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Astronomers study the oldest observable stars in the universe in much the same way that archaeologists study ancient artifacts on Earth. Here, Anna Frebel--who is credited with discovering several of the oldest and most primitive stars using the world's largest telescopes--takes readers into the far-flung depths of space and time to provide a gripping firsthand account of the cutting-edge science of stellar archaeology. Weaving the latest findings in astronomy with her own compelling insights as one of the world's leading researchers in the field, Frebel explains how sections of the night sky are \"excavated\" in the hunt for these extremely rare relic stars--some of which have been shining for more than 13 billion years--and how this astonishing quest is revealing tantalizing new details about the earliest times in the universe.--Jacket flap.

Advanced glycation end-products (AGEs) constitute a non-homogenous, chemically diverse group of compounds formed either exogeneously or endogeneously on the course of various pathways in the human body. In general, they are formed non-enzymatically by condensation between carbonyl groups of reducing sugars and free amine groups of nucleic acids, proteins, or lipids, followed by further rearrangements yielding stable, irreversible end-products. In the last decades, AGEs have aroused the interest of the scientific community due to the increasing evidence of their involvement in many pathophysiological processes and diseases, such as diabetes, cancer, cardiovascular, neurodegenerative diseases, and even infection with the SARS-CoV-2 virus. They are recognized by several cellular receptors and trigger many signaling pathways related to inflammation and oxidative stress. Despite many experimental research outcomes published recently, the complexity of their engagement in human physiology and pathophysiological states requires further elucidation. This review focuses on the receptors of AGEs, especially on the structural aspects of receptor–ligand interaction, and the diseases in which AGEs are involved. It also aims to present AGE classification in subgroups and to describe the basic processes leading to both exogeneous and endogeneous AGE formation.

Fighting the galactic bulge Observations show most dwarf galaxies to be almost 'bulgeless', consisting of a rotating stellar disc embedded in a massive near-constant-density core halo of cold dark matter. This sits uncomfortably with the predictions of models based on the dominance of cold dark matter, which invariably generate galaxies with dense stellar spheroidal bulges and steep central dark-matter profiles, as low-angular-momentum baryons and dark matter sink to the centre of galaxies through accretion and repeated mergers. Governato et al . report hydrodynamical simulations that resolve this paradox. Strong outflows from supernovae remove low-angular-momentum gas, thereby inhibiting the formation of bulges and decreasing dark-matter density around the centre of the galaxy. The properties of 'dwarf' galaxies have long challenged the cold dark matter (CDM) model of galaxy formation, as the properties of most observed dwarf galaxies contrast with models based on the dominance of CDM. Here, hydrodynamical simulations (assuming the presence of CDM) are reported in which the analogues of dwarf galaxies — bulgeless and with shallow central dark-matter profiles — arise naturally. For almost two decades the properties of ‘dwarf’ galaxies have challenged the cold dark matter (CDM) model of galaxy formation 1 . Most observed dwarf galaxies consist of a rotating stellar disk 2 embedded in a massive dark-matter halo with a near-constant-density core 3 . Models based on the dominance of CDM, however, invariably form galaxies with dense spheroidal stellar bulges and steep central dark-matter profiles 4 , 5 , 6 , because low-angular-momentum baryons and dark matter sink to the centres of galaxies through accretion and repeated mergers 7 . Processes that decrease the central density of CDM halos 8 have been identified, but have not yet reconciled theory with observations of present-day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different dark-matter particle candidate 9 . Here we report hydrodynamical simulations (in a framework 10 assuming the presence of CDM and a cosmological constant) in which the inhomogeneous interstellar medium is resolved. Strong outflows from supernovae remove low-angular-momentum gas, which inhibits the formation of bulges and decreases the dark-matter density to less than half of what it would otherwise be within the central kiloparsec. The analogues of dwarf galaxies—bulgeless and with shallow central dark-matter profiles—arise naturally in these simulations.

Galactic detritus around M31 A panoramic survey of the region around our nearest galactic neighbour, the well known Andromeda galaxy M31, has detected stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by M31's tidal field. The brightest companion, the Triangulum galaxy (M33), is surrounded by a previously unknown prominent stellar structure that provides evidence for a recent encounter with M31. This new view of galactic structures is consistent with hierarchical cosmological models in which galaxies grow in mass by the accretion of smaller ones. In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound and distant stars surrounding the galaxy. A panoramic survey of the Andromeda galaxy (M31) now reveals stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. In hierarchical cosmological models 1 , galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk 2 , 3 . The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history 4 , but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than M v = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

Early star formation: steady progress Recent observations suggest that the massive galaxies that were at the height of their star-forming activity in the young Universe ten billion years ago formed their stars at surprisingly high rates. While such rates are commonly attributed to violent galaxy mergers, many of these galaxies are rotating discs, as extended as today's Milky Way, a structure that is incompatible with such a history. A new cosmological simulation suggests that these galaxies were 'stream fed', acquiring the material that was needed to fuel star formation as a steady flow of cold gas from the extended dark-matter haloes surrounding the galaxies. It is the rarer submillimetre galaxies, which form stars even more intensely, that are largely merger-induced starbursts. Massive galaxies in the young universe (ten billion years ago) formed stars at surprising intensities. Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with mergers. This paper reports that they are 'stream-fed galaxies', growing via steady, narrow, cold gas streams. Unlike destructive mergers, the smoother flows are likely to keep the rotating disc configuration intact. Massive galaxies in the young Universe, ten billion years ago, formed stars at surprising intensities 1 , 2 . Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with such events, showing gas-rich, clumpy, extended rotating disks not dominated by spheroids 1 , 2 , 3 , 4 , 5 . Cosmological simulations 6 and clustering theory 6 , 7 are used to explore how these galaxies acquired their gas. Here we report that they are ‘stream-fed galaxies’, formed from steady, narrow, cold gas streams that penetrate the shock-heated media of massive dark matter haloes 8 , 9 . A comparison with the observed abundance of star-forming galaxies implies that most of the input gas must rapidly convert to stars. One-third of the stream mass is in gas clumps leading to mergers of mass ratio greater than 1:10, and the rest is in smoother flows. With a merger duty cycle of 0.1, three-quarters of the galaxies forming stars at a given rate are fed by smooth streams. The rarer, submillimetre galaxies that form stars even more intensely 2 , 12 , 13 are largely merger-induced starbursts. Unlike destructive mergers, the streams are likely to keep the rotating disk configuration intact, although turbulent and broken into giant star-forming clumps that merge into a central spheroid 4 , 10 , 11 . This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.

Distant star formation The individual star-forming regions of the massive galaxies in the early Universe, at redshifts of around z = 2, are beyond the reach of even the largest of today's telescopes. But with help from gravitational lensing by a massive intervening galaxy cluster, brightening the image 32-fold and making it look larger, intense star formation has been observed in the submillimetre galaxy SMMJ2135-0102 at redshift z = 2.3259. The results reveal luminosity densities comparable to those of the dense cores of giant molecular clouds in the local Universe, suggesting that the underlying physics of star formation is similar to that of nearby galaxies, though the regions in the distant galaxies are about 100 times larger and 10 7 times more luminous overall. Massive galaxies in the early Universe have been shown to be forming stars at high rates. Probing the properties of individual star-forming regions is beyond the resolution and sensitivity of existing telescopes. Here, however, observations are reported of the galaxy SMMJ2135–0102 at redshift z =2.3259, which has been gravitationally magnified by a factor of 32 by a galaxy cluster lens in the foreground. The physics underlying star formation here is similar to that in local galaxies, but the energetics are very different. Massive galaxies in the early Universe have been shown to be forming stars at surprisingly high rates 1 , 2 , 3 . Prominent examples are dust-obscured galaxies which are luminous when observed at sub-millimetre wavelengths and which may be forming stars at a rate of 1,000 solar masses ( M ⊙ ) per year 4 , 5 , 6 , 7 . These intense bursts of star formation are believed to be driven by mergers between gas-rich galaxies 8 , 9 . Probing the properties of individual star-forming regions within these galaxies, however, is beyond the spatial resolution and sensitivity of even the largest telescopes at present. Here we report observations of the sub-millimetre galaxy SMMJ2135-0102 at redshift z = 2.3259, which has been gravitationally magnified by a factor of 32 by a massive foreground galaxy cluster lens. This magnification, when combined with high-resolution sub-millimetre imaging, resolves the star-forming regions at a linear scale of only 100 parsecs. We find that the luminosity densities of these star-forming regions are comparable to the dense cores of giant molecular clouds in the local Universe, but they are about a hundred times larger and 10 7 times more luminous. Although vigorously star-forming, the underlying physics of the star-formation processes at z  ≈ 2 appears to be similar to that seen in local galaxies, although the energetics are unlike anything found in the present-day Universe.

Cool gas fuels star formation Although it is thought that some galaxies in the early Universe grew rapidly through violent mergers, the properties of many early galaxies are incompatible with that scenario. Cresci et al . now report chemical abundance data from three star-forming galaxies at redshift z = 3 — equivalent to only two billion years after the Big Bang — that support an alternative model: galactic growth through the accretion of cold gas. The central star-forming regions in these galaxies are found to have lower metallicity than the outer regions. This is opposite to what is seen in local galaxies and is consistent with the accretion of cold primordial (and hence low metallicity) gas. Galaxies in the early Universe might grow through the accretion of cold, primordial, low-metallicity gas. If such gas is funnelled to the centre of a galaxy, it will result in the central region having an overall lower metallicity than outer regions. These authors report such 'inverse' metallicity gradients in three rotationally supported, star-forming galaxies at redshift ∼3, and conclude that the central gas has been diluted by the accretion of primordial gas. It has recently been suggested 1 , 2 that galaxies in the early Universe could have grown through the accretion of cold gas, and that this may have been the main driver of star formation and stellar mass growth 3 , 4 , 5 . Because the cold gas is essentially primordial, it has a very low abundance of elements heavier than helium (referred to as metallicity). If funnelled to the centre of a galaxy, it will result in the central gas having an overall lower metallicity than gas further from the centre, because the gas further out has been enriched by supernovae and stellar winds, and not diluted by the primordial gas. Here we report chemical abundances across three rotationally supported star-forming galaxies at redshift z  ≈ 3, only 2 Gyr after the Big Bang. We find ‘inverse’ gradients, with the central, star-forming regions having lower metallicities than less active ones, which is opposite to what is seen in local galaxies 6 , 7 . We conclude that the central gas has been diluted by the accretion of primordial gas, as predicted by ‘cold flow’ models.

Using a millimetre-wave molecular line scan, a redshift has finally been determined for the extremely active star-forming galaxy HDF 850.1 in the Hubble Deep Field, which makes it younger than thought at 1.1 billion years after the Big Bang. Starburst object HDF 850.1 retains some mystery The brightest sub-millimetre radio source in the Hubble Deep Field view of the distant Universe, known as HDF 850.1, has proved enigmatic, evading detection in the optical and near-infrared ranges despite an intensive search. Without the discovery of a counterpart at shorter wavelengths, it has not been possible to estimate the source's redshift, size or mass directly. Now, by using a millimetre-wave molecular line scan, the redshift of HDF 850.1 has been determined. At z ≈ 5.2, it is much higher than expected and corresponds to a cosmic age of only 1.1 billion years after the Big Bang. Calculations from the new data suggest a high annual star formation rate of 850 solar masses and a mass of 1.3 × 10 11 solar masses. But as yet there is no sign of a starlight-emitting counterpart. The Hubble Deep Field provides one of the deepest multiwavelength views of the distant Universe and has led to the detection of thousands of galaxies seen throughout cosmic time 1 . An early map of the Hubble Deep Field at a wavelength of 850 micrometres, which is sensitive to dust emission powered by star formation, revealed the brightest source in the field, dubbed HDF 850.1 (ref. 2 ). For more than a decade, and despite significant efforts, no counterpart was found at shorter wavelengths, and it was not possible to determine its redshift, size or mass 3 , 4 , 5 , 6 , 7 . Here we report a redshift of z = 5.183 for HDF 850.1, from a millimetre-wave molecular line scan. This places HDF 850.1 in a galaxy overdensity at z  ≈ 5.2, corresponding to a cosmic age of only 1.1 billion years after the Big Bang. This redshift is significantly higher than earlier estimates 3 , 4 , 6 , 8 and higher than those of most of the hundreds of submillimetre-bright galaxies identified so far. The source has a star-formation rate of 850 solar masses per year and is spatially resolved on scales of 5 kiloparsecs, with an implied dynamical mass of about 1.3 × 10 11 solar masses, a significant fraction of which is present in the form of molecular gas. Despite our accurate determination of redshift and position, a counterpart emitting starlight remains elusive.

Turn back the cosmic clock Nature's series of review articles on astronomy, marking the International Year of Astronomy 2009, continues with a look at the 'final frontier' in observational astronomy: the formation of the first stars, galaxies and massive black holes. At present these objects can be studied only through simulations. Today's ground-based and space-borne telescopes have probed cosmic history back to a time and distance when the Universe was less than a tenth its present age. But the next generation of telescopes, with the latest theories as a benchmark, will be crossing the present high-redshift barrier to the first sources of light in the Universe. The remaining frontier in understanding the early Universe is the formation of the first stars, galaxies and massive black holes. The interplay of theory and upcoming observations promises to answer key open questions in this emerging field. Observations made using large ground-based and space-borne telescopes have probed cosmic history from the present day to a time when the Universe was less than one-tenth of its present age. Earlier still lies the remaining frontier, where the first stars, galaxies and massive black holes formed. They fundamentally transformed the early Universe by endowing it with the first sources of light and chemical elements beyond the primordial hydrogen and helium produced in the Big Bang. The interplay of theory and upcoming observations promises to answer the key open questions in this emerging field.

Galaxies and black holes The massive black holes found at the centre of most galaxies, including our own, release prodigious amounts of energy that power spectacular phenomena such as quasars and active galactic nuclei. If just a tiny fraction of that energy were absorbed into the host galaxy it could stop star formation in its tracks by heating and ejecting the ambient gas. The latest of our 'IYA 2009' reviews, marking the International Year of Astronomy and collected together on http://www.nature.com/astro09 , tackles one of the central questions in galaxy evolution — the degree to which black hole activity has limited star formation in large elliptical galaxies. These contain much less cool gas and fewer young stars than spiral galaxies, a contrast that could relate to how the central black hole interacts with its surroundings. Virtually all massive galaxies host central black holes, the growth of which releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. However, a tiny fraction of this energy could halt star formation by heating and ejecting ambient gas; a central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies. Virtually all massive galaxies, including our own, host central black holes ranging in mass from millions to billions of solar masses. The growth of these black holes releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. A tiny fraction of this energy, if absorbed by the host galaxy, could halt star formation by heating and ejecting ambient gas. A central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies, which typically have little cold gas and few young stars, unlike spiral galaxies.