Tag Archives: #helium

Supercomputers Dig Into First Star Fossils (Astronomy)

No one has yet found the first stars.

They’re hypothesized to have formed about 100 million years after the Big Bang out of universal darkness from the primordial gases of hydrogen, helium, and trace light metals. These gases cooled, collapsed, and ignited into stars up to 1,000 times more massive than our sun. The bigger the star, the faster they burn out. The first stars probably only lived a few million years, a drop in the bucket of the age of the universe, at about 13.8 billion years. They’re unlikely to ever be observed, lost to the mists of time.

‘Galactic archaeology’ refers to the study of second generation stars to learn about the physical characteristics of the first stars, which disappeared only tens of millions of years after the Big Bang. A computational physics study modeled for the first time faint supernovae of metal-free first stars, yielding carbon-enhanced abundance patterns for star formation. Slice of density, temperature, and carbon abundance for a 13 solar mass progenitor model at times (left-right) 0.41, 15.22, and 29.16 million years after the supernovae explosion in a box with a side 2 kpc. ©Chiaki, et al.

As the metal-free first stars collapsed and exploded into supernovae, they forged heavier elements such as carbon that seeded the next generation of stars. One type of these second stars is called a carbon-enhanced metal-poor star. They’re like fossils to astrophysicists. Their composition reflects the nucleosynthesis, or fusion, of heavier elements from the first stars.

“We can get results from indirect measurements to get the mass distribution of metal-free stars from the elemental abundances of metal-poor stars,” said Gen Chiaki, a post-doctoral researcher in the Center for Relativistic Astrophysics, School of Physics, Georgia Tech.

Chiaki is the lead author of a study published in the September 2020 issue of the Monthly Notices of the Royal Astronomical Society. The study modeled for the first time faint supernovae of metal-free first stars, which yielded carbon-enhanced abundance patterns through the mixing and fallback of the ejected bits.

Their simulations also showed the carbonaceous grains seeding the fragmentation of the gas cloud produced, leading to formation of low-mass ‘giga-metal-poor’ stars that can survive to the present day and possibly be found in future observations.

“We find that these stars have very low iron content compared to the observed carbon-enhanced stars with billionths of the solar abundance of iron. However, we can see the fragmentation of the clouds of gas. This indicates that the low mass stars form in a low iron abundance regime. Such stars have never been observed yet. Our study gives us theoretical insight of the formation of first stars,” Chiaki said.

NSF-funded XSEDE awarded scientists access to the Stampede2 supercomputer at the Texas Advanced Computing Center (left) and the Comet supercomputer at the San Diego Supercomputer Center (center). The authors utilized the Georgia Tech PACE Hive cluster (right). ©TACC/SDSC/Georgia Tech.

The investigations of Wise and Chiaki are a part of a field called ‘galactic archaeology.’ They liken it to searching for artifacts underground that tell about the character of societies long gone. To astrophysicists, the character of long-gone stars can be revealed from their fossilized remains.

“We can’t see the very first generations of stars,” said study co-author John Wise, an associate professor also at the Center for Relativistic Astrophysics, School of Physics, Georgia Tech. “Therefore, it’s important to actually look at these living fossils from the early universe, because they have the fingerprints of the first stars all over them through the chemicals that were produced in the supernova from the first stars.”

Animation shows the enrichment process of carbon and iron from the supernova of a first-generation of star of 50 solar masses. The four panels show density, temperature, carbon and iron abundances. First, metals are dispersed in the ambient region in the almost spherical manner (< 14 Myr after the explosion). Then, the metals expand in the horizontal direction, while the expansion halts in the vertical direction. Eventually, the metals return to the central region again, where the next generation of stars form. Credit: Chiaki, et al.

“These old stars have some fingerprints of the nucleosynthesis of metal-free stars. It’s a hint for us to seek the nucleosynthesis mechanism happening in the early universe,” Chiaki said.

“That’s where our simulations come into play to see this happening. After you run the simulation, you can watch a short movie of it to see where the metals come from and how the first stars and their supernovae actually affect these fossils that live until the present day,” Wise said.

The scientists first modeled the formation of their first star, called a Population III or Pop III star, and ran three different simulations that corresponded to its mass at 13.5, 50, and 80 solar masses. The simulations solved for the radiative transfer during its main sequence and then after it dies and goes supernova. The last step was to evolve the collapse of the cloud of molecules spewed out by the supernova that involved a chemical network of 100 reactions and 50 species such as carbon monoxide and water.

The majority of the simulations ran on the Georgia Tech PACE cluster. They were also awarded computer allocations by the National Science Foundation (NSF)-funded Extreme Science and Engineering Discovery Environment (XSEDE). Stampede2 at the Texas Advanced Computing Center (TACC) and Comet at the San Diego Supercomputer Center (SDSC) ran some of the main sequence radiative transfer simulations through XSEDE allocations.

“The XSEDE systems Comet at SDSC and Stampede2 at TACC are very fast and have a large storage system. They were very suitable to conduct our huge numerical simulations,” Chiaki said.

“Because Stampede2 is just so large, even though it has to accommodate thousands of researchers, it’s still an invaluable resource for us,” Wise said. “We can’t just run our simulations on local machines at Georgia Tech.”

Chiaki said he was also happy with the fast queues on Comet at SDSC. “On Comet, I could immediately run the simulations just after I submitted the job,” he said.

Wise has been using XSEDE system allocations for over a decade, starting when he was a postdoc. “I couldn’t have done my research without XSEDE.”

XSEDE also provided expertise for the researchers to take full advantage of their supercomputer allocations through the Extended Collaborative Support Services (ECSS) program. Wise recalled using ECSS several years ago to improve the performance of the Enzo adaptive mesh refinement simulation code he still uses to solve the radiative transfer of stellar radiation and supernovae.

Slice of density, temperature, and carbon abundance for a progenitor model with a mass Mpr = 13 solar masses at the time tSN = 0.41 Myr (column a), 15.22 Myr (column b), and 29.16 Myr (column c) after the supernova explosion in a box with a side 2 kpc centered on the centroid of the MH. Credit: Chiaki, et al.

“Through ECSS, I worked with Lars Koesterke at TACC, and I found out that he used to work in astrophysics. He worked with me to improve the performance by about 50 percent of the radiation transport solver. He helped me profile the code to pinpoint which loops were taking the most time, and how to speed it up by reordering some loops. I don’t think I would have identified that change without his help,” Wise said.

Wise has also been awarded time on TACC’s NSF-funded Frontera system, the fastest academic supercomputer in the world. “We haven’t gotten to full steam yet on Frontera. But we’re looking forward to using it, because that’s even a larger, more capable resource.”

Wise added: “We’re all working on the next generation of Enzo. We call it Enzo-E, E for exascale. This is a total re-write of Enzo by James Bordner, a computer scientist at the San Diego Supercomputer Center. And it scales almost perfectly to 256,000 cores so far. That was run on NSF’s Blue Waters. I think he scaled it to the same amount on Frontera, but Frontera is bigger, so I want to see how far it can go.”

The downside, he said, is that since the code is new, it doesn’t have all the physics they need yet. “We’re about two-thirds of the way there,” Wise said.

He said that he’s also hoping to get access to the new Expanse system at SDSC, which will supersede Comet after it retires in the next year or so. “Expanse has over double the compute cores per node than any other XSEDE resource, which will hopefully speed up our simulations by reducing the communication time between cores,” Wise said.

According to Chiaki, the next steps in the research are to branch out beyond the carbon features of ancient stars. “We want to enlarge our interest to the other types of stars and the general elements with larger simulations,” he said.

Said Chiaki: “The aim of this study is to know the origin of elements, such as carbon, oxygen, and calcium. These elements are concentrated through the repetitive matter cycles between the interstellar medium and stars. Our bodies and our planet are made of carbon and oxygen, nitrogen, and calcium. Our study is very important to help understand the origin of these elements that we human beings are made of.”

References: Gen Chiaki, John H Wise, Stefania Marassi, Raffaella Schneider, Marco Limongi, Alessandro Chieffi, Seeding the second star – II. CEMP star formation enriched from faint supernovae, Monthly Notices of the Royal Astronomical Society, Volume 497, Issue 3, September 2020, Pages 3149–3165, https://doi.org/10.1093/mnras/staa2144 link: https://academic.oup.com/mnras/article-abstract/497/3/3149/5875932?redirectedFrom=fulltext

Provided by University Of Texas

Astronomers Find Extremely Metal-Deficient Globular Cluster in Andromeda Galaxy (Planetary Science)

On the outskirts of the nearby Andromeda Galaxy, researchers have unexpectedly discovered a globular cluster (GC) – a massive congregation of relic stars – with a very low abundance of chemical elements heavier than hydrogen and helium (known as its metallicity), according to a new study. The GC, designated RBC EXT8, has 800 times lower abundance of these elements than the Sun, below a previously-observed limit, challenging the notion that massive GCs could not have formed at such low metallicities.

The globular cluster RBC EXT8. Image credit: ESASky / CFHT.

GCs are dense, gravitationally bound collections of thousands to millions of ancient stars that orbit in the fringes of large galaxies; many GCs formed early in the history of the Universe. Because they contain some of the oldest stars in a galaxy, GCs provide astronomers with a record of early galaxy formation and evolution. The most metal-poor GCs have abundances about 300 times lower than the Sun and no GCs with metallicities below that value were previously known. This was thought to indicate a limit to metal content – a metallicity floor – that was required for GC formation; several mechanisms have been proposed to explain this limit.

RBC EXT8 orbits the outskirts of the Andromeda Galaxy, which is located 2.5 million light-years from Earth. Image credit: ESASky / CFHT.

Søren Larsen and colleagues report the discovery of an extremely metal-deficient GC in the Andromeda Galaxy using the High-Resolution Echelle Spectrometer (HIRES) on the Keck I telescope at W. M. Keck Observatory and the Canada-France-Hawaii Telescope (CFHT). Spectral analysis of RBC EXT8 shows that its metallicity is nearly three times lower than the most metal-poor clusters previously known, challenging the need for a metallicity floor. “Our finding shows that massive globular clusters could form in the early Universe out of gas that had only received a small ‘sprinkling’ of elements other than hydrogen and helium. This is surprising because this kind of pristine gas was thought to be associated with proto-galactic building blocks too small to form such massive star clusters,” said Larsen.

References: Søren S. Larsen et al. An extremely metal-deficient globular cluster in the Andromeda Galaxy. Science, published online October 15, 2020; doi: 10.1126/science.abb1970 link: https://science.sciencemag.org/content/early/2020/10/14/science.abb1970

Provided by American Association For The Advancements Of Science

Astronomers Turn Up The Heavy Metal To Shed Light On Star Formation (Astronomy)

Astronomers from The University of Western Australia’s node of the International Centre for Radio Astronomy Research (ICRAR) have developed a new way to study star formation in galaxies from the dawn of time to today.

A selection of the 7,000 galaxies used by the researchers in this work. Credit: GAMA Survey Team, ICRAR/UWA.

“Stars can be thought of as enormous nuclear-powered processing plants,” said lead researcher Dr Sabine Bellstedt, from ICRAR.

“They take lighter elements like hydrogen and helium, and, over billions of years, produce the heavier elements of the periodic table that we find scattered throughout the Universe today.

“The carbon, calcium and iron in your body, the oxygen in the air you breathe, and the silicon in your computer all exist because a star created these heavier elements and left them behind,” Bellstedt said.

Artist’s impression of a galaxy. Credit: ICRAR.

“Stars are the ultimate element factories in the Universe.”

Understanding how galaxies formed stars billions of years ago requires the very difficult task of using powerful telescopes to observe galaxies many billions of light-years away in the distant Universe.

However, nearby galaxies are much easier to observe. Using the light from these local galaxies, astronomers can forensically piece together the history of their lives (called their star-formation history). This allows researchers to determine how and when they formed stars in their infancy, billions of years ago, without struggling to observe galaxies in the distant Universe.

Traditionally, astronomers studying star formation histories assumed the overall metallicity–or amount of heavy elements–in a galaxy doesn’t change over time.

But when they used these models to pinpoint when stars in the Universe should have formed, the results didn’t match up with what they were seeing through their telescopes.

Annotated graph showing the history of star formation from the Big Bang to now. Credit: ICRAR / UWA

“The results not matching up with our observations is a big problem,” Bellstedt said. “It tells us we’re missing something.”

“That missing ingredient, it turns out, is the gradual build-up of heavy metals within galaxies over time.”

Using a new algorithm to model the energy and wavelengths of light coming from almost 7000 nearby galaxies, the researchers succeeded in reconstructing when most of the stars in the Universe formed–in agreement with telescope observations for the first time.

The designer of the new code–known as ProSpect–is Associate Professor Aaron Robotham from ICRAR’s University of Western Australia node.

“This is the first time we’ve been able to constrain how the heavier elements in galaxies change over time based on our analysis of these 7000 nearby galaxies,” Robotham said.

“Using this galactic laboratory on our own doorstep gives us lots of observations to test this new approach, and we’re very excited that it works.

“With this tool, we can now dissect nearby galaxies to determine the state of the Universe and the rate at which stars form and mass grows at any stage over the past 13 billion years.

“It’s absolutely mind-blowing stuff.”

This work also confirms an important theory about when most of the stars in the Universe formed.

“Most of the stars in the Universe were born in extremely massive galaxies early on in cosmic history–around three to four billion years after the Big Bang,” Bellstedt said.

“Today, the Universe is almost 14 billion years old, and most new stars are being formed in much smaller galaxies.”

Based on this research, the next challenge for the team will be to expand the sample of galaxies being studied using this technique, in an effort to understand when, where and why galaxies die and stop forming new stars.

Bellstedt and Robotham, along with colleagues from Australia, the UK and the United States, are reporting their results in the scientific journal the Monthly Notices of the Royal Astronomical Society.

References: Sabine Bellstedt, Aaron S G Robotham, Simon P Driver, Jessica E Thorne, Luke J M Davies, Claudia del P Lagos, Adam R H Stevens, Edward N Taylor, Ivan K Baldry, Amanda J Moffett, Andrew M Hopkins, Steven Phillipps, Galaxy And Mass Assembly (GAMA): a forensic SED reconstruction of the cosmic star formation history and metallicity evolution by galaxy type, Monthly Notices of the Royal Astronomical Society, Volume 498, Issue 4, November 2020, Pages 5581–5603, https://doi.org/10.1093/mnras/staa2620

Provided by International Center For Radio astronomy Research

Why there is no speed limit in the superfluid universe? (Physics / Quantum)

Physicists from Lancaster University have established why objects moving through superfluid helium-3 lack a speed limit in a continuation of earlier Lancaster research.

Researchers found the reason for the absence of the speed limit: exotic particles that stick to all surfaces in the superfluid. Credit: Lancaster University

Helium-3 is a rare isotope of helium, in which one neutron is missing. It becomes superfluid at extremely low temperatures, enabling unusual properties such as a lack of friction for moving objects.

It was thought that the speed of objects moving through superfluid helium-3 was fundamentally limited to the critical Landau velocity, and that exceeding this speed limit would destroy the superfluid. Prior experiments in Lancaster have found that it is not a strict rule and objects can move at much greater speeds without destroying the fragile superfluid state.

Now scientists from Lancaster University have found the reason for the absence of the speed limit: exotic particles that stick to all surfaces in the superfluid.

The discovery may guide applications in quantum technology, even quantum computing, where multiple research groups already aim to make use of these unusual particles.

To shake the bound particles into sight, the researchers cooled superfluid helium-3 to within one ten thousandth of a degree from absolute zero (0.0001K or -273.15°C). They then moved a wire through the superfluid at a high speed, and measured how much force was needed to move the wire. Apart from an extremely small force related to moving the bound particles around when the wire starts to move, the measured force was zero.

Lead author Dr Samuli Autti said: “Superfluid helium-3 feels like vacuum to a rod moving through it, although it is a relatively dense liquid. There is no resistance, none at all. I find this very intriguing.”

PhD student Ash Jennings added: “By making the rod change its direction of motion we were able to conclude that the rod will be hidden from the superfluid by the bound particles covering it, even when its speed is very high.” “The bound particles initially need to move around to achieve this, and that exerts a tiny force on the rod, but once this is done, the force just completely disappears,” said Dr Dmitry Zmeev, who supervised the project.

This article is republished from science daily

References: S. Autti, S. L. Ahlstrom, R. P. Haley, A. Jennings, G. R. Pickett, M. Poole, R. Schanen, A. A. Soldatov, V. Tsepelin, J. Vonka, T. Wilcox, A. J. Woods, D. E. Zmeev. Fundamental dissipation due to bound fermions in the zero-temperature limit. Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-18499-1 link: https://www.nature.com/articles/s41467-020-18499-1

How Stars Fuse Their Fuels And How They die? (Astronomy)

When a star is born, it is because it has enough mass to create enough heat, gravity and pressure to sustain nuclear fusion. Fusing hydrogen atoms to helium gives off enormous amounts of energy, and the star spends its life quietly fusing away. This process takes four hydrogen atoms to fuse into one helium atom. Hydrogen has only one proton, while helium has two protons along with two neutrons. This means that two protons are missing. Matter cannot be created or destroyed—it can only be turned into something else.

In this case, the two missing protons have turned into two neutrons. This energy is what makes the star shine and give off heat. Well.. after a while, the star has built up quite a bit of helium. This helium has found its way to the star’s center to create a helium core. Since hydrogen only has one proton, and helium has two protons and two neutrons, it’s heavier. That means the star has a little more mass in its core, which generates more heat. This heat builds up more and more, until it’s hot enough and has enough pressure to start fusing helium to carbon. This process generates a little less energy than fusing hydrogen to helium, but it still produces energy.

As a guideline, a star that has about one half the mass of the sun is too small and cool to fuse helium to carbon. So it will end up as a white dwarf made of helium. Stars between one half to four times the mass of the sun are massive and hot enough to fuse carbon to oxygen. Carbon and oxygen are fused more or less at the same time, and you’ll end up with a white dwarf made out of carbon and oxygen. I want to jump off topic here for a moment and ask a basic chemistry/physics question to all you readers. What happens when you introduce large amounts of heat and pressure to carbon? ‘Diamonds’..

The star has died and it’s a white dwarf made out of carbon: a giant diamond in the sky. Stars with masses greater than four times the mass of the sun are massive and hot enough to fuse oxygen to silicon.

Order of Nuclear Fusion in Dying Stars (Source)

Stars that have earned the title of “supergiant” are so massive and so hot that they begin fusing silicon to a solid core of iron. Once the star starts fusing iron, that’s it– it’s doomed. Fusing silicon to iron takes more energy than it gives off. This means that the star is going to die soon; it is causing its own death by using more of its own energy than it is getting back from nuclear fusion.

When a star is fusing iron in its core, it’s still giving off insane amounts of energy. The helium, hydrogen, carbon, oxygen, and silicon are still there in the star in different shells. Hydrogen is at the surface, still fusing to helium; a little further down, helium fusing to carbon and oxygen; further down we have silicon until the core, where silicon fuses to iron. This is why the star still exists and doesn’t spontaneously explode the moment the first iron atom pops into existence.

At this point, the energy process is just no longer exothermic but endothermic. Iron cannot be fused into anything heavierbecause of the insane amounts of energy and force required to fuse iron atoms. The atomic structure of iron is very stable, more so than most other elements. I’m not saying all other elements are radioactive or unstable, just that iron is slightly more stable than the previous elements.

Stars this massive can turn into several things; it depends on how heavy it is. They can explode into supernova, collapse into various types of neutron stars, or even form a black hole. The iron in the star’s core isn’t the reason why the star went supernova, its overall mass made it explode. But, the iron in its core caused it to die.