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Nuclear Astrophysics

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As every year, today we celebrate the birthday of one of the founding fathers of the field of Nuclear Astrophysics.

The experimental nuclear physicist received the Nobel prize in physics in 1983 "for his theoretical and experimental studies of the nuclear reactions of importance in the formation of the chemical elements in the universe".
 
Happy Birthday, Willy!

One of the founding fathers of the field of Nuclear Astrophysics, he was born August 9th, 1911, in Pittsburgh, Pennsylvania. He received the Nobel Prize for Physics in 1983 for his theoretical and experimental studies of the nuclear reactions of importance in the formation of the chemical elements in the universe.
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Shapes and decays of nuclei tell us about symmetries in the universe
 
From Atoms To Everything

An atom is comprised of dense nucleus of protons and neutrons surrounded by a diffuse cloud of electrons. Since an atomic nucleus is held together by the strong force, similar to the way gravity holds stars and planets together, you would think nuclei would be basically spherical. This isn’t always the case, and a few such as barium-144 are even pear shaped. This has to do with a subtle way the Universe works, which could explain one of its biggest mysteries.

As Emmy Noether demonstrated in the early 1900s, symmetry is connected to the basic laws of physics. The presence of certain symmetries in the world is a property of certain unchanging quantity in the Universe. Noether’s theorem is an elegant demonstration of this connection, and is central to most modern physical theories. Noether showed us how some of the most powerful symmetries are those connected to space and time.

Take, for example, mirror symmetry, also known as parity. When you look in a mirror, the image you see is reversed. If you hold up your right hand, your mirror image would seem to hold up its left. Mirror image clocks would appear to move counter clockwise. So imagine a mirror universe. Not the evil Spock kind of mirror universe, but simply one in which the parity of everything is reversed. Americans would drive on the left side of the road, the Sun would rise in the west and set in the East, but fundamentally nothing would change. It would be no different than if we simply decided to switch the meanings of right and left.

At least that would be the case if parity symmetry were true. While usually things are symmetrical under a switch of parity, there are some cases were parity is violated. This was first demonstrated by the Wu experiment in 1956. Chien-Shiung Wu looked at the radioactive decay of cobalt-60 atoms. If parity was conserved, then mirror image decay experiments should behave in exactly the same way. What she found was that mirrored experiments of cobalt-60 decayed in opposite directions. Since radioactive decay is driven by the weak nuclear force, this meant the weak force violates parity.

Another symmetry is related to electromagnetic charge. In our Universe protons have a positive charge while electrons have a negative charge. Charge symmetry considers what would happen if these charges were reversed. Since antimatter particles have the opposite charge of their regular matter partners, this would be like replacing all matter with antimatter. Since positive charges interact with each other the same way as negative charges, you would think that charge symmetry would hold. After all, it’s why charge is conserved.

But it turns out charge symmetry can be violated in a subtle way, again connected to the weak interaction, specifically neutrinos. While neutrinos don’t have any charge, they do have a kind of rotation known as helicity. If charge symmetry were true, then matter and antimatter should produce neutrinos with the same helicity. But it turns out matter produces neutrinos of one helicity, while antimatter produces antineutrinos of the opposite helicity. So charge symmetry is violated as well. For a time it was thought that the symmetries of charge and parity could be combined into a more general CP symmetry that would be conserved, but there are radioactive particles that violate it as well.

So what does any of this have to do with pear-shaped atomic nuclei? The shape of a nucleus is determined by the various interactions that occur between the protons and neutrons (and quarks) within the nucleus. If those interactions were CP symmetric, there shouldn’t be a pear-shaped nucleus like barium-144. By studying odd nuclei like barium-144, we can gain clues about the ways CP-symmetry can be violated.

What does this have to do with astrophysics? Remember that charge symmetry is connected to matter and antimatter. Because charge is conserved, when any particle of matter is produced through some physical process, a corresponding particle of antimatter must also be produced. In the early moments after the big bang, when matter was being produced for the first time, there should have been an equal amount of matter and antimatter. But what we see today is a Universe dominated by matter. The origin of this matter-antimatter asymmetry is one of the great unanswered questions of cosmology. It’s been proposed that a violation of CP symmetry could have produced more matter than antimatter, but the currently known violations are not sufficient to produce the amount of matter we see. If there are other avenues of CP violation hidden within pear-shaped nuclei, they could explain this mystery after all.

Paper: C. S. Wu, et al. Experimental Test of Parity Conservation in Beta Decay. Physical Review 105 (4): 1413–1415. (1957)

Paper: B. Bucher et al. Direct Evidence of Octupole Deformation in Neutron-Rich 144Ba. Phys. Rev. Lett. 116, 112503 (2016) arXiv:1602.01485 [nucl-ex]

The pear-shaped nucleus of barium-144 could help solve one of the biggest mysteries in cosmology.
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We need 10 000 votes for the #VLT to become available as a #LEGO ® Kit! Image credit: ESO/F. Snik/M. Zamani
Vote here: http://socsi.in/cOmAP
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"High" Mass Stars
 
High Mass Stars: Crash Course Astronomy #31 with Phil Plait

Massive stars fuse heavier elements in their cores than lower mass stars. This leads to the creation of heavier elements up to iron. Iron robs critical energy from the core, causing it to collapse. The shock wave, together with a huge swarm of neutrinos, blast through the star’s outer layers, causing it to explode. The resulting supernova creates even more heavy elements, scattering them through space. Also, happily, we’re in no danger from a nearby supernova.

Watch here: http://buff.ly/1ZqZ5o9
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Three New Low-Energy Resonances in the Ne22(p,γ)Na23 Reaction

The Ne22(p,γ)Na23 reaction takes part in the neon-sodium cycle of hydrogen burning. This cycle affects the synthesis of the elements between Ne20 and Al27 in asymptotic giant branch stars and novae. The Ne22(p,γ)Na23 reaction rate is very uncertain because of a large number of unobserved resonances lying in the Gamow window. [...] In the present work, the first direct observations of the Ne22(p,γ)Na23 resonances at 156.2, 189.5, and 259.7 keV are reported. [...] Data are taken using a windowless Ne22 gas target and high-purity germanium detectors at the Laboratory for Underground Nuclear Astrophysics in the Gran Sasso laboratory of the National Institute for Nuclear Physics, Italy, taking advantage of the ultralow background observed deep underground. The new reaction rate is a factor of 20 higher than the recent evaluation at a temperature of 0.1 GK, relevant to nucleosynthesis in asymptotic giant branch stars.

Open access to article: http://arxiv.org/abs/1511.05329
The LUNA (Laboratory for Underground Nuclear Astrophysics) web site: http://luna.lngs.infn.it/
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Nobel prize for neutrino astrophysics
 
This deserves a Nobel prize

This is a picture of Super-Kamiokande, one of the neutrino detectors that won this year's physics Nobel prize.   It's a tank buried 1 kilometer deep in a mine in Japan.   The tank holds 50,000 tons of ultra-pure water, surrounded by 11,146 machines that can detect tiny flashes of light.  When a neutrino zipping through space happens to hit a water molecule, it makes a flash of light --- and Super-Kamiokande records it.

Here you see some people on a raft working on the detectors.  The winners of the Nobel prize were Takaaki Kajita and Arthur B. McDonald, who worked at another neutrino detector in Canada.   But these big experiments involve huge teams of people! 

These teams, and their machines, deserve a Nobel prize because they proved something we'd begun to suspect much earlier.

There are 3 different kinds of neutrinos: electron, muon and tau neutrinos.  Nuclear fusion in the Sun makes electron neutrinos... but we saw only about 1/3 as many as expected.  This made physicists suspect that electron neutrinos were turning into the other 2 kinds of neutrinos as they went from the Sun to Earth. 

But proving this was very hard.  And it's only possible if neutrinos have mass! 

You see, time doesn't pass for a massless particle, since special relativity says time slows down for you when you're moving fast, and it comes to a halt if you're moving at the speed of light.  So, a massless particle can't turn into something else until it hits another particle.

As early as the 1950s we knew that neutrinos were almost massless.  So, we thought they were massless.  But now, thanks to these experiments, we know neutrinos really do change from one kind into another.  So, we know they have a tiny but nonzero mass.

Here's what the Nobel prize committee says about it:

The discovery that neutrinos can convert from one flavour to another and therefore have nonzero masses is a major milestone for elementary particle physics. It represents compelling experimental evidence for the incompleteness of the Standard Model as a description of nature. Although the possibility of neutrino flavour change, i.e. neutrino oscillations, had been discussed ever since neutrinos were first discovered experimentally in 1956, it was only around the turn of the millennium that two convincing discoveries validated the actual existence of neutrino oscillations: in 1998, at Neutrino ’98, the largest international neutrino conference series, Takaaki Kajita of the Super-Kamiokande Collaboration presented data showing the disappearance of atmospheric muon-neutrinos, i.e. neutrinos produced when cosmic rays interact with the atmosphere, as they travel from their point of origin to the detector. And in 2001/2002, the Sudbury Neutrino Observatory (SNO) Collaboration, led by Arthur B. McDonald, published clear evidence for conversion of electron-type neutrinos from the Sun into muon- or tau-neutrinos. These discoveries are of fundamental importance and constitute a major breakthrough.

I would put it this way: in the old Standard Model, neutrinos were massless.  In the new improved Standard model, they have a nonzero mass. 

In fact, there's a whole 3 × 3 matrix of numbers, the neutrino mass matrix, which says what neutrinos do as they're flying through empty space.   These numbers actually say how the neutrinos interact with the Higgs boson.  This determines their masses, but also how the 3 kinds turn into each other.

We don't know why the numbers in this matrix are what they are.  We may never know.  But maybe someday someone will figure it out.  Physics is full of slow-burning mysteries like this. 

For the full story, go here:

http://www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.html

The neutrino mass matrix is also called the Pontecorvo–Maki–Nakagawa–Sakata matrix.  In 1962, right after the muon neutrino was discovered, Ziro Maki, Masami Nakagawa and Shoichi Sakata speculated that electron and muon neutrinos could turn into each other, and invented a 2 × 2 matrix to describe this.  And even earlier, in 1956, Bruno Pontecorvo had considered the possibility that neutrinos and antineutrinos could turn into each other. 

If you want to see the numbers in this matrix, go here:

https://en.wikipedia.org/wiki/PMNS_matrix

#physics  
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Nuclear Astrophysics

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Long-delayed nuclear center looks set for construction

After many years of delays, the €1.7 billion Facility for Antiproton and Ion Research, an extension of the GSI Helmholtz Center for Heavy Ion Research near Darmstadt, Germany, may finally get built. At a council meeting on 27 and 28 June, the partner countries—eight European Union members plus India and Russia—concluded that they have enough money to cover a €320 million budget gap; they will now seek building permits from the German government. Still, some countries have yet to commit their share of the missing cash, including Russia, which had agreed to bear about 18% of FAIR's total construction cost, the second largest contribution after Germany's 70%.

If construction proceeds now and goes as planned, first experiments can start as soon as 2022 and FAIR would be completed in 2025. This may still be optimistic, though.

One of the science directions at FAIR will be the study of matter at conditions found in the early universe and in stellar explosions.
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Investigation of Moon rocks points to nearby supernova explosion 2 million years ago
Approximately two million years ago a star exploded in a supernova close to our solar system: Its traces can still be found today in the form of an iron isotope found on the ocean floor. Now scientists at the Technical University of Munich (TUM), together with colleagues from the USA, have found increased concentrations of this supernova-iron in lunar samples as well. They believe both discoveries to originate from the same stellar explosion.
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18th Workshop on Nuclear Astrophysics at Ringberg Castle, Germany

The aim of the workshop, which will be held at Ringberg castle, is to bring together astrophysicists, nuclear physicists, and astro-particle physicists in order to discuss topics of common astrophysical interest.

currently ongoing...
The aim of the workshop, which will be held at Ringberg castle, is to bring together astrophysicists, nuclear physicists, and astro-particle physicists in order to discuss topics of common astrophysical interest such as. Hydrostatic and explosive nucleosynthesis; Chemical evolution of the Galaxy ...
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Cosmochemists show that the unstable isotope 247Cm was present in the Early Solar System

"This is particularly important because it indicates that as successive generations of stars die and eject the elements they produced into the galaxy, the heaviest elements are produced together, while previous work had suggested that this was not the case"
 
Cosmochemists find evidence for unstable heavy element at solar system formation
University of Chicago scientists have discovered evidence in a meteorite that a rare element, curium, was present during the formation of the solar system. This finding ends a 35-year-old debate on the possible presence of curium in the early solar system, and plays a crucial role in reassessing models of stellar evolution and synthesis of elements in stars. Details of the discovery appear in the March 4 edition of Science Advances.
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Could You Build A Helium Star?

Helium is the second most abundant element, making up roughly a quarter of the (baryonic) matter in the universe. Hydrogen is, of course, the most abundant element. When stars form, they have a similar ratio of hydrogen to helium. But suppose you were to gather a Sun-massed quantity of helium. Would it collapse into a star?

A while ago I wrote about a hypothetical situation where some advanced civilization might build a star out of hydrogen. We found that it was really just a matter of gathering enough mass. Once you reach about 90 Jupiter masses worth of hydrogen, it will become a small star. That’s the mass necessary for its interior to become hot enough and dense enough for hydrogen to start fusing. Adding more hydrogen would simply make it a larger, hotter, and brighter star. It’s tempting to think the same would be true for any type of matter. Gather enough stuff and you’ll eventually form a star. But that’s not necessarily true.

A star is typically defined as an object large enough for hydrogen to fuse in its core. Since our hypothetical helium star doesn’t have any hydrogen, let’s generalize this to be a point where nuclear fusion occurs. It turns out that hydrogen fuses fairly readily. Since its nucleus consists of a single proton, it’s the easiest material to fuse with itself (p-p chain), and it’s the easiest to fuse with other elements (CNO cycle). Helium is much more difficult to fuse. So difficult that the build up of helium in a star over time forms a fusion bottleneck that leads to a star’s demise. The only possible way to fuse pure helium is through a chain reaction known as the Salpeter process (after its discoverer Edwin Salpeter) or more commonly the triple-alpha process. Two helium nuclei collide to form beryllium, which quickly collides with another helium to produce carbon-12, which is stable. Once carbon is formed, helium can fuse with it to create heavier elements. But all of these require much more effort than fusing hydrogen.

Pure helium stars don’t exist in the universe, but we can get a good idea of what they might by like by looking at red giant stars. These are old stars that have used up most of their hydrogen. The core of a red giant consists mostly of “waste” helium produced over the course of its lifetime. In a real red giant, there’s a layer of hydrogen outside the helium core that fuses heavier elements. The energy production of the hydrogen layer is so intense that the outer layers of the star expand and cool, hence the red giant star.

Helium fusion can occur in stars with more than about 0.4 solar masses (420 Jupiter masses). Less than that and our ball of helium would never get hot enough to fuse. Technically it would be a large planet, but that’s a bit misleading. Large objects can create heat and light through gravitational collapse. The largest of helium “planets” would, for a time, shine as hot and bright as a small red dwarf star. Over time, our helium planet would continue to collapse as it loses heat. It would end its life either as a cold ball of helium (an old planet) or it would collapse into a white dwarf (like some old stars). The mass cut-off between the two is hard to determine. The smallest white dwarfs we’ve observed has about 17% the mass of our Sun, or about 175 Jupiter masses. So we can safely assume that anything larger than that will become a white dwarf.

Helium fusion into carbon produces plenty of energy, but it also uses up helium fairly quickly. In a red giant, the triple-alpha process only lasts for a few hundred thousand years at best. Compared to small red dwarfs that can shine for trillions of years, a small helium star would only last for a few moments of cosmic time. Stars with less than about 3 solar masses aren’t large enough to fuse carbon into higher elements. A helium star between 0.4 and 3 solar masses can fuse helium into carbon, but nothing more.

How our helium star ends its life depends upon its mass. On the smaller end of the range, a helium star would eventually cool and collapse into a white dwarf. But white dwarfs have an upper mass limit of 1.4 solar masses. More than that and the white dwarf will collapse into a neutron star. Theoretical estimates on neutron star’s upper limit put it at about 3 solar masses, but the largest observed neutron stars are about 2 solar masses. If the limit really is closer to 2, then it’s possible that stars on the larger end of this range would end their lives as black holes.

With more than 3 solar masses it’s possible to fuse carbon and helium into heavier elements, but by then you’ve reached a point of diminishing return. A star’s mass worth of carbon will be used up in a few hundred years, and the fusion of heavier elements would last for even shorter times. Like a normal star, a large helium star would likely expand into a red giant for a time, before collapsing to form a supernova, after which its core would collapse into a neutron star or black hole.

So it is possible to create a helium star but such stars would be short lived, and wouldn’t produce the same range of elements that real stars produce. Hydrogen is central to so many nuclear reactions in stars that without it the universe would be a much darker place.
Building a star isn't simply a matter of gathering enough mass. The type of matter you use greatly affects the behavior of your star.
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