Since iron is a stars waste could a star that goes Super nova eject pure iron to travel across the galaxy?
Sure. Iron is not made in significant quantities in the Big Bang, so 100% of the iron on Earth today was synthesized in a star. There's no other source.
It escapes in many ways:
- Mergers between compact stars like white dwarfs and neutron stars
- Stellar winds from more ordinary stars
Supernovae are the main source of modern-day iron, with r-process nucleosynthesis during the explosion producing a lot of iron and elements near iron.
Super-Earth discovered: Data will characterize planetary atmosphere models
Moments of the virtual journey, with overlaid astronomical data. Credit: RenderArea
During the past 25 years astronomers have discovered a wide variety of exoplanets, made of rock, ice and gas, thanks to the construction of astronomical instruments designed specifically for planet searches. Also, using a combination of different observing techniques they have been able to determine a large number of masses, sizes, and hence densities of the planets, which helps them to estimate their internal composition and raises the number of planets which have been discovered outside the Solar System.
However, to study the atmospheres of the rocky planets, which would made it possible to characterize fully those exoplanets which are similar to Earth, is extremely difficult with currently available instruments. For that reason, the atmospheric models for rocky planets remain untested.
So it is interesting that the astronomers in the CARMENES (Calar Alto high- Resolution search for M dwarfs with Exoearths with Near-infrared and optical échelle Spectrographs), consortium in which the Instituto de Astrofisica de Canarias (IAC) is a partner, have recently published a study, led by Trifon Trifonov, an astronomer at the Max Planck Institute for Astronomy at Heidelberg (Germany), about the discovery of a hot super-Earth in orbit around a nearby red dwarf star Gliese 486, only 26 light years from the Sun.
To do this the scientists used the combined techniques of transit photometry and radial velocity spectroscopy, and used, among others, observations with the instrument MuSCAT2 (Multicolour Simultaneous Camera for studying Atmospheres of Transiting exoplanets) on the 1.52m Carlos Sánchez Telescope at the Teide Observatory. The results of this study have been published in the journal Science.
This virtual journey to Gliese 486b begins with its position in the night sky. After focusing on the parent star Gliese 486b, the film depicts the measurements. Finally, we fly to the exoplanet Gliese 486b and explore its possible surface, which probably resembles Venus, with a hot and dry landscape interspersed with glowing lava flows. Credit: RenderArea
The planet they discovered, named Gliese 486b, has a mass 2.8 times that of the Earth, and is only 30% bigger. "Calculating its mean density from the measurements of its mass and radius we infer that its composition is similar to that of Venus or the Earth, which have metallic nuclei inside them," explains Enric Pallé, an IAC researcher and a co-author of the article.
Gliese 486b orbits its host star on a circular path every 1.5 days, at a distance of 2.5 million kilometers. In spite of being so near to its star, the planet has probably conserved part of its original atmosphere (the star is much cooler than our Sun) so that it is a good candidate to observe in more detail with the next generation of space and ground telescopes.The diagram provides an estimate of the interior compositions of selected exoplanets based on their masses and radii in Earth units. The red dot represents Gliese 486b, and the orange symbols represent planets around cool stars like Gliese 486. The gray dots show planets housed by hotter stars. The color curves indicate the theoretical mass radius relationships for pure water at 700 K (blue), for the mineral enstatite (orange), for Earth (green), and pure iron (red). By comparison, the diagram also highlights Venus and Earth. Credit: Trifonov et al./ MPIA Graphics Department.
For Trifonov, "the fact that this planet is so near the sun is exciting because it will be possible to study it in more detail using powerful telescopes such as the iminent James Webb Space Telescope and the ELT (Extremely Large Telescope) now being built."
Gliese 486b takes the same length of time to spin on its axis as to orbit its host star, so that it always has the same side facing the star. Although Gliese 486 is much fainter and cooler than the Sun, the radiation is so intense that the surface of the planet heats up to at least 700K (some 430 degrees C). Because of this, the suface of Gliese 486b is probably more like the surface of Venus that that of the Earth, with a hot dry landscape, with burning rivers of lava. However, unlike Venus, Gliese 486b may have a thin atmosphere.
Calculations made with existing models of planetary atmospheres can be consistent with both hot surface and thin atmosphere scenarios because stellar irradiation tends to evaporate the atmosphere, while the planet's gravity tends to hold it back. Determining the balance between the two contributions is difficult today.
"The discovery of Gliese 486b has been a stroke of luck. If it had been around a hundred degrees hotter all its surface would be lava, and its atmosphere would be vaporized rock," explains José Antonio Caballero, a researcher at the Astrobiology Centre (CAB, CSIC-INTA) and co-author of the article. "On the other hand, if Gliese 486b had been around a hundred degrees cooler, it would not have been suitable for the follow-up observations."Artist's impression of the atmosphere of Gliese 486b. Credit: RenderArea
Future planned observations by the CARMENES team will try to determine its orbital inclination, which makes it possible for Gliese 486b to cross the line of sight between us and the surface of the star, oculting some of its light, and producing what are known as transits.
They will also make spectroscopic measurements, using emission spectroscopy, when the areas of the hemisphere lit up by the star are visible as phases of the planet (analagous to the phases of our Moon), during the orbits of Gliese 486b, before it disappears behind the star. The spectrum observed will contain information about the conditions on the illuminated hot surface of the planet.
"We can't wait until the new telescopes are available," admits Trifonov. "The results we may obtain with them will help us to get a better understanding of the atmospheres of rocky planets, their extensión, their very high density, their composition, and their influence in distributing energy around the planets.
- Artistic impression of the surface of the newly discovered hot super-Earth Gliese 486b. With a temperature of about 700 Kelvin (430 °C), Gliese 486b possibly has an atmosphere. Credit: RenderArea
- Colorized 2D spectra of the star Gliese 486 as seen with MAROON-X. The two spectra are from the two camera arms of MAROON-X. Each spectrum covers the 500-670 nm wavelength range and the color-coding corresponds to how a human eye would perceive the colors. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/A. Seifahrt
The CARMENES project, whose consortium is made up by 11 research institutions in Spain and Germany, has the aim of monitoring a set of 350 red dwarf stars to seek planets like the Earth, using a spectrograph on the 3.5 m telescope at the Calar Alto Observatory (Spain). The present study has also used spectroscopic measurements to infer the mass of Gliese 486b. Observations were made with the MAROON-X instrument on Gemini North (8.1m) in the USA, and archive data were taken from the Keck 10 m telescope (USA) and the 3.6m telescope of ESO, (Chile).
The photometric observations come from NASA's TESS (Transiting Exoplanet Survey Satellite) space observatory, (USA), whose data were basic for obtaining the radius of the planet, from the MuSCAT2 instrument on the 1.52m Carlos Sánchez Telescope at the Teide Observatory (Spain) and from the LCOGT (Las Cumbres Observational Global Telescope) in Chile, among others.
When Did the Universe Have the Right Stuff for Planets?
To build a planet you need lots of rubble and that means lots of heavy elements – stuff more massive than atoms of hydrogen and helium. The elemental composition of the collapsing nebula that gave birth to the Sun and the planets of the Solar System included things like iron, silicon and magnesium that form the bulk of rocky planets, and carbon, oxygen, nitrogen, potassium and other such elements that are essential for life.
However, these materials were present in just trace quantities, amounting to no more than two percent of the entire nebula that was otherwise dominated by hydrogen (74 percent) and helium (24 percent). Yet this gaseous cloud was huge it is estimated that it harbored enough heavy elements to build at least thirty planets like Earth.
These heavy elements – 'metals' in astronomer-speak – don't just materialize out of nothing. They are the products of fusion power within stars, subsequently spewed out across the cosmos on the blast waves of supernovae, lacing the interstellar medium with the raw ingredients for planets. To build up enough of these materials, many stars must first live and die, each one contributing to the evolving chemistry of the universe, but how much material is really required to build a planet and how quickly did the universe accrue a sufficient level to do so?
Heavy Metal Planets
Earth was born out of the debris of a protoplanetary disc around a nascent Sun 4.54 billion years ago – a serious chunk of time in anybody's book. Yet the universe is 13.7 billion years old – the Solar System has been around for just the last third of cosmic history. Is it possible that rocky planets could have formed around other stars much earlier? Are we the new kids on the block by comparison?
Until recently, we didn't think so. The prevailing wisdom had been that the magic of stellar alchemy didn't produce enough useful "star-stuff" to build terrestrial worlds until at least six or seven billion years after the Big Bang. Initial studies of exoplanets backed this up, finding worlds around stars with a "metallicity" (i.e. a heavy element abundance) equal to or greater than our Sun. However, it turns out that the biases that affected our early planet hunting also skewed our understanding of the types of stars that could form planets. Until 2009 and the launch of NASA's Kepler mission, the vast majority of exoplanets known to exist were gas giants close to their stars, simply because these were the easiest to detect. These planets seemed to prefer higher metallicity stars.
Kepler, however, has changed the way we view exoplanets. Simply by observing so many all at once in its field-of-view, the space telescope is taking an unprecedented census of alien worlds. It has found 2,321 candidate planets to date, over a third of which are smaller, rocky planets (Jupiter-sized gas giants or larger make up just 11 percent, with the rest being Neptune-sized worlds of indeterminate nature), whereas before Kepler you could count the number of rocky exoplanets discovered on one hand. Follow-up studies of their host stars have since revealed a surprising discovery. [Gallery: A World of Kepler Planets]
"We found that the existence of small planets does not depend as strongly on the metallicity of their star as is the case for the larger planets," says Lars Buchhave of the Niels Bohr Institute at the University of Copenhagen. Buchhave is lead author of a new study involving a multinational group of astronomers investigating the spectra of 150 stars that play host to 226 candidate planets found by Kepler. Their research was initially presented at the 220th meeting of the American Astronomical Society in Anchorage, Alaska this June, followed by a paper in Nature.
"At first glance it appears very counter-intuitive that gas giants should be the ones caring about metallicity and terrestrial planets less so," says Anders Johansen of Lund Observatory in Sweden, who was a co-author on the Buchhave paper. Only when you stop to consider how planets are constructed does it begin to make sense. The process of accreting hierarchically from smaller building blocks is termed core accretion, but there has been something of a debate surrounding gas giants like Jupiter. Can they condense straight out of the gas of the solar nebula like a star, or do they need a large seed around which to grow by rapidly gathering gas from the protoplanetary disc in a runaway process?
The preference of gas giants for higher metallicity stars indicates that they formed through core accretion, building up a central rocky core ten times the mass of Earth that could dominate the protoplanetary disc and sweep up much of the gas before it dissipates after around ten million years. In lower metallicity systems there would not be enough heavy elements to build up large cores, leaving only small rocky worlds. As such, Johansen suggests that one way of looking at terrestrial planets is to see them as failed gas giant cores.
Limits to Life
Planetary systems around stars possessing a deficiency in heavy elements might prove to be attractive locales to search for life because, without the presence of gas giants, life might have an easier time of it. Most of the extra-solar gas giants that we have discovered are so-called 'hot Jupiters' located very close to their stars and completing an orbit in just a few days. These planets were not born this close, instead they migrated in-system from their birth orbits. Johansen says that more and more astronomers are coming around to the idea that such migration is forced by the gravitational pull and dynamical friction of the gas, or by close encounters with other planets. These interactions with fellow constituents of the protoplanetary disc removed angular momentum from the planets, often causing them to spiral towards their stars. Any smaller planets unfortunate to be in their way are thrown out of the system by the marauding gas giant.
"If a Jupiter-type planet migrates and in the process scatters all the smaller planets away, one should probably look for terrestrial planets elsewhere," says Buchhave. Life may have had a more pleasant ride in the early universe when, thanks to the lower metallicity, there were no gas giants – and the argument that Jupiter-sized planets are needed as a shield against comet impactors no longer holds water either. Life can do without gas giant planets.
If Earth-sized planets do not require stars with high abundances of heavy elements, then that has huge implications, expanding the possible abodes for life throughout both space and time. Consider: galaxies tend to evolve chemically from the inside out, with the highest abundances of heavy elements closer to the galactic center than in the outskirts of the spiral arms. Under the previous paradigm, the outer regions of the spiral arms were effectively the badlands, incapable of building planets or life. Yet when metallicity is no longer such a big issue, the galactic habitable zone – a region where environmental conditions including the metallicity and the rate of supernovae conspire to make habitable planets possible – suddenly widens to encompass much wider swathes of a galaxy.
Now consider that the abundance of heavy elements in the universe has grown over history. In the past the average metallicity would be quite a bit less. Again, under the previous paradigm this had been assumed to preclude rocky planet formation early in the universe, but now we know that such planets could have been constructed in environments that contained much poorer levels of heavy elements. This means that planets that could potentially have supported life may have formed eight, ten, maybe even twelve billion years ago.
Surveys do detect a decrease in the number of planet-hosting stars with decreasing metallicity, but this drop is much shallower for terrestrial planets than it is for gas giants. Of course, the presence of some heavy elements during the planet-building phases is required, but the minimum level has not yet been determined.
"I expect there will be a lower limit," says Johansen. "Simply because below a threshold metallicity there is not enough building material to form Earth-mass planets." Clearly, a heavy element abundance a tenth of the Sun's or less would struggle to build any planets. However, each galaxy evolves differently and there is no way to say for sure when the Milky Way crossed this threshold, although it is likely to have been early in the history of the universe, for the young cosmos was particularly adept at producing multiple generations of stars in quick succession. Star-formation rates of 4,000 solar masses per year have been measured less than a billion years after the Big Bang, compared to the paltry ten solar masses of gas converted into stars each year in the Milky Way.
"A typical massive star that exploded and released heavy elements 10 to 12 billion years ago had a metallicity of about a tenth of the Sun," adds Johan Fynbo, Professor of Cosmology at the Niels Bohr Institute. "But whenever you have a new generation of stars then you start enriching the interstellar gas with heavy elements." [The Strangest Alien Planets (Gallery)]
The Fermi Paradox
So, rocky planets around more stars, across greater expanses of the Milky Way and going back deeper in time than we had ever dreamt adds more fuel to the fire of the Fermi Paradox. First voiced by the brilliant nuclear physicist Enrico Fermi in 1950, the Fermi Paradox questions why, given all the stars and planets out there coupled with the huge age of the universe, have no alien civilizations encountered Earth yet? Where are they all?
The problem is made even worse when you consider that the first term in the Drake Equation – Frank Drake's method for estimating the number of intelligent civilizations in the Galaxy – is the star formation rate, which on average was much higher in the universe 10 to 13 billion years ago when it seems planets could first begin forming. In the Milky Way today the average annual star formation rate is ten solar masses an order of ten or one hundred greater has the effect of bumping up the product of the equation: the estimated number of civilizations.
One of the favorite counter-arguments to the Fermi Paradox was that the threshold metallicity takes time to build up, resulting in the Sun being one of the first stars at the required level and hence Earth would be one of the first planets with life. Now we see that planets and possibly life could have arisen at practically any point in cosmic history, undermining this counter-argument and once again forcing us to ask, where is everybody? If life did first appear on worlds 12 to13 billion years ago, then intelligent civilizations (if indeed they survived all this time) would now billions of years ahead of us and their concerns may no longer include the happenings on a damp mudball somewhere in the galactic hinterlands. Perhaps civilizations that are many billions of years old instead spend their time siphoning energy from black holes or living inside Dyson Spheres.
There are, however, some twists in the tale. In 2010 researchers at the Max Planck Institute for Astronomy in Heidelberg, Germany, found a gas giant planet around a star so lacking in heavy elements that it must have formed very early in the history of the universe. To add to the intrigue, the star, known as HIP 13044 and located 2,000 light years away, is part of a stellar stream that is all that remains of a dwarf galaxy that has been cannibalized by the Milky Way. This year, the same researchers found another low metallicity star with two gas giants. Based on its abundance of hydrogen and helium the star, known as HIP 11952, was born 12.8 billion years ago, a mere 900 million years after the Big Bang. Why gas giants have been able to form around these heavy-metal deficient stars is unknown, perhaps hinting at an alternative process for gas planet formation.
On the other hand new results suggest that, in some regions of the universe at least, gas giants have been able to form all along.
For some faint galaxies in the distant universe, whose light is too feeble to allow a measurement of their spectra, it is possible to cheat by making use of natural backlights such as highly luminous quasars to probe faint foreground galaxies. When taking advantage of this method to study the chemical composition of a galaxy that existed 12 billion years ago, a team of astronomers including Johan Fynbo made a rather surprising revelation.
"We looked at a background quasar whose light was passing through a galaxy in front of it, where the light of the quasar was absorbed," says Fynbo. "This allowed us to see the absorption lines from oxygen, sulphur, carbon and all the elements that have been synthesized in the galaxy."
Twelve billion years ago the chemistry of galaxies should have been fairly primitive, yet in this one particular galaxy Fynbo and his colleagues, who reported on their findings in Monthly Notices of the Royal Astronomical Society, found abundances of heavy elements equivalent to the abundance in the Sun. Such finds at high distances are not unusual in themselves, but they tend to occur within the hearts of quasars, across a very small area of a galaxy. In this instance, however, the quasar light was shining through the disc of the foreground galaxy revealing the solar levels of heavy elements 52,000 light years from the center, right in the outskirts. Even today our own Milky Way isn't so heavily chemically processed to the edge of its spiral arms, so how did this distant galaxy become so enriched throughout its full extent so quickly?
The best explanation so far is that a starburst – a ferociously rapid bout of star formation – within the inner regions of the galaxy has blown the heavy elements into the galactic outlands. This can be done simply though the gale force stellar winds of radiation emanating from hot, massive stars, or riding on the shock waves of supernovae. Furthermore, the quasar light was reddened by intervening dust in the galaxy. Dust is the most basic building block of planet formation, coming together in conglomerations and clumps that build up into protoplanets. Dust is also a product of the violent bombardment phase endured by young planetary systems and is copiously manufactured in supernovae.
"In order to make planets you clearly need metals and that seems to be possible quite far out in a galaxy at a very early time, which is what surprised us," says Fynbo. However, such high metallicities enables gas giant planets to also form but, although Lars Buchhave has mentioned what difficulties gas giants can cause for habitable planets, they don't necessarily have to be a show-stopper and our Solar System with Jupiter and Saturn is not the only exception.
"In the Kepler-20 planetary system there are five planets," he says, "Three are Saturn-sized planets and two are terrestrial-sized, with the order being big–small–big–small–big. If the Saturn-mass planets migrated in, how can the small planets be in-between the larger ones?'
Regardless, one thing is becoming clear: that sufficient raw materials for building terrestrial planets were available very soon after the Big Bang, raising the possibility that there could be life in the universe far older than we. Perhaps they reside around long-lived red dwarf stars, or have moved on from their home system after their star expired. Or, perhaps, we really are the first, which means that if life has happened just once throughout the entire history of the universe, our existence must be a fluke and our planet very, very special indeed.
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Lattice Structure: Body-Centered Cubic
Lattice Constant: 2.870 Å
Debye Temperature: 460.00 K
Iron is vital to plant and animal life. Iron is the active part of the hemoglobin molecule our bodies use to transport oxygen from the lungs to the rest of the body. Iron metal is widely alloyed with other metals and carbon for a multiple commercial uses. Pig iron is an alloy containing about 3-5% carbon, with varying quantities of Si, S, P, and Mn. Pig iron is brittle, hard, and fairly fusible and is used to produce other iron alloys, including steel. Wrought iron contains only a few tenths of a percent of carbon and is malleable, tough, and less fusible than pig iron. Wrought iron typically has a fibrous structure. Carbon steel is an iron alloy with carbon and small amounts of S, Si, Mn, and P. Alloy steels are carbon steels that contain additives such as chromium, nickel, vanadium, etc. Iron is the least expensive, most abundant, and most used of all metals.
Following pioneering research into the Big Bang and the formation of helium in stars, an unknown process responsible for producing heavier elements found on Earth from hydrogen and helium was suspected to exist. One early attempt at explanation came from Chandrasekhar and Louis R. Henrich who postulated that elements were produced at temperatures between 6×10 9 and 8×10 9 K. Their theory accounted for elements up to chlorine, though there was no explanation for elements of atomic weight heavier than 40 amu at non-negligible abundances.  This became the foundation of a study by Fred Hoyle, who hypothesized that conditions in the core of collapsing stars would enable nucleosynthesis of the remainder of the elements via rapid capture of densely packed free neutrons. However, there remained unanswered questions about equilibrium in stars that was required to balance beta-decays and precisely account for abundances of elements that would be formed in such conditions. 
The need for a physical setting providing rapid neutron capture, which was known to almost certainly have a role in element formation, was also seen in a table of abundances of isotopes of heavy elements by Hans Suess and Harold Urey in 1956.  Their abundance table revealed larger than average abundances of natural isotopes containing magic numbers [b] of neutrons as well as abundance peaks about 10 amu lighter than stable nuclei containing magic numbers of neutrons which were also in abundance, suggesting that radioactive neutron-rich nuclei having the magic neutron numbers but roughly ten fewer protons were formed. These observations also implied that rapid neutron capture occurred faster than beta decay, and the resulting abundance peaks were caused by so-called waiting points at magic numbers.  [c] This process, rapid neutron capture by neutron-rich isotopes, became known as the r-process, whereas the s-process was named for its characteristic slow neutron capture. A table apportioning the heavy isotopes phenomenologically between s-process and r-process isotopes was published in 1957 in the B 2 FH review paper,  which named the r-process and outlined the physics that guides it. Alastair G. W. Cameron also published a smaller study about the r-process in the same year. 
The stationary r-process as described by the B 2 FH paper was first demonstrated in a time-dependent calculation at Caltech by Phillip A. Seeger, William A. Fowler and Donald D. Clayton,  who found that no single temporal snapshot matched the solar r-process abundances, but, that when superposed, did achieve a successful characterization of the r-process abundance distribution. Shorter-time distributions emphasize abundances at atomic weights less than A = 140 , whereas longer-time distributions emphasized those at atomic weights greater than A = 140 .  Subsequent treatments of the r-process reinforced those temporal features. Seeger et al. were also able to construct more quantitative apportionment between s-process and r-process of the abundance table of heavy isotopes, thereby establishing a more reliable abundance curve for the r-process isotopes than B 2 FH had been able to define. Today, the r-process abundances are determined using their technique of subtracting the more reliable s-process isotopic abundances from the total isotopic abundances and attributing the remainder to r-process nucleosynthesis.  That r-process abundance curve (vs. atomic weight) has provided for many decades the target for theoretical computations of abundances synthesized by the physical r-process.
The creation of free neutrons by electron capture during the rapid collapse to high density of a supernova core along with quick assembly of some neutron-rich seed nuclei makes the r-process a primary nucleosynthesis process, meaning a process that can occur even in a star initially of pure H and He, in contrast to the B 2 FH designation as a secondary process building on preexisting iron. Primary stellar nucleosynthesis begins earlier in the galaxy than does secondary nucleosynthesis. Alternatively the high density of neutrons within neutron stars would be available for rapid assembly into r-process nuclei if a collision were to eject portions of a neutron star, which then rapidly expands freed from confinement. That sequence could also begin earlier in galactic time than would s-process nucleosynthesis so each scenario fits the earlier growth of r-process abundances in the galaxy. Each of these scenarios is the subject of active theoretical research. Observational evidence of the early r-process enrichment of interstellar gas and of subsequent newly formed of stars, as applied to the abundance evolution of the galaxy of stars, was first laid out by James W. Truran in 1981.  He and subsequent astronomers showed that the pattern of heavy-element abundances in the earliest metal-poor stars matched that of the shape of the solar r-process curve, as if the s-process component were missing. This was consistent with the hypothesis that the s-process had not yet begun to enrich interstellar gas when these young stars missing the s-process abundances were born from that gas, for it requires about 100 million years of galactic history for the s-process to get started whereas the r-process can begin after two million years. These s-process–poor, r-process–rich stellar compositions must have been born earlier than any s-process, showing that the r-process emerges from quickly evolving massive stars that become supernovae and leave neutron-star remnants that can merge with another neutron star. The primary nature of the early r-process thereby derives from observed abundance spectra in old stars  that had been born early, when the galactic metallicity was still small, but that nonetheless contain their complement of r-process nuclei.
Either interpretation, though generally supported by supernova experts, has yet to achieve a totally satisfactory calculation of r-process abundances because the overall problem is numerically formidable, but existing results are supportive. In 2017, new data about the r-process was discovered when the LIGO and Virgo gravitational-wave observatories discovered a merger of two neutron stars ejecting r-process matter.  See Astrophysical sites below.
Noteworthy is that the r-process is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most neutron-rich isotopes of each heavy element.
There are three candidate sites for r-process nucleosynthesis where the required conditions are thought to exist: low-mass supernovae, Type II supernovae, and neutron star mergers. 
Immediately after the severe compression of electrons in a Type II supernova, beta-minus decay is blocked. This is because the high electron density fills all available free electron states up to a Fermi energy which is greater than the energy of nuclear beta decay. However, nuclear capture of those free electrons still occurs, and causes increasing neutronization of matter. This results in an extremely high density of free neutrons which cannot decay, on the order of 10 24 neutrons per cm 3 ),  and high temperatures. As this re-expands and cools, neutron capture by still-existing heavy nuclei occurs much faster than beta-minus decay. As a consequence, the r-process runs up along the neutron drip line and highly-unstable neutron-rich nuclei are created.
Three processes which affect the climbing of the neutron drip line are a notable decrease in the neutron-capture cross section in nuclei with closed neutron shells, the inhibiting process of photodisintegration, and the degree of nuclear stability in the heavy-isotope region. Neutron captures in r-process nucleosynthesis leads to the formation of neutron-rich, weakly bound nuclei with neutron separation energies as low as 2 MeV.   At this stage, closed neutron shells at N = 50, 82, and 126 are reached, and neutron capture is temporarily paused. These so-called waiting points are characterized by increased binding energy relative to heavier isotopes, leading to low neutron capture cross sections and a buildup of semi-magic nuclei that are more stable toward beta decay.  In addition, nuclei beyond the shell closures are susceptible to quicker beta decay owing to their proximity to the drip line for these nuclei, beta decay occurs before further neutron capture.  Waiting point nuclei are then allowed to beta decay toward stability before further neutron capture can occur,  resulting in a slowdown or freeze-out of the reaction. 
Decreasing nuclear stability terminates the r-process when its heaviest nuclei become unstable to spontaneous fission, when the total number of nucleons approaches 270. The fission barrier may be low enough before 270 such that neutron capture might induce fission instead of continuing up the neutron drip line.  After the neutron flux decreases, these highly unstable radioactive nuclei undergo a rapid succession of beta decays until they reach more stable, neutron-rich nuclei.  While the s-process creates an abundance of stable nuclei having closed neutron shells, the r-process, in neutron-rich predecessor nuclei, creates an abundance of radioactive nuclei about 10 amu below the s-process peaks after their decay back to stability. 
The r-process also occurs in thermonuclear weapons, and was responsible for the initial discovery of neutron-rich almost stable isotopes of actinides like plutonium-244 and the new elements einsteinium and fermium (atomic numbers 99 and 100) in the 1950s. It has been suggested that multiple nuclear explosions would make it possible to reach the island of stability, as the affected nuclides (starting with uranium-238 as seed nuclei) would not have time to beta decay all the way to the quickly spontaneously fissioning nuclides at the line of beta stability before absorbing more neutrons in the next explosion, thus providing a chance to reach neutron-rich superheavy nuclides like copernicium-291 and -293 which should have half-lives of centuries or millennia. 
The most probable candidate site for the r-process has long been suggested to be core-collapse supernovae (spectral types Ib, Ic and II), which may provide the necessary physical conditions for the r-process. However, the very low abundance of r-process nuclei in the interstellar gas limits the amount each can have ejected. It requires either that only a small fraction of supernovae eject r-process nuclei to the interstellar medium, or that each supernova ejects only a very small amount of r-process material. The ejected material must be relatively neutron-rich, a condition which has been difficult to achieve in models,  so that astrophysicists remain uneasy about their adequacy for successful r-process yields.
In 2017, entirely new astronomical data about the r-process was discovered in data about the merger of two neutron stars. Using the gravitational wave data captured in GW170817 to identify the location of the merger, several teams    observed and studied optical data of the merger, finding spectroscopic evidence of r-process material thrown off by the merging neutron stars. The bulk of this material seems to consist of two types: hot blue masses of highly radioactive r-process matter of lower-mass-range heavy nuclei ( A < 140 such as strontium)  and cooler red masses of higher mass-number r-process nuclei ( A > 140 ) rich in actinides (such as uranium, thorium, and californium). When released from the huge internal pressure of the neutron star, these ejecta expand and form seed heavy nuclei that rapidly capture free neutrons, and radiate detected optical light for about a week. Such duration of luminosity would not be possible without heating by internal radioactive decay, which is provided by r-process nuclei near their waiting points. Two distinct mass regions ( A < 140 and A > 140 ) for the r-process yields have been known since the first time dependent calculations of the r-process.  Because of these spectroscopic features it has been argued that such nucleosynthesis in the Milky Way has been primarily ejecta from neutron-star mergers rather than from supernovae. 
These results offer a new possibility for clarifying six decades of uncertainty over the site of origin of r-process nuclei. Confirming relevance to the r-process is that it is radiogenic power from radioactive decay of r-process nuclei that maintains the visibility of these spun off r-process fragments. Otherwise they would dim quickly. Such alternative sites were first seriously proposed in 1974  as decompressing neutron star matter. It was proposed such matter is ejected from neutron stars merging with black holes in compact binaries. In 1989  (and 1999  ) this scenario was extended to binary neutron star mergers (a binary star system of two neutron stars that collide). After preliminary identification of these sites,  the scenario was confirmed in GW170817. Current astrophysical models suggest that a single neutron star merger event may have generated between 3 and 13 Earth masses of gold. 
How Much Would It Cost to Build the Death Star from Star Wars?
Let’s say you’re an evil Imperial overlord in the Star Wars universe, and you want to keep the rebels in line. You should build something they fear, say, a Death Star. How much will advancing your evil plans set you back? Short answer: it’s more expensive than you can possibly imagine.
Even if you can imagine quite a bit, Centives, the economics blog of students of Lehigh University, says it would cost “$852,000,000,000,000,000. Or roughly 13,000 times the world's GDP” to build the Death Star…and that’s just the cost of steel production.
It turns out that it would take more than money to build a fully operational battle station. It would take cold, hard steel, made from hot molten iron. Although the Earth’s crust doesn’t have enough, the Earth’s core, on the other hand, has iron aplenty. In fact, Earth has enough iron at its core to produce 2 billion Death Stars—a veritable Death Galaxy.
But let you think it’s a simple task of Force-Lifting the molten liquid and molding it into shape, consider this:
Firstly, the two billion death stars is mostly from the Earth's core which we would all really rather you didn't remove. And secondly, at today's rate of steel production (1.3 billion tonnes annually), it would take 833,315 years to produce enough steel to begin work. So once someone notices what you're up to, you have to fend them off for 800 millennia before you have a chance to fight back.
So instead of constructing a technological terror or two, it might be better to try actual diplomacy. If nothing else, it's absolutely more cost effective.
How Is Iron Extracted From the Earth?
Iron ores in the form of hematite (ferrous oxide) and magnetite are removed from the earth through mining. The use of heavy mining equipment is necessary to dig out large pits in an area with a large deposit of iron ore however, because iron does not occur naturally, it is necessary to use a blast furnace to separate or refine iron from the other substances in the iron ore.
Magnetite and hematite are iron oxides however, the extraction of iron from an iron oxide involves a series of steps that begins when mined iron ore is crushed into smaller pieces by a crusher and then washed. The second step in the process is calcination or roasting of the ore, which removes certain impurities, carbon dioxide and other substances. Through this process, ferrous oxide also oxidizes to ferric oxide.
The blast furnace reduces iron from the iron oxide, but the complete reduction reaction requires the addition of coke and limestone to the roasted ore. After the completion of different processes in the blast furnace, molten iron flows from the bottom of the blast furnace. This molten iron may be utilized as cast iron.
A high purity iron, such as wrought iron, requires the complete removal of carbon from this iron. Likewise, the steel-making process requires the removal of other impurities like sulfur and phosphorus from molten iron. Many other different types of steel also are fabricated from this molten iron.
The Cooling of a Star
As the hydrogen fuel in a star gets converted to helium, and to some heavier elements, it takes more and more heat to cause the nuclear fusion. The mass of a star plays a role in how long it takes to "burn" through the fuel. More massive stars use their fuel faster because it takes more energy to counteract the larger gravitational force. (Or, put another way, the larger gravitational force causes the atoms to collide together more rapidly.) While our sun will probably last for about 5 thousand million years, more massive stars may last as little as 1 hundred million years before using up their fuel.
As the star's fuel begins to run out, the star begins to generate less heat. Without the heat to counteract the gravitational pull, the star begins to contract.
All is not lost, however! Remember that these atoms are made up of protons, neutrons, and electrons, which are fermions. One of the rules governing fermions is called the Pauli Exclusion Principle, which states that no two fermions can occupy the same "state," which is a fancy way of saying that there can't be more than one identical one in the same place doing the same thing. (Bosons, on the other hand, don't run into this problem, which is part of the reason photon-based lasers work.)
The result of this is that the Pauli Exclusion Principle creates yet another slight repulsive force between electrons, which can help counteract the collapse of a star, turning it into a white dwarf. This was discovered by the Indian physicist Subrahmanyan Chandrasekhar in 1928.
Another type of star, the neutron star, come into being when a star collapses and the neutron-to-neutron repulsion counteracts the gravitational collapse.
However, not all stars become white dwarf stars or even neutron stars. Chandrasekhar realized that some stars would have very different fates.
Metal, Iron, & Nickel
Metal grains reflecting light in a sawn slice of Tafassasset (primitive achondrite). Note the saw marks in the metal grains. Saw marks are a good way to distinguish metal from shiny sulfide minerals like pyrite – sulfide grains will not look so severely scraped.
Some rare meteorites do not contain any appreciable metal and, consequently, they have low concentrations of Ni. Unbrecciated achondrites are poor in metal. In other words, many of the rarest types of meteorites contain little or no metal and have low nickel concentrations, just like Earth rocks.
Iron Meteorites and Pallasites
Iron meteorites, of course, are nearly 100% metal, although many contain the iron sulfide mineral troilite. Pallasites, a rare type of stony-iron meteorite, consist of olivine grains embedded in an iron-nickel metal matrix. Because they contain much iron-nickel metal, all metal-bearing meteorites are attracted to a magnet. The concentration of nickel in iron meteorites and the metallic part of pallasites, typically 5-30%, is much greater than that in industrial metals except for high-nickel steels. The concentration of nickel in industrial iron is usually <1%.
Left: A softball-size piece of the 2-ton Campo del Cielo (IAB) iron meteorite. Right: Sawn, polished, and etched slab of the meteorite. Notice the coarse Widmanstätten pattern. Among collectors, Campo del Cielo is known as a “ruster” because of its tendency to easily rust (right).
Sawn, polished, and etched slab of the Gibeon (IVA) iron meteorite. Gibeon has a finer Widmanstätten pattern than does Campo del Cielo. Widmanstätten patterns only occur on sawn, polished, and etched faces of an iron meteorite. They do not occur in stony meteorites. The linear lamellae are intergrowths of crystals of kamacite and taenite. The crystals are large because they cooled slowly over millions of years in the core of an asteroid. One consequence of this slow cooling is that iron meteorites do not easily bend or break unless they are badly rusted.
Widmanstätten patterns do not occur in stony meteorites. They only seen in iron meteorites that have been cut, polished, and etched.
A sawn, polished, etched slab of the Canyon Diablo iron (IAB) meteorite showing the Widmanstätten pattern and large, round troilite (iron sulfide) inclusions. The meteorite specimen is the property of the Collection of the Arizona State University Center for Meteorite Studies.
Most collectors would agree that the most attractive iron meteorite is Sikhote-Alin (IIAB). At least 23 tons in mass, the meteorite is the largest to fall in historical times. It fell in the Sikhote-Alin mountains of eastern Russia on February 12th, 1947.
Thousands of pieces of Sikhote-Alin have been recovered. This is a collection of Sikhote-Alin “shrapnel.” Sikhote-Alin is not a “ruster.”
An iron meteorite or pallasite that has been buried in the ground for a long time is unspectacular – until someone hits it with a plow. This is the Conception Junction pallasite found by a landowner in Missouri in 2006.
A sawn slice of Conception Junction. It has experienced considerable weathering and rusting, at least near the exterior. The rounded dark material is the silicate mineral olivine. The lighter gray material is iron-nickel metal.
A slice of the Brahin pallasite. The meteorites was found in 1968 by a school girl in the Republic of Belarus. Again, the rounded dark materials are grains of olivine. The lighter gray material is iron-nickel metal.
Gujba, a CB chondrite, fell in Nigeria in 1984. Only about 21 CB chondrites are known. Gujba and some other CB chondrites have rounded metal grains, which are otherwise very rare in meteorites. Like Canyon Diablo (above), it also has rounded blebs of troilite. Notice that this specimen has not been polished thoroughly so the saw marks are still very evident in the metal. Thanks to Karl for loan of the Gujba specimen.
Rounded metal blebs usually mean that the “rock” is a piece of slag. In slags, the metal will be dispersed less evenly than in a meteorite and they are usually contain vesicles (gas bubbles) in the matrix because the matrix was molten. Thanks to Jeff for the sample.
With a few rare and exceptions, naturally occurring terrestrial rock do not contain iron metal or iron-nickel metal. There are two reasons. First, the Earth formed from the same kind of material as the asteroids but early in Earth’s history the iron-nickel metal that it contained sank to form the Earth’s core. Second, any metal that did not sink has oxidized (rusted) over Earth’s long history. The Earth’s environment is far more oxidizing (oxygen atmosphere and water) than space, where meteorites originate. Earth rocks do contain iron and nickel, but only in oxidized (non-metallic) form. Therefore, if you find a rock that contains iron-nickel metal, then it is almost certainly a meteorite.
When someone walked into my office with this chunk of metal, I thought “Whoa, this could be an iron meteorite!” The “hole” at the lower left was suspicious, however, and when we sawed into it (inset, upper right), it was full of vesicles. Iron meteorites are cores of asteroids that cooled from liquids over millions of years. They do not contain vesicles. (The elongated “hole” in Gibeon, above, is not a vesicle, it is a void where a troilite grain was “plucked” during sawing of the meteorite.) Also, when we analyzed this chunk of metal, there was much less that 1% nickel. I do not know how this thing formed, but it is man-made.