While it is unimaginatively named, PSR J0737-3039 just might be one of the most exotic objects in the galaxy. Discovered in 2003 at Australia’s Parkes Observatory, the observatory made famous by the film ‘’The Dish’’, PSR J0737-3039 is the only known double pulsar.
A pulsating star, or pulsar for short, is a rapidly spinning ball of neutrons that’s about the size of a small town. Although they are small in volume as stellar objects go, pulsars are unimaginably dense. A single coffee mug full of pulsar tips the scales at a hefty 10^17 kg – which is roughly the weight of 50 billion Boeing 747’s.
A pulsar’s density isn’t the only thing that makes them extraordinary, they also spin quickly. Really quickly. In 2005, a pulsar was found whose equator was rotating at 25% the speed of light, or a nimble 70,000 km/s.
A pulsar has such excessively violent tendencies because they are the tombs of dead stars. When a star of mass between 12 to 20 times the mass of our sun explodes at the end of its life, it will leave behind a neutron star. These neutron stars emit two jets of electromagnetic radiation from either end of their magnetic poles, which unlike the Earth, don’t always align up with their axis of rotation. The result is a beam of radiation that swings about through space like a rotating light house illuminating a point periodically only when the light is being shone onto it.
This is what makes a pulsar ‘pulse’. The neutron star isn’t changing in any sense; it’s simply that the electromagnetic beam that it shoots out only appears in our line of sight every so often – every few milliseconds in this case since they rotates so quickly. In fact, the ‘pulse’ of a pulsar is so regular that their time keeping accuracy rivals that of our most modern atomic clocks.
In 1967, Jocelyn Bell Burnell discovered the first pulsar. While remaining sceptical, the thought had crossed Burnell’s mind that the regular pulses were signs of extraterrestrial life. As such, her team jokingly named the pulsing object LGM-1 which stood for ‘’Little Green Men’’. After similar pulsing objects were found in different parts of the sky, it became clear that pulsars weren’t the work of alien civilisations after all.
While it mightn’t be home to alien life, what still makes PSR J0737-3039 so special is that it is the only known double pulsar. As the name suggests, this means that we are looking at two neutron stars in orbit around one another whose beams both coincidentally happen to pass through the Earth. Remarkably fortuitous as this may be, it’s not the end of it. By sheer coincidence, the two neutron stars orbit in a plane nearly edge on to Earth and the signal from one is eclipsed when it passes directly behind the other. In short, PSR J0737-3039 is an astronomer’s delight (an Earthian astronomer anyway).
Whilst being smaller than the size of Sydney, each of these neutron stars both weigh more than our sun and dance around each other every two and a half hours. It just so happens that these extreme conditions are what is necessary to test some of the predictions made by Einstein’s Theory of General Relativity. For instance the common orbit of both the pulsars is decreasing at a rate of 7mm per day (an amazingly precise observation when you consider that the system is around 1800 light years away from Earth). This rate fits with Einstein’s model which predicts that the orbits should shrink very slowly over time due to the emission of gravitational waves. At this rate, the two pulsars should collide in about 85 million years where they’re likely to go on to form a black hole.
While both beams are aligned so we Earthian astronomers can see them now, General Relativity predicts that both neutrons stars will wobble on their axis in a way similar to a spin top. Eventually each pulsar will ‘precess’ far enough that the electromagnetic jets will no longer be aimed at Earth and they will fade out of sight.
If Einstein was right, we should see mankind’s first known binary pulsar fade away over the next 15 to 20 years. However, like any interesting life, it will leave those who saw it with an awful lot to contemplate.