Radiative levitation is the name given to a phenomenon that causes the spectroscopically-derived abundance of heavy elements in the photospheres of hot

stars A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth ma ...

to be very much higher than solar abundance or than the expected bulk abundance; for example, the spectrum of the star Feige 86 has

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...

and

platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Pla ...

abundances three to ten thousand times higher than solar norms. The mechanism is that heavier elements have large photon absorption cross-sections when partially ionized (see opacity), so efficiently absorb photons from the radiation coming from the core of the star, and some of the energy of the photons gets converted to outward momentum, effectively 'kicking' the heavy atom towards the photosphere. The effect is strong enough that very hot white dwarfs are significantly less bright in the

EUV Extreme ultraviolet radiation (EUV or XUV) or high- energy ultraviolet radiation is electromagnetic radiation in the part of the electromagnetic spectrum spanning wavelengths from 124 nm down to 10 nm, and therefore (by the Planck ...

and

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

bands than would be expected from a

black-body A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...

model. The countervailing process is gravitational settling, where, in very high gravitational fields, the effects of

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...

even in a hot atmosphere are cancelled out to the point that the heavier elements will sink unobservably to the bottom and lighter elements settle on the top.

References

{{reflist Stellar phenomena

See also

References