Features

Scientists have identified a new type of supernova or exploding star which is ten times brighter than any other type of stellar explosion.

Astrophysicist Dr Mark Sullivan of Oxford University’s Department of Physics is among researchers reporting the discovery in this week's Nature.

Until now scientists have been aware of two basic types of supernova – ‘Type Ia supernovae’, which are thermonuclear explosions of small and very dense stars called white dwarfs – and slightly fainter ‘core collapse supernovae’, brought about by the deaths of very massive stars (possibly with masses up to 50-100 times that of the sun). This second type of supernova was featured in the BBC’s Wonders of the Universe.

The new research is published by 27 academics from Oxford, Caltech, the universities of California and Toronto, the Weizmann Institute and other leading institutions. 

It demonstrates the existence of a new, exceptionally bright, kind of supernova, 10 times more luminous than most Type Ia supernovae and with unique characteristics.

These new supernovae show no trace of hydrogen or helium, and emit significant ultraviolet flux for long periods, which means the exploding star was probably low in metals and from a very faint galaxy with few stars. Its light output decays in a way that suggests something other than radioactivity - which powers the light from all other supernova explosions - is the cause of its deterioration.

‘The mechanism of the explosion is unknown, which is the most exciting part,’ Dr Sullivan says. ‘The high luminosity means that these supernovae can be used to probe the very distant universe.’

Because the new supernovae are rare, it took a new type of search to find them - the Palomar Transient Factory [PTF]. The PTF is a systematic search for cosmic explosions and uses the Palomar Observatory in San Diego.

Oxford University is helping fund the project via the John Fell OUP Research Fund and the research is providing material for Galaxy Zoo Supernova which featured on the BBC’s Bang Goes the Theory.

‘Practically every chemical element in the universe (other than hydrogen and helium) gets made in stars,’ Dr Sullivan says. ‘This research has implications for both the study of stellar evolution and the way in which elements in the Universe were formed.’

Erasing data, rather like cleaning a house, should be hard, hot work.

But now a team including Oxford University’s Vlatko Vedral have shown that, in the quantum world at least, it doesn’t always have to be.

They report in this week’s Nature that, under certain conditions, deleting data held in a quantum computer could actually cool the environment down rather than heating it up.

‘Everyone who has ever worked with a computer knows that they get hotter the more we use them,’ Vlatko writes in Scientific American.

‘Physicist Rolf Landauer argued that this needs to be so, elevating the observation to the level of a principle,’ Vlatko comments. ‘The principle states that in order to erase one bit of information, we need to increase the entropy of the environment by at least as much. In other words we need to dissipate at least one bit of heat into the environment (which is just equal to the bit of entropy times the temperature of the environment).’

This heat threshold is something that could potentially hold supercomputers back: that there comes a point when deleting data to perform fresh computations will create so much heat that any system will no longer be able to cool itself down.

Saying that you can erase information from a system and cool the environment at the same time is, he says, a bit like telling a physicist that perpetual motion is possible.

However, the team’s new theoretical study has turned up an exception based on the idea that entropy, the measure of disorder of a system which should always increase, can be ‘negative’ in quantum mechanics.

Vlatko writes: ‘Adding negative entropy is the same as taking entropy away. The key phenomenon behind it is the spookiest of all quantum phenomena, entanglement.’

In a Nature commentary Patrick Hayden describes the team’s findings like this, that: ‘instead of having to invest work to erase the qubit [quantum bit], the process of erasing the qubit can actually generate work, like a tiny quantum-logical wind turbine.’

Vlatko adds: ‘The implications of our result could be important for superfast and superefficient computers. Current computers waste about 10,000 units of heat per computational step. If we can somehow control and manipulate entanglement between the microprocessor and the computer memory, then we could erase computations to make room for new ones, but keep the environment cool.’

More about this work in Vlatko Vedral's article for Scientific American.

Professor Vlatko Vedral is based at Oxford University's Department of Physics.

Stellate cells, a type of cell in the pancreas which normally helps the body respond to damage or disease of the pancreas, can act as a double agent when it comes to cancer.

These mysterious cells become ‘partners in crime’ with pancreatic cancer cells, Oxford University researchers have shown, stimulating growth of the cancer cells and protecting them against radiotherapy.

The research, led by Professor Thomas Brunner at the Gray Institute for Radiation Oncology and Biology, suggests that developing drugs to remove specific communication lines between the pancreatic cancer cells and the stellate cells could improve patients’ response to radiotherapy in the future.

Most people diagnosed with pancreatic cancer are told that they may have less than 1 year to live. Part of the reason is that by the time someone is diagnosed, the cancer is often quite advanced. Cancer Research UK figures show that around 20 in every 100 people diagnosed with pancreatic cancer live for 1 year or more, and that only 5 out of every 100 people live for more than 5 years.

In terms of treatments, surgery is currently the only way to cure the disease – but less than 20% of all patients can be operated on, and only 5% of these patients will be alive 5 years later. Chemotherapy helps to prolong survival after an operation, and is also used when the cancer has spread elsewhere. Radiotherapy is used along with chemotherapy in patients without spread of the disease to other organs and where surgery isn’t an option.

Stellate cells – so-called because they are star shaped – normally make up around 4% of the cells in the pancreas. But upon any type of trauma (pancreatitis as well as cancer) these cells can drive an inflammatory reaction that leads to the formation of a fibrous mass. It can be up to 90% of the mass of a pancreatic tumour, for example.

‘It’s like a non-healing wound,’ says Thomas Brunner. His group has just published the first paper demonstrating the influence of the pancreatic stellate cells on how effective radiotherapy is in destroying the cancer cells. The results can be found in the journal Cancer Research.

‘We’ve tended to be so focused on the cancer that we’ve neglected what’s around,’ he adds. ‘Sherlock Holmes would not be impressed. We have forgotten there may be more to the disease in the environment surrounding the tumour.’

The group looked at the survival of pancreatic cancer cells in the lab when dosed with radiation. When the cancer cells were co-cultured with the noncancerous stellate cells, the radiation had far less effect in killing off the cancer cells.

In mouse models, tumour growth was faster with the pancreatic stellate cells present and the stellate cells provided something of a protective shield, reducing the effect of radiotherapy on the cancer.

‘It turns out that stellate cells are partners in crime with the cancer cells,’ says Professor Brunner. ‘They actively help the tumour cells and have a protective effect against radiotherapy.

‘While they normally help defend the pancreas against injury – wound healing is very critical – this response needs to stop at some point or it is harmful. In pancreatic cancer, this wound-healing response becomes active forever and that’s counterproductive in the end.’

The researchers looked at a number of signalling pathways that might be responsible for this effect by enabling the cancer cells and the stellate cells to communicate. They found that some molecules on the surface of the cells called integrins were likely to be involved.

‘Blocking the integrin signalling gets rid of any protective effect against radiotherapy,’ says Thomas Brunner. ‘By finding the mechanism behind this effect, we ultimately may be able to develop a drug to target this process and improve the outcome of radiotherapy.’

Life as a seed isn’t easy: you need to be tough enough to deter all but the most muscular-jawed predators but not so hard that you can’t germinate.

A new study published this week in Journal of the Royal Society Interface shows just how fine this evolutionary balance between protection and reproduction is.

A team, including Susan Cheyne of Oxford University’s WildCRU, analysed the properties of the seeds of the plant Mezzettia parviflora (Annonaceae) and the effort that seed predators, such as orangutans, have to put into cracking them open.

‘The intricate architecture of the Mezzettia parviflora seed allows its germination while impeding both small predators such as weevils and large ones like orangutans,’ Susan tells us.

‘Orangutans open the seed by biting into the germination bank and cracking the wooden plug, the weaker part of the seed through which the stem of the germinating seedling emerges.’

Field observations by Susan and colleagues of orangutans in Sabangau, Borneo, show that whilst orang-utans consume an average of about 120 seeds per day (up to a maximum of 1001) the jaw strength they have had to evolve to accomplish this task is formidable: the force their jaws deliver is equal to the weight of up to six people bearing down on the seed.

So is all this effort worth it? ‘The seeds contain a small amount of a lipid-rich substance which is very high in energy, so worth the effort to break not only the seed but the hard outer shell of the fruit,’ Susan explains. ‘The toughness of this fruit and seed prevents consumption by other primates, for example gibbons, who lack the jaw strength to open the seeds.’

The research is thought to be the first to show that the mechanical properties of a seed play a central role in stabilising the arms race between seeds evolving armour for protection and the predators evolving a way to open a nutritious snack.

Dr Susan Cheyne is a member of WildCRU, part of Oxford University's Department of Zoology.

It may be our home but just how special is the Milk Way?

That’s the question a team including Oxford University scientists have been looking to answer using simulations of our galaxy and our neighbours, the Magellanic Clouds.

Their findings, reported in a paper in The Astrophysical Journal could help in the hunt for dark matter. I asked one of the paper’s authors, Phil Marshall of Oxford University’s Department of Physics, about Universal assumptions, starless galaxies, and telltale gamma rays…

OxSciBlog: What made cosmologists assume that the Milky Way is an 'ordinary' galaxy?
Phil Marshall: Basically, we had to start somewhere! The cosmological principle states that we do not live in a special place in the Universe, one that has a special viewpoint. Asserting this principle allows us to make many wide-ranging inferences about the Universe, even though we can only observe it from one location (Earth). But it's important to test our assumptions, so we asked whether the galaxy we live in was, in fact, special - at least in one respect.

OSB: How can the Magellanic Clouds reveal if the Milky Way is special?
PM: A galaxy's neighbours - its ‘satellite galaxies’ - are one of its observable features. We wondered if having these two very nearby neighbours, the Magellanic Clouds, made the Milky Way special.

So we looked in the Sloan Digital Sky Survey [SDSS] sky survey at thousands of galaxies that have the same brightness as the Milky Way, and asked how many of them have two nearby neighbours like our Magellanic Clouds. It turns out that only about 4% of them do - so the Milky Way is a little unusual, but not very unusual. It's a one-in-twenty-five galaxy, rather than one in a million.

OSB: How did you use simulations to see how the Milky Way relates to its neighbours?
PM: We did the same thing in a simulated sky survey, counting neighbouring objects around Milky Way-like objects. If the simulated Milky Way galaxies don't have as many satellites as the SDSS galaxies, then the simulation needs more work.

We used a simulation called ‘Bolshoi’ that followed the formation of about 100,000 galaxies, and picked out the ones that were about as bright as the Milky Way. This is tricky to do actually, because the simulated galaxies don't have any simulated stars in them! They are just dark matter ‘halos’ - blobs of dark matter that would contain gas and stars in real life. The simulation doesn't include stars and gas, because it's too difficult to simulate them. Dark matter structures are easier to model - for them, it's only gravity you have to understand, and not the complicated physics and chemistry of how stars are made.

What we do is match the simulated dark matter halos to the real SDSS galaxies, one by one, most massive halo to most luminous galaxy and so on. You end up with a model Universe full of dark matter halos with bright galaxies ‘painted on’ - and it turns out this painted Universe looks very similar to the real one indeed. Then we can select all the model galaxies that are as bright as the Milky Way, and count their neighbours.

OSB: What can you infer about how 'odd' our home galaxy is?
PM: We found that, just like in the real Universe, Magellanic Clouds occur in about 5% of Milky Way galaxies. So the simulation matches the SDSS sky survey very well, right down to the smallest galaxies it contains, the Magellanic Cloud-like satellites.

Actually we can say quite a lot about our home galaxy without doing all the matching I just described: If we just look in the simulation for halos that have 2 subhalos that are the same mass as the Magellanic Clouds, and that are at the same distance from their host galaxy as our Magellanic Clouds are from us, and that are moving at the same speeds as our Magellanic Clouds are, we can collect a group of model halos that really resemble our own halo very closely.

We call these halos ‘analogs’, and they show us some possibilities for what our own dark matter halo is like. For example, they weigh about a trillion solar masses each, so we can say that this is probably what our halo weighs. Likewise, looking at the formation histories of each our analogs, we can infer that our Magellanic Clouds probably arrived quite recently (within the last billion years), and they probably arrived together.

OSB: How might such simulations help in the hunt for dark matter?
PM: Understanding the distribution of dark matter in our own galaxy is very important, especially when searching for the very faint glow expected if dark matter turns into something else.

The idea is that dark matter particles in our galaxy could, very occasionally, collide with each other, and ‘annihilate’, in a very faint flash of gamma rays. These flashes may be so faint that knowing where the dark matter is likely to be, ahead of time, from its gravity, would really help in interpreting the gamma rays that telescopes, like Fermi, detect.

Dr Phil Marshall is based at Oxford University’s Department of Physics