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Stellar Structure
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Légende : Stars are balls of plasma. For most of a star’s life it burns hydrogen into helium in its core. This phase of a star’s life is known as the main sequence. Burning hydrogen into helium produces heat, that heat travels out of the star’s core eventually reaching the star’s photosphere (often referred to as the “surface” of the star). From here the heat can radiate into space as various forms of electromagnetic radiation. However, how heat travels from the core to the photosphere depends on the star’s mass.
Imagine a parcel of gas rising inside a star. As it rises, it moves into an area of lower pressure, so it cools down and expands. If the parcel is still hotter, and therefore less dense than its surroundings, it keeps moving upward due to buoyancy. Eventually, it will rise far enough to cool and sink back down. This rising and sinking cycle is called convection. Whether convection occurs depends on how quickly temperature changes as you move away from the star’s core. If the temperature in a star drops rapidly, rising parcels of gas are more likely to stay hotter than their surroundings, so convection dominates as the mode of energy transfer in this part of the star. Conversely if the temperature drops more slowly (i.e. if the temperature gradient is small) then heat will mostly be transferred by radiation (photons).
In the most massive main sequence stars (more massive than about 1.5 times the mass of the Sun, seen here on the left), hydrogen is burned into helium using the CNO cycle. This is highly temperature dependent and thus energy production is concentrated near the center of the star. This leads to a larger temperature gradient and thus a convective core. Further out the temperature gradient becomes smaller and heat transport is dominated by radiation. This is called the radiative zone.
For lower mass stars like the Sun (between 0.3 and 1.5 solar masses, seen here in the middle) hydrogen is burned to helium using a different process (the pp chain). This depends less on the internal temperature than the CNO cycle and so energy production is more distributed in the star’s core. This leads to a smaller temperature gradient and thus a radiative core where convection occurs surrounded by a radiative zone. Going further out the gas becomes cool enough for some elements to hang to on some of their electrons, i.e. not being completely ionised. This partially ionised gas is more opaque to photons, trapping heat. This leads to a large temperature gradient and thus convection.
The lowest mass stars (below 0.3 solar masses, seen here on the right) have no radiative zone and are fully convective.
The arrows in the radiative zone are shown as wavy lines heading out of the star. However, a photon’s journey out of a star is much more complex with each individual photon travelling only a short distance before being deflected by some of the charged particles that make up the plasma of the star’s interior. This leads to a long and winding road that takes millennia instead of the few seconds it would take if the photon did not interact with particles in the plasma.
Crédit : Based on a vector diagram by Wikimedia user Д.Ильин which itself is based on a diagram from sun.org
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Termes du glossaire:
Rayonnement électromagnétique , Fusion de l'hydrogène , Photosphère , Zone radiative , Structure stellaire , Zone de convection , noyau stellaire
Catégories :
Étoiles
Licence : Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Icônes
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Also available in black and white
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Close-up view of a sunspot
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Légende : This image of a sunspot was taken by the Daniel K Inouye Solar Telescope (DKIST), operated by the U.S. National Science Foundation. It was taken only in light with a wavelength of 530 nanometers, within the greenish-yellow part of the visible spectrum.
The picture reveals the detail of the spot's structure and the Sun’s photosphere. The dark central region, known as the umbra, is surrounded by a lighter area called the penumbra with radially elongated features stretching towards the umbra. Note that the umbra and penumbra here are not the same as the umbra and penumbra that occur during an eclipse.
The sunspot measures approximately 5000 kilometres in diameter, roughly equivalent to the east-west span of China. While the umbra appears black, it is actually hot and bright. It only appears dark because it is a few thousand kelvin cooler than the surrounding solar photosphere. Surrounding the sunspot, granulation patterns of plasma are visible on the photospheric surface of the Sun.
Crédit : NSO/NSF/AURA
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Termes du glossaire:
Photosphère , Soleil , Tache solaire , Granulation
Licence : Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Icônes
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H-alpha image of the Sun's chromosphere
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Légende : This false-color image was captured with a 10-cm telescope at the Big Bear Solar Observatory (BBSO) in the United States in July 2002. It depicts the full disk of the Sun using the H-alpha emission line of hydrogen. When observed in this spectral line, the Sun's chromosphere appears particularly prominent due to hydrogen atoms emitting light at the specific wavelength. This emission produces a distinctive red color, making features such as spicules (jets of plasma that look hair-like) and plage (bright patches in the chromosphere) highly visible. Several small solar prominences can be seen protruding from the edge of the solar disk. When prominences (also known as filaments) cross the face of the disk they appear as dark threads caused by the cooler material in the prominence absorbing light. The chromosphere is also visible in the violet part of the solar spectrum due to ionized calcium showing emissions in these wavelengths.
Crédit : Big Bear Solar Observatory (BBSO)/New Jersey Institut of Technology (NJIT)
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Termes du glossaire:
Chromosphère , Protubérance solaire , Soleil , Raie spectrale
Catégories :
Le Soleil
Licence : Domaine Public Domaine Public Icônes
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High-resolution image of the surface of the Sun
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Légende : This high-resolution image of a small portion of the outermost visible "surface" of the Sun (the photosphere) covers an area 36,500 by 36,500 km. It was one of the first images taken by the Daniel K. Inouye Solar Telescope, as part of the telescope's Science Verification Phase. Each of the cells that are visible is about the size of the US state Texas, or of France, or of Afghanistan, or of Somalia. In the brighter centers of these cells, plasma from the underlying regions rises to the surface, cools off, and then sinks down again at the location of the darker lanes delineating the cells. In these dark lanes we can also see the tiny, bright markers of magnetic fields.
Crédit : NSO/NSF/AURA
Lien du crédit
Termes du glossaire:
Photosphère , Granulation
Catégories :
Le Soleil
Licence : Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Icônes
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The Sun in our box Activités pédagogiques de astroEDU (liens vers le site astroEDU) Description : Comparing the size of the Sun and the Earth building and using a pinhole camera.
Termes du glossaire:
Soleil
Catégories :
Mathématiques
, Physique
, Le Soleil
Tranches d'âge :
10-12
, 12-14
Niveau scolaire :
Collège
Domaines d'apprentissage :
Groupes de discussion
, Basée sur l'observation
, Apprentissage par projet
, Apprentissage par enquête
Coûts :
Faible
Durée :
3h
Taille du groupe :
Par groupe
Compétences :
Développer et utiliser des modèles
, S'engager dans des arguments fondés sur des preuves
, Utiliser les mathématiques et la pensée computationnelle
Auteurs : Aysegul Yelkenci, Korhan Yelkenci, Mert Koçer
Licence : Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Creative Commons (CC) Attribution 4.0 International (CC BY 4.0) Icônes
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