Two White Dwarfs and a Pulsar: The Mysterious Triple Stellar System PSR J0337+1715

Artist's impression of the triple stellar system

They're stars, but not as we know them

Last Sunday evening astronomers announced the discovery of a star system so strange that it made the BBC news website. Three technically dead, but very much still kicking, stars that could allow us to explore one of the most important gaps in our knowledge of the Universe.

The paper is surprisingly readable, although I realise I may not be the best judge! If you don't want to slog through it, read on...

The system, with the exciting name of , is made of three stars all orbiting each other over 4000 light years from the Earth. These stars aren't like our Sun: instead they show two of the different things that can happen to a star when it runs out of the hydrogen fuel that it needs to produce energy.

At the centre of the system is a pulsar, the aftermath of the explosive death of a truly massive star. It's a neutron star, an object so dense that it has one and a half times as much mass as our Sun packed into a ball just a dozen or so kilometres across.

Stranger still, this object is spinning at a rate of 366 times every second, sending out a beam of radio light from its poles that can be picked up by telescopes here on Earth. These are know as millisecond pulsars, or MSPs. The telescopes are so good that this beam can be measured to within an accuracy of one ten-thousandth of a second every half an hour.

This precision allowed the observers to spot tiny variations in the arrival time of the beams, suggesting that there was another object close by that was affecting the path of the beam. Looking through old telescope data allowed the culprit to be spotted. It was a faint, blue dot. Another dead star.

This time it was a white dwarf, formed from a star very similar to our Sun. These are the end-points of the evolution of smaller stars, a slowly cooling hot ball about the size of the Earth, with a fifth of the Sun's mass packed in. The white dwarf is orbiting so close to the pulsar that they spin around each other in just a day and a half.

(My PhD involves studying what's left of any planets that were around white dwarf stars. But that's a subject for another day!)

However, the white dwarf couldn't explain all of the timing variations, meaning that there must be a third object, invisible to our telescopes, orbiting the pulsar-white dwarf pair. Only one object fits the bill: A second white dwarf, larger than the first but older and dimmer, orbiting the inner pair at a distance slightly less than that of the Earth from the Sun.

With three dead stars, this system must have had a turbulent history. It would have started as three normal, main sequence stars like the Sun. Two would have been just a bit bigger than the Sun, but they would have orbited around a monster ten times the mass of our star.

The more massive a star is, the shorter it lives. As the central star quickly burned through its fuel it would have swelled up, encasing the other two stars in a common envelope of gas and plasma, before exploding in a supernova to leave behind the spinning pulsar.

About a billion years later the outer star would also run out of fuel. Smaller stars don't blow up. Instead they swell up to form red giants, before blowing away their outer layers and shrinking down into white dwarf stars. As it grew in size material from the outer star fell onto the inner pair, changing the dynamics of the system so that now the stars all orbit each other in almost perfect, flat circles.

After another billion years the inner star would also have died. This time its outer layers flowed onto the pulsar, speeding it up until it was spinning around once every 2.73 milliseconds. The inner star then finished evolving into a white dwarf, forming the system as we see it today.     

The history of the triple star system . Starting as three normal stars, the central star exploded to form a pulsar. The two other stars then became white dwarfs, to leave the system as it is today.

This history is fascinating in itself, but what makes the triple star system even more interesting is its potential to test our understanding of how the entire universe works.

Since 1917 our knowledge of the night sky has been underpinned by Albert Einstein's Theory of General Relativity, an explanation of how gravity holds all of the stars, planets and galaxies in the universe together. General Relativity has passed every test that we've thrown at it and explained everything that we can see.

It's also wrong.

We know that it is wrong because it doesn't fit together with the other great theory of modern physics, Quantum Theory. This explains the physics of the smallest building blocks of the universe, the particles and atoms that make up us and everything around us. Like General Relativity, it has been tremendously successful at explaining the world around us.

But put the two together and it all falls apart. General Relativity is hopeless at explaining the very small, Quantum Theory doesn't show any way to build a galaxy. The search for a theory that would explain both of them, the semi-mythical quantum gravity, is one of the biggest challenges of modern physics.

One of the problems when searching for quantum gravity is that it's hard to find places where the effects of both Quantum Theory and General Relativity apply, a laboratory where we could test them both together. PSR J0337+1715 could provide such a laboratory.

A key feature of General Relativity is the Strong Equivalence Principle. This says that all objects behave the same in another objects gravity, regardless of their own gravity. Showing this is easy: Find two objects that are the same shape, but with different weights. Drop them from the same height, and they will hit the ground at exactly the same time, every time.

This means that two objects orbiting another should orbit in exactly the same way, regardless of how much gravity they have. Planet Earth orbits the Sun in the same way as a tiny satellite. 

But Quantum Theory disagrees. It suggests that there should be very small changes between how objects with different gravity react to something else's gravity.     

We can't test this in our solar system, or in any others that we've seen. The differences in gravity between the planets, moons and asteroids are only spread over a factor of a thousand or so, too small to see any difference that might provide clues to quantum gravity.

The newly discovered triple star system is different. There, the gravity of the pulsar in the centre is roughly one hundred thousand times bigger than that of the white dwarf next to it. By seeing how they react to the gravity of the third white dwarf, we just might be able to see the tiny cracks in General Relativity that would help us to build a new theory of the universe.    

There are some strange thing out there among the stars. But the stranger the things we find become, the closer we get to understanding it all.
 
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P.S. Some of you may be wondering what happened to my last blog, Realising We're in the Future. It's still there, but I just haven't had the time to research topics that aren't related to my work in enough detail. Who knows, it may be back if something particularly interesting happens!