**2. Observing the sun and the stars**

The closest of the stars, the Sun, is a G-IV, main-sequence star, and is located in the center of the Hertzsprung-Russell (HR) diagram [7] classification (**Figure 5**). Its next evolutions would be to leave the main sequence, that is the oblique line crossing **Figure 5** toward the upper right. It is then going to inflate and shift to become eventually a red giant, moving further up in the upper right arm of the HR diagram. Once reach maximum inflation, a cascade of gravitational collapses will happen, ejecting material by major explosions. Gravity will compact the remaining into a white dwarf, a neutron star, in the midst of its ejecta, witnessed as a *(super) nova* remnant, a nebula. The transition from the super-giant, the (super) nova reducing into the remaining white dwarf, will take the star rapidly across the HR diagram, from the upper right to the mid-upper left, and crossing down in the visible arc in the bottom left side of **Figure 5**.

The central part of the Sun is composed of a core (a fourth of its radius) where thermonuclear reactions generate energy. It has an average density ten times that of lead, and a temperature of 15 10 K. The radiative zone of the Sun is about one-third of the Sun's radius. Both transfer energy by radiative forces of photons. The pathways undergo a random walk and act as an apparent solid body [8].

The photosphere is 300–500 Km deep, and the light is emitted from there before the plasma becomes opaque. This is also the layer that defines the effective temperature of the Sun, about 5.8 10 K, plasma convection is visible there under the form of granules of sizes measured in Mm (10 m).

**Figure 5.** *HR diagram, classification of stars evolution. Image credits: Wikimedia.*

#### **Figure 6.**

*High-resolution image of the sun from solar orbiter, showing magnetically bound plasma. Credit: ESA & NASA/ solar orbiter/EUI team; data processing: E. Kraaikamp (ROB). [9].*

The Sun's "atmosphere" (starting from **Figure 6**) is composed of the chromosphere and the corona (in that order). The chromosphere is 2000 Km deep and is the Sun's eclipse "red ring of fire." It has a steep drop in material density, and an initial temperature drop from 5.8 103 K to 3.5 10<sup>3</sup> K to eventually reach 35 10<sup>3</sup> K.

The corona is a very large volume above the chromosphere, vastly warmer too, made of ionized plasma of about 1106 K, with a majority of emission coming from Fe-XIV and Fe-X. It is the origin of the solar winds. Some areas with open magnetic fields yield faster solar winds (about 0.7 10<sup>6</sup> m/s).

We remotely sense the Sun (Solar Orbiter imagery in **Figure 6**), by analyzing electromagnetic spectra emitted from its activity. Thermodynamics and hydrodynamics applied to plasma with magnetic fields are all needed to study the radiative, convective, and exo-atmospheric conditions of the Sun energy transport.

More generally, stellar objects of different characteristics have long been observed and many physical theories have been developed relating observations and life cycles, that is, HR diagram in **Figure 5** and equations of stellar structure, respectively. Stellar oscillations [10], spherical harmonics, and resonance patterns analysis belong to geophysics and are now in common use to study and classify stars.
