Jocelyn Bell and the Breakthrough prize 2018

Pulsars were first detected in 1967 by a research student called Jocelyn Bell when she was taking observations for her PhD thesis. Her supervisor, Anthony Hewish, went on to win the Nobel prize in 1974 for the discovery, and her contribution was overlooked. Many at the time felt that Jocelyn Bell should have been given at least a share in the prize, since she was the person who had initially spotted the signal from the first pulsar.

Jocelyn Bell 

This was finally addressed this month when she was awarded the $2.3 million Physics Breakthrough Prize for the discovery. Previous recipients include Stephen Hawking, researchers at Cern who discovered the Higgs boson, and physicists on the Ligo experiment who detected gravitational waves.

According to the BBC website, Bell has decided to donate her entire winnings to set up a fund to help women, under-represented ethnic minority and refugee students to become physics researchers.

Bell who is now 75 years old told the BBC:

‘I don’t want or need the money myself and it seemed to me that this was perhaps the best use I could put it to.’

If you would like to know more, please see my post from last year on the discovery of pulsars below.

———–

In 1967 Jocelyn Bell, a 24-year-old student from Cambridge University, was doing the research for her PhD. She was using a radio telescope to study radio waves emitted from compact astronomical objects known as quasars, and when she analysed the data, she noticed a signal which appeared to pulse on and off every 1.3 seconds. After doing this continuously for about an hour, the pulsing signal would stop altogether, but it would start again precisely 23 hours 56 minutes later after it had first started. As Jocelyn Bell – and indeed all astronomers – knew, the Earth rotates on its axis once every 23 hours 56 minutes, a period of time called a ‘sidereal day’, and the fact that the pulsating signal was detected at the same time in each sidereal day meant that it must almost certainly be coming from space rather than being man-made as shown below.

 

Not only was the time interval between each pulse very short at only 1.3  seconds, but this time interval was completely constant and didn’t vary to any significant degree.  The fact that the pulses were very short in duration and so regular meant that whatever was emitting the pulses must be extremely small in astronomical terms.  A larger object – something, for example, the size of the Sun – would not be able to generate such precise pulses. For a while Bell and her PhD supervisor Antony Hewish considered the possibility that the mysterious signals were generated by a signaling beacon left by an alien civilisation. For this reason, they briefly nicknamed  the unknown object ‘Little Green Men-1’. However, this explanation was rejected when it became clear that the pulses contained no information and when they also discovered additional pulsing sources in other parts of the sky.

Pulsars

The origin of the mysterious signals turned out to be a hitherto unknown class of astronomical objects, known as neutron stars. Neutron stars are very small, typically around 20 km in diameter, but have an enormous mass – between 1.1 and 3 times the mass of the Sun.  Having such a large mass squeezed into a small volume means that their density is incredibly high.  A cubic centimetre of neutron star material would weigh about 500 million tons. They are called neutron stars because they consist mainly of neutrons, which are subatomic particles found in the nucleus of ordinary atoms. In a neutron star the neutrons are so tightly squeezed together that they are touching each other. Neutron stars also have very strong magnetic fields – around 1 trillion times stronger than the Earth’s.

Neutron stars which rotate extremely rapidly, around once a second, are known as pulsars, and these are the objects which Bell and Hewish had detected. This rotation causes electrically charged particles around the neutron star to move rapidly in the intense magnetic fields. This causes electromagnetic radiation (such as radio waves) to be emitted in two cone-shaped beams along the magnetic North and South poles of the pulsar, shown as B in the diagram below.

In the diagram above A is the axis around which the pulsar rotates and B is the magnetic axis.

As you can see, the magnetic poles are at an angle to the line through the North and South poles of the pulsar about which it rotates. This means that the cone-shaped beams will rotate around the axis of the pulsar and only when either of the beams is pointing directly at the Earth will it be detected. A pulsar behaves like a lighthouse – the light is on all the time but appears to a viewer to switch on when it is pointing towards them and switch off when it is pointing away.

Neutron stars are formed when a large massive star explodes at the end of its life in a violent event known as a supernova. Most of the outer parts of the star are blown out into space by the force of the explosion. The remaining material in the star’s core collapses, forming a neutron star.  In fact, if the remaining material from the star’s core is more than three times the mass of the Sun, a neutron star won’t be formed at all.  Instead, an even more compact object called a black hole will result.

Large stars with a diameter of tens of millions of kilometres rotate relative slowly, taking around one year to complete one rotation. As the star collapses into a small massive object, millions of times smaller in diameter, a law of physics called the conservation of angular momentum causes its rotation to speed up massively.  A  more familiar example of this is from the world of ice skating: ice skaters spin more rapidly when they pull in their arms.

Why this knowledge has been useful to astronomy

In the last 50 years more than 2000 pulsars have been detected. Understanding the properties of pulsars has led to further discoveries in various areas.  For example, it has led to greater understanding of the diffuse gas between stars known as the interstellar medium, and it has also been used to test Einstein’s theory of general relativity. I will discuss these advances in more detail in future posts.

Knowledge of pulsars was also used in a very interesting way in the 1970s.  Four spacecraft were launched, destined to the leave the Solar system and head out into interstellar space. The missions are described in more detail in my previous post Artefacts from Earth.  On each spacecraft there was a diagram devised by the American astronomer, Frank Drake (1930-), consisting of a circle with 15 lines coming out of it.

The centre of the diagram, from which the lines radiate, represents the Sun.  The right-hand end of the longest line (at 3 o’clock) represents the centre of the galaxy. The end of each of the remaining line represents a pulsar and the length of the line between the Sun and the pulsar represents the distance to the pulsar.

The diagram relies on the fact that each pulsar has its own distinct period between pulses. So if, in the far distant future, an alien civilisation were to recover the spacecraft they would be able to identify the pulsars from their periods.  To enable this, each of the lines depicts the length of the pulsar’s period not in seconds (which as a man-made unit would be meaningless to an alien race) but in multiples of a ‘fundamental time unit’ that an alien might understand.  The alien civilisation could then have sufficient information to identify the fourteen pulsars and the distance of the Sun from each pulsar, and thus work out the location of our Solar system within the galaxy.

The Nobel prize controversy

Jocelyn Bell and Antony Hewish published their results in February 1968 (Hewish et al 1968). The discovery of this entirely new type of astronomical object was a major advance. Interestingly, it had been suggested as long ago as 1934 by the astronomers Baade and Zwicky that neutron stars would be the end result of supernova explosions. However, this prediction was ignored. Before 1967 most astronomers regarded neutron stars as hypothetical objects which might or might not actually exist in reality.

Such was the impact of the discovery that it led to the Nobel prize for physics in 1974. Two astronomers were awarded the prize – but Jocelyn Bell was not one of them. Antony Hewish and Martin Ryle were the recipients, the former for the discovery of pulsars and the latter for his work in radio astronomy.  Jocelyn Bell’s contribution was not recognised.

Antony Hewish (1924- ) 

Many at the time felt that Jocelyn Bell should have been given at least a share in the prize, since she was the person who had initially spotted the signal from the first pulsar. The British astronomer Fred Hoyle was particularly vocal on the issue and stated publicly  that Bell should have been given a major share of the prize to acknowledge her contribution. Bell herself said very little about the controversy in the years immediately afterwards. The few statements she made were, in general, supportive of the Nobel prize committee’s decision. In the 1960’s and 1970’s it was commonplace for the senior person leading a team of scientific researchers to get the credit for a major discovery on behalf of the entire team. This is largely still true today.

Bell went on to have a successful academic career and always has been a passionate advocate for getting women more involved in science. From 2002 to 2004 she served as the president of the Royal Astronomical Society, the organisation for British Astronomers. She was the first ever woman to hold that role.  She later served as the first ever female president of the Institute of Physics. In 2006, nearly 40 years after the discovery, she said in an  interview:

In those days, it was believed that science was done, driven by great men . . . And that these men had a fleet of minions under them who did their every bidding, and did not think. It also came at the stage where I had a small child and I was struggling with how to find proper childminding, combine a career, and before it was acceptable for women to work. And so I think at one level it said to me ‘Well men win prizes and young women look after babies.’

Postscript: a different kind of pulsar discovered in 2016

Stars such as the Sun do not end their lives in violent supernova explosions resulting in neutron stars. Instead, when they have used up all their nuclear fuel, the outer layers of the star are blown away into space and form a bright glowing shell of gas called a planetary nebula, shown below.

Planetary Nebula

The remnants of the star’s core collapse into a dense hot star called a white dwarf, an object which is roughly the same size as the Earth.  For the last 50 years astronomers have predicted that some white dwarfs might also form pulsars, although because white dwarfs are much larger and rotate more slowly than neutron stars, the radiation would be much weaker than neutrons star pulsars, making them harder to detect and the pulses would be much longer.  This prediction was finally confirmed in 2016 when a team led by Tom Marsh from Warwick University discovered that the white dwarf star AR Scorpii was a pulsar with a period of about 2 minutes.

References

Baade, W.and Zwicky, F. (1934) ‘Remarks on supernovae and cosmic rays’, Physical Review, 46(1), pp. 76-77.

Hewish, A., Bell. S. J., Pilkington, J. D. H, Scott P. F. and Collins, A (1968) ‘ Observation of a rapidly pulsating radio source’, Nature, 217(), pp. 709-713.

 

The Rare Earth hypothesis

Ever since the pioneering work of Frank Drake (1930-) in 1960, astronomers have been looking for radio signals from extraterrestrial civilisations and have failed to find anything. This could be because Earth-like planets containing complex life forms (such as ourselves) are rare in the Universe and only a series of highly improbable events led to the evolution of intelligent life on Earth. In 2000 geologist and palaeontologist Peter Ward and astronomer Donald Brownlee published a book in which they explained the term ‘Rare Earth Hypothesis’ which they had coined to describe this viewpoint.

However, they were not the first people to arrive at this conclusion. These ideas had been circulating for decades before the publication of their book. For example, the astronomers John Barrow and Frank Tipler discussed them in detail in their 1987 book ‘The Anthropic Cosmological Principle’.

A little background – the Drake Equation

Back in 1961, Frank Drake invented an equation to estimate the number of intelligent civilizations within our galaxy with whom we could potentially communicate, to which he gave the symbol N.  His equation, known as the Drake equation, consists of seven numbers multiplied together:

N=  R* x FP x NE x FL x FI x FC x L

in which

  • R* is the average number of stars formed per year in our galaxy. Current estimates are that this has a value of around 10.
  • FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1. A value of 1 means that all stars have planets. 0 would mean that no stars have planets. Current estimates are that FP is very close to 1.
  • NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life. For this to happen liquid water must exist somewhere on the planet.  A reasonable value which many astronomers would agree with is 0.4, meaning that out of every 10 stars which have planets, 4 have bodies which could support life.
  • FL is the fraction of bodies with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1.  A value of 1 means that on all  planets with the right conditions life will evolve. There is no consensus among astronomers about the value of FL.
  • FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, again expressed on a scale of 0 to 1. Again, there is no consensus among astronomers about what this value is.
  • FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space. For example, on Earth TV and radio signals escape into space and could be picked up by a nearby alien intelligence with a sensitive enough receiver tuned to the right frequency. No one knows what the value of FC is.
  • L is the average lifetime of a civilisation in years.  Again, there is no consensus on this point.

As the values of many of the terms in the Drake Equation are not known to any degree of accuracy, it cannot be used to provide a meaningful estimate of N.  However, it is still very useful to illustrate the factors involved

Could we be alone? Could FI could be very very low?

Perhaps the term with the biggest degree of uncertainty in the Drake equation is FI, the fraction of bodies having life on which life has evolved into intelligent civilisations. I will outline the arguments for believing that this number is very low indeed.

The Earth is roughly 4.6 billion years old. Simple single-celled lifeforms emerged 300 million years later, a relatively short time after the Earth had cooled enough for liquid water to exist. These simple cells, called prokaryotes, cannot form complex organisms where different types of cells perform different functions. However, individual prokaryotes  can group together in colonies, forming a kind of slime.

Colonies of prokaryotes

All complex life on Earth is based upon cells called eukaryotes. These cells have a nucleus (containing the genetic material of the cell), structures called mitochondria, which regulate the cell’s energy and other specialised units known as organelles.

The first eukaryotes didn’t emerge until 2 billion years after prokaryotes, indicating that it was a much bigger step in evolution than the emergence of the first simple cells. It could well be the case that the large number of sub-steps needed in the evolution from prokaryotes to eukaryotes means that even where simple lifeforms have emerged,  in the vast majority of cases there has been no further evolution beyond this stage.

Even after the emergence of these complex eukaryotes, it would take over one billion years before multi-cellular life forms such as the first plants and animals appeared. In these organisms cells are specialised, so different types of cells perform different functions within the organism. Given the large amount of time taken to move from eukaryotes to complex organisms, it might well be the case that even if something akin to eukaryotes emerge on a planet, which have the potential to eventually evolve into multi-celled organisms, evolution proceeds no further. The average time taken to evolve from complex cells to multi-celled organisms given favourable conditions might be 2, 5 or even 10 billions years.

The role of mass extinction events

It has taken 600 million years from the appearance of the first animals to the emergence of Homo sapiens in Africa, around 200,000 years ago. Over this vast amount of time the vast majority of species have disappeared and have been replaced by other species which are better suited to the changing environment. However, the disappearance of species and emergence of new ones doesn’t occur at an even rate. Every 50-100 million years there have been catastrophic events which have caused periodic mass extinctions when a large number of species failed to survive. The most dramatic of these was at the end of the Permian period, around 230 million years ago, when 95% of land and 70% of sea species became extinct.

Fossil of a Trilobite – one of many species to disappear in the mass extinction at the end of the Permian Period

Perhaps the best known mass extinction occurred 65 million years ago when a massive comet or asteroid 10 km in diameter hit the Earth at a speed of up 50,000 km per hour. Its high speed coupled with its huge mass meant that it smashed into the Earth in the area now known as the Yucatan peninsula with an energy 6 billion times greater than the atomic bomb dropped on Hiroshima at the end of the Second World War.

KT extinction asteroid

The impact melted much of the local crust and blasted molten material outwards. Any object near to the impact site would have been instantly vapourised. Such was the energy of the impact that some of the Earth’s crust was thrown upwards with so much velocity that it went out into space. Over the next few hours molten rock, dust and ash rained down on an area millions of square kilometres in area. This hot material would have ignited fires, destroying plant and animal life within a large area.

Kt ejecta map

The impact 65 million years ago occurred in what is now the Yucatan peninsula in Mexico.  The outer ring shows the area which become covered in debris.

A fine cloud of dust from the impact circled the entire Earth. This blocked out sunlight, causing the Earth’s temperature to fall by about 15 degrees Celsius. The sudden drop in temperatures and lack of sunlight getting through meant that plant growth stopped for several months.  As a result, many species of herbivorous animals which fed on those plants became extinct and the carnivores which fed on the herbivores became extinct too. Those species which did survive had their numbers drastically reduced.  Nearly 75% per cent of the species of animals alive before the impact became extinct, including every species of dinosaur. After the disappearance of the large reptiles, mammals became the dominant land animals.

Although we as a species are now probably sufficiently advanced as to be able to survive a mass extinction event, even if only in small numbers, this would not have been the case if one had occurred earlier in our development. If one had happened 150,000 years ago, when Homo sapiens were few in number and less equipped to withstand a major famine – those hunter gatherers would have had nothing to hunt and nothing to gather for many months – we would have become extinct long before we could develop civilisation.

Indeed,  if mass extinctions were to occur every million years, rather than every 50 million years, it is difficult to see how any intelligent species could ever evolve. It would be wiped out before it became fully established. The reason why mass extinctions occur infrequently is due to the unusual layout of the Solar System, and I’ll talk about this next.

Our special Solar System

The Sun is a single star in an uncrowded region of space, in the outer regions of the Milky Way galaxy. The nearest star to the Sun is over 4 light years (40 trillion km) away, more than ten thousand times further than the outermost planet Neptune. If the Sun were in a more crowded region of the galaxy, a passing star’s gravity could easily disrupt the Earth into a different orbit, which might be closer to Sun, making it too hot to support life, or further away, making it too cold to support life.

The giant planet Jupiter is more than 300 times the mass of the Earth. As comets enter the inner Solar System from its outer reaches, Jupiter’s gravity slings most of these fast-moving ice balls out of the Solar System before they can get close to Earth.  Without Jupiter, comets like the one which hit the Earth 65 million years ago would collide with our planet much more frequently.

Image from NASA

Observations of planets detected around other stars have shown that arrangements of planets similar to our Solar System, with small inner rocky planets surrounded by massive giants in the outer reaches, are relatively rare.

Another factor which may be essential to the emergence of complex lifeforms is the presence of a large moon close to the planet. The Moon is 25% of the Earth’s diameter and is only 400 000 km away, a very short distance in astronomical terms.

Relative sizes of the Earth and the Moon

The Moon  is much larger in comparison to its parent planet than any other moon in the Solar System. This means that the Moon’s gravity stabilises the Earth’s tilt so that it doesn’t vary too much from its current value. Without the Moon there would be massive swings in the tilt between 0 degrees, when there would be no seasons, and 50 degrees, where there would be extreme seasons where much of the planet would be in darkness or full daylight for months at a time.

The Earth’s magnetic field

The Earth’s magnetic field is generated by convection currents stirred up by rotation in its liquid outer core, which is made out of iron. In fact for any planet to have a strong magnetic field, part of its interior must consist of a liquid which conducts electricity and it must be rotating rapidly enough to generate convection currents.

Earth Interior

Without a magnetic field the planet would not have a protective ozone layer shielding its surface from deadly ultraviolet radiation. It would be very difficult for advanced lifeforms to exist on the surface.  It is not known how many planets around other stars satisfy the criteria to have a magnetic field. However, the Earth is the only one of the inner planets in our Solar System (Mercury, Venus, Earth and Mars) to have a strong magnetic field.

Planetary size

In order for life to evolve, the planet must be large enough to retain a significant  atmosphere, without which liquid water cannot exist. Mars is roughly one tenth the mass of the Earth and its gravity is so low that its atmospheric pressure is only 0.6% of that of Earth – too low for liquid water to exist.

Man has been lucky to survive

In addition to periodic mass extinctions which wipe out most species, all populations come under pressure due to sudden changes in climate, such as ice ages, major volcanic eruptions, loss of habitat, disease and competition from other species for food and shelter. The first Homo sapiens were confined to a single region of Africa and were few in number, and thus vulnerable to becoming extinct. 70,000 years ago a super-volcano erupted in Indonesia causing the Sun to be partially blocked out and resulting in a fall in global temperatures. Only an estimated 15,000 humans are thought to have survived (Edwards 2010).

Summary

If we take all the factors I’ve talked about into account, for single-cell organisms to evolve into intelligent lifeforms capable of building civilisations, all of the following must happen.

  • Single cell creatures must have evolved in into complex cells, something akin to eukaryotes.
  • To reduce the frequency of mass extinctions, the planetary system must have a large giant planet in its outer reaches.
  • To ensure the planet remains at a habitable temperature, the parent star must be a single star in a less crowded region of the galaxy.
  • To ensure the planet remains at a habitable temperature over a long period of time, the parent star’s energy output must not vary too much.  This constraint rules out advanced life around the most common type of star in the galaxy, red dwarfs .
  • To reduce the frequency of dramatic changes in climate leading to mass extinctions, the planet must have a large satellite.
  • The planet must rotate rapidly enough to generate a magnetic field, as without this complex life would not be possible on its surface.
  • The planet must be large enough to hold an atmosphere.
  • Multi cellular organisms must have evolved.
  • The planet must have a significant fraction of its surface as dry land. The assumption is that although aquatic creature such as dolphins are undoubtedly intelligent, only land based creatures could build civilisations.
  • A land-based intelligent species has emerged during a survival of the fittest.
  • The intelligent species has managed to survive major environmental changes such as ice ages and has built a civilisation.

Putting all these together, followers of the Rare Earth hypothesis believe that the probability of all the above happening on a planet on which simple life has evolved could be as low as a billion to one.

If it were this low, then the expected number of intelligent communicating civilisations in a galaxy such as ours would be very low.  If the disputed figures within the Drake equation were guessed to be on the low side, N could be as low as 0.000016.  This would mean that not only are we the only intelligent communicating civilisation in our own Milky Way galaxy, but also that there are none in any galaxies within tens of millions of light years of us.

If this is the case then the Earth would not be just be an ordinary planet orbiting an ordinary star in an ordinary galaxy.  It would be a very special place indeed!


I hope you have enjoyed this post. To find out more about the Science Geek’s blog, click here or at the Science Geek Home link at the top of this page.


Reference

Edwards, L. (2010) Human were once an endangered species, Available at: https://phys.org/news/2010-01-humans-endangered-species.html#jCp (Accessed: 7 April 2018).

 

 

A Christmas gift from The Science Geek 2017

 

Christmas is almost upon us. Once again I’m offering my e-books for free during the first five days of December!  Just call me Father Christmas :-).

Is Anyone Out There?” is about the likelihood of there being extraterrestrial intelligent life.  It is based on a number of posts from my blog.  For readers based in the UK the book is available to download from Amazon in Kindle format by clicking here and for readers in the US by clicking here. If you’re based outside the UK or US , see the notes at the end of the post.

Is Anyone Out There Cover

The Moon” is also based on a series of posts from my blog and you can guess what it is about.  UK readers download here, US readers here and anyone else please see the notes below.

Moon Cover

How to download the books if you’re based outside the UK or US.

There are threeways of doing this.

Option 1 if you go into the Amazon Kindle store and search for “The Science Geek” as the author you should find my books.

Option 2  I have created a page on my website where you will be able to download either of books for free.
https://thesciencegeek.org/e-books/

I’ll put them there until at least the end of the year.

Option 3  If your country is listed below, I have added some links  to allow you to download the books by just clicking on the link for your country.

Is There Anyone Out There?

Australia http://www.amazon.com.au/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Brazil http://www.amazon.com.br/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Canada http://www.amazon.ca/gp/product/B0131LVNW8?*Version*=1&*entries*=0

France http://www.amazon.fr/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Germany http://www.amazon.de/gp/product/B0131LVNW8?*Version*=1&*entries*=0

India http://www.amazon.in/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Italy http://www.amazon.it/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Japan http://www.amazon.co.jp/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Mexico http://www.amazon.com.mx/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Netherlands http://www.amazon.nl/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Spain http://www.amazon.es/gp/product/B0131LVNW8?*Version*=1&*entries*=0

USA http://www.amazon.com/gp/product/B0131LVNW8?*Version*=1&*entries*=0

The Moon

Australia http://www.amazon.com.au/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Brazil http://www.amazon.com.br/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Canada http://www.amazon.ca/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

France http://www.amazon.fr/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Germany http://www.amazon.de/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

India http://www.amazon.in/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Italy http://www.amazon.it/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Japan http://www.amazon.co.jp/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Mexico http://www.amazon.com.mx/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Netherlands http://www.amazon.nl/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Spain http://www.amazon.es/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

USA http://www.amazon.com/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

The discovery of pulsars 1967

In 1967 Jocelyn Bell, a 24-year-old student from Cambridge University, was doing the research for her PhD.

She was using a radio telescope to study radio waves emitted from compact astronomical objects known as quasars, and when she analysed the data she had collected, she noticed a signal which appeared to pulse on and off every 1.3 seconds. After doing this continuously for about an hour, the pulsing signal would stop altogether, but it would start again precisely 23 hours 56 minutes later after it had first started. As Jocelyn Bell – and indeed all astronomers – knew, the Earth rotates on its axis once every 23 hours 56 minutes, a period of time called a ‘sidereal day’, and the fact that the pulsating signal was detected at the same time in each sidereal day meant that it must almost certainly be coming from space rather than being man-made. The diagram below shows how the radio telescope can only pick up the signal when the pulsing object is in its field of vision.

 

Not only was the time interval between each pulse very short at only 1.3  seconds, but this time interval was completely constant and didn’t vary to any significant degree.  The fact that the pulses were very short in duration and so regular meant that whatever was emitting the pulses must be extremely small in astronomical terms.  A larger object – something, for example, the size of the Sun – would not be able to generate such precise pulses. For a while Bell and her PhD supervisor Antony Hewish considered the possibility that the mysterious signals were generated by a signaling beacon left by an alien civilisation. For this reason, they briefly nicknamed  the unknown object ‘Little Green Men-1’. However, this explanation was rejected when it became clear that the pulses contained no information and when they also discovered additional pulsing sources in other parts of the sky.

Pulsars

The origin of the mysterious signals turned out to be a hitherto unknown class of astronomical objects, known as neutron stars. Neutron stars are very small, typically around 20 km in diameter, but have an enormous mass – between 1.1 and 3 times the mass of the Sun.  Having such a large mass squeezed into a small volume means that their density (mass divided by volume) is incredibly high.  A cubic centimetre of neutron star material would weigh about 500 million tons. They are called neutron stars because they consist mainly of neutrons, which are subatomic particles found in the nucleus of ordinary atoms. In a neutron star the neutrons are so tightly squeezed together that they are touching each other. Neutron stars also have very strong magnetic fields – around 1 trillion times stronger than the Earth’s.

Neutron stars which rotate extremely rapidly, around once a second, are known as pulsars, and these are the objects which Bell and Hewish had detected. This rotation causes electrically charged particles around the neutron star to move rapidly in the intense magnetic fields. This causes electromagnetic radiation (such as radio waves) to be emitted in two cone-shaped beams along the magnetic North and South poles of the pulsar, shown as B in the diagram below.

In the diagram above A is the axis around which the pulsar rotates and B is the magnetic axis.

As you can see, the magnetic poles are at an angle to the line through the North and South poles of the pulsar about which it rotates. This means that the cone-shaped beams will rotate around the axis of the pulsar and only when either of the beams is pointing directly at the Earth will it be detected. A pulsar behaves like a lighthouse – the light is on all the time but appears to a viewer to switch on when it is pointing towards them and switch off when it is pointing away.

Neutron stars are formed when a large massive star explodes at the end of its life in a violent event known as a supernova. Most of the outer parts of the star are blown out into space by the force of the explosion. The remaining material in the star’s core collapses, forming a neutron star.  In fact, if the remaining material from the star’s core is more than three times the mass of the Sun, a neutron star won’t be formed at all.  Instead, an even more compact object called a black hole will result.

Large stars with a diameter of tens of millions of kilometres rotate relative slowly, taking around one year to complete one rotation. As the star collapses into a small massive object, millions of times smaller in diameter, a law of physics called the conservation of angular momentum causes its rotation to speed up massively.  A much more familiar example of this is from the world of ice skating: ice skaters spin more rapidly when they pull in their arms.

Why this knowledge has been useful to astronomy

In the last 50 years more than 2000 pulsars have been detected. Understanding the properties of pulsars has led to further discoveries in various areas.  For example, it has led to greater understanding of the diffuse gas between stars known as the interstellar medium, and it has also been used to test Einstein’s theory of general relativity. I will discuss these advances in more detail in future posts.

Knowledge of pulsars was also used in a very interesting way in the 1970s.  Four spacecraft were launched, destined to the leave the Solar system and head out into interstellar space. The missions are described in more detail in my previous post Artefacts from Earth.  On each spacecraft there was a diagram devised by the American astronomer, Frank Drake (1930-), consisting of a circle with 15 lines coming out of it.

The centre of the diagram, from which the lines radiate, represents the Sun.  The right-hand end of the longest line (at 3 o’clock) represents the centre of the galaxy. The end of each of the remaining line represents a pulsar and the length of the line between the Sun and the pulsar represents the distance to the pulsar.

The diagram relies on the fact that each pulsar has its own distinct period between pulses. So if, in the far distant future, an alien civilisation were to recover the spacecraft they would be able to identify the pulsars from their periods.  To enable this, each of the lines depicts the length of the pulsar’s period not in seconds (which as a man-made unit would be meaningless to an alien race) but in multiples of a ‘fundamental time unit’ that an alien might understand.  The alien civilisation could then have sufficient information to identify the fourteen pulsars and the distance of the Sun from each pulsar, and thus work out the location of our Solar system within the galaxy.

The Nobel prize controversy

Jocelyn Bell and Antony Hewish published their results in February 1968 (Hewish et al 1968). The discovery of this entirely new type of astronomical object was a major advance. Interestingly, it had been suggested as long ago as 1934 by the astronomers Baade and Zwicky that neutron stars would be the end result of supernova explosions. However, this prediction was ignored. Before 1967 most astronomers regarded neutron stars as hypothetical objects which might or might not actually exist in reality.

Such was the impact of the discovery that it led to the Nobel prize for physics in 1974. Two astronomers were awarded the prize – but Jocelyn Bell was not one of them. Antony Hewish and Martin Ryle were the recipients, the former for the discovery of pulsars and the latter for his work in radio astronomy.  Jocelyn Bell’s contribution was not recognised.

Antony Hewish (1924- ) 

Many at the time felt that Jocelyn Bell should have been given at least a share in the prize, since she was the person who had initially spotted the signal from the first pulsar. The British astronomer Fred Hoyle (who readers of a previous post may recall was a founder of the steady state theory of the origin of the Universe) was particularly vocal on the issue and stated publicly many times that Bell should have been given a major share of the prize to acknowledge her contribution. Bell herself said very little about the controversy in the years immediately afterwards. The few statements she made were, in general, supportive of the Nobel prize committee’s decision. In the 1960’s and 1970’s it was commonplace for the senior person leading a team of scientific researchers to get the credit for a major discovery on behalf of the entire team. This is largely still true today.

Bell went on to have a successful academic career and always has been a passionate advocate for getting women more involved in science. From 2002 to 2004 she served as the president of the Royal Astronomical Society, the organisation for British Astronomers. She was the first ever woman to hold that role.  She later served as the first ever female president of the Institute of Physics. In 2006, nearly 40 years after the discovery, she said in an  interview:

In those days, it was believed that science was done, driven by great men . . . And that these men had a fleet of minions under them who did their every bidding, and did not think. It also came at the stage where I had a small child and I was struggling with how to find proper childminding, combine a career, and before it was acceptable for women to work. And so I think at one level it said to me ‘Well men win prizes and young women look after babies.’

Postscript: a different kind of pulsar discovered only last year

Stars such as the Sun do not end their lives in violent supernova explosions resulting in neutron stars. Instead, when they have used up all their nuclear fuel, the outer layers of the star are blown away into space and form a bright glowing shell of gas called a planetary nebula, shown below.

Planetary Nebula

The remnants of the star’s core collapse into a dense hot star called a white dwarf, an object which is roughly the same size as the Earth.  For the last 50 years astronomers have predicted that some white dwarfs might also form pulsars, although because white dwarfs are much larger and rotate more slowly than neutron stars, the radiation would be much weaker than neutrons star pulsars, making them harder to detect and the pulses would be much longer.  This prediction was finally confirmed in 2016 when a team led by Tom Marsh from Warwick University discovered that the white dwarf star AR Scorpii was a pulsar with a period of about 2 minutes.

References

Baade, W.and Zwicky, F. (1934) ‘Remarks on supernovae and cosmic rays’, Physical Review, 46(1), pp. 76-77.

Hewish, A., Bell. S. J., Pilkington, J. D. H, Scott P. F. and Collins, A (1968) ‘ Observation of a rapidly pulsating radio source’, Nature, 217(), pp. 709-713.

 

Voyager 40th anniversary

Nearly 40 years ago, on 20 August 1977, the Voyager 2 space probe was launched from Cape Canaveral, Florida, on a mission to study the Solar System’s four outermost planets. It was followed 15 days later by the launch of an identical spacecraft, Voyager 1.

The Voyager spacecraft -Image from NASA

Although Voyager 1 was launched after Voyager 2, it followed a different path which meant that it actually visited its first target, the planet Jupiter, four months earlier.

Image from Wikimedia Commons

The diagram above shows the trajectory of the Voyager spacecraft. The first targets, the giant planets Jupiter and Saturn, had previously been visited by NASA’s Pioneer 10 and 11 spacecraft, but the Voyagers had better instruments and were able to take more accurate observations. Among the discoveries made by Voyager was that Io, one of Jupiter’s moons, has a number of active volcanoes. This made Io the first place other than the Earth where volcanoes had been seen to erupt.

Jupiter’s moon Io – Image from NASA

Voyager 1 passed close to Saturn’s giant moon Titan. Titan is 5,150 km in diameter, nearly twice as large as the Moon, and is in fact slightly larger than the planet Mercury. It is the only moon in the Solar System with a thick atmosphere, which surrounds it with a dense haze hundreds of kilometres deep, making it impossible to see any surface details. At Titan’s surface the atmospheric density is roughly 5 times greater than the Earth’s at sea level.  Voyager 1 discovered that it consists mainly of nitrogen (98.4%) and methane (1.4%) with trace amounts of other gases such as ethane.

Saturn’s moon Titan – Image from NASA

These discoveries led to speculation that, like Earth, Titan has complex weather patterns. Being so far from the Sun, Titan’s surface is very cold, around minus 180 degrees.  This temperature is near the boiling point of hydrocarbons such as ethane and methane, which on Earth are found in natural gas. Voyager 1’s discoveries suggested that on Titan it rains liquid hydrocarbons in a similar way to how it rains water on Earth and that there were lakes of liquid hydrocarbons on Titan’s surface. A later mission by the spacecraft Cassini to Saturn and its moons did indeed detect many lakes of liquid methane on Titan (NASA 2016).

After passing Saturn, Voyager 2 visited the two outermost planets Uranus and Neptune (shown below).

Neptune – image from NASA

Uranus and Neptune are of similar size and are both much smaller than the gas giants Jupiter and Saturn. They are mainly composed of frozen water, ammonia and methane and are sometimes called the “ice giants”  So far Voyager 2 is the only mission to have visited the ice giants and most of what we know about Uranus and Neptune came from this spacecraft. No further missions to Uranus or Neptune are planned. Although it is possible that a mission to the ice giants could be launched by NASA in the the 2030s (Clark 2015), this is very much at the proposal stage. I think that this won’t happen, given the high cost of space missions and other higher priority targets such as exploring Mars.

After completing their primary mission to study the outer planets, the Voyagers left the Solar System. They are still in contact with Earth and are now sending back data about interstellar space. For more details on this part of the Voyagers’ mission click here.

The golden record

Each Voyager contained a golden record, the cover of which is shown below.

Voyager Record Cover

Image from NASA

The purpose of the golden record was twofold. If either of the Voyager spacecraft were ever found by an alien intelligence in the far future then, as I’ll explain later, from the golden record cover they would be able to work out the location of the Sun within the galaxy and, by playing the record, get some information about the sounds and sights of Earth in the 1970s.

If you look at the bottom left hand corner of the golden record cover there is a small circle with 15 lines coming out of it. This is shown in more detail in the diagram below.

This diagram was also put on a plaque on the earlier Pioneer probes and was devised by Frank Drake (1930-), who has been heavily involved in the search for extraterrestrial intelligence (SETI). The centre of the diagram, from which the lines radiate, represents the Sun.  The right-hand end of the longest line (at 3 o’clock) represents the centre of the galaxy.  The end of each of the remaining lines represents an object called a pulsar and the length of the line between the Sun and the pulsar represents the distance to the pulsar.

Pulsars are rapidly rotating objects which emit radio waves in regular pulses a few seconds or fractions of a second apart. Each pulsar has its own distinct interval or period between pulses and the marks drawn on the lines depict the length of the pulsar period not in seconds but in multiples of a fundamental time unit that an alien might understand (see note below).  The alien civilisation could then have sufficient information to identify the fourteen pulsars and the distance of the Sun from each pulsar, and thus work out the location of our Solar system.

On the upper part of the golden record cover is a picture of the record itself plus instructions for playing it.

Voyager Record

Image from NASA

The content of the record was agreed by a committee chaired by Carl Sagan (1934-1996) itself, and includes:

  • greetings in 55 different languages,
  • natural sounds and some music from Earth
  • 116 pictures of a variety of objects such as the planets in the solar system, human anatomy, groups of children, important landmarks, interesting places, and man-made structures such as airports, large telescopes, the Golden Gate Bridge in San Francisco, and an American highway in the rush-hour.
  • a printed message from the then United States president Jimmy Carter, part of which is given below:

“…This is a present from a small different world, a token of our sounds, our science, our images, our music, our thoughts, and our feelings.  We are attempting to survive in our time so that we we may live into yours. We hope someday, having solved the problems we face, to join a community of galactic civilisations. This record represents our hope and our determination and our goodwill in a vast and awesome universe.”

Where are the space probes now?

As well as the two Voyagers, three other spacecraft have been launched on a trajectory to take them out of the Solar System. They are New Horizons, which passed the dwarf planet Pluto in 2015 and Pioneer 10 and 11, which were the first spacecraft launched to leave the solar system. As you can see from the table, the Pioneer spacecraft  are no longer in contact with Earth.  There are two possible reasons for this: either their power supply has run out, or the radio transmitter is no longer lined up with the Earth and is beaming out signals in a completely different direction. The Voyager spacecraft are still in contact and should have enough power to continue transmitting messages back to the Earth until the mid 2020s.

 

In the table, the distance of the probes is given both in kilometers and astronomical units (AU). 1 AU is the average distance from the Earth to the Sun and is equal to just under 150,000,000 km. The outermost planet Neptune is about 30 AU from the Sun and the nearest star about 250,000 AU away. The speed column shows how fast the spacecraft is moving away from the Sun in AU per year.

Will the Voyagers ever be found?

Even astronomers who think that alien life is widespread within our galaxy think that it is very unlikely, given the vastness of space, that the Voyager spacecraft will ever be found.  The spacecraft are small and their power sources will have long died, thus making them unable to transmit a signal which could be picked up. However if they were ever to be intercepted by an alien civilisation it is fascinating to think what they would make of them.

Note about the pulsar diagram

Clearly units of time such as hours, minutes and seconds, which have been invented by humans, would be meaningless to an alien. Atomic hydrogen is the most common element in the Universe and Drake used the transition of a hydrogen atom, shown on the bottom right hand corner of the record cover, to create a “fundamental time unit” which he hoped an extraterrestrial race would understand.  When a hydrogen atom flips from one state to another it emits radio waves at a frequency of 1,420,406,000 waves per second, a fact that any advanced civilisation would be well aware of. So each individual wave corresponds to a time interval of 0.000 000 000 704 seconds. All the pulsar periods are expressed in multiples of this fundamental time interval.

References

Clark, S (2015) Uranus, Neptune in NASA’s sights for new robotic mission, Available at: https://spaceflightnow.com/2015/08/25/uranus-neptune-in-nasas-sights-for-new-robotic-mission/ (Accessed: 15 June 2017).

NASA (2016) Cassini explores a methane sea on Titan, Available at: https://www.nasa.gov/feature/jpl/cassini-explores-a-methane-sea-on-titan (Accessed: 15 June 2017).

 

Life in our galaxy?

With the recent discovery of three planets orbiting the red dwarf star Trappist-1 which have a similar size, mass and average surface temperature as the Earth, there has been considerable speculation as to whether one or more of these planets supports life.

What the surface of Trappist 1f, one of the planets orbiting Trappist 1, might look like – Image from NASA

Although there are challenges to complex lifetime forms  evolving on a planet around a red dwarf – which I discussed in a previous post – red dwarfs are the most common type of star in our galaxy. In this post I’ll discuss the likelihood that life has evolved in other places within our galaxy, including on planets around red dwarfs.

The Drake Equation

Frank Drake (1930-) is an American astronomer who is known as the ‘father of SETI’ – the Search for Extra Terrestrial Intelligence. Beginning in 1960, he was the first person to search for radio signals from aliens.

.  FrankDrake

Frank Drake- Image from Wikimedia Common 

In 1961 he invented an equation to estimate the number of intelligent civilizations within our galaxy with whom we could potentially communicate, to which he gave the symbol N.  This equation, which is known as the Drake equation, consists of seven numbers multiplied together:

N=  R*  x  FP  x  NE x FL x FI x FC x L

As I’ll explain below, some of these numbers are known to a reasonable accuracy, whereas others are not well known and astronomers differ widely their views of what the values should be.

  • R* is the average number of stars formed per year in our galaxy.  Current estimates are that this has a value of around 10.
  • FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1. A value of 1 means that all stars have planets. 0 would mean that no stars have planets. Planets are difficult to detect around other stars, because they are far too faint to be seen directly and have to be detected by other techniques. In 1961 Drake estimated that FP lay in the region of 0.2 to 0.5 (i.e between 20% and 50% of stars had planets). Current estimates are somewhat higher and that FP is very close to 1.
  • NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life. Current estimates for this vary considerably.  If most stars were like Trappist-1 then this value would be as high as 3. A reasonable value, which many astronomers would agree with, is 0.4, meaning that out of every 10 stars which have planets, 4 have bodies which could support life.

The Trappist-1 system – image from NASA

  • FL is the fraction of bodies, with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1.  A value of 1 means that on all  planets with the right conditions life will evolve. There is no consensus among astronomers about the value of FL. If, in the future, life is found in many other places in our solar system which have the right conditions  e.g. Mars, or in the warm underground oceans of Saturn’s moon Enceladus (see here for more information) then it would be reasonable to assume that, given the right conditions, in general life will evolve and FL is nearly 1 (see note 1).

Enceladus Ice Volcano

A geyser of warm water erupting from an underground ocean on Enceladus. Image from NASA

  • FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, expressed on a scale of 0 to 1. Again, there is no consensus among astronomers about what this value should be. Enthusiasts for extra terrestrial intelligence such as Drake believe that the value is close to 1, meaning that intelligent life will always evolve. Others, who believe that it was a highly improbable chain of events which led to the eventual evolution of man from single celled creatures, believe the value is very low.
  • FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space. For example, on Earth TV and radio signals escape into space and could be picked up by a nearby alien intelligence with a sensitive enough receiver tuned to the right frequency. No one knows what the value of FC is, but current estimates are around 0.2.
  • L is the average lifetime of a civilisation in years. This could be very short if civilisations end up destroying themselves once they have discovered nuclear weapons – or it could be hundreds of millions of years.

The Optimists’ View.

As said previously, no one really knows what the values of most of the terms in the Drake equation are. If we go for values at the high end (FP= 1, NE=0.4, FL=1, FI=1, FC=0.2, L= 100 million) then we get the following:

N= 10 x 1 x 0.4 x 1 x 1 x 0.2 x 100,000,000

which works out as 80 million intelligent communicating civilisations in our galaxy!

One of the problems with such a large number is that we would expect a significant fraction of civilisations to be more advanced than us. Humans have only been civilised for a few thousand years and have already travelled into space.  If a civilisation had been around for more than 1 million years, for example, it is likely that they would have developed the ability to travel the vast distances to other planetary systems and would have already attempted to make contact with us. The fact that they haven’t may mean that civilisations much more advanced than us are rare.

It is also possible (although in my opinion extremely unlikely) that intelligent civilisations do exist and have been observing our planet for a long period of time. They deliberately do not contact us to avoid interfering with our development although they may decide to reveal themselves to us when we reach a certain level of development. This is known as the zoo hypothesis and has appeared in many science fiction stories.

What is clear is that for nearly 60 years, since the pioneering work of Drake in 1960, astronomers have been looking for radio signals from nearby civilisations over a wide range of radio frequencies and have failed to find anything.

Could we be alone ?

Other astronomers believe that some of the values in the Drake equation are very low. There are a large number of steps which occurred between the emergence of the first primitive single-celled life forms and the evolution of man. Each of the individual steps may have a very low probability. So FI the probability of life evolving into intelligent civilisations would be extremely small. For most of the Earth’s lifetime there were only single-celled organisms and, perhaps on most places where life emerges, it never gets beyond this point.

Another point is that mammals only become became the dominant life form after the extinction of the dinosaurs 65 millions years ago. Before that large small-brained reptiles were the dominant life form. Having greater intelligence does not always give an advantage over other traits such as size, speed and physical strength in the survival of the fittest.  There is therefore no guarantee that evolution will result in life forms with the intelligence necessary to develop civilisations.

In addition, dramatic events such as sudden changes in climate can cause any species to become extinct. Roughly 70,000 years ago, an enormous eruption occurred in what is now Sumatra, leaving behind Lake Toba. This triggered a major environmental change which caused the near extinction of the human race.  Humanity could have easily disappeared at this point. Although this has been recently disputed (BBC 2010).

Lake_Toba

Lake Toba, site of a supervolcano eruption 70,000 years ago – Image from Wikimedia Commons

For these reasons some scientists, such as the British theoretical physicist and popular science writer John Barrow, believe that FI could be around 0.000000001 or even lower. If it were this low, and we take the low end values for for the other parameters, then the expected number of intelligent communicating civilizations in the galaxy would be 0.000016. What this means that if we took 60,000 galaxies similar to our own Milky Way we would on average expect to find only one communicating civilisation. Ourselves!

If this is the case then the Earth would not be just be an ordinary planet orbiting an ordinary star in an ordinary galaxy.  It would be a very special place indeed, for it would be the only place for tens of millions of light years where intelligent life exists.

Notes

1 The Earth was formed about 4.6 billion year ago, and at first its temperature was thousands of degrees – far too hot for life to exist.  The first life forms appeared relatively early in the Earth’s history, when it was less than 1 billion years old when conditions became cool enough for life to exist.  This might seem to indicate that, if conditions are right, then life will evolve relatively quickly. Indicating that, perhaps, FL is close to 1.

Reference

BBC (2010) Toba super-volcano catastrophe idea ‘dismissed’, Available at:http://www.bbc.co.uk/news/science-environment-22355515 (Accessed: 15 Apr 2015).

A Christmas present from The Science Geek

 

Now that we are into December, Christmas is almost upon us. So, as I did last year, I’d like to give my readers an early Christmas present, by letting you download my short e-books for free during the first five days of December!

Is Anyone Out There?” is about the likelihood of there being extraterrestrial intelligent life.  It is based on a number of posts from my blog.  For readers based in the UK the book is available to download from Amazon in Kindle format by clicking here and for readers in the US by clicking here. If you’re based outside the UK or US , see the notes at the end of the post.

Is Anyone Out There Cover

The Moon” is also based on a series of posts from my blog and you can guess what it is about.  UK readers download here, US readers here and anyone else please see the notes below.

Moon Cover

How to download the books if you’re based outside the UK or US.

There are two ways of doing this.

Firstly, if you go into the Amazon Kindle store and search for “The Science Geek” as the author you should find my books.

Secondly, I have added some links below to allow you to download the book by just clicking on the link for your country.

Is There Anyone Out There?

Australia http://www.amazon.com.au/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Brazil http://www.amazon.com.br/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Canada http://www.amazon.ca/gp/product/B0131LVNW8?*Version*=1&*entries*=0

France http://www.amazon.fr/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Germany http://www.amazon.de/gp/product/B0131LVNW8?*Version*=1&*entries*=0

India http://www.amazon.in/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Italy http://www.amazon.it/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Japan http://www.amazon.co.jp/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Mexico http://www.amazon.com.mx/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Netherlands http://www.amazon.nl/gp/product/B0131LVNW8?*Version*=1&*entries*=0

Spain http://www.amazon.es/gp/product/B0131LVNW8?*Version*=1&*entries*=0

USA http://www.amazon.com/gp/product/B0131LVNW8?*Version*=1&*entries*=0

The Moon

Australia http://www.amazon.com.au/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Brazil http://www.amazon.com.br/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Canada http://www.amazon.ca/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

France http://www.amazon.fr/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Germany http://www.amazon.de/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

India http://www.amazon.in/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Italy http://www.amazon.it/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Japan http://www.amazon.co.jp/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Mexico http://www.amazon.com.mx/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Netherlands http://www.amazon.nl/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

Spain http://www.amazon.es/gp/product/B00WRPR2S4?*Version*=1&*entries*=0

USA http://www.amazon.com/gp/product/B00WRPR2S4?*Version*=1&*entries*=0