Jeans instability

At the center of a galaxy, where a supermassive black hole spins a disk of scorching gas around itself, giant stars sometimes form. They are unlike ordinary stars: the disk continuously feeds them with matter, and a star can grow to hundreds of solar masses, staying bright as long as the gas flow lasts.

In the first half-billion years after the Big Bang, galaxies grew like crazy. Gas fell in rapidly and collapsed into stars without any resistance. This period is called 'unimpeded star formation.' It's like frying pancakes on a scorching skillet without a lid: splatters fly, but nothing holds them back.

Every pulsating object has a moment when its rhythm falters. For boson stars, hypothetical clumps of ultralight particles, that moment turns out to be the point where mass stops growing. The authors calculated the lowest radial mode (the fundamental 'breathing' tone) for a whole family of such stars, including models with self-interaction.

Light from quasar J0744+2052 traveled to us for almost 12 billion years. Along the way, it bumped into hydrogen clouds that snatched individual lines from the spectrum — like a comb sifting through delicate frequencies. Astronomers call this pattern the Lyman-alpha forest in spectroscopy.

The collapse of a star's core is like compressing a spring: matter falls inward, and a neutron star is born at the center. But the fall is never perfectly smooth. In the boiling outer layers, random eddies transfer angular momentum to the infalling gas, and a disk swirls around the central object. This disk is no calm river. It pulses, strengthening and fading.

A campfire burns steadily and long. But when the wind blows, the flame flickers out, leaving only embers. A similar story plays out with the gravastar — a hypothetical object that was supposed to replace the black hole. An ordinary black hole hides a singularity — a point where the laws of physics break down.

In the search for the nature of dark matter, one of the most intriguing hypotheses traces back to the first moments after the Big Bang. Back then, quantum fluctuations of the inflation field gave rise to clumps of matter that could collapse into primordial black holes.

A black hole is like a spinning drop of viscous liquid. Normally, such a drop tends to stay whole: splitting would reduce its surface area, which contradicts the law of non-decrease of entropy (a measure of disorder, like a broken cup doesn't glue itself back together).

LIGO and Virgo detectors catch ripples in spacetime, but some signals force us to rewrite the scripts of stellar death. GW190814 — the merger of a black hole of 23 solar masses with a compact object of 2.6 solar masses — is a discordant chord in the symphony of standard models.

A giant cloud of invisible matter acts like a funhouse mirror — distorting the light from a distant galaxy. From these distortions, astronomers can weigh the cloud and peer into its depths.

Dark matter is the invisible scaffold of the universe. It doesn't shine, but its gravity holds stars in galaxies. In some models, dark matter particles can collide and exchange energy, like molecules in warm air.

A figure skater pulls their arms in, and their spin suddenly accelerates. When a massive star exhausts its fuel and its core collapses, something similar happens, but on a cosmic scale. The core shrinks to a city-sized sphere, and if it is spinning rapidly, a peculiar instability can arise.

A distant galaxy always looks redder than it really is. That's the work of cosmic dust — tiny solid particles that scatter blue light more than red, like haze at sunset. Astronomers measured this effect for tens of thousands of galaxies using two powerful instruments.

A drop of honey spreads slowly across a table, while water spreads quickly. The resistance to flow, viscosity, differs from one fluid to another. But what if the viscosity changes on the fly, as the substance compresses or expands?

Back in the 1960s, physicist Joseph Weber tried to catch gravitational waves using aluminum cylinders: they were supposed to tremble like tuning forks from the ripples of spacetime.

When hot soup cools, clumps form on the surface — similarly, as the early universe cooled, it gave birth to threadlike clumps of energy: cosmic strings. Some of them are like a stretched rubber band: as long as it's intact, it's stable. But if you nick the band, it snaps.

In a packed hall where couples are spinning, a wave sometimes ripples through: some pairs break apart, others form, and the motion sweeps everyone along. A dense neutrino cloud behaves similarly: these are the tiniest particles that barely interact with matter.

After the Big Bang, dark matter and ordinary matter (mostly hydrogen) went their separate ways in terms of speed. Their gravitational pull acts like a bow gliding across a string: when frequencies match, resonance occurs, and sound waves start to rapidly grow in ordinary matter.

A balloon in your hand springs back equally in all directions. But a bottle of soda behaves differently: along the neck it is stiff, while across it is easily squeezed. This is roughly how pressure works inside some stars: it can be unequal in different directions. Such stars are called anisotropic, and their behavior has so far been described by disparate methods.

A star weighing a hundred thousand Suns lives bright and brief. Its finale is a rapid collapse into a black hole. But just before the end, it manages to wrap itself in a dense gas cocoon.

A river flows toward a whirlpool, but a dam with a notch blocks the way. Most water piles up before the dam, but some, gaining height, leaps over it and falls straight into the funnel. So it is with gas in a galaxy: it streams toward the center but hits a ring of stars and gas, like a dam. Usually, everything stops there, swirling into a dense vortex.

In the distant universe, when it was just a couple of billion years old, many galaxies resembled boiling cauldrons: gas clumped so quickly that stars ignited one after another, and dust heated up and shone brightly in the submillimeter range.

A stream of electrons enters plasma like a jet of water hitting a calm pond. The jet disturbs the surface, giving rise to waves that grow quickly and mix the water. In plasma, this process is called the Buneman instability: the electron beam excites oscillations that then scatter its energy.

A stream of scorching gas races away from a star, and suddenly stalls, as if it hit an invisible wall. That's exactly what researchers saw when they tried to reproduce a stellar eruption on a laboratory scale. They used a laser to create a plasma jet and shot it through a magnetic field — first weak, then strong, like that of many stars.

Hidden in cold interstellar gas clouds are long, thin filaments, like threads in a tangle. It is within these filaments, as they collapse, that new stars are born. Astronomers selected seven such clouds, located between 140 and 920 light-years away, and counted how many filaments of various thicknesses each contains.

In a disk of gas and dust around a young star, dust particles behave like sand grains in a stream of water: the gas flows faster, the dust lags behind, and friction arises between them. This friction can rock the magnetic waves that run through the disk. The galaxy is full of such disks: it is precisely in them that planets are born.

Two stars orbit a common center, accreting gas from a surrounding cloud. Rotating disks of gas form around each star and around the pair as a whole. From these disks, jets shoot out like hoses. Inside the disks, gas is constantly churned — that's the magnetorotational instability at work.

In cold clouds of interstellar gas, long, dense filaments stretch out. Inside them, like overcrowded incubators, future stars are maturing. For a long time, it was thought that within such filaments, everything was governed by gravity: matter smoothly flows toward the centers, where density increases and eventually a star ignites.

In the cold gas cloud where a star is born, chemistry sleeps. Molecules barely collide, reactions stall. But when a charged particle zips through the cloud, it knocks electrons off atoms — ionizing them. That's how active ions appear in a dormant cloud, ready to build new molecules.

In a dark cloud where no star has yet ignited, gas begins to pull together toward a common center. This is how the most massive stars in the Galaxy are born: those tens of times heavier than the Sun. Astronomers pointed radio telescopes at the cloud SDC335.579-0.292 and saw gas molecules lining up into streams falling inward.

A piece of dough, when pulled from the sides, doesn't stretch evenly: in one spot it thins and tears. Living tissue behaves similarly, but instead of a tear, shear bands appear — narrow zones where cells begin to slide past each other, like cards in a deck when cut.

At the center of a vast city of galaxies hangs a thick cloud of gas. It has cooled to the point where, by all the rules, it should be collapsing into new stars, like steam turning to droplets on a cold window. But the window stays dry. Astronomers aimed the Chandra X-ray telescope at this cloud and watched it for nearly three straight days.

In a tokamak, plasma is like a crowd on a narrow street: once someone starts running, everyone takes off at once. In a stellarator with a non-circular cross-section, the street is winding, with dead ends and pockets. The crowd can't gather into a single wave — people get stuck in different corners.

A galaxy cluster is like a busy intersection where galaxy-cars race through a headwind of hot gas. This wind, called ram pressure, strips galaxies of their cold gas — the fuel for star formation. Astronomers from the SAMI survey traced how this happens in hundreds of galaxies across eight nearby clusters.

At the heart of nearly every large galaxy lurks a supermassive black hole. As it devours gas, it blasts powerful outflows back into space, like a dragon that can't swallow all its prey and breathes fire. These 'winds' accelerate to thousands of kilometers per second and can sweep away the gas reservoirs from which stars are born.

In the dark cloud G318.049+00.086, where no star has yet formed, dust and gas have arranged themselves into four long filaments. They converge into a dense central hub, like four rivers emptying into a lake. Gas flows along these filaments toward the center: the flow speed reaches 10–20 kilometers per second per parsec of length.

A young star is surrounded by a flat, rotating disk of gas and dust — the construction site for future planets. The disk lives its own life: gas flows toward the star, dust clumps together, and from outside, harsh ultraviolet light from neighboring hot stars presses down on it all, like wind blowing leaves off a path.

Just as in a crowded room each new person sharply increases the pressure on the walls, in a neutron star increasing density leads to a rise in internal resistance. But even this resistance has an absolute ceiling.

A water droplet falls onto a lotus leaf and bounces off without leaving a trace. Nature has perfected this self-cleaning marvel over millions of years, and engineers have long tried to replicate it. But if the droplet hits too fast, it shatters into splashes, losing mass and leaving wet marks on the surface.