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

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

s of hot

stars A star is a luminous spheroid of plasma held together by self-gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night; their immense distances from Earth make them appear as fixed points of ...

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; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

and

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

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 A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that ...

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 A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place in a white dwarf; what ...

are significantly less bright in the EUV and

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

bands than would be expected from 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 chemical p ...

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