The photosphere is the visible "surface" of a star: a relatively thin region in which the stellar gas becomes transparent enough that most of the outgoing photons escape to space. It is not a solid surface but a layer defined by optical depth, and it largely determines a star's apparent brightness, color and effective temperature. For the Sun the photosphere has an effective temperature of roughly 5,800 K and produces the familiar solar disk seen in white light.

Structure and physical properties

By convention the photosphere is located near the depth where the optical depth is of order unity (commonly approximated by tau ≈ 2/3 in the Rosseland mean). The layer is thin compared with the star's radius — for a solar-type star it extends over a few hundred kilometres — and shows a negative temperature gradient outward. Different wavelengths emerge from slightly different heights because of wavelength-dependent opacity; this means spectral lines and continuum sample overlapping but distinct layers.

Observable features

The photosphere displays a variety of phenomena visible in high-resolution images and spectra:

  • Granulation: a mottled pattern produced by convective cells carrying heat from the interior, with bright upflows and darker downflows.
  • Limb darkening: the apparent darkening toward the stellar edge because light from the limb comes from higher, cooler layers.
  • Magnetic features: sunspots (cool, dark regions), faculae (bright patches) and small-scale magnetic elements that alter local brightness and spectral lines.
  • Spectral lines: absorption features (often called Fraunhofer lines on the Sun) formed in the photosphere provide chemical composition, surface gravity and velocity diagnostics.

How it is studied

Astronomers study photospheres using imaging, high-resolution spectroscopy, polarimetry and time-series observations. Spectral line profiles reveal thermal broadening, Doppler shifts from motions, and Zeeman splitting from magnetic fields. Interferometry and eclipses or transits measure a star's apparent photospheric radius and limb-darkening laws. Techniques such as helioseismology and asteroseismology infer interior structure by analysing oscillations that interact with the photospheric layer.

Context and importance

The term photosphere derives from Greek roots meaning "light sphere." It should be distinguished from overlying layers such as the chromosphere and corona, which are hotter and more tenuous and emit different kinds of radiation (including ultraviolet and X-rays). Because most observational data about distant stars come from photospheric light and spectra, understanding photospheric physics — radiative transfer and opacity — is essential for determining stellar temperatures, compositions and magnetic activity; see radiative transfer and opacity and transparency. Photons interact with matter in wavelength-dependent ways; for more on those interactions and the heights from which photons of different wavelengths emerge see photon interactions and wavelength-dependent formation. For introductory material and further reading consult general resources on stellar atmospheres and observation introductory resources.