spinning neutron star which gives out huge amount of radio radiation at extremely regular intervals (constant to within 1 part in 10 million). They were accidentally discovered in 1967 by a then graduate student Jocelyn Bell. Today we know of more than 400, including the well known Crab nebula pulsar with a revolution period of 1,800rpm.
They are believed to work as follows. If you imagine the Earth did not rotate about it's axis but rather it's axis actually rotated (which it does every 26,000 years) but at the same rate as today's axial period of 24hrs. Then from space you would see the North and South poles rotating once every 24hrs pointing toward you and then moving away. This is believed to be the cause of a pulsars emissions, but at a much higher rate. If you now imagine that the (north and south poles so to speak) were actually the magnetic poles of the neutron star, then as the star rotates the magnetic field would sweep out a beam of highly energetic radiation along the direction of these magnetic poles, in periods as short as 1/1000th of a second. If the Earth lies in the direction of one of these beams then we will receive the beams every time they pass. The frequency of the radiation the star emits varies from pulsar to pulsar, but can be easily calculated because it is proportional to the rotational frequency. i.e. the frequency of the Crab nebula is 30hz because the rotational frequency of the pulsar is 1/30th of a second.
The above paragraphs explain that if a gas becomes degenerate then the only thing stopping the atoms from destroying each other are their electrons. If the mass increases further, then the electrons are squeezed into the nucleus of the atom where they react (so to speak) with the protons and become neutrons and neutrinos. At this stage the gas becomes neutron degenerate (instead of electron degenerate) and only the neutrons of the atoms are left. The density of a neutron star at its core is immense, 1cm3 can weight almost 1 billion tons. If the mass of the star increases even further, then even this neutron degeneracy cannot support the force of gravity and the weight of the star. It is at this point that there is a final collapse, and a black hole is born.
At the centre of the black hole is a point, almost but not quite infinitely small and infinitely dense called the singularity. No physicist on the planet knows exactly what happens here, but we do know that the laws of physics don't apply. They can have the mass of many stars, yet this incredible weight is compressed into a size smaller than an atom. This unbelievable density gives rise to an enormous gravitational field.
To try and understand this a little better, imagine a large square rubber sheet about 5x5m with lines drawn onto it to form a grid. This is a model of what space-time looks like. Now place a marble (earth) in the middle, obviously the sheet (space-time) will be warped a bit as the lines are curved around the object. Now place a larger object onto it, say a bowling ball (the Sun). Again the sheet (space-time) is warped, but by a much larger factor. The larger the object placed on the sheet, the larger that so called space-time is affected. This is due to gravity. Gravity is the effect of the curvature of space time around an object. The space-time around a black hole could be demonstrated by holding the sheet above your head, and then pulling the centre of the sheet down to the floor. You would be left with an inverted cone shape. This would demonstrate the enormous gravity of a black hole.
How do we know Black holes exist?
The picture above is a Hubble image of the core of Galaxy NGC 4261 in Virgo. The image shows large amounts of gas and dust swirling into the centre of the galaxy. The disk shown is 800 light years in diameter, over 700,000 times bigger than our solar system. When the velocity of the matter swirling around in the centre of the disk was measured, astronomers have calculated that the object at the centre must weigh almost 1.2 billion times the mass of the Sun. This mass is simply inferred by rearranging Newton's law of gravity for orbiting bodies. At the centre the mass is concentrated into an area no bigger than our solar system and has a central object that is almost invisible. The enormous mass, lack of visible radiation, and tiny volume lead to only one possible answer......Black Hole. The disk of gas and dust surrounding the black hole is enough to make 100,000 stars. Many examples of the these disks at the centre of galaxies are known, of which most also posses large jets of material spewing out from the black holes at close to the speed of light. NGC 4261 has a pair spanning 88,000 light years.
It is evidence like this that leads astronomers to the overwhelming conclusion that black holes must exist as there is no other explanation to the observations. But until we see one up close (not recommended) black holes still remain a theory.
How much does a black hole weigh?
It all depends on the mass of the original star. First of all it is worth noting that there is no limit as to how much mass a black hole can have. If you think about it, the more mass a black hole has, then the more gravity it has to attract more mass, the bigger black holes just get bigger quicker.
The mass of a black hole can vary enormously, from 0.0001g up to theoretically anything. Black holes left over from ordinary high mass stars are known as galactic wonderers. However, in the last 10 years astronomers have found large black holes at the centre of almost every galaxy we know of. These are called supermassive black holes and can weight up to 1039 kg, or 100 million billion billion billion tons.
How big are Black Holes?
The Schwarzschild radius (or radius of the event horizon) is directly proportional to the mass of the black hole i.e. if the mass doubles, so does the radius. The density of a black hole is inversely proportional to the square of the mass i.e. as the mass increases the average density actually decreases. If a black hole weighed 1 solar mass (as much as the sun) it's event horizon would have a radius of around 1.8 miles (not much). It is not hard to therefore workout that a 10 solar mass black hole would have an event horizon of around 18.6 miles, & a 1 million solar mass black hole about 1.8 million miles. It has been estimated that M104 (Spiral galaxy in Virgo) may indeed harbor a 1 billion solar mass black hole at it's centre. That equates to a 180 million mile radius. Interestingly that is more or less that same diameter as Earth's orbit around the Sun.
What would happen if I went near a black hole?
First of all, DON'T, but if you did then I shall attempt to explain.
Lets suppose you point yourself toward a 1 million solar mass black hole and let gravity do the rest. Initially you feel no different at all to when you were in Earth orbit, but as you get closer something known as "tidal gravitational forces" start to effect you. Depending on which part of your body is closest to the black hole (lets say your head) you will start to feel a slightly stronger gravitational pull from your head than your feet. The closer you get to the black hole the more you will feel stretched. As you approached the event horizon the gravitational forces would pull you apart like play-doh.
In a small (say 30 solar mass ) black hole, the gravity at the event horizon would exert upwards of 1 million g, although after 10 you would probably be unconscious.
From a fair distance before you would be killed, you would notice a few visual distortions. For example, you would be able to see stars that were behind you, in-front of you. Objects to your right would also appear on your left, and objects above the black hole would seem to appear below, as the light would be bent around the back of the black hole. This effect is called gravitational lensing.
Lets say you were still alive as you crossed the event horizon, what would you see? Obviously looking into the black hole you would see nothing. If you were to look back the way you came in, you would encounter a form of tunnel vision. All the light entering the black hole would gradually (but extremely quickly, considering you are travelling close to the speed of light) converge toward your centre of vision more and more the further in you went. This is because all the light coming from 360o around you and from behind you is being bent by gravity all around you, to a point of origin in-front of you. Due to relativistic beaming and gravitational tidal distortions, the in-falling light would form a kidney shape oval at around 0.35 Schwarzschild radii, and flattens out into a curved line as you approach the singularity.
If I saw somebody fall into a black hole, What would I see?
As you would expect, you would initially see them falling away from yourself and toward the black hole. They would appear to get faster and faster, but then something peculiar happens. For the person falling, they would be torn to pieces while travelling close to the speed of light, but for you they would appear to slow down and eventually stop just before falling in. They would appear to remain motionless for ever... Why?
You must remember that the gravity within/around a black hole is so strong that the escape velocity is that of light. This means that any photons (light particles) that are constantly being reflected off him/her and toward you, start to travel much slower because the gravity of the black hole is attempting to pull them back. As the person crosses the event horizon, they cross the point where light can no longer escape out of the black hole. But instead of them becoming invisible, their photons that escaped the split second they crossed over the event horizon are now stuck. They are travelling at the escape velocity of the black hole, no faster. This means that those photons will remain motionless, suspended above the event horizon. Only when the mass of the black hole increases will the photons be consumed.
