The Structure and Layers of the Sun: A Comprehensive Guide, Study notes of Acting

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The Structure of the Sun

CESAR’s Booklet

How stars work

In order to have a stable star, the energy it emits must be the same as it can produce. There must be an equilibrium. The main source of energy of a star it is nuclear fusion, especially the proton-proton chain, which can transform hydrogen into Helium. The energy generated in the core is transported outside by two main mechanisms: radiation and convection. The radiation process consists just of photons emitted by the Sun, and the convection means huge movements of material all throughout its interior, as you can see on the cover. Scientists call this balance “ hydrostatic equilibrium ”. There are two main forces acting in a star:  Gravitational contraction: it is due to the higher layers, this force pushes mass to the center.  Radiation pressure: it is produced by the inner layers, and it forces material upwards.

The different layers of the Sun

The Sun, like other stars, is a huge spherical object made of hydrogen and helium. Its diameter reaches

1.400.000 km, or 109 times the Earth’s diameter; but is 4 times less dense than the Earth due to its composition. The Sun is not only made of the glowing gas that we see with a telescope. It has, exactly like the Earth, different layers at different temperatures. Every layer has its own features which makes them interesting. Below is a figure of the structure of the Sun with all the different layers and components named. Figure 1 : A slice of the Sun. The nuclear fusion reactions occur i n its center Credit: NASA

vi. The corona: It is the biggest and least dense structure of the Sun and it surrounds it. Composed of plasma escaping from the Sun that reaches 1.000.000 kelvin, but with a density even lower than the chromosphere. Furthermore, the solar wind transports the material of the corona out to the interplanetary medium. From Earth, the corona is only visible during a total solar eclipse. Type of radiation Temperature Thickness of the layer Density Photosphere 8000 – 4500 K 500 km (^) ~ 10 -^4 kg/ m^3 Chromosphere 4500 - 20000 K 1600 km^ ~ 10 -^8 kg/ m^3 Transition Zone 20000 – 106 K 100 km^ ~ 10 -^10 kg/ m^3 Corona 106 - 3x 106 K >^107 km^ ~ 10 -^13 kg/ m^3 Table 1 : Temperature, density and size of every layer of the Sun Credit: (CESAR) Neither their temperature nor their composition are the same, so the solar activity on these layers is very different. These different features are:

1. The sunspots: When looking at the Sun through an adapted telescope (or by projection), dark spots can sometimes be observed. These dark, continuously changing areas are called sunspots and they are generated on the photosphere. They appear darker to the eye for the reason that their temperature is not as high as the neighbouring gas. This happens because the temperature of the sunspots vary from 3000 to 4500 K, meanwhile in the rest of the photosphere the temperature is usually around 5780K. Moreover, a very high concentration of magnetic field lines go outside and inside the Sun through the sunspots. Sunspots usually pop up in pairs or in groups, and they usually appear in belts to the north and south of the Sun’s equator. Figure 2 : Magnetic Field’s lines of the sun visualized (Credit: CESAR)

2. The flares: Not only the sunspots indicate the Sun’s activity, also solar flares do. The Sun usually ejects material from its chromosphere and corona. This material contains a huge amount of energy that the Sun releases in form of flares. From Earth, these flares are observed as a flash of light that increase the brightness of the Sun in that region. Sometimes, these flares are extremely powerful and the material ejected (typically electrons and hydrogen atoms) escape from the Sun´s gravitational field so they are free to travel through the Solar System. Figure 3 : Solar “coronal rain” – some of the material released from a solar flare – falling back down to the Sun. Earth to Scale (Credit: NASA) 3. The prominences: They are also called filaments, and they are formed in the corona. A prominence is a huge structure of gas at a very high temperature that is held up by magnetic field lines above the surface of the Sun. We often observe them with long, twisted shape. 4. The coronal mass ejections (CMEs): It may occur that the twisted magnetic field holding up a prominence becomes unstable and rises up very suddenly and quickly. If that occurs, the material of the prominence could be released out of the Sun, reaching a speed of 1000 km/s. This is known as a CME. They sometimes happen at the same time as flares, but where the flare releases light, a coronal mass ejection releases material. They usually last several hours, until all the twisted magnetic field lines are finally broken and rearranged. These ejections release huge amount of matter and electromagnetic energy (EM radiation) when this occurs, and if the ejection is pointed at the Earth, in 2 – 5 days an intense flux of particles will arrive to Earth. The CME also brings twisted magnetic field with it, which can play a little with the magnetic field of the Earth, causing aurorae and other effects. All the phenomena mentioned are closely related to the solar cycle.

Besides the techniques mentioned above, three more methods have been proven to be useful.  One is to relate the connection between the number of days the Earth is affected by geomagnetic turbulence from the sunspot cycle and the amplitude of the coming maximum.  The second method is to develop an index that helps us conclude the value of geomagnetic fields at sunspot minimum which the geomagnetic field correlates to during the (following) minimum.  And the third method is to create/develop a geomagnetic index which has one component in phase with the sunspot number, and an additional component which stays as the signal and happens as a magnetic maximum close to the sunspot minimum.

How astronomers study the Sun

Astrophysicists study the Sun in very different ways, and they are complementary: they use ground- based and satellite telescopes. Every layer of the Sun is very different , and requires a distinct instrument, and as they study it in all the electromagnetic spectrum, scientists obtain as much information as possible from the Sun. Just as we previously said we can get information from:

• Inner Sun
• Photosphere
• Chromosphere
• Corona Figure 5 : Different views of the Sun Credit: SOHO (NASA/ESA)

1. Inner Sun The core of the Sun is the most dense part of it. It is so dense that photons take more than 200 0 years, following the “random walk” movement, until reaching the convection region. Scientists use helioseismology to study this part of the Sun, just as in Earth we study the interior by measuring earthquakes (seismology). Most of the information we know about the core of the Sun has been studied with ESA’s SOHO mission. In particular with GOLF ( G lobal O scillations at L ow F requencies) instrument. Figure 6 : Artist's impression of SOHO. (Credit: ESA/ATG medialab)
2. Photosphere The photosphere is also known as the Sun’s surface, so it was the first part scientists started to study, as it can be seen even with our eyes (WARNING: ALWAYS WEAR PROTECTION WHEN LOOKING AT THE SUN). Everything that occurs in this layer is strongly responsible for what happens in the atmosphere above, so is vital to study the photosphere. ESA also study this region of the Sun with SOHO mission, but with other instruments: MDI ( M ichelson D oppler I mager) and VIRGO ( V ariability of Solar IR radiance and G ravity O scillations) are in charge of the photosphere. There are many ground telescopes pointing at the Sun studying the photosphere as well. For example, ESA uses the HELIOS Observatory placed on ESAC (Madrid, Spain) and fully operated by CESAR. Figure 7 : Sun photos from CESAR observatory, processed by Abel de Burgos (ESA/CESAR). The image on the left shows the photosphere, while the image on the right shows the chromosphere, which looks red