When there’s a clear night, see if you can find these three constellations, all next to each other, and not that far from the pole star: they are called Cygnus (the swan), Lyra (the lyre) and Vulpecula (the little fox). Each of the first two has one very bright star, as you see from this photo. This story is about a very special kind of star, and each of those three constellations has one: these extraordinary objects are known as PULSARS.
There was a king of a tribe, called Ixion, who behaved so badly that Zeus, the king of the ancient Greek gods, struck him with one of his thunderbolts and ordered Hermes to tie Ixion to a burning solar wheel spinning round and round for ever and ever. In this drawing of those constellations, you can see the lyre which Hermes made and gave to a poet and prophet called Orpheus, who apparently was the greatest musician ever – but that’s another story. I wonder if Zeus knew that each pulsar, like the one in the Lyre constellation, spins on its axis very fast, much faster than the earth or any normal star could possibly do.
Here, pictured on an Ancient Greek vase, are some ancient pictures of poor Ixion tied for ever to his fiery wheel:
The story of Zeus’s thunderbolts and supernovas outlined how stars that are massive enough achieve high enough temperatures to create chemical elements in increasing order of mass, up to the point where iron atoms are made by nuclear fusion. If at that point the star expodes as a SUPERNOVA, heavier atoms can me synthesized by …. Well….. in this story, we’ll read how very recently, astronomers were able to solve one of the big puzzles about the synthesis of the atoms that are the heaviest that exist, such as lead and gold.
Pulsars turned out to provide an important clue to the solution to this puzzle. This clue developed from a very surprising finding in 1967. Quite often, a big discovery in science can happen when something comes up that seems to make no sense at all, like a question that at first is completely impossible to answer! Such a question turned up in 1967 when Jocelyn Bell Burnell, a graduate student in Cambridge, was searching through some recordings of radio signals from stars, which she had picked up using the “5-kilometre radio telescope” that had been built for the university using a stretch of the old railway line that used to run from Oxford to Cambridge. One of the radio signals, coming from the constellation Vulpecula (the little fox) that in shown in those images, went “tick tick tick tick….” on and on indefinitely, the ticks sounding absolutely regularly, every three-quarters of a second (more precisely, one tick every 1.3373 seconds).
Vulpecula, or the little fox with the goose, represented a fox carrying a goose
to Cerberus, the dog that guarded the entrance to the Underworld in Greek mythology
We don’t of course know what that Cambridge research team said to each other when they were arguing about what these ticks could mean. But we can think of the kind of imaginary conversation that might have been quite likely in the lab after that very puzzling signal had been picked up – such as:
- Perhaps so. But then, remember that for days and days we haven’t detected the slightest slowing-down of these ticks. The mysterious source MUST be enormously massive, as massive as the sun, so as to contain enough energy to power the radio beam indefinitely, without it running out of fuel and slowing down. Last week we’ve found that whatever it was, it kept time to better than 1 part in 10 million, either rotating or pulsating (we’re not sure which) more than 60,000 times per day. On top of this, it’s emitting enough power across interstellar distances to be detected even by our instruments here.
This imaginary conversation ends with an idea that since those days, has many times been confirmed by phenomena that it has predicted and found to agree with astronomical measurements. So to explain what pulsars could be, first astronomers had to think of what could possibly be that dense. You may know that the nucleus (the massive part) of any atom is made of protons and neutrons, but that almost all the rest of the atom is empty space. Calculations forced the astronomers to work out a model of what is now called a NEUTRON STAR, of amazingly small size and huge mass, consisting entirely of neutrons squashed together, with no empty space between them: Next, the physicists had to work out how such an object could be formed. Well, a very massive star may end up as a “supergiant”, and Deneb, shown in the first picture here, is one of them. An earlier story mentions how the nuclear physicists were able to work out what happens in the most massive stars of all, the supernovae, which for several days, after outshining all the other stars in our galaxy, explode in a matter of seconds, and by a very complicated series of nuclear reactions, collapse and are ripped apart. A neutron star is the collapsed core of just such a star, which originally had a total mass of between 10 and 25 times the mass of our sun and its core, after the collapse – the neutron star - could still have thrice the sun’s mass.
Another conspicuous supergiant is the bright red star Antares, sometimes mistaken for the planet Mars. So what causes the tick tick tick signal? It seems clear to astronomers now that neutron stars have a magnetic field, as indeed does the earth does as we know when we use a compass. The difference is that the magnetic field strength of a neutron star can be a quadrillion times Earth’s, far exceeding anything scientists can create in a laboratory. This mega-magnetism whips up charged particles in the vicinity, causing them to swarm around the magnetic poles and emit beams of intense radiation. These beams become a sort of lighthouse beacon, sweeping across the universe as the neutron star spins. On the off chance that Earth happens to lie in the beam’s path, we see a quick blip with every rotation.
There’s a quite different dramatic example of a pulsar, located in a constellation that you may well be familiar with. It’s Taurus ("the Bull"), a large and prominent constellation in the northern hemisphere’s winter sky. It’s the constellation that contains the Pleiades,
and the very bright red giant star Aldebaran.
In the northwest part of Taurus, near the southern horn of the celestial bull, is the remains of a supernova which exploded in 1054. It was visible to the naked eye for almost two years. In 1731 British astronomer William Parsons sketched this “nebula”, or “cloud”, around 1844 after observing it though a 36-inch telescope. The image of the object resembled a crab, which is how the nebula got its name.
These remains, known as the Crab Nebula, can be seen with 7×50 or 10×50 binoculars. Below is an image of this nebula as seen with the Hubble space telescope. In the middle is a relatively young neutron star. You remember about Jocelyn Bell Burnell – well, she told how in the late 1950s a member of the public, who was a pilot, sadly missed another big discovery: she was looking at the Crab Nebula through the University of Chicago's telescope, which was open to the public. She told the astronomer there that day, saying “doesn’t the nebula seem to be flashing?” The astronomer said no, it’s just scintillation (a type of twinkling), despite the woman's protesting that as a qualified pilot she understood scintillation and this was something else. Years later, in 1968, this was discovered to be a pulsar, the first to be connected with a supernova remnant and one of very few pulsars to be identified optically rather than using a radio telescope. It pulses much faster than the one discovered by Jocelyn, 30 times per second. It is running out of energy, as is clear from the fact that each day its pulsing slows down by more than a ten-thousandth of one percent.
In the supergiants story, we were told thatthe first atoms in the universe were those of hydrogen, the simplest and lightest of all. Once the first stars had lived through their life cycle, eventually they explode or blow off their outer layers, filling the universe with dust. Because of gravity, clouds of dust and hydrogen gas start to collapse and heat up: this collapse means that it can get so hot that a star is formed, in which the nuclei of the hydrogen atoms can combine (“FUSION”), to form nuclei of heavier atoms, namely helium. Once the hydrogen is used up, the star’s core contracts due to gravity and heats up even more, triggering fusion of helium to produce eventually the nuclei of heavier elements, up to iron for the most massive stars. But…. until recently the difficulty for astronomers was that no-one could work out how such stars could possibly reach temperatures high enough for fusion to produce even heavier atoms like lead, gold or uranium.
It was the physicists whose experiments showed that the massive nucleus of any atom other than hydrogen contains one or more neutrons as well as protons, When astronomers have a question that is hard to answer, they often turn for help to the nuclear physics experts who can calculate what nature may be able to perform. An example of this is that in the last stages of the life of a supernova, protons are released that in turn react with electrons to form neutrons. There's another supernova, in the constellation Cassiopaea, called Tycho’s nova, spotted 1572 by the Danish astronomer Tycho Brahe:
The first observation of a collision between neutron stars, detected in August 2017, created gravitational waves, light and heavy elements like gold and platinum. But astronomers have realized they also witnessed what is defined as a kilonova (a term defined on the next page) being that is kind of explosion that creates gold and platinum, the year before as well. Now, we have the first observational proof for neutron star mergers as sources; in fact, they could well be the main source of the elements heavier than iron, like gold and platinum.
The 2017 observation offered evidence for the theory that such massive explosions in space are responsible for creating large amounts of heavy elements. All of the gold and platinum found on Earth was likely created by ancient kilonovae that resulted from neutron star collisions.
Alternatively, the violent supernova blast produces powerful shock waves, analogous to the bangs made by supersonic flight, which create regions so dense and hot that they fuse some of the star's heavy elements into still heavier elements. It is in these supernova shocks that all natural elements heavier than iron are created, including uranium, the heaviest natural element found on Earth. Supernova shocks create even heavier elements (as do experiments with supercolliders), but they decay much more quickly than uranium.
We now know that the sudden brightness of what Tycho saw was the explosion of a particular type of supernova, which occurs when a small extremely hot star merges with a nearby companion star until it is is obliterated, sending its debris hurtling into space. That image is composed of x-rays of different energies, each characteristic of a particular element.
You can just see that at the centre, there is the tiny speck of the remnant neutron star which was left after the explosion. During the final destruction of the star in a supernova explosion like this, a high flux of neutrons is released as iron nuclei are ripped apart. We saw in story 3 that these neutrons can be captured by unstable nuclei before the nuclei have had a chance to decay. In this way nuclei of elements such as lead, gold and all the way up to uranium can be synthesised. The gold in jewellery here on Earth was in fact formed during the fiery last seconds of a massive star.
(an artist’s impression of two neutron stars colliding and merging -
– not an actual telescope image!)
Another mechanism is the result of neutron stars colliding with each other. Like normal stars, two neutron stars can orbit one another. If they are close enough, they can even spiral inwards to their doom in an intense phenomenon known as a “kilonova”.
The first solid evidence that neutron-star collisions are the source of much of the universe's gold, platinum and other heavy elements heavier than iron, came from precise determination of X-ray energies.
During a supernova explosion, a massive star ejects gas out into space. The force of the explosion also sends a shock wave into space. As this shock wave interacts with the ejected gas, the gas heats up and emits x-rays. Because the gas is hot, some of the electrons are stripped from the atoms in the gas, and the atoms becomes ionized. The energies of the electrons remaining in the atom are raised, and then decay again, emitting light at specific energies. By determining the energy of X-rays very precisely, astronomers can identify the elements in supernova remnants.
But that’s not the only alternative! There is a very fundamental principle in physics which among other things, tells us that NO two neutrons having the same energy (and spin) can occupy the same location in space. This means that generally, a neutron star cannot go on collapsing once it has reached a few miles in size. HOWEVER - sometimes a star is so massive that the mass of the material left after a neutron star is formed, means that the material keeps collapsing inwards until all the mass becomes concentrated at a single point, a singularity. That is called a “black hole”……
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