ONE MORE CLUE FOR MOON’S ORIGIN
ETH Zurich
Humankind has maintained an enduring fascination with the Moon. It was not until Galileo's time, however, that scientists really began study it. Over the course of nearly five centuries, researchers put forward numerous, much debated theories as to how the Moon was formed. Now, geochemists, cosmochemists, and petrologists at ETH Zurich shed new light on the Moon's origin story. In a study published in the journal, Science Advances, the research team reports findings that show that the Moon inherited the indigenous noble gases of helium and neon from Earth's mantle. The discovery adds to the already strong constraints on the currently favoured "Giant Impact" theory that hypothesizes the Moon was formed by a massive collision between Earth and another celestial body. The team analysed six samples of lunar meteorites from an Antarctic collection, obtained from NASA. The meteorites consist of basalt rock that formed when magma welled up from the interior of the Moon and cooled quickly. They remained covered by additional basalt layers after their formation, which protected the rock from cosmic rays and, particularly, the solar wind. The cooling process resulted in the formation of lunar glass particles amongst the other minerals found in magma. The team discovered that the glass particles retain the chemical fingerprints (isotopic signatures) of the solar gases: helium and neon from the Moon's interior. Their findings strongly support that the Moon inherited noble gases indigenous to the Earth. Without the protection of an atmosphere, asteroids continually pelt the Moon's surface. It likely took a high-energy impact to eject the meteorites from the middle layers of the lava flow similar to the vast plains known as the Lunar Mare. Eventually the rock fragments made their way to Earth in the form of meteorites. Many of these meteorite samples are picked up in the deserts of North Africa or in, in this case, the "cold desert" of Antarctica where they are easier to spot in the landscape.
In the Noble Gas Laboratory at ETH Zurich resides a state-of-the-art noble gas mass spectrometer named, "Tom Dooley. The instrument got its name, when earlier researchers, at one point, suspended the highly sensitive equipment from the ceiling of the lab to avoid interference from the vibrations of everyday life. Using the Tom Dooley instrument, the research team was able to measure sub-millimetre glass particles from the meteorites and rule out solar wind as the source of the detected gases. The helium and neon that they detected were in a much higher abundance than expected. The Tom Dooley is so sensitive that it is, in fact, the only instrument in the world capable of detecting such minimal concentrations of helium and neon. It was used to detect these noble gases in the 7 billion years old grains in the Murchison meteorite -- the oldest known solid matter to-date. Knowing where to look inside NASA's vast collection of some 70,000 approved meteorites represents a major step forward. While such gases are not necessary for life, it would be interesting to know how some of these noble gases survived the brutal and violent formation of the Moon. Such knowledge might help scientists in geochemistry and geophysics to create new models that show more generally how such most volatile elements can survive planet formation, in our solar system and beyond.
BETELGEUSE SLOWLY RECOVERS AFTER BLOWING ITS TOP
NASA/Goddard Space Flight Center
The star Betelgeuse appears as a brilliant, ruby-red, twinkling spot of light in the upper right shoulder of the winter constellation Orion the Hunter. But when viewed close up, astronomers know it as a seething monster with a 400-day-long heartbeat of regular pulsations. This aging star is classified as a supergiant because it has swelled up to an astonishing diameter of approximately 1 billion miles. If placed at the centre of our solar system it would reach out to the orbit of Jupiter. The star's ultimate fate is to explode as a supernova. When that eventually happens it will be briefly visible in the daytime sky from Earth. But there are a lot of fireworks going on now before the final detonation. Astronomers using Hubble and other telescopes have deduced that the star blew off a huge piece of its visible surface in 2019. This has never before been seen on a star. Our petulant Sun routinely goes through mass ejections of its outer atmosphere, the corona. But those events are orders of magnitude weaker than what was seen on Betelgeuse. The first clue came when the star mysteriously darkened in late 2019. An immense cloud of obscuring dust formed from the ejected surface as it cooled. Astronomers have now pieced together a scenario for the upheaval. And the star is still slowly recovering; the photosphere is rebuilding itself. And the interior is reverberating like a bell that has been hit with a sledgehammer, disrupting the star's normal cycle. This doesn't mean the monster star is going to explode any time soon, but the late-life convulsions may continue to amaze astronomers.
Analyzing data from NASA's Hubble Space Telescope and several other observatories, astronomers have concluded that the bright red supergiant star Betelgeuse quite literally blew its top in 2019, losing a substantial part of its visible surface and producing a gigantic Surface Mass Ejection (SME). This is something never before seen in a normal star's behaviour. Our Sun routinely blows off parts of its tenuous outer atmosphere, the corona, in an event known as a Coronal Mass Ejection (CME). But the Betelgeuse SME blasted off 400 billion times as much mass as a typical CME! The monster star is still slowly recovering from this catastrophic upheaval. These new observations yield clues as to how red stars lose mass late in their lives as their nuclear fusion furnaces burn out, before exploding as supernovae. The amount of mass loss significantly affects their fate. However, Betelgeuse's surprisingly petulant behavior is not evidence the star is about to blow up anytime soon. So the mass loss event is not necessarily the signal of an imminent explosion. The titanic outburst in 2019 was possibly caused by a convective plume, more than a million miles across, bubbling up from deep inside the star. It produced shocks and pulsations that blasted off the chunk of the photosphere leaving the star with a large cool surface area under the dust cloud that was produced by the cooling piece of photosphere. Betelgeuse is now struggling to recover from this injury. Weighing roughly several times as much as our Moon, the fractured piece of photosphere sped off into space and cooled to form a dust cloud that blocked light from the star as seen by Earth observers. The dimming, which began in late 2019 and lasted for a few months, was easily noticeable even by backyard observers watching the star change brightness. One of the brightest stars in the sky, Betelgeuse is easily found in the right shoulder of the constellation Orion. Even more fantastic, the supergiant's 400-day pulsation rate is now gone, perhaps at least temporarily. For almost 200 years astronomers have measured this rhythm as evident in changes in Betelgeuse's brightness variations and surface motions. Its disruption attests to the ferocity of the blowout. The star's interior convection cells, which drive the regular pulsation may be sloshing around like an imbalanced washing machine tub. TRES and Hubble spectra imply that the outer layers may be back to normal, but the surface is still bouncing like a plate of jelly as the photosphere rebuilds itself. Though our Sun has coronal mass ejections that blow off small pieces of the outer atmosphere, astronomers have never witnessed such a large amount of a star's visible surface get blasted into space. Therefore, surface mass ejections and coronal mass ejections may be different events.
STARS DETERMINE THEIR OWN MASSES
Northwestern University
Last year, a team of astrophysicists launched STARFORGE, a project that produces the most realistic, highest-resolution 3D simulations of star formation to date. Now, the scientists have used the highly detailed simulations to uncover what determines the masses of stars, a mystery that has captivated astrophysicists for decades. In a new study, the team discovered that star formation is a self-regulatory process. In other words, stars themselves set their own masses. This helps explain why stars formed in disparate environments still have similar masses. The new finding may enable researchers to better understand star formation within our own Milky Way and other galaxies. Understanding the stellar initial mass function is such an important problem because it impacts astrophysics across the board -- from nearby planets to distant galaxies. This is because stars have relatively simple DNA. If you know the mass of a star, then you know most things about the star: how much light it emits, how long it will live and what will happen to it when it dies. The distribution of stellar masses is thus critical for whether planets that orbit stars can potentially sustain life, as well as what distant galaxies look like. Outer space is filled with giant clouds, consisting of cold gas and dust. Slowly, gravity pulls far-flung specks of this gas and dust toward each other to form dense clumps. Materials in these clumps fall inward, crashing and sparking heat to create a newborn star. Surrounding each of these "protostars" is a rotating disk of gas and dust. Every planet in our solar system was once specks in such a disk around our newborn sun. Whether or not planets orbiting a star could host life is dependent on the mass of the star and how it formed. Therefore, understanding star formation is crucial to determining where life can form in the Universe.
Every subfield in astronomy depends on the mass distribution of stars -- or initial mass function (IMF) -- which has proved challenging for scientists to model correctly. Stars much bigger than our Sun are rare, making up only 1% of newborn stars. And, for every one of these stars there are up to 10 Sun-like stars and 30 dwarf stars. Observations found that no matter where we look in the Milky Way these ratios (i.e., the IMF) are the same, for both newly formed star clusters and for those that are billions of years old. This is the mystery of the IMF. Every population of stars in our galaxy, and in all the dwarf galaxies that surround us, has this same balance -- even though their stars were born under wildly different conditions over billions of years. In theory, the IMF should vary dramatically, but it is virtually universal, which has puzzled astronomers for decades. The new simulations, however, showed that stellar feedback, in an effort to oppose gravity, pushes stellar masses toward the same mass distribution. These simulations are the first to follow the formation of individual stars in a collapsing giant cloud, while also capturing how these newly formed stars interact with their surroundings by giving off light and shedding mass via jets and winds -- a phenomenon referred to as "stellar feedback."
STAR WRECK IS SOURCE OF EXTREME COSMIC PARTICLES
NASA/Goddard Space Flight Center
Astronomers have long sought the launch sites for some of the highest-energy protons in our galaxy. Now a study using 12 years of data from NASA's Fermi Gamma-ray Space Telescope confirms that one supernova remnant is just such a place. Fermi has shown that the shock waves of exploded stars boost particles to speeds comparable to that of light. Called cosmic rays, these particles mostly take the form of protons, but can include atomic nuclei and electrons. Because they all carry an electric charge, their paths become scrambled as they whisk through our galaxy's magnetic field. Since we can no longer tell which direction they originated from, this masks their birthplace. But when these particles collide with interstellar gas near the supernova remnant, they produce a tell-tale glow in gamma rays -- the highest-energy light there is. Trapped by chaotic magnetic fields, the particles repeatedly cross the supernova's shock wave, gaining speed and energy with each passage. Eventually, the remnant can no longer hold them, and they zip off into interstellar space. Boosted to some 10 times the energy mustered by the world's most powerful particle accelerator, the Large Hadron Collider, PeV protons are on the cusp of escaping our galaxy altogether. Astronomers have identified a few suspected PeVatrons, including one at the centre of our galaxy. Naturally, supernova remnants top the list of candidates. Yet out of about 300 known remnants, only a few have been found to emit gamma rays with sufficiently high energies.
One particular star wreck has commanded a lot of attention from gamma-ray astronomers. Called G106.3+2.7, it's a comet-shaped cloud located about 2,600 light-years away in the constellation Cepheus. A bright pulsar caps the northern end of the supernova remnant, and astronomers think both objects formed in the same explosion. Fermi's Large Area Telescope, its primary instrument, detected billion-electron-volt (GeV) gamma rays from within the remnant's extended tail. (For comparison, visible light's energy measures between about 2 and 3 electron volts.) The Very Energetic Radiation Imaging Telescope Array System (VERITAS) at the Fred Lawrence Whipple Observatory in southern Arizona recorded even higher-energy gamma rays from the same region. And both the High-Altitude Water Cherenkov Gamma-Ray Observatory in Mexico and the Tibet AS-Gamma Experiment in China have detected photons with energies of 100 trillion electron volts (TeV) from the area probed by Fermi and VERITAS. This object has been a source of considerable interest for a while now, but to crown it as a PeVatron, astronomers have to prove it's accelerating protons. The catch is that electrons accelerated to a few hundred TeV can produce the same emission. Now, with the help of 12 years of Fermi data, we think we've made the case that G106.3+2.7 is indeed a PeVatron. The pulsar, J2229+6114, emits its own gamma rays in a lighthouse-like beacon as it spins, and this glow dominates the region to energies of a few GeV. Most of this emission occurs in the first half of the pulsar's rotation. The team effectively turned off the pulsar by analyzing only gamma rays arriving from the latter part of the cycle. Below 10 GeV, there is no significant emission from the remnant's tail.Above this energy, the pulsar's interference is negligible and the additional source becomes readily apparent. The team's detailed analysis overwhelmingly favours PeV protons as the particles driving this gamma-ray emission. So far, G106.3+2.7 is unique, but it may turn out to be the brightest member of a new population of supernova remnants that emit gamma rays reaching TeV energies. More of them may be revealed through future observations by Fermi and very-high-energy gamma-ray observatories.
FIRST STARS AND BLACK HOLES
University of Texas at Austin, Texas Advanced Computing Center
Just milliseconds after the Big Bang, chaos reigned. Atomic nuclei fused and broke apart in hot, frenzied motion. Incredibly strong pressure waves built up and squeezed matter so tightly together that black holes formed, which astrophysicists call primordial black holes. Did primordial black holes help or hinder formation of the Universe's first stars, eventually born about 100 million years later? Supercomputer simulations helped investigate this cosmic question, thanks to simulations on the Stampede2 supercomputer. In the early Universe, the standard model of astrophysics holds that black holes seeded the formation of halo-like structures by virtue of their gravitational pull, analogous to how clouds form by being seeded by dust particles. This is a plus for star formation, where these structures served as scaffolding that helped matter coalesce into the first stars and galaxies. However, a black hole also causes heating by gas or debris falling into it. This forms a hot accretion disk around the black hole, which emits energetic photons that ionize and heat the surrounding gas. And that's a minus for star formation, as gas needs to cool down to be able to condense to high enough density that a nuclear reaction is triggered, setting the star ablaze. Depending on which effect wins over the other, star formation can be accelerated, delayed or prevented by primordial black holes. Regarding the importance of primordial black holes, the research also implied that they interact with the first stars and produce gravitational waves. "They may also be able to trigger the formation of supermassive black holes. These aspects will be investigated in follow-up studies. For the study, astronomers used cosmological hydrodynamic zoom-in simulations as their tool for state-of-the-art numerical schemes of the gravity hydrodynamics, chemistry and cooling in structure formation and early star formation. They then added a sub-grid model for black hole accretion and feedback.
The model calculates at each timestep how a black hole accretes gas and also how it heats its surroundings. The simulations use data from the universe's initial conditions to high precision based on observations of the cosmic microwave background. Simulation boxes are then set up that follow the cosmic evolution timestep by timestep. But the challenges in computational simulation of structure formation lie in the way large scales of the Universe -- millions to billions of light years and billions of years -- mesh with the atomic scales where stellar chemistry happens. The structures that emerged from the Big Bang were driven by the dynamical importance of dark matter. The clues of this hypothetical yet unobservable substance are undeniable, seen in the impossible rotational speeds of galaxies. The mass of all the stars and planets in galaxies like our Milky Way do not have enough gravity to keep them from flying apart. The 'x-factor' is called dark matter, yet laboratories have not yet directly detected it. However, gravitational waves have been detected, first by LIGO in 2015. Supercomputers are enabling unprecedented new insights into how the universe works. The universe provides us with extreme environments that are extremely challenging to understand. This also gives motivation to build ever-more-powerful computation architectures and devise better algorithmic structures. There's great beauty and power to the benefit of everyone.
Topic: Early September Astronomy Bulletin (Read 385 times)