## Concept explanation A star stays stable through **hydrostatic equilibrium**, which means two opposing effects are in balance. **Gravity** pulls the star’s gas inward, trying to compress it, while **pressure** from the star’s intensely hot interior pushes outward. When those two forces match, the star keeps nearly the same size. If the outward pressure drops below gravity, the star contracts; if the pressure rises above gravity, the star expands. ## What you see You’re looking at a glowing spherical star with blue arrows pointing inward to show gravity and teal arrows pointing outward to show interior pressure. The star’s size changes slowly so you can see how it responds over time rather than jumping instantly. The state card and balance gauge help you compare the current `core pressure` value with the fixed pull of gravity. ## Try it yourself - **Set the slider to `50`** and notice that the inward and outward effects balance, so the star stays steady. - **Drag the core pressure slider below `50`** and watch the star slowly contract as gravity becomes stronger than the outward push. - **Raise the core pressure slider above `50`** and see the star expand slightly because the hot interior is pushing outward more strongly. - **Use the preset menu** to jump quickly between a balanced star, a low-pressure case, and a high-pressure case. - **Move the slider back and forth across `50`** to see how a small change around the balance point switches the star between contracting, stable, and expanding. ## Concept explanation A massive star stays stable because **fusion** in its core keeps adding thermal energy, which maintains hot gas pressure pushing outward against gravity. When the **core fuel** runs low, fusion can no longer replenish that energy fast enough, so **pressure support** weakens. Gravity then begins to win, and the core starts to contract first while the surrounding layers respond more slowly. ## What you see You’re looking at a cutaway cross-section of a layered massive star. The bright central region is the core, the colored rings are surrounding shells, and the teal arrows show outward pressure from the core. As fuel decreases, the core glow fades, those pressure arrows shorten, and the core shrinks sooner than the outer layers, showing how collapse begins in the center before the rest of the star fully follows. ## Try it yourself - **Drag the `remaining core fuel` slider downward slowly** and watch the core dim before the shells noticeably move. - **Pause around mid fuel levels** and notice that the outward pressure arrows are already shorter even though the star’s outer layers still look relatively extended. - **Lower the fuel close to `0%`** and see the core contract strongly while collapse indicators appear from the outside. - **Compare the core and shell motion** to see that the center responds first and the outer layers lag behind. - **Press `Refuel core`** to restore pressure support and reset the star to its high-fuel state. ## Concept explanation When a stellar core loses **pressure support**, gravity is no longer balanced, so the core begins a **runaway collapse**. As the core radius shrinks, the same mass is squeezed into a smaller volume, causing **density** to rise very quickly. A more massive core has stronger gravity, so infalling material speeds up more rapidly and the collapse steepens sooner. ## What you see You can watch the inner sphere represent the collapsing core while the surrounding concentric shells show layers of matter falling inward. The arrows point toward the center to emphasize the inward pull of gravity. Beside the star, the graph tracks how density changes with time, so you can compare a gentler rise for lower core mass with a much steeper climb when the core mass is larger. ## Try it yourself - **Press the `Start collapse` button** and watch the core begin to shrink while the shells start moving inward. - **Move the `core mass` slider to a low value** and notice the shells fall inward more gradually and the density curve rises less sharply. - **Increase the `core mass` slider after collapse begins** and see how the shells speed up and the density graph bends upward more steeply. - **Compare two runs mentally: low mass vs high mass** and look for how stronger gravity leads to faster infall and a more dramatic density increase. - **Keep raising the mass while watching the center** to see how shrinking radius and increasing density reinforce one another during runaway collapse. ## Concept explanation The fate of a collapsing stellar core depends mainly on its **core mass**. If the core stays below a **critical mass**, **neutron degeneracy pressure** can resist gravity and hold the remnant up as a neutron star. If the core is more massive than that limit, gravity wins: pressure is no longer enough to stop the squeeze, so collapse continues and can lead toward black hole formation. ## What you see You are looking at a horizontal mass scale with a movable marker for the core's mass. The blue-toned region represents masses where a compact neutron star can remain stable, while the red-toned region marks masses where collapse keeps going. The highlighted remnant card shows which outcome matches the current marker position, and the central caption updates to show whether degeneracy pressure can still halt the collapse. ## Try it yourself - **Drag the core-mass marker** left and right across the scale and notice where the remnant switches from a neutron star to continued collapse. - **Use the core-mass slider** to move more precisely and compare values just below and just above the critical threshold. - **Pause near the dashed critical line** and watch how a small change in mass produces a different final fate. - **Press the reset button** to return to a lower-mass example and compare the stable case again. - **Look at the highlighted remnant panel** and connect the visual change to the idea that pressure support only works below the critical mass. ## Concept explanation A massive star can collapse so violently that its **core** rebounds and launches a shock wave outward, but that shock does not always have enough energy to throw off the star’s **outer envelope**. When the **explosion strength** is high, the shock keeps moving outward and leaves behind a small compact remnant such as a neutron star. When the explosion is weak or fails, the outward motion stalls, the envelope loses the battle against gravity, and material falls back onto the center. That extra infalling mass can push the remnant past the point where it can support itself, leading to **black hole** formation. ## What you see You are looking at a cross-section of the collapsing star. The blue inner region marks the bounce zone where the core briefly pushes outward, while the layered orange-red arcs represent the overlying stellar envelope. The curved shock front and arrows show whether material is escaping or falling back. As the strength changes, the lower readouts summarize how strongly the envelope is moving outward and how much mass is being added back onto the center. ## Try it yourself - **Move the `explosion strength` slider to a high value** and notice how the shock keeps advancing while the envelope layers are driven outward. - **Lower the slider below the weak-explosion threshold** and watch the outward motion stall before the layers reverse and fall back inward. - **Compare the central accretion bar at low and high strength** to see why fallback adds mass to the compact center. - **Toggle `Show labels`** to test whether you can still identify the bounce region, shock, and envelope from their motion alone. - **Press `Restart collapse`** after changing the slider so you can replay the sequence and compare successful ejection with fallback-driven black hole formation. ## Concept explanation A **black hole** forms when enough mass is squeezed into a small enough space that its surface lies inside the **Schwarzschild radius**. That radius marks the size of the **event horizon**, the boundary beyond which even light cannot escape. If the collapsing object is still larger than this critical boundary, light emitted nearby can get away. Once the object shrinks inside it, the escape paths close off and spacetime bends so strongly that outgoing light is pulled back inward. ## What you see You’re looking at a dark collapsing sphere and a fixed circular marker showing the Schwarzschild radius. The slider changes the sphere’s **collapsed radius**. When the sphere is larger than the marked boundary, the blue light rays pass across the scene and escape. When the sphere shrinks inside the boundary, the circle is highlighted and the rays turn gold, curve back, and fall inward to show that an event horizon has appeared. ## Try it yourself - **Drag the `collapsed radius` slider slowly left** and watch for the moment the dark object becomes smaller than the marked circle. - **Pause right near the boundary** to see the transition point where escape changes into trapping. - **Move the slider back to a larger radius** and notice that the event horizon highlight disappears and the light rays escape again. - **Toggle `Pulse horizon`** to make the critical boundary easier to track visually once the black hole has formed. - **Click `Reset rays`** after changing the radius to replay the light paths and compare the two regimes more clearly. ## Concept explanation Astronomers do not literally watch an **event horizon** appear. Instead, they infer black hole formation from changing messengers that escape before the center becomes hidden. A collapsing star can produce a burst of **neutrinos**, a changing **light curve**, and sometimes **fallback accretion**, where some material launched outward later falls back toward the center. When these signals shut off early, fade unusually fast, or show strong returning material, they point to a collapse outcome that is different from an ordinary successful explosion. ## What you see You are looking at three linked views of the same stellar collapse. The top panel shows how the star’s brightness changes with time, the middle panel shows the neutrino signal, and the bottom panel shows the star and its envelope as material either escapes or falls back inward. A moving vertical marker keeps the timing synchronized, so when you change the scenario or fallback amount, all three panels update together. ## Try it yourself - **Switch the scenario dropdown** between `Successful supernova` and `Failed supernova` to compare a strong explosion with a stalled one. - **Increase the fallback amount** and notice how the brightness becomes weaker, the neutrino signal cuts off differently, and more material returns to the center. - **Lower the fallback amount** to see a cleaner explosion with less returning mass and a more persistent outward envelope. - **Drag across the panels** to scrub through time and match features in the light curve, neutrino burst, and envelope motion. - **Press `Restart cycle`** and watch the full sequence again from core collapse onward.