# Formation of the First Stars

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I've got a few questions about the first stars to form in the universe. First off how might metalicity have impacted the formation of the first stars and also what effect would the absence of metals impacted the evolution of these stars?

Why were these stars able to be so massive? I've heard that metallicity impacts opacity so would this maybe inhibit mass loss from radiation pressure?

Would the Eddington limit come into play with stars these large?

## Light From Universe's First Stars Seen

Astronomers have spotted light from the very first stars in the universe, which are almost as old as time itself.

Shortly after the Big Bang 13.7 billion years ago, the universe cooled enough to let atoms form, which eventually clumped together to create the first stars. Ever since these stars ignited, their light has been filling the universe, creating a pervasive glow throughout space that each successive generation of stars adds to.

Now, astronomers have detected this glow &mdash called the extragalactic background light, or EBL &mdash and have separated out the light from later stars, isolating the contribution from the first stars that ever existed.

"The EBL is the ensemble of photons generated by all the stars and also all the black holes in the universe," said astrophysicist Marco Ajello of the SLAC National Accelerator Laboratory in California, who led the research. "The EBL also includes the light of the first massive stars that ever shone. We have a fairly good knowledge of the light emitted by 'normal' stars. Thus, by measuring the EBL we are able to constrain the light of the first stars."

Ajello and his team did not measure the EBL directly, but they detected it by analyzing measurements of distant black holes made by NASA's Fermi Gamma-Ray Space Telescope. Fermi studied light from objects called blazars, which are giant black holes that release copious amounts of light while gobbling up large meals of matter.

"We use [blazars] as cosmic lighthouses," Ajello said. "We observe their dimming due to the EBL 'fog'. This allows us to quantify how much EBL there is between us and the blazars. As blazars are distributed across the universe, we can measure the EBL at different epochs."

The study was able to probe light emitted by stars that existed when the universe was just 0.6 billion years old or so &mdash relatively an infant.

These first stars are thought to have been quite different from stars that form today. In general, they were much more massive, containing up to hundreds of times the mass of our sun, and burned hotter, brighter, and for shorter lifetimes than stars today. [Gallery: History & Structure of the Universe]

The new measurements should help astronomers answer some of their most basic questions about the first generations of stars, such as how quickly they formed, and how soon after the birth of the universe the first stars came to be, researchers said.

"We really need to understand this period," said Volker Bromm, an astronomer at the University of Texas at Austin who did not participate in the research, during a NASA press conference announcing the results. "At this point we have theoretical models, but we need to test them and constrain them."

"With Fermi we have the first step into this cosmic frontier," Bromm added.

Already, the scientists have found that the first stars' peak formation rate must have been lower than previously thought.

Ultimately, the researchers would like to constrain this parameter further, and eventually to glimpse these ancient stars themselves. Future technology, such as NASA's successor to the Hubble Space Telescope, called the James Webb Space Telescope (expected to come online by 2018), should come closer to doing the job.

"Detecting these stars is very important, but currently impossible," Ajello said. "The Webb Telescope in a few years might be able to see the first galaxies (not the first stars though). In this way we are already able to set constraints on the amount and role of these stars in the early universe."

## First-Generation Stars Formed More Quickly than Previously Thought

An international team of astronomers has discovered a massive cloud of gas that formed just 850 million years after the Big Bang. The chemical composition of the object reveals that the first generation of stars formed quickly and enriched the Universe with the elements they synthesized.

The newly-discovered gas cloud formed just 850 million years after the Big Bang. Image credit: Max Planck Society.

The Big Bang started the Universe as a hot, murky soup of extremely energetic particles that was rapidly expanding. As this material spread out, it cooled, and the particles coalesced into neutral hydrogen gas.

The Universe stayed dark, without any luminous sources, until gravity condensed matter into the first stars and galaxies.

All stars, including this first generation, act as chemical factories, synthesizing almost all of the elements that make up the world around us.

When the original stars exploded as supernovae, they spewed out the elements that they created, seeding the surrounding gas.

Subsequent generations of stars incorporated these elements and steadily increased the chemical abundances of their surroundings.

But the first stars formed in a still pristine, cold Universe. Consequently these initial stars produced elements in different proportions than those synthesized by younger stars, which were formed in an environment that was already enriched by earlier generations.

“Looking back in time far enough, one may expect cosmic gas clouds to show the tell-tale signature of the peculiar element ratios made by the first stars,” said Dr. Michael Rauch, an astronomer in the Observatories of the Carnegie Institution for Science.

“Peering even further back, we may ultimately witness the disappearance of most elements and the emergence of pristine gas.”

Astronomers have long used quasars to learn about the chemical composition of cosmic gas over time, showing how different generations of stars enrich their surroundings.

“We found this ancient gas cloud when following up on an inventory of very distant quasars using the Magellan telescopes at Carnegie’s Las Campanas Observatory in Chile,” said Dr. Eduardo Bañados, from the Observatories of the Carnegie Institution for Science and the Max-Planck-Institut für Astronomie.

Because the gas cloud exists between a quasar called P183+05 and us on Earth, the quasar’s incredibly bright light must pass through it to get to us and astronomers can take advantage of this to understand the cloud’s chemistry.

The team found that the cloud’s chemical makeup was quite modern, and not as primitive as expected if dominated by the first stars.

Although it formed only 850 million years after the Big Bang, its chemical abundances were already as high as those typically seen in cosmic gas clouds that were formed several billion years later.

“Apparently, the first generation of stars had already expired by the time the cloud formed,” Dr. Rauch said.

“This shows that the Universe was rapidly swamped by the chemical products of later generations of stars, even before most of the present-day galaxies were in place.”

The findings were published in the Astrophysical Journal.

Eduardo Bañados et al. 2019. A Metal-poor Damped Lyα System at Redshift 6.4. ApJ 885, 59 doi: 10.3847/1538-4357/ab4129

## An Extreme Simulation of the Universe’s First Stars

For astronomers, astrophysicists, and cosmologists, the ability to spot the first stars that formed in our Universe has always been just beyond reach. On the one hand, there are the limits of our current telescopes and observatories, which can only see so far. The farthest object ever observed was MACS 1149-JD, a galaxy located 13.2 billion light-years from Earth that was spotted in the Hubble eXtreme Deep Field (XDF) image.

On the other, up until about 1 billion years after the Big Bang, the Universe was experiencing what cosmologists refer to as the “Dark Ages” when the Universe was filled with gas clouds that obscured visible and infrared light. Luckily, a team of researchers from Georgia Tech’s Center for Relativistic Astrophysics recently conducted simulations that show what the formation of the first stars looked like.

The study that describes their findings, published in the Monthly Notices of the Royal Astronomical Society, was led by Gen Chiaki and John Wise – a post-doctoral researcher and associate professor from the CfRA (respectively). They were joined by researchers from the Sapienza Università di Roma, the Astronomical Observatory of Rome, the Istituto Nazionale di Astrofisica (INAF), and the Istituto Nazionale di Fisica Nucleare (INFN).

The Hubble Extreme Deep Fields (XDF) image. Credit: NASA/ESA/UCSC/Leiden University/ HUDF09 Team

Based on the life and death cycles of stars, astrophysicists theorize that the first stars in the Universe were very metal-poor. Having formed about 100 million years after the Big Bang, these stars formed from a primordial soup of hydrogen gas, helium, and trace amounts of light metals. These gases would collapse to form stars that were up to 1,000 times more massive than our Sun.

Because of their size, these stars were shortlived and probably only existed for a few million years. In that time, they new and heavier elements in their nuclear furnaces, which were then dispersed once the stars collapsed and exploded in supernovae. As a result, the next generation of stars would heavier elements contain carbon, leading to the designation of Carbon-Enhanced Metal-Poor (CEMP) stars.

The composition of these stars, which may be visible to astronomers today, is the result of the nucleosynthesis (fusion) of heavier elements from the first generation of stars. By studying the mechanism behind the formation of these metal-poor stars, scientists can infer what was happening during the cosmic “Dark Ages” when the first stars formed. As Wise said in a Texas Advanced Computer Center (TACC) press release:

“We can’t see the very first generations of stars. 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.”

“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.”

Density, temperature, and carbon abundance (top) and the formation cycle of Pop III stars (bottom). Credit: Chiaki, et al.. Credit: Chiaki et al.

For the sake of their simulations, the team relied predominantly on the Georgia Tech PACE cluster. Additional time was allocated by the National Science Foundation’s (NSF) Extreme Science and Engineering Discovery Environment (XSEDE), the Stampede2 supercomputer at TACC and NSF-funded Frontera system (the fastest academic supercomputer in the world), and the Comet cluster at the San Diego Supercomputer Center (SDSC).

With the massive amounts of processing power and data storage these clusters provided, the team was able to model the faint supernova of the first stars in the Universe. What this revealed was that the metal-poor stars that formed after the first stars in the Universe became carbon-enhanced through the mixing and fallback of bits ejected from the first supernovae.

Their simulations also showed the gas clouds produced by the first supernovae were seeding with carbonaceous grains, leading to the formation of low-mass ‘giga-metal-poor’ stars that likely still exist today (and could be studied by future surveys). Said Chiaki of these stars:

“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.”

A new study looked at 52 submillimeter galaxies to help us understand the early ages of our Universe. Credit: University of Nottingham/Omar Almaini

These investigations are part of a growing field known as “galactic archaeology.” Much like how archaeologists rely on fossilized remains and artifacts to learn more about societies that disappeared centuries or millennia ago, astronomers look for ancient stars to study in order to learn more about those that have long since died.

According to Chiaki, the next step is to branch out beyond the carbon features of ancient stars and incorporate other heavier elements into larger simulations. In so doing, galactic archaeologists hope to learn more about the origins and distribution of life in our Universe. 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.”

## Formation of the First Stars - Astronomy

We describe results from a fully self-consistent three-dimensional hydrodynamical simulation of the formation of one of the first stars in the Universe. In current models of structure formation, dark matter initially dominates, and pregalactic objects form because of gravitational instability from small initial density perturbations. As they assemble via hierarchical merging, primordial gas cools through ro-vibrational lines of hydrogen molecules and sinks to the center of the dark matter potential well. The high-redshift analog of a molecular cloud is formed. As the dense, central parts of the cold gas cloud become self-gravitating, a dense core of

100 M solar (where M solar is the mass of the Sun) undergoes rapid contraction. At particle number densities greater than 10 9 per cubic centimeter, a 1 M solar protostellar core becomes fully molecular as a result of three-body H 2 formation. Contrary to analytical expectations, this process does not lead to renewed fragmentation and only one star is formed. The calculation is stopped when optical depth effects become important, leaving the final mass of the fully formed star somewhat uncertain. At this stage the protostar is accreting material very rapidly (

10 -2 M solar year -1 ). Radiative feedback from the star will not only halt its growth but also inhibit the formation of other stars in the same pregalactic object (at least until the first star ends its life, presumably as a supernova). We conclude that at most one massive (M >> 1 M solar ) metal-free star forms per pregalactic halo, consistent with recent abundance measurements of metal-poor galactic halo stars.

## Astronomers Estimate the Time Scale for the Formation of Stars

Map of the galactic landscape: ATLASGAL covers approximately two thirds of the whole region of the Milky Way within 50,000 light years of the galactic center. The image shows the area covered by ATLASGAL in the region between the two huge molecular cloud complexes W33 and M17 in the Sagittarius constellation. The two enlarged sections show radiation in the medium infrared range of the GLIMPSE survey with the Spitzer telescope in blue and green, and radiation in the submillimeter range from ATLASGAL in red, with contour lines added. The left detail shows a cold, massive clump in which no star has yet formed, the right detail shows a young massive star. At the bottom right is a schematic image of the galaxy with the “Solar Circle” (green circle) and the whole region covered by ATLASGAL within the Milky Way (shaded area). Credit: ATLASGAL-Team

Using a map of the galactic landscape that covers approximately two thirds of the whole region of the Milky Way, astronomers estimate the time scale for the formation of stars.

Astronomers have undertaken a new survey of the plane of our Milky Way and discovered a large quantity of cold, dense clumps of gas and dust, apparently the cradles of massive stars. A team headed by Timea Csengeri from the Max Planck Institute for Radio Astronomy in Bonn has now used the map, which was obtained by the APEX telescope at a wavelength of 0.87 millimeters, to estimate the time scale for the formation of stars. The result: the process seems to proceed very rapidly, with massive stars taking only 75,000 years to form on average, a significantly shorter time than less massive stars.

Stars with a much larger mass than our Sun finish their fast and furious lives with huge supernova explosions, thereby producing the heavy elements in the universe. Before they do this they emit powerful stellar winds and high-energy radiation. Consequently, the stars not only impact on their local environment, but also on the appearance and future evolution of the galaxy as a whole.

These stars form in the coldest regions of the Milky Way – deep inside clouds of dust which are so dense that they almost completely swallow up the radiation of the young stars hidden inside. It is here that a new generation of massive stars forms, embedded in the dense gas and dust clouds. Observations at wavelengths longer than those in the visible or infrared range are necessary if researchers want to follow the earliest stages of the birth.

This is where the 12-metre APEX telescope, which operates in the submillimeter range, comes into play. A team of astronomers has used the telescope in conjunction with the LABOCA camera built at the Max Planck Institute for Radio Astronomy to track down the cradles of the most massive stars. APEX stands on the Chajnantor Plain in the Chilean Atacama Desert at an altitude of 5,100 meters, one of the few locations on Earth which allows observations at submillimeter wavelengths at all.

ATLASGAL (APEX Telescope Large Area Survey of the Galaxy) covers a region of more than 420 square degrees in the galactic plane and thus 97 percent of the inner Milky Way within the so-called Solar Circle. This contains large regions of all four spiral arms and around two thirds of the complete molecular portion of the galaxy. The data cover most of the nurseries of massive stars. And it is also to help the researchers to produce a three-dimensional map.

The sky over APEX: The image shows the southern region of the Milky Way with the stars Alpha and Beta Centauri, the Crux, and the region around Eta Carinae (bright reddish nebula above left of the image center). The ATLASGAL survey with the APEX telescope covers the galactic plane up to the Carina region. Credit: ESO/Y. Beletsky (sky photo) / ESO (APEX telescope) / image collation by C. Urquhart

“Our team has used the ATLASGAL data to generate the most comprehensive sample of the previously hidden birthplaces of massive stars,” says Timea Csengeri from the Max Planck Institute for Radio Astronomy, the lead author of the study. “We found a large number of new potential locations where these stars are currently forming in our Milky Way.”

This comprehensive statistical record has enabled the researchers to show that the processes which give rise to the cold, dense clouds in which the most massive stars are born must proceed extremely rapidly – on a time scale of only 75,000 years. This is significantly shorter than the corresponding time scales for the formation of lower mass stars such as our Sun.

“Researchers already knew about the fast and furious life of the most massive stars in our Milky Way. But now we have been able to show that it is also accompanied by a correspondingly short formation period in their birth cocoons,” says Max Planck astronomer James Urquhart. In fact, less massive, Sun-like stars live around 1,000 times longer than massive ones. And the new results show that the massive stars also form on a very short time scale in a star formation process which is much more dynamic.

According to Friedrich Wyrowski, ATLASGAL also provides charts with data for the most extreme dust clouds: “The star formation processes taking place inside these dust clouds can then be investigated with the aid of the new ALMA telescope network at much higher angular resolution,” is what the APEX project scientist at the Max Planck Institute for Radio Astronomy has to say about future projects.

Publication: T. Csengeri, et al., “The ATLASGAL survey: a catalog of dust condensations in the Galactic plane,” A&A 565, A75 (2014) doi:10.1051/0004-6361/201322434

Images: ATLASGAL-Team ESO/Y. Beletsky (sky photo) / ESO (APEX telescope) / image collation by C. Urquhart

## Cosmic Ray Propagation can affect Star Formation in Galaxies

Cosmic rays play a significant role in controlling thermal balance in thick molecular clouds, where the majority of stars form, and may also play a role in driving galactic winds, controlling star formation, and even determining the character of the intergalactic medium.

Because of the realization that cosmic rays can efficiently accelerate galactic winds, the role of cosmic rays generated by supernovae and young stars has only recently begun to receive significant attention in studies of galaxy formation and evolution. The efficiency of cosmic-ray wind driving is determined by microscopic cosmic-ray transport processes.

Young massive stars in galaxies that inject energy and momentum into the interstellar medium control the triggering and quenching of star formation. Feedback from supermassive black holes at galaxies’ nuclei is also important. Massive gas outflows observed in galaxies, for example, are driven by these processes. However, the specifics, such as how they work and the relative roles of the various feedback processes, are hotly debated.

Cosmic rays in particular are accelerated in strong shocks formed by supernova explosions and stellar winds and generate considerable pressure in the interstellar medium.

Cosmic rays, in particular, are accelerated by supernova explosions and stellar winds (both aspects of star formation) and generate significant pressure in the interstellar medium. They are crucial in regulating thermal balance in dense molecular clouds, where most stars form and they may also play a role in regulating star formation, driving galactic winds, and even determining the character of the intergalactic medium.

Astronomers believe that the ability of cosmic rays to propagate out of the sites where they are produced into the interstellar medium and beyond the disk is a key property limiting their influence, but the details are not well understood.

Vadim Semenov of CfA and two collaborators used computer simulations to investigate how such a variation in cosmic ray propagation can affect star formation in galaxies, which was motivated by recent observations of gamma-ray emission from nearby cosmic ray sources such as star clusters and supernova remnants.

Cosmic ray influences on star formation in galaxies

The observations investigate the propagation of cosmic rays because cosmic rays are thought to produce a significant fraction of gamma-ray emission when they interact with interstellar gas. The observed gamma-ray fluxes imply that cosmic ray propagation near such sources can be locally suppressed by a significant factor, potentially by several orders of magnitude. According to theoretical studies, such suppression can be caused by nonlinear interactions of cosmic rays with magnetic fields and turbulence.

Although it is widely assumed that Galactic cosmic rays (protons and nuclei) are primarily accelerated by the winds and supernovae of massive stars, definitive evidence of this origin remains elusive nearly a century after their discovery. Starburst galaxies have exceptionally high rates of star formation, and their large size, more than 50 times the diameter of comparable Galactic regions, allows for reliable calorimetric measurements of their potentially high cosmic-ray density.

Cosmic rays emitted during the formation, life, and death of their massive stars are expected to eventually produce diffuse gamma-ray emission through interactions with interstellar gas and radiation.

Active star formation powers the emission of radiation in the nuclear regions of starburst galaxies, both directly by supernova (SN) explosions and indirectly by SN shock heating of interstellar gas and dust, as well as radiative processes involving SN shock-accelerated electrons and protons.

The simulations were used by the scientists to investigate the effects of suppressing the transport of cosmic rays near the sources. They discover that suppression causes a local pressure buildup and strong pressure gradients, which prevent the formation of massive clumps of molecular gas that form new stars, qualitatively changing the global distribution of star formation, particularly in massive, gas-rich galaxies prone to clump formation. They conclude that the cosmic-ray effect regulates the development of the structure of the galaxy’s disk and is an important complement to the other processes involved in the galaxy’s formation.

## Study of young, chaotic star system reveals planet formation secrets

Using gas velocity data, scientists observing Elias 2-27 were able to directly measure the mass of the young staru2019s protoplanetary disk and also trace dynamical perturbations in the star system. Visible in this paneled composite are the dust continuum 0.87mm emission data (blue), along with emissions from gases C18O (yellow) and 13CO (red). Credit: ALMA (ESO/NAOJ/NRAO)/T. Paneque-Carreño (Universidad de Chile), B. Saxton (NRAO)

A team of scientists using the Atacama Large Millimeter/submillimeter Array (ALMA) to study the young star Elias 2-27 have confirmed that gravitational instabilities play a key role in planet formation, and have for the first time directly measured the mass of protoplanetary disks using gas velocity data, potentially unlocking one of the mysteries of planet formation. The results of the research are published today in two papers in The Astrophysical Journal.

Protoplanetary disks—planet-forming disks made of gas and dust that surround newly formed young stars—are known to scientists as the birthplace of planets. The exact process of planet formation, however, has remained a mystery. The new research, led by Teresa Paneque-Carreño—a recent graduate of the Universidad de Chile and Ph.D. student at the University of Leiden and the European Southern Observatory, and the primary author on the first of the two papers—focuses on unlocking the mystery of planet formation.

During observations, scientists confirmed that the Elias 2-27 star system—a young star located less than 400 light-years away from Earth in the constellation Ophiuchus—was exhibiting evidence of gravitational instabilities which occur when planet-forming disks carry a large fraction of the system's stellar mass. "How exactly planets form is one of the main questions in our field. However, there are some key mechanisms that we believe can accelerate the process of planet formation," said Paneque-Carreño. "We found direct evidence for gravitational instabilities in Elias 2-27, which is very exciting because this is the first time that we can show kinematic and multi-wavelength proof of a system being gravitationally unstable. Elias 2-27 is the first system that checks all of the boxes."

Elias 2-27's unique characteristics have made it popular with ALMA scientists for more than half a decade. In 2016, a team of scientists using ALMA discovered a pinwheel of dust swirling around the young star. The spirals were believed to be the result of density waves, commonly known to produce the recognizable arms of spiral galaxies—like the Milky Way Galaxy—but at the time, had never before been seen around individual stars.

Elias 2-27 is a young star located just 378 light-years from Earth. The star is host to a massive protoplanetary disk of gas and dust, one of the key elements to planet formation. In this graphic illustration, dust is distributed along a spiral-shaped morphology first discovered in Elias 2-27 in 2016. The larger dust grains are found along the spiral arms while the smaller dust grains are distributed all around the protoplanetary disk. Asymmetric inflows of gas were also detected during the study, indicating that there may still be material infalling into the disk. Scientists believe that Elias 2-27 may eventually evolve into a planetary system, with gravitational instabilities causing the formation of giant planets. Because this process takes millions of years to occur, scientists can only observe the beginning stages. Credit: B. Saxton NRAO/AUI/NSF

"We discovered in 2016 that the Elias 2-27 disk had a different structure from other already studied systems, something not observed in a protoplanetary disk before: two large-scale spiral arms. Gravitational instabilities were a strong possibility, but the origin of these structures remained a mystery and we needed further observations," said Laura Pérez, Assistant Professor at the Universidad de Chile and the principal investigator on the 2016 study. Together with collaborators, she proposed further observations in multiple ALMA bands that were analyzed with Paneque-Carreño as a part of her M.Sc. thesis at Universidad de Chile.

In addition to confirming gravitational instabilities, scientists found perturbations—or disturbances—in the star system above and beyond theoretical expectations. "There may still be new material from the surrounding molecular cloud falling onto the disk, which makes everything more chaotic," said Paneque-Carreño, adding that this chaos has contributed to interesting phenomena that have never been observed before, and for which scientists have no clear explanation. "The Elias 2-27 star system is highly asymmetric in the gas structure. This was completely unexpected, and it is the first time we've observed such vertical asymmetry in a protoplanetary disk."

Cassandra Hall, Assistant Professor of Computational Astrophysics at the University of Georgia, and a co-author on the research, added that the confirmation of both vertical asymmetry and velocity perturbations—the first large-scale perturbations linked to spiral structure in a protoplanetary disk—could have significant implications for planet formation theory. "This could be a 'smoking gun' of gravitational instability, which may accelerate some of the earliest stages of planet formation. We first predicted this signature in 2020, and from a computational astrophysics point of view, it's exciting to be right."

Paneque-Carreño added that while the new research has confirmed some theories, it has also raised new questions. "While gravitational instabilities can now be confirmed to explain the spiral structures in the dust continuum surrounding the star, there is also an inner gap, or missing material in the disk, for which we do not have a clear explanation."

One of the barriers to understanding planet formation was the lack of direct measurement of the mass of planet-forming disks, a problem addressed in the new research. The high sensitivity of ALMA Band 6, paired with Bands 3 and 7, allowed the team to more closely study the dynamical processes, density, and even the mass of the disk. "Previous measurements of protoplanetary disk mass were indirect and based only on dust or rare isotopologues. With this new study, we are now sensitive to the entire mass of the disk," said Benedetta Veronesi—a graduate student at the University of Milan and postdoctoral researcher at École normale supérieure de Lyon, and the lead author on the second paper. "This finding lays the foundation for the development of a method to measure disk mass that will allow us to break down one of the biggest and most pressing barriers in the field of planet formation. Knowing the amount of mass present in planet-forming disks allows us to determine the amount of material available for the formation of planetary systems, and to better understand the process by which they form."

Although the team has answered a number of key questions about the role of gravitational instability and disk mass in planet formation, the work is not yet done. "Studying how planets form is difficult because it takes millions of years to form planets. This is a very short time-scale for stars, which live thousands of millions of years, but a very long process for us," said Paneque-Carreño. "What we can do is observe young stars, with disks of gas and dust around them, and try to explain why these disks of material look the way they do. It's like looking at a crime scene and trying to guess what happened. Our observational analysis paired with future in-depth analysis of Elias 2-27 will allow us to characterize exactly how gravitational instabilities act in planet-forming disks, and gain more insight into how planets are formed."

## Lecture 1 : From the Big Bang to Stars

There is persuasive evidence that the Universe was created in the Big Bang'', in which space and time were created in a simple hot energetic, state, about 15 billion (15 x 10 9 ) years ago.

### 1.1.1 Five steps from the Big Bang to Stars

During the first 10 -43 seconds the four fundamental forces are unified (although no complete physical description of this era yet exists). Temperature 10 32 Kelvin. 10 -43 seconds defines the time when gravity splits from the other forces (weak, strong and Electro-Magnetic).

These steps are illustrated schematically in figure 1.1.

Figure 1.1: Schematic illustration of the steps leading from the Big Bang to the present-day Universe. Light from the first stars re-illuminated the Universe some 1-5 Gyr after the Big Bang. Source: Encyclopedia of Applied Physics, Vol. 23

### 1.1.2 Structure formation - stars and galaxies are born

About 300,000 years after the Big Bang, there was the era of recombination in which protons and electrons combined to form neutral Hydrogen. At this point, baryonic matter in the Universe consisted of about 75% Hydrogen and 25% Helium (by mass), with some small amounts of heavy elements (elements starting from Lithium). The distribution of this material was very close to, but not quite, uniform. These slight over- and under-densities were observed for the first time by the COBE satellite (launched in 1989) and amount to only a few parts in 100,000. The variations were mapped out over the whole sky (see figure 1.2) on scales of greater than about 7 degrees.

Figure 1.2: Map of the entire sky at a resolution of about 7 degrees, showing vary small variations in the background microwave radiation. These small variations indicate lesser and greater density regions in the early Universe, which would have led via gravity to the structures we see today. Source: Encyclopedia of Applied Physics, Vol. 23 (Page 47 - 81), 1998 WILEY-VCH Verlag GmbH, ISBN: 3-527-29476-7.

After recombination, the Universe entered a period called the Dark Ages'', until gravitational attraction had operated on very slight over-densities in the matter distribution, leading to the formation of light emitting stars and galaxies. The Universe was optically observable again!

Exactly how stars and galaxies formed, when the process started and how long it took is currently a major area of research. A simple picture runs like this: about 1 billion years after the big bang the first star forming regions, conglomerates of perhaps 10 6 to 10 9 solar masses began to develop. Over the next several billion years, most of these merge to form larger units or are partially destroyed by the energetic supernovae which develop as a natural part of star formation. Within a few billion years most of these have developed into stable configurations of stars and gas and are recognisable as galaxies''.

The faintest galaxies so far observed were seen in the Hubble Deep Field'', a tiny patch of sky which was imaged for more than a week by the Hubble Space Telescope (figure 1.3).

Figure 1.3: Part of the Hubble Deep Field, the deepest image of the sky ever taken. The image contains thousands of galaxies and a handful of stars. Some of the galaxies in the image are so distant that the light has taken more than 10 billion years to reach us.

Galaxies have been identified in this image with redshifts which indicate that the light reaching us has been traveling for about 90 % of the age of the Universe - or about 12-15 billion years. One such galaxy is shown in figure 1.4. Such galaxies (and the stars in them) are likely part of the very first generation of stars and galaxies to have formed.

Figure 1.4: One of the most distant galaxies known appears as little more than a coloured spot on this section of the Hubble Deep Field. Such distant, high red-shift galaxies give interesting information about how fast the Universe changed from an almost uniform state to the clumpy galaxy distribution we see today, and also allow its minimum age to be determined.

These galaxies are at present too few in number and too faint to be study in the kind of detail which we would like - to allow us to answer basic questions concerning exactly when and and what conditions the stars formed. However, a new generation of 10 meter class telescopes is currently coming on line all around the world so these issues are among those which these telescopes will concentrate.

The last 10 billion years or so of the developments in galaxies and in their stellar content is now quite well studied because of the Hubble Space Telescope, which was able to obtain clear images of these distant galaxies for the first time. Figure 1.5 shows early galaxies in the (confused) process of forming. One shouldn't forget that these knots of light are due to billions of stars forming, more or less at the same time.

Figure 1.5: Snapshot of very distant galaxies apparently in the early stages of formation. The galaxies have not yet settled into mostly regular forms we see in nearby galaxies, but are very irregular in morphology.

The space telescope has recently allowed the construction of a sequence of typical galaxy images over time (see figure 1.6). The general picture is that galaxies have been forming over quite a few billion years, are continuing to form and develop still, and seem to have been assembled from many smaller sub-galaxies.

Figure 1.6: Sequence of images of galaxies over the last 12 billion years. For the last 5 billion years or so, almost all galaxies can be classified neatly into two types - spiral and elliptical. In the early times, the irregular galaxies'' (only rarely seen today) become dominant, indicating that the galaxies are still forming.

### 1.1.3 Star formation

In the proto-galactic units the dominant process is that of star formation itself. We know that star formation takes place in giant collapsing clouds of gas, and can take place under a wide range of circumstances which result in stars being formed at a slow rate (such as most nearby regions of star formation in our own galaxy) or hundreds to thousands of times faster (such as in the compressed gas clouds which result when galaxies are disturbed or actually collide). However, the process by which clouds actually fragment and collapse into individual stars has long been a very poorly understood area of Astronomy, and remains almost as obscure today as it was 30 years ago. This is an area of research whose time has not yet come!

Hubble has revealed higher resolution images of star forming regions, which at least show some of the complexities of the physical processes involved.

Figure 1.7: Part of the M16 nebula, showing gas and dust clumps cocooning new born stars. The stars will take several million years to emerge from their incubation sites

Figure 1.7 shows columns of cool interstellar hydrogen gas and dust that are also incubators for new stars. The pillars protrude from the interior wall of a dark molecular cloud like stalagmites from the floor of a cavern. They are part of the Eagle Nebula'' (also called M16), a nearby star-forming region 7,000 light-years away in the constellation Serpens. The pillars of gas are dense gas which has survived being eroded'' away by the light of hot UV stars nearby, and the small blobs are even denser regions where it is very likely that stars themselves are actually forming. They'll emerge from the cocoons in which they are incubating millions of years from now, and for the moment are so heavily shrouded by dust that we cannot see them at all with optical light. Infrared light (heat) which does escape from the regions is the main clue that energetic processes are taking place. Figure 1.8 shows how different images in optical (on the left) and infrared (on the right) of the same region in the Orion Nebula can be.

Figure 1.8: Optical (left) and infrared (IR) image (right) of a region of the Orion nebula, taken with the Space Telescope. In the infrared, stars are seen which are quite invisible in the optical. IR radiation can penetrate the clouds of dust surrounding the young stars which are forming in Orion

The star forming regions can be very large, sometimes occupying a major section of an entire galaxy, such as seen in figure 1.9.

Figure 1.9: Star forming region in the galaxy NGC2363. The region occupies a major fraction of the entire galaxy, showing that the processes by which galaxies develop need not be smooth and regular. Interaction between galaxies is one cause of major star formation events like this

Out of these star forming regions star eventually emerge - often en mass'' in giant clusters of up to a million stars at once. An example of this is seen in the giant star forming complex 30Dor in the Large Magellanic Cloud, a galaxy nearby to our own (see figure 1.10).

Figure 1.10: A cluster of several thousand new-born stars has recently emerged from a star forming gas cloud in 30 Dor in the Large Magellanic Cloud.

Clusters of stars which we can see forming like this might end up as what we know today as globular clusters'' of our own Galaxy - millions of stars tightly bound by their own gravity which have survived from the earliest times when our galaxy was forming.

### 1.1.4 Timeline

A timeline of the processes of gravitational collapse in the Universe, the formation of galaxies and stars, and the formation of our own Galaxy (the Milky Way), Sun and Earth is shown in figure 1.11.

## Astronomers see first hint of the silhouette of a spaghettified star

For decades astronomers have been spotting bursts of electromagnetic radiation coming from black holes. They assumed those are the result of stars being torn apart, but they have never seen the silhouette of the actual material ligaments. Now a group of astronomers, including lead author Giacomo Cannizzaro and Peter Jonker from SRON Netherlands Institute for Space Research/Radboud University, has for the first time observed spectral absorption lines caused by strands of a spaghettified star. Publication in Monthly Notices of the Royal Astronomical Society.

Most stars in our universe die of natural causes. They either blow off their outer shells, or simply cool down due to fuel shortage, or they could go out with a bang in a giant supernova explosion. But stars living in the inner region of their galaxy might not be so lucky. They are in danger of getting torn into slim filaments by the supermassive black hole that lurks in the center of most galaxies. The extreme gravity of the black hole pulls so much harder at one side of the star than at the other side that it rips the star apart. Astronomers like to call this process spaghettification, but in scientific publications they reluctantly stick with the official term Tidal Disruption Event.

After a star has transformed into a spaghetti strand, it falls further into the black hole, emitting a short burst of radiation. Astronomers have spotted these bursts for decades now, and based on the theory they assumed that they were looking at Tidal Disruption Events. But they have never seen the actual material ligaments, as in a physical object that not only emits but also blocks light. Now an international team of astronomers has for the first time observed spectral absorption lines while looking at one of the poles of a black hole. It was already evident that black holes can have a disk of accreted material around their equator, but absorption lines above a black hole's pole suggest there is a long strand wrapped many times all around the black hole, like a yarn ball: the actual material ligament from a freshly torn star.

The researchers know the black hole is facing them from its pole because they detect X-rays. The accretion disk is the only part of a black hole system that emits this type of radiation. If they were looking edge-on, they wouldn't see the accretion disk's X-rays. "Moreover the absorption lines are narrow," says lead author Giacomo Cannizzaro (SRON/Radboud University). "They are not broadened by the Doppler effect, like you'd expect when you would be looking at a rotating disk."

More information: G Cannizzaro et al. Accretion disc cooling and narrow absorption lines in the tidal disruption event AT 2019dsg, Monthly Notices of the Royal Astronomical Society (2021). DOI: 10.1093/mnras/stab851