Background on Accretion Disks

An accretion disk forms when gas spirals toward a central body, such as a star, white dwarf, neutron star, or black hole. Conservation of angular momentum causes the inflowing material to spread out into a disk, gradually losing energy through friction and radiation as it moves inward. This process converts gravitational potential energy into heat, often making the disk one of the brightest objects in the system. Accretion disks power phenomena ranging from protostellar growth to quasars that outshine their host galaxies. Estimating the disk temperature helps astronomers interpret observations across the electromagnetic spectrum.

Thin Disk Approximation

The simplest analytical model for an accretion disk is the thin disk approximation developed by Shakura and Sunyaev. It assumes that the disk height is much smaller than its radius and that energy released through viscous dissipation is radiated locally. The effective temperature at a given radius can then be written as

$T={\frac{3GM\mathrm{\dot{}M}}{8\pi \sigma r^3}}^{1/4}$

where $G$ is the gravitational constant, $M$ is the mass of the central object, $\mathrm{\dot{}M}$ is the mass accretion rate, $\sigma$ is the Stefan-Boltzmann constant, and $r$ is the radial distance from the center. This expression assumes Newtonian gravity and ignores relativistic corrections, which become important near black holes. The temperature typically peaks close to the inner edge of the disk and decreases with radius.

Choosing Units

To keep calculations straightforward, our form requests mass in solar masses, accretion rate in solar masses per year, and radius in kilometers. Internally, the script converts these values to SI units: kilograms, kilograms per second, and meters. The solar mass is $1.989\times {10}^{30}\mathrm{kg}$, and one year equals $3.154\times {10}^{7}s$. Plugging the converted values into the thin disk formula yields a temperature in kelvin.

Using the Calculator

Enter the mass of the central object, the steady-state accretion rate, and the radius at which you want the temperature estimate. The script computes the temperature according to the formula above. Because accretion disks often span a wide range of radii, you may want to try several values to see how temperature falls off with distance. Keep in mind that close to the innermost stable circular orbit of a black hole, relativistic effects can raise the temperature beyond the simple prediction.

Physical Interpretation

The computed temperature represents an effective blackbody temperature of the disk surface at the specified radius. In reality, the disk may not radiate exactly as a blackbody, and additional processes like Compton scattering or magnetic turbulence can modify the spectrum. Nevertheless, the thin disk temperature provides a first-order estimate that guides observational strategies. For instance, an inner disk temperature of tens of thousands of kelvin suggests that the disk emits strongly in the ultraviolet, whereas a temperature of millions of kelvin would produce X-rays.

Astrophysical Examples

In an accreting binary system where a white dwarf pulls matter from a companion star, typical mass accretion rates might be 10⁻⁹ to 10⁻⁸ solar masses per year. At a radius of 10,000 km around a one-solar-mass white dwarf, the thin disk formula gives temperatures of a few tens of thousands of kelvin. In contrast, a supermassive black hole accreting a solar mass per year near its event horizon can reach millions of kelvin, producing intense X-ray emission. Exploring how mass and accretion rate affect temperature helps explain why different astronomical sources have widely varying spectra.

Limitations

While the thin disk model captures many qualitative features, real accretion disks can deviate significantly due to magnetic fields, radiation pressure, and general relativity. For black holes, the inner edge of the disk is usually taken to be a few times the Schwarzschild radius, inside of which material plunges rapidly toward the event horizon. Viscous processes that transport angular momentum are often parameterized with an "alpha" prescription, which we do not explicitly include here. Thus, the calculator should be viewed as a tool for gaining intuition rather than a substitute for detailed numerical simulations.

Conclusion

Accretion disks convert gravitational potential energy into radiant heat, powering some of the most luminous objects in the universe. By estimating the disk temperature at a given radius, astronomers can predict the wavelengths where the disk shines brightest and compare theoretical models with observations. Use this calculator to explore how varying mass, accretion rate, and radius influence the thermal structure of these fascinating cosmic engines.