--- title: "Why gas around a black hole shines" description: "Orbiting gas gives angular momentum outward and settles inward. The difference in orbital energy heats the disk and powers its radiation." canonical_url: "https://sandboxphysics.com/stories/accretion-disks/why-accretion-disks-shine/" language: "en" page_type: "story" alternate_languages: zh-CN: "https://sandboxphysics.com/zh/stories/accretion-disks/why-accretion-disks-shine/" last_modified: "2026-11-12" --- L1 · Concept # Why gas around a black hole shines Sandbox Physics · 2026-11-12 The bright gas around a black hole releases energy before it falls in. As the gas slowly settles into smaller orbits, some of its orbital energy becomes heat. The hot gas radiates, and that light can make the dark object at the centre conspicuous. Strong gravity alone does not make orbiting gas drop straight down. The Moon feels Earth's gravity throughout its orbit, yet it keeps circling. Gas around a black hole also has angular momentum, the quantity that describes its orbital motion. To move into a smaller, nearly circular orbit, it must give some of that angular momentum away. A supply of orbiting gas can form an **accretion disk**: a thin layer that circles the centre and slowly flows inward. The source of its heat becomes clear when we look at two neighbouring rings. ## The inner ring turns faster An inner ring makes its way around the centre faster than the ring just outside it. If the rings never interacted, the gas could keep orbiting without the sustained inward flow we want to explain. Internal forces let the rings exchange angular momentum. Physicists describe the pull between neighbouring parts of the gas as a **stress**. It drags the inner ring backward and pulls the outer ring forward. Angular momentum moves outward, allowing the inner gas to settle inward. How the disk produces these stresses is a further question. For now, we can ask what an existing stress does to the gas. This is the thin-disk picture used in [Zimmerman and colleagues' section 2](https://arxiv.org/pdf/astro-ph/0408209v1). The rings exchange equal amounts of angular momentum, but the energy exchange is unequal. The inner ring turns faster, so the drag takes orbital energy from it at a higher rate. The slower outer ring receives orbital energy at a lower rate. The difference can become heat within the gas. Gravity provides access to lower-energy orbits. The interaction between rings lets gas reach them and divides the released energy between transport and heating. A steady supply of gas can therefore sustain both inward flow and radiation. ## The hottest ring can lie outside the inner edge Heating generally becomes more concentrated toward the centre. Its behaviour near the inner edge also depends on how strongly the neighbouring gas still pulls there. A common simplification sets the torque, the turning effect of that pull, to zero at the inner edge. The local heating then falls to zero at the edge itself. Moving outward from that edge, we first reach hotter gas, pass through a temperature maximum, and then reach progressively cooler gas. A picture that simply gets hotter all the way to the centre misses this boundary effect. An additional torque at the inner edge changes the heating. [Figures 1 and 2 of the Zimmerman paper](https://arxiv.org/pdf/astro-ph/0408209v1#page=7) compare two choices of boundary condition: the temperature profiles differ, and so does the combined radiation. For the teaching example used here, the maximum effective temperature is about 2.18 million kelvin, roughly 121 kilometres from the centre. Effective temperature means the temperature a perfect blackbody would need to emit the same total energy per unit surface area. Much of the thermal emission from gas this hot lies at X-ray energies. The chosen central mass is $2.00\times10^{31}\,\mathrm{kg}$; changing the model parameters changes these temperatures and lengths. ## Many rings contribute to one spectrum A hot surface emits light across a range of frequencies. A hotter surface supplies more high-frequency radiation. A cooler one contributes more toward lower frequencies. A **spectrum** shows how the radiation is distributed over frequency. A disk contains a whole sequence of temperatures. Its hot inner rings and cool outer rings contribute together, producing a broad thermal spectrum. Assigning one temperature to the whole disk loses this radial structure. [Appendix E of Li and colleagues' paper](https://arxiv.org/pdf/astro-ph/0411583v2#page=54) distinguishes local temperature, local emission and the radiation received far away. The following chapters build these steps in order. The orbiting motion of the gas and the bending of light add another layer. One side of the image can appear brighter, and light from the far side can curve into our view. The shape of the bright region contains information about both the emitting gas and the path its light takes. The companion experiment prescribes Newtonian thin-disk thermal emission locally, then propagates the light through the Schwarzschild spacetime of a nonrotating black hole. Its colours encode the specified radiation intensity. These false colours do not predict the view through a human eye. Brighter tones represent greater radiation intensity. ## A question to take with you Suppose every ring could orbit perfectly and neighbouring rings exchanged no angular momentum. Would the gas still settle steadily inward? In this ideal picture, the mechanism for that inward flow is missing. Gas must give angular momentum to something else before it can enter a smaller near-circular orbit. That exchange connects the disk's inward flow to its heat and light. The next chapter, Where a thin disk heats, turns the energy difference between neighbouring rings into a temperature profile. ## Sources 1. [Zimmerman and colleagues' section 2](https://arxiv.org/pdf/astro-ph/0408209v1) 2. [Figures 1 and 2 of the Zimmerman paper](https://arxiv.org/pdf/astro-ph/0408209v1#page=7) 3. [Appendix E of Li and colleagues' paper](https://arxiv.org/pdf/astro-ph/0411583v2#page=54) ## Keep reading How accretion disks shine · Reading path ### How accretion disks shine 1. [You are here **Why gas around a black hole shines**](https://sandboxphysics.com/stories/accretion-disks/why-accretion-disks-shine/) 2. [Article **Where a thin disk heats**](https://sandboxphysics.com/stories/accretion-disks/thin-disk-torque-temperature/) 3. [Article **Adding the spectra of differently heated rings**](https://sandboxphysics.com/stories/accretion-disks/adding-disk-blackbody-spectra/) 4. [Article **How frequency and brightness change on the way out**](https://sandboxphysics.com/stories/accretion-disks/photon-frequency-and-brightness/) 5. [Article **Which disk surface a light path meets first**](https://sandboxphysics.com/stories/accretion-disks/first-opaque-disk-crossing/) 6. [Article **Recompute a disk image and spectrum from saved intersections**](https://sandboxphysics.com/stories/accretion-disks/recompute-frozen-disk-light/) 7. [Article **How to read a thin-disk spectrum paper**](https://sandboxphysics.com/stories/accretion-disks/reading-thin-disk-spectrum-paper/) 1 more group ### Other articles that use the same experiments Each experiment from this article that another series also uses, followed by the articles there. 1. [Lab **Black Hole Light Lab**](https://sandboxphysics.com/labs/black-hole-light/) 2. [Article **Why collapse does not need a perfectly spherical star**](https://sandboxphysics.com/stories/astronomy-gravity/penrose-trapped-surfaces/)