Stellar atmosphere
The stellar atmosphere is the outer region of the volume of a star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

, lying above the stellar core
Solar core
The core of the Sun is considered to extend from the center to about 0.2 to 0.25 solar radius. It is the hottest part of the Sun and of the Solar System. It has a density of up to 150 g/cm³ and a temperature of close to 15,000,000 kelvin...

, radiation zone
Radiation zone
The radiation zone is the middle zone in the Sun's interior. Energy travels out of the core into the radiation zone. Energy that travels through the radiation zone is in the form of electromagnetic radiation. The radiation zone is so dense that the waves bounce around...

 and convection zone
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...

. It is divided into several regions of distinct character:
  • The photosphere
    The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

    , which is the atmosphere's lowest and coolest layer is normally its only visible part. Light
    Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

     escaping from the surface of the star stems from this region and passes through the higher layers. The Sun
    The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

    's photosphere has a temperature
    Effective temperature
    The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...

     in the 5,770 K
    The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

     to 5,780 K range. Starspot
    Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness...

    s, cool regions of disrupted magnetic field
    Magnetic field
    A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

     lie on the photosphere.

  • Above the photosphere lies the chromosphere
    The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....

    . This part of the atmosphere first cools down and then starts to heat up to about 10 times the temperature of the photosphere.

  • Above the chromosphere lies the transition region, where the temperature increases rapidly on a distance of only around 100 km.

  • The outermost part of the stellar atmosphere, is the corona
    A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...

    , a tenuous plasma
    Plasma (physics)
    In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

     which has a temperature above a million Kelvin. While all stars on the main sequence
    Main sequence
    The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

     feature transition regions and coronae, not all evolved stars
    Stellar evolution
    Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

     do so. It seems that only some giants
    Giant star
    A giant star is a star with substantially larger radius and luminosity than a main sequence star of the same surface temperature. Typically, giant stars have radii between 10 and 100 solar radii and luminosities between 10 and 1,000 times that of the Sun. Stars still more luminous than giants are...

    , and very few supergiant
    Supergiants are among the most massive stars. They occupy the top region of the Hertzsprung-Russell diagram. In the Yerkes spectral classification, supergiants are class Ia or Ib . They typically have bolometric absolute magnitudes between -5 and -12...

    s possess coronae. An unresolved problem in stellar astrophysics
    Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...

     is how the corona can be heated to such high temperatures. The answer lies in magnetic field
    Magnetic field
    A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

    s, but the exact mechanism remains unclear.

During a total solar eclipse
Solar eclipse
As seen from the Earth, a solar eclipse occurs when the Moon passes between the Sun and the Earth, and the Moon fully or partially blocks the Sun as viewed from a location on Earth. This can happen only during a new moon, when the Sun and the Moon are in conjunction as seen from Earth. At least...

, the photosphere of the Sun is obscured
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...

, revealing its atmosphere's other layers. Observed during eclipse, the sun's chromosphere appears (briefly) as a thin pinkish arc
Arc may refer to:-Mathematics:*Arc , a segment of a differentiable curve*Arc , a particular type of set of points of a projective plane*Arcminute, a measure used for angles, equal to 1/60th of a degree...

, and its corona is seen as a tufted halo
Halo (optical phenomenon)
A halo from Greek ἅλως; also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky...

. The same phenomenon in eclipsing binaries can make the chromosphere of giant stars visible.
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