Pan Galactic Gargle Blaster!

by Nicholas Mee on August 21, 2013

Neutron stars are the incredibly compact and bizarre remnants of very massive stars. They have greater mass than the Sun compressed into a ball with a diameter of around 20-30 kilometres. They are as dense as an atomic nucleus. (See my article Fermions, Atoms and Neutron Stars)

Cosmic Lighthouses

Neutron stars spin at an unbelievable rate typically completing a full rotation in a fraction of a second. This generates a huge magnetic field that is thought to be responsible for two intense beams of radiation that are emitted from their poles. These beams are known as pulsars. Neutron stars are like cosmic lighthouses. The pulsar beams are not perfectly aligned with the axis of the neutron star, so as the star spins the beams sweep across the heavens. On Earth radio astronomers detect a pulse of radio waves once every rotation when the pulsar beam points in our direction, which may be several times a second.

This image is a composite of X-ray, infrared and optical data. The diffuse area in magenta is an X-ray image of a supernova remnant. In the bottom right the comet-like object in green contains a pulsar that is racing away and appears to have been ejected by the supernova. The two bright stars in the image are in the same line of sight, but are thought to be unrelated to the supernova event.

In 1974 astronomers Joseph Taylor and Russell Hulse searched systematically for pulsars with the giant Arecibo radio telescope in Puerto Rico and found many examples of these remarkable objects. Amidst the data was one whose behaviour seemed unusual. The pulses of pulsar beams are received with incredibly regularity. But this one example was curiously different. The intervals between the pulses would increase for a while and then decrease again. There was a regular pattern to this behaviour which would repeat every seven and three quarter hours.

Hulse and Taylor had found one member of a binary neutron star system. The system consists of two neutron stars  orbiting around each other and the pulsar belongs to one of these neutron stars. As the neutron star approaches the Earth the interval between the arrival of its pulses steadily decreases, then as the neutron star recedes from Earth the interval increases again. This is the well known Doppler effect that we often hear with moving sirens. Incidentally, if the other neutron star also has a pulsar it never points in our direction, as it has not been detected.

Artist’s impression of a binary neutron star system (copyright Nicholas Mee)

The binary neutron star is a great find because the regular pulses have enabled astronomers to study the motion of the neutron stars and measure their properties with great precision. We know that the two neutron stars have about the same mass, which is around one and a half times the mass of the Sun. They complete one orbit every 7.75 hours. This means that the orbit is quite small by cosmic standards. At their closest the distance between the neutron stars is slightly more than the radius of the Sun. At their furthest it is almost five times this distance. It is amazing to have such detailed information about the paths followed by objects that are an immense 20,000 light years away, which is about a billion times further away from us than the Sun.

But the Hulse-Taylor neutron star has much more to offer. The solar system is quite sedate by cosmic standards. The planets serenely orbit the Sun and Newton’s theory of gravity explains their motion to great accuracy. Only in the case of Mercury was there a small hiccough that required a better theory than Newton’s. The resolution was provided in 1915 by Einstein’s theory of general relativity. Einstein’s theory gives different predictions for the shape of planetary orbits, but the differences are tiny unless the planets are moving rapidly in a very strong gravitational field. As Mercury is the closest planet to the Sun the relativistic corrections are much bigger than for the other planets. But the effect is still very small.

The Laboratory at the End of the Universe

The Hulse-Taylor neutron star system is a much more extreme gravitational environment than the solar system, so the relativistic effects are expected to be much greater. And the pulsar offers us a built-in precision time-keeper. This gives astronomers a great physics laboratory in which they can put Einstein’s theory to the test.

When the paths of the neutron stars were mapped out in detail, as expected their orbit didn’t quite match the Newtonian prediction, but it agreed with Einstein’s theory exactly. This was a great confirmation of our modern understanding of gravity and it is still one of the greatest triumphs of Einstein’s theory of general relativity. Even so, this effect had already been seen with Mercury many years earlier, so it wasn’t quite headline news.

But there was something else that was even more exciting. Einstein’s investigations that led to his theories of relativity were triggered by his desire to provide a better understanding of electromagnetism. His general theory of relativity, which explains gravity as the warping of space and time by massive objects, has many features that are analogous to electromagnetic effects.

For instance, when an alternating current is passed through an aerial the electrons in the aerial, which is just a long thin piece of metal, will rapidly move up and down. The acceleration of the electric charges of the electrons generates electromagnetic waves that we know as radio waves. This effect was first observed in the laboratory by Heinrich Hertz in 1887. We perform the same experiment every day when we turn on the radio or television or use our mobile phone.

Ripples in the Fabric of Space

In Einstein’s theory of gravity mass plays a similar role to charge. One of the surprising predictions of the theory is that if large masses are whirled around vigorously then gravitational waves will be emitted. As the theory explains gravity in terms of the warping and curvature of space, gravitational waves are simply ripples in the fabric of space that emanate away from the gravitating system. (A schematic description of how this comes about is shown below.)

This is a schematic representation of the shape of space around two very massive compact objects in orbit around each other. (copyright Nicholas Mee)

Gravity is incredibly weak compared to electromagnetism. This might seem surprising, but think of a fridge magnet. A tiny magnet such as this can defy the gravity of the entire Earth. The weakness of gravity means that a huge mass must be given an enormous shaking to produce even the tiniest ripple of a gravitational wave. For this reason physicists have so far been unable to construct instruments that are sensitive enough to detect gravitational waves.

Shrinking Orbits

We can now return to the Hulse-Taylor binary neutron star system. The orbital period of the binary system can be measured with great precision. As mentioned above, it is about seven and three quarter hours. But it is decreasing by 76.5 microseconds every year.

General relativity provides the explanation. As the two neutron stars whirl around each other at high speed they generate gravitational waves. The energy that is radiated into space in this way causes the orbit of the neutron stars to shrink. When physicists calculated the rate at which the orbit of the neutron stars would shrink due to the emission of gravitational waves the prediction of general relativity matched the observations to perfection.

So although physicists have never detected gravitational waves their existence has been confirmed by this extraordinary star system. Einstein’s theory has been proved right again.

The size of the neutron stars’ orbit is shrinking by a few metres every year due to the emission of gravitational waves. In 300 million years the two neutron stars are scheduled for a close encounter. This meeting is sure to be an incredibly violent one.

A Blast From the Past

The Nuclear Test Ban Treaty was signed in 1963. To monitor compliance with the treaty in 1967 the United States launched a series of satellites that could detect gamma rays, which are the tell-tale signatures of nuclear explosions. These satellites began to detect occasional flashes of gamma rays, or Gamma Ray Bursts, as they are known. They were immediately put under investigation.

By 1973 it was clear that the gamma rays originated in deep space and the research was declassified by the military. Since this time astronomers have worked steadily to uncover the secrets of these events. As the Gamma Ray Bursts typically last just a few seconds this has proved quite a challenge. Success has depended on the rapid deployment of telescopes to study the afterglow following the detection of a burst by a gamma ray detector aboard a satellite. It is now clear that these events are the product of the most violent cataclysms in the universe. They are rare, but the events are so powerful that we can detect them from the other side of the universe thousands of millions of light years away.

Gamma Ray Bursts have been divided into two categories. Most of the events last for a few seconds and are known as long Gamma Ray Bursts. The other category is the short gamma ray bursts that last for less than two seconds.

Into the Jaws of Hell

The long Gamma Ray Bursts are thought to be due to the gravitational collapse of a huge star into a black hole at the end of its life. As the star collapses it spins ever faster, such that when the black hole forms it is just a few kilometres across and spinning at almost the speed of light.

Black holes are often portrayed as gaping like the jaws of Hell. It is certainly true that once inside it is impossible to escape. But getting inside in the first place may not be that easy. Black holes are very messy eaters. They are tiny by cosmic standards, so squeezing an entire star into one proves to be difficult. Much of the material shoots out at the poles of the black hole rather than entering the abyss. This material is compressed beyond nuclear densities and focused into two beams that shoot outwards at almost the speed of light. Much of this material is converted into intense gamma ray beams that race across the universe.

Staring Down the Barrel of a Ray-gun

A civilization on the other side of the universe that happens to be looking down the barrel of this gamma ray-gun may eventually pick up a brief trace of radiation signalling the death of a mighty star and the formation of a black hole.

Short Gamma Ray Bursts have proved to be even more difficult to study because they are so short-lived and much less powerful. But recently there has been an important breakthrough.

On 3rd June the gamma ray telescope aboard NASA’s Swift satellite, shown above, picked up a Gamma Ray Burst that lasted just a tenth of a second. Nine days later the Hubble Space Telescope took up the search for the origin of the gamma radiation. Hubble found a faint glow in a galaxy four thousand million light years distant, so the Gamma Ray Burst had been produced in an event that occurred when the Earth was in its infancy.

Analysis of Hubble’s images has shown that the short Gamma Ray Burst was generated by a type of stellar explosion called a kilonova, so named because they are around a thousand times as bright as a nova. Nevertheless they are just a hundredth to a tenth of the brilliance of a supernova. (For more details above novas and supernovas, see my article Apocalypse Now!)

Last Tango in Deep Space

What sort of drama produces a kilonova, you might ask?

They are generated by the merger of two neutron stars that have reached the climax of their cosmic dance. In 300 million years time this will be the ultimate fate of the neutron star system discovered by Hulse and Taylor.

Further Information

The Pan Galactic Gargle Blaster Science News article Hubble Sees the Fireball from a Kilonova

Previous post:

Next post: