Editing Poynting–Robertson effect (section)

== Cause ==

The effect can be understood in two ways, depending on the [[Frame of reference|reference frame]] chosen.

[[Image:Poynting-Robertson effect.svg|thumb|312px|Radiation from a star (S) and thermal radiation from a particle seen (a) from an observer moving with the particle and (b) from an observer at rest with respect to the star.]]
From the perspective of the grain of dust circling a star (panel (a) of the figure), the star's radiation appears to be coming from a slightly forward direction ([[aberration of light]]). Therefore, the absorption of this radiation leads to a force with a component against the direction of movement. The angle of aberration is extremely small since the radiation is moving at the [[speed of light]] while the dust grain is moving many orders of magnitude slower than that.

From the perspective of the star (panel (b) of the figure), the dust grain absorbs sunlight entirely in a radial direction, thus the grain's angular momentum is not affected by it. But the re-emission of photons, which is isotropic in the frame of the grain (a), is no longer isotropic in the frame of the star (b). This [[anisotropic]] emission causes the photons to carry away angular momentum from the dust grain.

The Poynting–Robertson drag acts in the opposite direction to the dust grain's orbital motion, leading to a drop in the grain's angular momentum. While the dust grain thus spirals slowly into the star, its [[orbital speed]] increases continuously.

The Poynting–Robertson force is equal to
: <math>F_\text{PR} = \frac{v}{c^2} W = \frac{r^2 L_{\odot}}{4 c^2} \sqrt{\frac{G M_{\odot}}{R^5}},</math>
where ''v'' is the grain's velocity, ''c'' is the [[speed of light]], ''W'' is the power of the incoming radiation, ''r'' the grain's radius, ''G'' is the universal [[gravitational constant]], ''M''<sub>☉</sub> the [[Sun]]'s mass, ''L''<sub>☉</sub> is the solar luminosity, and ''R'' the grain's orbital radius.