# How do stars or galaxies get their spin?

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It is my understanding that when a star, a planetary disk, or a galaxy forms, the rotational momentum of the whole system is conserved.

Due to the smaller size of the resulting object, it will spin with a significantly higher speed than the original nebula.

What I do not understand is where the original rotation comes from. Why should a random dust cloud have an overall spin? Wouldn't the impulses of all particles in the cloud tend to average each other out?

Is there some alternative source of spin or a reason why nebula have an inherent spin?

You could start from the premise that there was no net angular momentum in the universe at all; but it would still be the case that everything of interest was spinning.

On the scales of stars and planets there are (at least) two important mechanisms that result in individual systems having angular momentum. The first is turbulence. If you take a parcel of turbulent gas from a giant molecular cloud it will always possess some angular momentum, even if the total cloud does not. As the parcel collapses to form a star/planets conservation of angular momentum $J$ and dissipative interactions result in an ncrease in spin rate and collapse towards a planar geometry.

Second, stars form in clusters. There is interaction between stellar systems early in their lives. Again, the cluster may have little net J, but groups of stars can, relative to their own centre of mass frame.

On bigger scales (galaxies) the second of these explanations becomes more important. The interaction and accretion of galaxies is what gives individual galaxies a spin, even if the clusters they are born in have much less or even no net angular momentum.

As an example of how turbulent velocity fields lead to gravitational condensations containing angular momentum you could do worse than study the star formation simulation performed by Matthew Bate and collaborators. These simulations start off in clouds with zero net angular momentum, yet produce a host of stars with swirling accretion disks, binary systems of all shape and sizes etc. An example journal paper can be found here: http://adsabs.harvard.edu/abs/2009MNRAS.392… 590B Here is a web page where you can download the animations and study them at length http://www.astro.ex.ac.uk/people/mbate/Cluster/cluster500RT.html

Turbulent clouds are by their nature random and stochastic in terms of their motions. Often the velocity field is defined in terms of a power law dependence on spatial scale. The formation of vortices is a characteristic of turbulent media. They can be produced in the absence of external forces. The vortices contain angular momentum.

It is also worth noting that not all galaxies have an appreciable spin. Spiral galaxies do, but many elliptical galaxies have little net rotation. See https://physics.stackexchange.com/questions/93830/why-the-galaxies-forms-2d-plane-or-spiral-like-instead-of-3d-ball-or-spherica

Any gaseous object has some spin, usually acquired by interactions with other objects. For example, (proto-)galaxies torque each other to acquire a low rate of angular momentum. Initially, this spin is rather low in the sense that it does not dominate the dynamics: the energy in rotational motion is small compared to other energies, typically by a factor $sim100$.

However, energy can be lost via dissipation (and ultimately radiated away), while angular momentum (spin) is much harder to get rid off. That's why rotating gaseous objects eventually form a disc-like configuration (galactic and proto-stellar discs). In these discs, the kinetic energy is dominated by the rotation. Such systems can only significantly evolve if angular momentum can be exchanged and/or transported. For example, the formation of a star from a proto-stellar disc is promoted by (outwards) angular-momentum transport within the disc. The newly born star retains some residual spin, but that is no longer dominating its energy (otherwise the star wouldn't be near-spherical by disc-like). The same essentially holds for planets.

## A new spin on star-forming galaxies

Spiral Galaxy V Clumpy Galaxy. Credit: International Centre for Radio Astronomy Research

Australian researchers have discovered why some galaxies are "clumpy" rather than spiral in shape—and it appears low spin is to blame.

The finding challenges an earlier theory that high levels of gas cause clumpy galaxies and sheds light on the conditions that brought about the birth of most of the stars in the Universe.

Lead author Dr Danail Obreschkow, from The University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR), said that ten billion years ago the Universe was full of clumpy galaxies but these developed into more regular objects as they evolved.

He said the majority of stars in the sky today, including our five billion-year-old Sun, were probably born inside these clumpy formations.

"The clumpy galaxies produce stars at phenomenal rates," Dr Obreschkow said.

"A new star pops up about once a week, whereas spiral galaxies like our Milky Way only form about one new star a year."

The research team—a collaboration between ICRAR and Swinburne University of Technology—focused on a few rare galaxies, known as the DYNAMO galaxies.

They still look clumpy even though they're seen "only" 500 million years in the past.

Dr Obreschkow said looking at galaxies 500 million years ago was like looking at a passport photo taken a year ago.

"We see that galaxy the way it probably looks now… something could have happened to it but it's very unlikely," he said.

"The galaxies that are 10 billion light years away, that's comparable to a picture from when you were three or four years old, that's very different."

The team used the Keck and Gemini observatories in Hawaii to measure the spin of the galaxies, along with millimetre and radio telescopes to measure the amount of gas they contained.

Dr Obreschkow said the DYNAMO galaxies had a low spin and this was the dominant cause of their clumpiness, rather than their high gas content as previously thought.

"While the Milky Way appears to have a lot of spin, the galaxies we studied here have a low spin, about three times lower," he said.

Swinburne University astronomer Professor Karl Glazebrook, co-author and leader of the survey team, said the finding was exciting because the first observation that galaxies rotate was made exactly 100 years ago.

"Today we are still revealing the important role that the spin of the initial cloud of gas plays in galaxy formation," he said.

"This novel result suggests that spin is fundamental to explaining why early galaxies are gas-rich and lumpy while modern galaxies display beautiful symmetric patterns."

## The secrets of 3000 galaxies laid bare

Credit: ARC Centre of Excellence for All Sky Astrophysics in 3D (ASTRO 3D)

The complex mechanics determining how galaxies spin, grow, cluster and die have been revealed following the release of all the data gathered during a massive seven-year Australian-led astronomy research project.

The scientists observed 13 galaxies at a time, building to a total of 3068, using a custom-built instrument called the Sydney-AAO Multi-Object Integral-Field Spectrograph (SAMI), connected to the 4-meter Anglo-Australian Telescope (AAT) at Siding Spring Observatory in New South Wales. The telescope is operated by the Australian National University.

Overseen by the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3-D), the project used bundles of optical fibers to capture and analyze bands of colors, or spectra, at multiple points in each galaxy.

The results allowed astronomers from around the world to explore how these galaxies interacted with each other, and how they grew, sped up or slowed down over time.

No two galaxies are alike. They have different bulges, haloes, disks and rings. Some are forming new generations of stars, while others haven't done so for billions of years. And there are powerful feedback loops in them fuelled by supermassive black holes.

"The SAMI survey lets us see the actual internal structures of galaxies, and the results have been surprising," said lead author Professor Scott Croom from ASTRO 3-D and the University of Sydney.

"The sheer size of the SAMI Survey lets us identify similarities as well as differences, so we can move closer to understanding the forces that affect the fortunes of galaxies over their very long lives."

The survey, which began in 2013, has already formed the basis of dozens of astronomy papers, with several more in preparation. A paper describing the final data release—including, for the first time, details of 888 galaxies within galaxy clusters—was published today on the arxiv pre-print server and in the journal Monthly Notices of the Royal Astronomical Society.

"The nature of galaxies depends both on how massive they are and their environment," said Professor Croom.

"For example, they can be lonely in voids, or crowded into the dense heart of galactic clusters, or anywhere in between. The SAMI Survey shows how the internal structure of galaxies is related to their mass and environment at the same time, so we can understand how these things influence each other."

Research arising from the survey has already revealed several unexpected outcomes.

One group of astronomers showed that the direction of a galaxy's spin depends on the other galaxies around it, and changes depending on the galaxy's size. Another group showed that the amount of rotation a galaxy has is primarily determined by its mass, with little influence from the surrounding environment. A third looked at galaxies that were winding down star-making, and found that for many the process began only a billion years after they drifted into the dense inner-city regions of clusters.

This three-and-a-half minute video shows highlights of researchers adjusting and deploying the SAMI instrument at the Anglo Australian Telescope at Siding Spring University in New South Wales, Australia. Featuring: Luca Cortese (ICRAR-UWA), Jesse van de Sande (University of Sydney) and Steve Chapman (Night Assistant at the AAT)SAMI plugger: Ángel R. López-Sánchez (AAO/MQU)Music: It's personal (World in Flames, 2011), Celestial Aeon Project. Credit: Ángel R. López-Sánchez (Australian Astronomical Observatory / Macquarie University)

"The SAMI Survey was set up to help us answer some really broad top-level questions about galaxy evolution," said co-author Dr. Matt Owers from Macquarie University in Australia.

"The detailed information we've gathered will help us to understand fundamental questions such as: Why do galaxies look different depending on where they live in the Universe? What processes stop galaxies forming new stars and, conversely, what processes drive the formation of new stars? Why do the stars in some galaxies move in a highly ordered rotating disk, while in other galaxies their orbits are randomly oriented?"

Professor Croom added, "The survey is finished now, but by making it all public we hope that the data will continue to bear fruit from many, many years to come."

Co-author Associate Professor Julia Bryant from ASTRO 3-D and the University of Sydney said: "The next steps in this research will make use of a new Australian instrument—which we've called Hector—that will start operation in 2021, increasing the detail and number of galaxies that can be observed."

When fully installed in the AAT, Hector will survey 15,000 galaxies.

The full data set is available online through Australian Astronomical Optics (AAO) Data Central.

## How do stars and galaxies get their magnetic fields?

One of the longstanding mysteries of astronomy—how stars and galaxies acquire their magnetic fields—may now be one step closer to being solved, thanks to the efforts of researchers from the US Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) .

In a recent edition of the journal Physical Review Letters , PPPL researchers Jonathan Squire and Amitava Bhattacharjee reported that they found that small magnetic disturbances can combine to form larger-scale magnetic fields similar to those found around objects across the universe.

Squire and Bhattacharjee analyzed the behavior of dynamos, which occur when an electrically-charged fluid such as plasma swirls so that a magnetic field is created and then amplified. Experts knew that plasma turbulences could create multiple small magnetic fields, but how those fields combined to produce a larger one remained unknown.

“We can observe magnetic fields all over the universe, but we currently lack a sound theoretical explanation for how they are generated,” said Squire. At the heart of the puzzle was the unlikely concept of smaller disturbances combining to form something larger and more organized.

Simulations suggest that small magnetic fields can combine

The study authors explained that the phenomenon is like a tornado, which forms when several atmospheric disturbances occurring during a storm combine to form one giant vortex. In a like manner, large-scale magnetic fields around galaxies and stars seem to form from a multitude of smaller disturbances, but unlike tornados, they persist instead of disappearing.

“Something is holding up the universe’s magnetic fields for billions of years,” Bhattacharjee, head of PPPL’s Theory Department and co-author of the study, said in a statement. “But how exactly does the universe get these persistent magnetic properties?” To find out, he and Squire conducted a series of statistical and numerical simulations using computers at PPPL.

They found that, under certain conditions, small magnetic fields can combine into one larger one. Specifically, when this occurs, there is a large amount of velocity shear (which occurs when two areas of a fluid move at different speeds). Their simulations indicate that the larger fields are able to persist, but to confirm their findings, they would need to run simulations for very low levels of dissipation (a measure of energy loss).

“It is impossible to run simulations for dissipation as low as those of real astrophysical plasmas,” said Bhattacharjee, “but our analytical and computational results, in the range in which they are done, strongly suggest that such dynamo action is possible.”

## Why do galaxies spin?

Do galaxies turn clockwise or anticlockwise? What's the difference between dark matter and dark energy? Why do older people have quivering voices? Why is our planet called Earth? How do bionic eyes work? Dr Chris Smith and Eusebius McKaiser answer the science questions you call in.

Eusebius - He loves us so much he's on holiday but still will be imparting his knowledge with you and I. Hello Chris. Are you well?

Chris - Yeah, I'm very well thank you Eusebius. How are you?

Eusebius - I'm extremely well. I can't complain. High spirits this morning. We had a question left over from last week and I think we might be able to help the gentleman out. You did give us a caveat it will depend on whether some of your other clever colleagues were available to answer. I want to replay it and to see whether we can help out the gentleman from last week who gave us some homework.

Paul welcome to the show. What is your question.

Paul - Yeah hi, Chris. Hi, Eusebius. Okay, my question is this: when you take clear slate glass,to toughen it is put through a furnace, but prior to this process if you need holes or you need notches made you need to do it before it's fired. Recently I ordered a 8 millimetre piece of shower glass shattered glass, just one panel, and I requested 8 millimetre holes in the panel to take a towel rail. I was told I can have 6 millimetere or I can have 10 millimetre, but I can't have 8 millimetre because the diameter of the hole can't be the same size as the thickness of the glass as it tends to shatter in the furnace. They don't know why. I don't know why. And I wonder whether Chris knows why?

Eusebius - What a lovely question Chris. Okay, it feels like deja vu Chris.

Chris - Right, okay. I did my research because I'm not a material scientist and I had a feeling this had to do with something to do with the way in which glass has different parts of the material being in either compression or tension. So I called up a friend of mine called Howard Stone who's at Cambridge University. He's a materials scientist he works as part of his research with Rolls Royce helping to design alloys that go into jet engines. Into the parts of jet engines that have to survive at extremes of temperature. So he's very familiar with how materials change and respond to changes in temperature like putting glass in a furnace, and also how materials respond to stresses and strains. Now he said "I don't know for sure because I don't know what the composition of the glass is," but what he points out is that when you take toughened glass you end up with a situation where the center of the glass material is in tension - it's being pulled towards the edges, and the surface of the glass is in compression - its squeezing in. When you put the hole through this, obviously you change the way in which forces are transmitted through the glass and how those areas of tension and areas of compression exchange that force or transmit that force through themselves. Therefore, you will fundamentally, if you put a hold of a certain size affect, in perhaps a critical way, how that force is distributed through that patch of glass. It will also make a difference how far from the edge the hole is. So he thinks that it's likely, in this case, that it's that there is a critical size here that if you make the size of the hole a certain dimension you will end up focusing force between those interfaces of the area of compression and the area of tension, and this will result in it breaking because they'll be uneven uneven distribution of the forces in the glass. So I think this is probably where it's coming from, but without doing our own experiment we wouldn't know for sure, but I think that's a very plausible answer. Why the size of the hole should be so critical? I'm not sure and whether it's a red herring that it's the same thickness of the glass or not, but without doing some experiments we couldn't tell. But I think that sounds pretty plausible.

Eusebius - Fascinating. Yohan, good morning to you. What is your question for Chris?

Yohan - Hi, good morning. My question is do galaxies turn clockwise or anticlockwise?

Eusebius - Chris, you get that?

Chris - Yes, hello Yohan. The answer is they do both. And the reason galaxies turn at all is because they have angular momentum. The material that was formed in the early universe and then splurged out into space by the evolution of stars and prior galaxies, that material all had spin and embodied momentum. There is nothing to stop it turning, so if you add something that's turning to something else that's turning, the net resultant momentum or angular momentum would be the sum of the two. So if the material that got together to make a galaxy happened to en mass be on average spinning anticlockwise you'll get to galaxy that's going anticlockwise. So you will get, you would expect on the basis of chance to have equal numbers of clockwise and anticlockwise rotating galaxies, and they're spinning because the material that made them was spinning in the first place. So our Milky Way galaxy, the galaxy itself is a spiral. It is turning, there's a central black hole in the middle of the galaxy, there's dark matter throughout this galaxy that helps holdeverything together, and the planets and the start of the stars that are in the galaxy are going around the galaxy. And there are planets in say our system which are going around our star. They're all turning and they're all turning because the material that made them was turning.

Eusebius - Okay. I'm going to take one from Twitter. This what's interesting here from Gustaf. Gustaf says "hi guys great show. How does the - and I'm going to mangle this - Bosset river flow in both directions west to east in the morning and vice versa in the afternoon? Apparently they say that it is the only river with that particular characteristic. And Professor Google tells me this is somewhere in Eastern Croatia. Have you heard of that?

Chris - I must admit I haven't heard of that river. I have been to Croatia. A very beautiful country. I'm not familiar with the river. The usual reason why rivers flow in two directions is because they're tidal. I don't know about that river. I don't know therefore if it has tides or if rivers it is connected to have tides and so there may therefore be a consequent effect of that. I don't know for sure but it would most the most likely explanation would be that there is some kind of tidal influence over the water in the river. We'd have to look it up. If we can have a few more details, or anyone else is more familiar with that river, or knows about this story please please come back to us and we'll see what we can do.

Eusebius - Let me pretend to be the Naked Scientists for one second with the help of Wikepedia. It says here: the river is known as meandering and extrmeley slow, Chris. And it has a very small inclination in it's basin, less than 10 meters from somewhere until its mouth that is known for a phenomenon of being the river that flows backwards. But it sees like that's an illusion. Sometimes with strong winds and being so slow it appears as if the water is flowing backwards. Did I just sound clever?

Chris - You sounded fantastic because you answered the question as well. So another another 'ding ding' we have solved another one.

Eusebius - Well there you have it, yeah. Sven, good morning to you.

Sven - Morning. How are you guys doing?

Eusebius - Extremely well thank you. What questions have you got for us? Can you stump Chris?

Sven - I don't know if I'll stump him but I'm curious about a question and I've been thinking about.

Sven - In the universe right, we try to understand what the universe is made up of, but I want to understand what the difference is between dark matter and dark energy?

Eusebius - Fantastic. Jeanie, welcome to the show. Let your question out.

Jeanie - Good morning to you. I'd just like to ask the Naked Scientist why is it that when older people speak their voices quiver? What makes it quiver?

Eusebius - What a lovely question.

Eusebius - Thank you Jeanie.

Chris - Hello Jeannie. I think that not everything improves with age, unfortunately. And as we get older we all become a bit saggier, a bit more wrinkly,and a bit shakier. And as you get older you might, for instance, find that the muscles that you use to control your vocal cords weaken a bit, the nerves supply to them weakens a bit, and so they're not as easily controllable as they once were. And also as you age you might spend less time talking. I mean a person who's say in Eusebius' job, their job is to talk all the time and so their voice is getting a lot of exercise. A person who is a professional singer, their voice is getting a lot of exercise and they have developed very good control of the breathing and the processes that we use to make sound. A person who doesn't spend a lot of time, especially as they get older,speaking and chatting and interacting socially with people in the same way that if you don't go to the gym so often, your muscles do get a bit weaker because you don't need to have these enormous muscles. An older person's voice will become a bit thinner and reedier if they don't use it so much. So I think as a consequence of the ageing process naturally making tissue a bit less elastic and springy. Secondly the fact that if we use it a bit less it doesn't retain its strength and vigor. And, in fact, if you practice and you do more talking and more singing and that kind of thing you probably will preserve those things into your old age better than someone who doesn't.

Eusebius - Rammi, good morning. Welcome to the show.

Rammi - Good morning. How are you?

Eusebius - I'm well thank you. What is your question?

Rammi - My question is who named our planet Earth and why is it inconsistent with other planets that have been named after Gods? And why is the Moon not being set with a name like all other celestial objects?

Eusebius - I don't know if on a theme high Chris, but I'm loving some of these question today.

Chris - Yeah, I like the space theme. I don't know why we called the Earth the Earth but it certainly has been called that for a long time. The Romans called it Terrar, which is ground ,probably because it was the ground we lived on. The Moon isn't called the Moon in ancient parlance the Moon had a range of different names. The Romans called it Lunar for Moon. So it hasn't always been called the Moon. But who came up with those names in the first place? I dont know. I'd have to have to engage a historian for their help. But obviously people have been obsessed with these things for a very long time because they really meant something to them. The Moon was a very visible daily presence. You know every day you'd see the moon rise and sink, apart from when you have a new Moon, and it would do it regularly. So people spotted those patterns and they attributed enormous significance to it and the ground beneath your feet decided whether you lived or died. And so I think probably for those reasons they gave them very highly important names. People didn't know that the Earth wasn't the only place in the universe until relatively recently. If you think in the 15/16 hundreds, people began to realize there were other planets. Galileo invented the telescope and began to look into the heavens. People like Copernicus began to be daring enough to suggest that the Earth wasn't at the center of the universe, and at that point people then began this whole business of spotting other celestial objects. They realized that stars included planets. The planets weren't just other stars, there were there were other bodies like the Earth out there. So we began to grow our knowledge and out of that knowledge came a much better understanding of the universe. So probably part of it was that we were extremely proud of ourselves in the early days and attached enormous significance because we thought we were the center of the universe, and then realized later that were not.

Eusebius - Menacha, you've been holding on. Thank you for being patient. What is your question for us?

Menacha - Hi, good morning. Good morning Chris and Eubi. My question is that some time ago at the University of St. Andrews in Scotland an experiment was carried out where they took a sphere and rotated it at a very very high speed. Now that sphere disapeared. So I want to know from Chris, can you just tell us what has actually happened there and what was the explanation?

Chris - I'm not familiar with this experiment. So they took a sphere and rotated it at very high speed and it vanished?

Menacha - Yes. They rotated it at I think were some of the fastest speeds thet they've known on Earth, and then that sphere just disappeared.

Chris - Yeah. I'm not familiar with that experiment. If you can send me a reference to it. If you can just tweet @nakedscientists a reference to the study that you're referring to I can take a look into it because it sounds a little bit fishy that we're not getting the whole story here. So if we can have a few more details, I'll certainly come back next week and tell you a bit more about it.

Eusebius - Okay, tweet us or just call us back or e-mail us [email protected] and then Chris will come back to that particular story.

Teluses, good morning to you.

Teluses - Good morning to you. I need to find out, I became blind about a year ago and I've heard someone talk about a bionic eye. I just wondered if the Naked Scientists would know anything about it or you know?

Chris - Yes, good morning. I'm sorry to hear that you had a problem with your sight. The bionic eye refers to people developing devices that can take over the role of your eyes at the moment. What does the eye do? Well the eye is a posh camera which is interfaced with your nervous system. It's got at the back of the eye a structure called the retina. And in front of that retina is a focusing system, a bit like the one in your camera, which takes light and focuses it onto the retina, and the retina is this sheet of tissue which converts light waves into brain waves. Basically it's layers of cells that are light sensitive. When light falls on them it changes their electrical activity and those changes in electrical activity are then sent down an optic nerve to the back of the brain, and they are compiled into the image that we see in front of us. It's bizarre isn't it to think that what you're seeing in front of you is being decoded on the back of your brain. But when the eye goes wrong it can go wrong for many reasons. And it can be a problem with the front part of the eye, the focusing system. It can be a problem with the retina that decodes the light it comes in and turns it into nerve signals. Or it can be a problem with the optic nerve getting the signal into the brain. Or it can be a problem with the brain itself. So there's a range of different reasons why things go wrong and a bionic eye will only be able to work for some of those problems. Usually there's something wrong with the eye itself or the retina because what most of these systems rely on is that you put into the eye a light sensitive device which sits on the retina that's no longer working. Convert the light that's coming in and being focused onto it into electrical signals which are then injected into the healthy optic nerves that can carry the signals to the brain. We're not yet at the stage where we can replace the optic nerve connections to the brain. If a person therefore has a healthy optic nerve and you can electrically stimulate the nerve cells that go into that optic nerve with one of these devices you can begin to replace vision. And scientists at Oxford University and in Germany and other many other places are doing pioneering experiments now and getting quite a lot of good success where you can take people who have got blindness and can't see a thing, and you can get them being able to see, in low resolution admittedly, but see things again with these techniques. So it's coming along very fast and it's very exciting.

Eusebius - Evander talk to us. What's your question?

Evander - I would like to know why is that the righthanded peoples seem to be more intelligent than those that use the left?

Eubesius - Really! What do you base that on?

Evander - I want to encourage my baby to be using the right hand

Eusebius - Evander, you sound to me like you might be doing a bit of left-handed thinking! Chris?

Chris - Oh dear. Oh dear, Eusebius. careful! Don't tell my daughter either because she's left-handed and she's pretty intelligent!

Eusebius - Is there a correlation?

Chris - Yeah. Let's just demystify this one right away. There is no evidence that people who are lefthanded are less intelligent than people who are righthanded. What they do have to do, in fact, is struggle with a righthanded dominated world, because if your lefthanded you'll make up fewer than 10 percent of the world population. And because the world is dominated by righthanders, then righthanders have made the world for righthanders. So pairs of scissors, tin openers, calculators, everything's for righthanders, so lefthanders actually have to be much more adaptable in order to cope in that environment, which some people argue makes them even more intelligent and able to cope. Probably stretching the truth a bit far with that one. But it's certainly true that people who are lefthanded do cope admirably well and they certainly don't suffer from any intellectual detriment. It may well be though that they are better at sport. And the reason is that a righthander spends the vast majority of their time competing against other righthanders. When they meet a lefthander on the tennis court or the cricket field, the lefthander will have spent the vast majority of their time competing against righthanders, but the righthanders won't have spent a lot of time competing against them so the left handers are at an advantage. So, as a result, it's not a bad thing to be lefthanded and you should certainly not try and encourage your children to go against their natural preference for their handedness because you're not going to change that. I used to do my own little experiment with my daughter because from a very young age I could tell she seemed to prefer using her left hand. So I would see what would happen if I would take the spoon out of her left hand and put it in her right hand and then later on it was crayons and pens. And I'd just do it subtly without telling her what I was doing and see what would happen. This is when she was about one and a half two. And very quickly she would just quickly transfer the device back to the other hand and it was clear from a very early age that she was going to be a lefty. So let your kids use the hand that they prefer using. The days of banning banning people from using the wrong hand are over, thank goodness, and it is very very bad for the people that that happened to.

Eusebius - Actually, this is quite fascinating now that you tease it out as wonderful as you always do because there is a competitive advantage sometimes. I'm thinking immediately, I love watching cricket and I know if you do, and sometimes when you have a left/righthanded combination of batsmen at the crease it can often immediately cause technical woes for the other side because they've got to deal with an orthodox situation.

Chris - Yes, exactly right. And we think that probably buildings have been manipulated because of righthandedness as well. If you think of castles and things that people use to build defensive structures and they had spiral staircases. The reason spiral staircases were probably invented, apart from efficiency of space, is that righthanders because the spiral staircases all rotate to favour the righthanders who could hide up the stairs and round the corner and then fight round the bend with the sword in their right hand, so any lefthanded swordsman, in those days, were at a real disadvantage because they sword was in the wrong hand wasn't it trying to defend their castle? So you can see these sorts of impacts of left and right handedness going back thousands of years.

Eusebius - Okay, Martham. I fele guilty, we have run out of time. But very quickly give us your question and go straight for it.

Martham - On social media there was a strange post not so long ago. Later this month, I think on the 27th August, there'll be a what appears to be two Moons in our sky and they say this phenomenen has happened, or happens once every two and a half thousand years or something like that. Is this true?

Eusebius - Okay. Can we deal with that one quickly Chris? Did you hear it clearly enough?

Chris - Yes I did. I've not come across the idea that there are going to be multiple Moons, so unless this is some funny optical illusion I'm not aware of that story. But again if anyone has a reference for me and they could send me this. It may be that it's got some sound science behind an optical illusion or something. I'll look into it, but I haven't come across any stories to suggest the Moon is going to clone itself and have a twin.

Eusebius - Okay. Thank you Chris. Have a wonderful weekend. We'll do this again next week Friday.

## Astronomers discover galaxies spin like clockwork

Astronomers have discovered that all galaxies rotate once every billion years, no matter how big they are.

The Earth spinning around on its axis once gives us the length of a day, and a complete orbit of the Earth around the Sun gives us a year.

"It's not Swiss watch precision," said Professor Gerhardt Meurer from the UWA node of the International Centre for Radio Astronomy Research (ICRAR).

"But regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round."

Professor Meurer said that by using simple maths, you can show all galaxies of the same size have the same average interior density.

"Discovering such regularity in galaxies really helps us to better understand the mechanics that make them tick-you won't find a dense galaxy rotating quickly, while another with the same size but lower density is rotating more slowly," he said.

Professor Meurer and his team also found evidence of older stars existing out to the edge of galaxies.

"Based on existing models, we expected to find a thin population of young stars at the very edge of the galactic disks we studied," he said.

"But instead of finding just gas and newly formed stars at the edges of their disks, we also found a significant population of older stars along with the thin smattering of young stars and interstellar gas."

"This is an important result because knowing where a galaxy ends means we astronomers can limit our observations and not waste time, effort and computer processing power on studying data from beyond that point," said Professor Meurer.

"So because of this work, we now know that galaxies rotate once every billion years, with a sharp edge that's populated with a mixture of interstellar gas, with both old and young stars."

Professor Meurer said that the next generation of radio telescopes, like the soon-to-be-built Square Kilometre Array (SKA), will generate enormous amounts of data, and knowing where the edge of a galaxy lies will reduce the processing power needed to search through the data.

"When the SKA comes online in the next decade, we'll need as much help as we can get to characterise the billions of galaxies these telescopes will soon make available to us."

## Astronomers discover galaxies spin like clockwork

This Hubble image reveals the gigantic Pinwheel galaxy, one of the best known examples of "grand design spirals", and its supergiant star-forming regions in unprecedented detail. The image is the largest and most detailed photo of a spiral galaxy ever released from Hubble. Credit: ESA/NASA

Astronomers have discovered that all galaxies rotate once every billion years, no matter how big they are.

The Earth spinning around on its axis once gives us the length of a day, and a complete orbit of the Earth around the Sun gives us a year.

"It's not Swiss watch precision," said Professor Gerhardt Meurer from the UWA node of the International Centre for Radio Astronomy Research (ICRAR).

"But regardless of whether a galaxy is very big or very small, if you could sit on the extreme edge of its disk as it spins, it would take you about a billion years to go all the way round."

Professor Meurer said that by using simple maths, you can show all galaxies of the same size have the same average interior density.

"Discovering such regularity in galaxies really helps us to better understand the mechanics that make them tick-you won't find a dense galaxy rotating quickly, while another with the same size but lower density is rotating more slowly," he said.

Professor Meurer and his team also found evidence of older stars existing out to the edge of galaxies.

"Based on existing models, we expected to find a thin population of young stars at the very edge of the galactic disks we studied," he said.

Astronomers have discovered that all galaxies rotate once every billion years, no matter how big they are. Credit: ICRAR

"But instead of finding just gas and newly formed stars at the edges of their disks, we also found a significant population of older stars along with the thin smattering of young stars and interstellar gas."

"This is an important result because knowing where a galaxy ends means we astronomers can limit our observations and not waste time, effort and computer processing power on studying data from beyond that point," said Professor Meurer.

"So because of this work, we now know that galaxies rotate once every billion years, with a sharp edge that's populated with a mixture of interstellar gas, with both old and young stars."

Professor Meurer said that the next generation of radio telescopes, like the soon-to-be-built Square Kilometre Array (SKA), will generate enormous amounts of data, and knowing where the edge of a galaxy lies will reduce the processing power needed to search through the data.

"When the SKA comes online in the next decade, we'll need as much help as we can get to characterise the billions of galaxies these telescopes will soon make available to us."

## New Method Helps Detect Signs of Primordial Galaxies

An international team of scientists has generated the most accurate statistical description yet of early galaxies as they existed in the Universe about a half billion years after the Big Bang. In a paper published online in the journal Nature Communications, they describe the use of a novel statistical method to analyze data captured by the NASA/ESA Hubble Space Telescope during deep-sky surveys.

These panels show different components of near-infrared background light detected by the NASA/ ESA Hubble Space Telescope in deep-sky surveys. The one on the upper left is a mosaic of images taken over a ten-year period. When all the stars and galaxies are masked, the background signals can be isolated, as seen in the second and third panels. The one on the upper right reveals ‘intrahalo light’ from rogue stars torn from their host galaxies, and the lower panel captures the signature of the first galaxies formed in the Universe. Image credit: Ketron Mitchell-Wynne / University of California, Irvine.

The method enabled the astronomers to parse out signals from the noise in Hubble’s images, providing the first estimate of the number of primordial galaxies in the early Universe.

The researchers concluded that there are close to ten times more of these galaxies than were previously detected in deep Hubble surveys.

“The time period under investigation is known as the epoch of reionization,” said lead author Ketron Mitchell-Wynne of the University of California, Irvine.

Coming after the Big Bang and a few hundred million years in which the Universe was dominated by photon-absorbing neutral hydrogen, the epoch of reionization was characterized by a phase transition of hydrogen gas due to the accelerated process of star and galaxy formation.

“It’s the furthest back you can study with Hubble,” Mitchell-Wynne said. “Hubble’s cameras utilize charge-coupled devices, high-quality electronic image sensors first used in astronomy that later were employed in professional video cameras.”

Mitchell-Wynne and co-authors looked at data spanning optical and infrared wavelengths. Photons in the infrared spectrum come directly from stars and galaxies.

Co-author Prof Asantha Cooray, also from the University of California, Irvine, pointed to recent probes into extragalactic infrared background light by the California Institute of Technology’s CIBER instrument.

“CIBER measured the infrared background at two wavelengths, 1.1 and 1.6 microns. These measurements led the CIBER group to confirm the existence of ‘intrahalo light’ from stars distributed outside galaxies,” Prof Cooray said.

“We believe it’s true that there is intrahalo light, but we made a new discovery by looking at five infrared bands with Hubble,” he said.

“We sort of overlap with CIBER and then go into short optical wavelengths, and we see in addition to intrahalo light a new component – stars and galaxies that formed first in the Universe.”

“From the CIBER analysis, we knew there would be a detection of intrahalo light in the infrared bands. We didn’t really know what to expect in the optical ones,” Mitchell-Wynne said.

“With Hubble data, we saw a large drop in the amplitude of the signal between the two. With that spectra, we started to get a little more confident that we were seeing the earliest galaxies.”

Prof Cooray added: “for this research, we had to look closely at what we call ‘empty pixels,’ the pixels between galaxies and stars.”

“We can separate noise from the faint signal associated with first galaxies by looking at the variations in the intensity from one pixel to another. We pick out a statistical signal that says there is a population of faint objects. We do not see that signal in the optical wavelengths, only in infrared. This is confirmation that the signal is from early times in the Universe.”

“These primordial galaxies were very different from the well-defined spiral and disc-shaped galaxies currently visible in the Universe. They were more diffuse and populated by giant stars.”

Ketron Mitchell-Wynne et al. 2015. Ultraviolet luminosity density of the Universe during the epoch of reionization. Nature Communications 6, article number: 7945 doi: 10.1038/ncomms8945

## Old galaxies spin in sync

The rate at which galaxies transform gas into stars as a function of time gives astronomers insight into the way galaxies formed and evolved. By using the SDSS spectra one can infer the past star formation history of a galaxy. We have been doing this using sophisticated statistical tools, take a look here. Much has been learned about the formation of galaxies using their star formation history, for example we know that the most massive galaxies assemble their stars early on, about 1-2 Gyr after the big-bang while small mass galaxies (100 to 1000 times smaller than the milky way) do it during the whole age of the universe. What we have done in our recent paper is to look at how the star formation history of galaxies correlates to the rotation direction of galaxies as measured by the galaxy zoo project. What we have found is that galaxies that had lots of star formation in the past do tend to rotate in the same direction in groups with lengths of about 10 to 20 Mpc.

Although this might sound surprising, it is not! If one reviews very old papers, almost 40-50 years ago, where people like Andrei Doroskievich worked out the way galaxies should rotate based on how they were formed in the past, one realizes that the correlation we have found arises naturally in these models of galaxy formation, so-called hierarchical models. What is happening is that in the past the cluster of galaxies was not yet formed and the spiral galaxies that the galaxy zoo has been classifying by morphology were coming down the filamentary structure into the proto-clusters. Because the proto-cluster already contains the big elliptical galaxies, they provide the same “pull” on all the spiral galaxies in the filament. So it is quite exciting to see this result from the galaxy zoo and the MOPED/VESPA catalogs. Now it is time to go back to theory and numerical simulations and understand better what it means for galaxy formation and evolution. This is something we will do next.

The paper has been submitted to MNRAS, and the pre-print is available for download on astro-ph.