Astrophysics
Stars, galaxies, black holes, cosmic explosions and planets, and the physics that powers them.
arXiv: astro-ph.HE, astro-ph.SR, astro-ph.GA, astro-ph.EP
32 topics
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Gas spiraling into black holes and neutron stars must shed angular momentum (rotation) to fall in; on the way it switches between distinct states, flickers with near-regular rhythms, and part of the released energy is shot out as jets near light speed. Which processes move the angular momentum, set the states and launch the jets is only partly understood.
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Life on a distant planet might reveal itself through gases in its atmosphere that are hard to make without biology, such as oxygen together with methane. Every claimed detection so far is disputed, and whether planets around small red stars can even keep atmospheres is unknown.
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Galaxies are surrounded by large halos of thin gas, the circumgalactic medium, that feed and receive gas from the galaxy. Most of the ordinary matter that should belong to a galaxy like ours is not found in its stars and disk gas.
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Gravitational-wave detectors have recorded hundreds of mergers of black holes and neutron stars. How these pairs formed, and why some masses appear where theory forbids them, is open.
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Core-collapse supernova explosion mechanism
7 problemsWhen the iron core of a massive star collapses into a neutron star, the rest of the star is blown off in a supernova. Computer models say neutrinos from the hot core reheat the stalled shock wave, but whether this works with the right energy for every star is unsettled.
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Galactic cosmic-ray spectra and antimatter
5 problemsSpace detectors measure cosmic rays, fast charged particles from the Galaxy, with percent precision. The spectra show unexpected bends and more antimatter than simple models predict.
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Atomic nuclei reach Earth with energies up to $3e20\,\mathrm{eV}$, the kinetic energy of a thrown tennis ball packed into one particle. Which objects in our galaxy and beyond accelerate them is mostly unknown.
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Dynamics of galactic disks
5 problemsDisk galaxies like the Milky Way show spiral arms, central bars and ripples in the motions of their stars. How long these patterns last and what made them is still argued over.
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Origin of fast radio bursts
6 problemsFast radio bursts (FRBs) are millisecond flashes of radio waves from other galaxies, each releasing in a millisecond as much energy as the Sun emits in days to years. Some sources flash repeatedly, most have been seen only once, and what makes them is still debated.
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Black holes of a billion solar masses already powered quasars less than a billion years after the Big Bang, and JWST now sees many smaller ones even earlier. Nobody knows what the first seed black holes were or how they grew so fast.
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Galactic center GeV excess and 511 keV line
5 problemsThe center of our galaxy emits more gamma rays at a few billion electron volts than known sources explain, and a strong line from electrons annihilating with positrons (their antiparticles). Neither signal has an agreed source.
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Galaxy quenching and black hole feedback
5 problemsMany massive galaxies stopped forming stars billions of years ago even though gas is still around them. Something, probably energy from the central black hole or the heating of gas falling into the galaxy, keeps that gas from cooling into new stars.
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Gamma-ray burst central engines and jets
7 problemsGamma-ray bursts are the brightest explosions known: beams of matter moving at over $99.99\,\text{percent}$ of light speed, launched when a massive star collapses or two neutron stars merge. What powers the beam and what it is made of are unsettled.
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Neutrinos (nearly massless particles that pass through matter almost untouched) arrive from space with energies from about 1e13 eV to about 2e17 eV. Only a few sources have been identified, and most of the flux has no known origin.
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Origin of hot Jupiters
5 problemsHot Jupiters are gas giants that orbit their stars in a few days, far inside the region where giant planets are thought to form. Whether they moved inward through the gas disk, were thrown inward onto stretched orbits that tides then made circular, or formed in place is still argued.
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Three bodies from other planetary systems have been seen passing through the Solar System: 1I/'Oumuamua in 2017, 2I/Borisov in 2019 and 3I/ATLAS in 2025. They are samples of other planetary systems, but the first moved in a way no visible gas explains, and how common they are is uncertain.
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Interstellar medium and astrochemistry
6 problemsThe gas and dust between stars contain hundreds of molecules and absorb starlight in characteristic ways. Several absorption features and emission signals still have no identified source, and some molecules appear where chemistry models say they should not form.
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Neutron stars are city-size balls of neutrons whose interiors are expected to be superfluid, a frictionless quantum liquid. Sudden spin-ups called glitches, the cooling of young stars and the absence of certain gravitational waves test that picture.
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Planets grow from microscopic dust in disks of gas around young stars, but grains that reach centimeter to meter size break up in collisions or spiral into the star within a few hundred orbits. How solids get past this to kilometer-size bodies and then to planet cores is not settled.
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Young stars are surrounded by flat disks of gas and dust that feed the star and build planets, and the disks vanish within a few million years. What makes the gas fall onto the star, how much material the disks really hold, and what clears them away are all uncertain.
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Pulsar radio emission and magnetar origin
6 problemsPulsars are spinning neutron stars that sweep radio beams across Earth like a lighthouse, and magnetars are neutron stars with fields a hundred to a thousand times stronger still. How the radio beam is made, and how magnetars get their fields, are both open.
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About half of the atoms of elements heavier than iron, and nearly all of the gold and uranium, form when nuclei absorb neutrons faster than they can decay, the r-process. Neutron-star mergers are one confirmed site, but whether they make most of it is debated.
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The most common planets in the Galaxy are between Earth and Neptune in size, a kind the Solar System lacks, and few of them are 1.5 to 2 times Earth's radius. Whether this gap comes from planets losing their atmospheres or from how they formed, and what these planets are made of, is unsettled.
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When galaxies merge, their central black holes should pair up and eventually collide, emitting gravitational waves with periods of years. Pulsar timing arrays (networks of precisely clocked spinning neutron stars) now detect a background of such waves, but its source and how the black hole pairs get close enough to merge are uncertain.
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Type Ia supernovae are thermonuclear explosions of white dwarfs, the dense leftover cores of Sun-like stars, and they serve as standard candles for measuring the expansion of the universe. Which binary systems produce them, and several puzzles in how white dwarfs cool and become magnetic, are still unresolved.
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Solar corona heating and solar wind
5 problemsThe Sun's outer atmosphere (corona) is over a million kelvin while its surface is about 6000 K, and it blows a continuous wind of charged particles into space. How the corona is heated and how the wind is launched are not fully explained.
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Solar dynamo, activity cycle and composition
5 problemsThe Sun's magnetic activity rises and falls in an 11-year cycle that sometimes stops for decades, and its measured surface composition disagrees with what sound waves inside it imply. Neither has a complete explanation.
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Formation of massive star clusters
3 problemsMassive star clusters pack up to millions of stars born at nearly the same time in a small region. How the oldest of them, the globular clusters, formed, and why their stars differ in chemistry and spin, are still open.
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Clouds of cold gas turn only a few percent of their mass into stars before they are dispersed, and the new stars come out in a nearly fixed mix of light and heavy ones, called the initial mass function. Why the efficiency is so low and whether the mass mix is the same everywhere, including in the first galaxies, are unsolved.
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Stellar interiors: transport and mixing
3 problemsStars are not fully understood inside: models must guess how gas mixes and how spin is passed between the core and the surface. Measurements of star vibrations and of double stars show that these guesses fail in several places.
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Architectures of planetary systems
4 problemsPlanetary systems around other stars are arranged very differently from ours: many have several planets bigger than Earth packed inside Mercury's orbit. Why systems take the shapes they do, and why ours is unusual, is not explained.
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Tidal disruption events
6 problemsWhen a star passes too close to a giant black hole, tidal forces tear it apart and part of it falls in, producing a flare that lasts months. Where the light comes from and why less energy than expected comes out are still debated.