Wednesday, 29 January 2014

The M82 Supernova SN2014J

Something went bang in the night...

I thought I was doing well last week. Tuesday evening I had my Rosetta blog finished, ready to publish the next morning. I was sure people would still be interested in it two days after the event. I mean, it's not as if the closest supernova since 1987 was going to go off during the night?


At 7.20pm last Tuesday evening a group of students together with their Professor, Steve Fossey, were in the UCL observatory for a practical astronomy class. It was a cloudy night, and the only bright thing left to look at in the sky was the Cigar Galaxy, officially known as M82. Turning their telescope on it, they noticed something odd: A point of light near the centre of the image so bright , it was outshining the rest of the galaxy. Quickly turning another telescope on it, they saw the same thing. A star had exploded in one of the most violent and energetic event in the Universe. A supernova.

The discovery of the supernova (bottom image) compared with an older image of M82 above it.
Fossey contacted the Palomar Transient Factory (PTF), a collection of linked telescopes in the USA that specialise in studying supernova and other unique events. They confirmed that it was a supernova and, at just twelve million light years away, the closest one to Earth since 1987. As the tenth supernova discovered so far this year, it was christened SN2014J.

They then took a spectra, splitting the light from the explosion into its separate wavelengths, or colours. Different materials absorb or release light at different wavelengths, so by looking for dark gaps or bright spikes in the spectra the PTF team could see what the exploding star was made of.

Although they are all exploding stars, the type of star which causes the supernova can vary. Many are the deaths of huge stars, eight times the mass of the Sun or more, collapsing in on themselves then blowing to pieces as they run out of the hydrogen fuel they need to hold themselves up. It was a supernova of this kind, known as core-collapse supernovae, that produced the millisecond pulsar I talked about in the first post. SN1987A, the closest recent supernova, was one of these.

The closest recent Supernova, which went off in 1897 in the Large Magellanic cloud, a dwarf galaxy orbiting our own Milky Way. This picture  was taken by the Hubble Space Telescope in 2006, showing the still-expanding cloud of debris from the massive explosion.

The spectra of the M82 supernova showed that it was something different. SN2014J is a Type 1A supernova, the explosion of a White Dwarf star.

White dwarfs are the leftover cores of stars like our Sun. Stars are held in a delicate balance, stopped from collapsing under their own weight by the energy generated by hydrogen fusion in their cores. When this fuel runs out the core starts to collapse, whilst the rest of the star expands into a red giant. The outer layers are gradually blown away, leaving behind a hot, dense core of carbon and oxygen.

White dwarfs are strange objects. They are so dense that they have around the same mass as the Sun squeezed in to an object no bigger than the Earth. They are held up only by something called electron degeneracy pressure, the fundamental rule that two things can't be in the same place at once.

This pressure can only hold up so much. If the mass of the white dwarf goes over 1.44 times that of the Sun then it collapses, exploding so violently that it releases more energy than the entirety of the galaxy that contains it. These are known as supernova progenitors.

How to get a white dwarf above that 1.44 Solar mass limit is a subject of debate. For decades the accepted model was that the white dwarfs are in binary systems with another star. When that star turns into a red giant, its outer layers get close enough to the white dwarf that the gravity of the white dwarf beings to pull the material of the red giant onto itself. If this carries on for too long the white dwarf slips over the mass limit and it's all over.
Artists impression of the traditional model for the cause, or progenitor, of a Type 1A supernova. Material from a Red Giant flows onto a White Dwarf, causing it to grow above 1.44 Solar masses and explode. 
Although that model makes sense, actually getting the physics to work has been tricky. Few computer simulations can get the white dwarf to explode. In many of them the material being dumped onto the white dwarf is simply blown away before it can build up to the limit.

This has lead to suggestions that instead of a white dwarf taking matter from a red giant, Type 1A supernovae may be caused by the collision of two white dwarfs. This model works better in simulations, as well as providing a better explanation for some observations.

Working out which of these options cause Type1A supernovae is important because of one crucial difference. In the white dwarf-red giant model, every white dwarf explodes at exactly 1.44 Solar masses. With two white dwarfs, the double degenerate model, that's not necessarily true. And that has implications for the spookiest ghost in physics: Dark Energy.

If white dwarfs always explode at 1.44 solar masses, then the explosion will always have roughly the same brightness. This means that if we see two Type 1A supernova, one of which is brighter than the other, then the dimmer supernova must be further way. Objects with that behaviour are known as standard candles, and are our main way of measuring distances in space. By observing Type 1A supernova in far away galaxies, we can measure the size of the Universe.

In the late 1990s Type1A supernovae were used to investigate how quickly the universe was expanding. The expectation was that as time went on the expansion of the Universe will slow down, as gravity begins to overcome the initial outwards explosion of the Big Bang. The Type1A measurements showed the opposite. The expansion of the Universe is accelerating, and we don't know why. The cause of the acceleration is known as Dark Energy, although the only things we know about it are that it isn't dark and it probably isn't energy.

Should Type1A supernovae turn out not to be standard candles then this whole measurement and much of modern cosmology would be thrown into doubt. What makes SN2014J so interesting then is that it is close enough to investigate what the progenitors were. 

Already the Hubble Space Telescope has swung round to take a look, joining a host of other telescopes in space and around the world to study the new supernova. Meanwhile astronomers are scouring older pictures of M82, trying to spot what stars were at the exact spot where the supernova went off.

Hubble image of the new supernova. Astronomers everywhere have been shocked by the appearance of a giant arrow over M82...

My own experience with the supernova has been quite interesting. The news was breaking as I woke up last Wednesday. I tweeted about it, and a few hours later was emailed by a Spanish science news website asking for more details! The article is in Spanish, but Google Translate does a fair job on it.

Because this supernova is so close, anyone can take a look. It's not quite bright enough to see with the naked eye, but a small telescope should be able to spot it. Over the next week or so it it should brighten so much that it will be visible through binoculars. M82 is just above the Plough, near the North Star.

I hope you get a chance to go out and see it. Space science is often a long process, with missions and observations taking years to come to fruition and gives results. Events like this, coming out of nowhere, are relatively rare, but have the potential to reshape our knowledge of the Universe.

For new blogs and various other science news (including any more nearby supernovae), follow me on Twitter

UPDATE: On 27th February, I finally managed to get the telescope out and have a look. I only have a small telescope so M82 wasn't much more than a fuzzy blob, but there was definitely a small point of light just off centre. Success!

Wednesday, 22 January 2014

Esa's Comet Hunting Rosetta Probe Wakes Up

Rosetta deploys Philae for the first ever landing on a comet in this artist's impression
In space, no one can hear your alarm clock.

At exactly 10am GMT on Monday morning, 807 million kilometres from Earth, an alarm clock went off. For the first time in two and a half years, a spacecraft stirred into life. Heaters switched on, warming up star trackers so that the spacecraft could find out where it was. Thrusters fired, stopping a slow spin that had kept its solar panels facing the Sun during it's long sleep. Six hours after the first alarm, enough systems were active for Rosetta to point at it's distant home and tell its creators that it was awake.

At 6.18 that evening, 48 minutes into the one hour window that Rosetta had to contact Earth, the message was received. A ten year voyage is about to come to a dramatic conclusion.

Launched in 2004, Rosetta's mission looks fairly straightforwards on paper. Fly out to Comet Churyumov-Gerasimenko, a four kilometre wide ball of ice and dust, go into orbit around it, and deploy a small lander called Philae to touchdown on the surface. Sounds simple enough.

(Comet Churyumov-Gerasimenko also rejoices in the the much more pronounceable but less impressive name of 67P)

This mission would however require one on the most complex journeys ever taken by a spacecraft, a six billion kilometre quest around the inner solar system in order to catch up with Churyumov-Gerasimenko.

Comets travel around the Sun in wildly different orbits to planets like the one we live on. Churyumov-Gerasimenko orbits the Sun once every six and a half years, sweeping out a huge ellipse between the mostly circular orbits of Earth and Jupiter. At closest approach it is three times closer to the Sun than when it reaches the edge of it's orbit. 

This means that, at any given point in its orbit, Churyumov-Gerasimenko is travelling at a very different speed to the planets around it, as well as any spacecraft that launch off those planets. Previous missions to comets, such as the rather spectacular Deep Impact, haven't had to worry about that, simply flying past the comets at tremendous speed in one brief encounter, desperately snapping pictures as they went.

However, for Rosetta to go into orbit around Churyumov-Gerasimenko it will have to be able to match speeds with the comet, slowing down enough to be captured by the comet's weak gravity. No rocket on Earth was (or is) capable of providing the change in velocity, or delta-V, needed to accomplish this feat. 

Instead Rosetta has spent the last ten years looping backwards and forwards between the inner solar system. Flying past first Earth, then Mars, then twice back around Earth, Rosetta has used the planets' gravity to sling it out into the orbit it needs to catch Churyumov-Gerasimenko at just the right moment. 

As Rosetta swung by Earth for the last time in 2009, it took this rather spectacular picture.

Speeding away from the inner solar system, flying past and imaging two asteroids on the way, Rosetta's orbit carried it out beyond the orbit of Jupiter. 800 million kilometres from the Sun, the spacecraft's two 14 meter long solar panels could no longer provide enough power to keep Rosetta alive. Shutting down everything except for the main computer and a few heaters, Rosetta went to sleep for two and a half years.  

Rosetta's awakening on Monday was both a technological and media triumph.Every major new website was running with the story. It made the front page of the Guardian and the Independent, #WakeUpRosetta trended on Twitter and Newsnight gave Jeremy Paxman an excuse to show off his scientific ignorance.

It's easy to see why. Esa could have talked about a preprogrammed reactivation sequence and establishing contact. Instead they described it as an alarm clock going off, as Rosetta waking up and calling home.

By talking about it in these very human terms, people have connected with the space mission far more than if it had just been seen as a technological marvel. Rosetta waking up has reminded people that our spacecraft aren't just computers and wires. They're us, an extension of our desire to explore, to investigate our home around us. Robots that we cast out into space to go where we can't go (yet).

Rosetta may be awake, but its real mission hasn't even started. The next few months will be spent testing the eleven scientific instruments on board, ready for when it finally catches up with Comet Churyumov-Gerasimenko in May.

Once there Rosetta will study the comet as it plunges towards the sun, watching it change from a dormant ball of gas and dust to one of the most active objects in the Solar System. By August it will have closed into within twenty five kilometres: close enough to enter orbit. Throughout the approach it will have to dodge eruptions of gas and rock as the comet heats up and begins to develop a tail. Any impact to Rosetta's delicate instruments or solar panels could prove fatal.  

Rosetta will map the comet in detail, providing a new understanding of the processes that take place on these tiny objects. It will sample the gas and dust streaming off the comet, studying it's chemistry and behaviour.

Churyumov-Gerasimenko represents a unique laboratory, a sample of the primordial building blocks of the planets untouched since the dawn of the solar system. By understanding this tiny world we will be able to shed light on how our world and everything around it came into being.

Perhaps the most interesting results will come when Rosetta samples the water that will start to pour off the comet, eventually creating a massive tail behind it. Our current ideas about how Earth was formed suggest that almost all of the water that life relies on was delivered by impacting comets (A theory backed up by some work in the area I'm studying for my PhD). Rosetta's discoveries at Churyumov-Gerasimenko could help see if that really was the case or not.

Rosetta's images of the comets will provide more than just pretty pictures. They will also be scouting the comet out for a suitable landing site. Sometime in November a small spacecraft called Philae will detach from Rosetta to attempt the most difficult part of the mission: The first spacecraft to land on a comet.

If all goes to plan, Philae will drift down from a height of around one kilometre, using a harpoon to drag itself onto the comet's surface. Churyumov-Gerasimenko's gravity is too small to hold it in place, so once down Philae will drill into the surface, clinging onto the side of the increasing volatile comet. There it will deploy nine experiments of it's own, capturing hopefully superb close up images and taking samples from under the surface. Philae is expected to last around a week, but could return data for many months.

If everything goes to plan, by the time the mission finishes in 2015 Rosetta and Philae will have revolutionised our knowledge of comets and answered some of the biggest gaps in our knowledge of how our solar system, our planet and everything alive on it came to exist. Not only that, but the images of a comet exploding into life as it reaches the heat of the inner solar system should be spectacular.

Comet ISON grows a spectacular tail. Soon Rosetta will show all of us how this happens from the inside.
Rosetta is now awake, hurtling towards it's distant target. The Rosetta stone revealed the secrets of ancient Egypt, but the spacecraft that it gives its name to is ready to unlock even greater mysteries.

New blogs will be posted on Twitter, which is also the best place to keep up to date with Rosetta's progress.


Thursday, 9 January 2014

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!