chapter 15.1; the structure and composition of the Sun

features of the Sun

• does not have a solid surface or continents

• does not have a solid core

• Sun’s makeup is quite typical of stars

• sun’s temperature mean many of the atoms are ionised (stripped of one or more of their electrons)

  • means there is a large quantity of free electrons and positively charged ions in the Sun (the Sun this thus an electrically charged environment)

  • hot ionised gas: plasma

composition of the sun’s atmosphere

• 73% Sun’s mass = hydrogen

• 25% Sun’s mass = helium

• 2% Sun’s mass = oxygen, nitrogen, carbon

• most of the elements found in the Sun are in the form of atoms, all in gas form

• sun’s temperature mean many of the atoms are ionised (stripped of one or more of their electrons)

layers of the Sun

1. sun’s core

• extremely dense

• source of all the sun’s energy

• nuclear energy is released

• approx. 20% of the size of the solar interior

• temperature of 15 million K

  • hottest part of the Sun

2. radiative zone

• named for the primary mode of transporting energy across it

• stars at 25% of the distance to the solar surface and extends up to about 70% of the way to the surface

• ight generated at the core is transported through the radiative zone very slowly, since the high density of matter in this region means a photon cannot travel too far without encountering a particle, causing it to change direction and lose some energy

3. convective zone

• outermost layer of the solar interior

• thick layer of approx. 200,000km deep that transports energy from the edge of the radiative zone to the surface through giant convection cells

• the plasma at the bottom of the convective zone is extremely hot, and it bubbles to the surface where it loses its heat to space

• once the plasma cools, it sinks back to the bottom of the convective zone

solar photosphere

• layer where the Sun becomes opaque and marks the boundary past which we cannot see

• temperature of approx. 5800K

• energy that emerges from the photosphere was originally generated deep inside the Sun

  • this energy is in the form of photons, which make their way slowly toward the solar surface

  • outside the Sun, we can only observe those photos that are emitted into the solar photosphere (density of atoms is sufficiently low and the photos can escape without colliding)

• the thin region in which the solar atmosphere changes from almost transparent to almost opaque in a distance of just over 400km

• at a typical point of the photosphere, the pressure is less than 10% of Earth’s pressure at sea level, and the density is about 1/10,000 of Earth’s atmospheric density at sea level

granulation of photosphere

• photosphere has a mottled appearance

• granulation

• granules (700-1000km in diameter)

  • lifetime of each granule is 5-10 minutes

  • supergranules (35,000km diameter) last about 24 hours

• motion of granules can be studied by examining the Doppler shifts in the spectra of gases just above them

• the bright granules are columns of hotter gases rising at speeds of 2 to 3 kilometers per second from below the photosphere

  • As this rising gas reaches the photosphere, it spreads out, cools, and sinks down again into the darker regions between the granules

• Measurements show that the centers of the granules are hotter than the intergranular regions by 50 to 100 K.

chromosphere

• region of the Sun’s atmosphere that lies immediately above the photosphere

• visible when the photosphere is concealed by the Moon during a total solar eclipse

• 2000-3000km thick

• 10,000K

• spectrum consists of bright emission lines; this layer is composed of hot gases emitting light at discrete wavelengths

  • The reddish color of the chromosphere arises from one of the strongest emission lines in the visible part of its spectrum - the bright red line caused by hydrogen

transition region

• the part of the Sun where rapid temperature rise occurs between the chromosphere to the corona

corona

• temperature of >1 million degrees

• outermost part of the Sun’s atmosphere

• can be observed during a total eclipses

• extends millions of kms above the photosphere and emits about half the light of a full moon

• very low density

• 10^90 atoms per m³

solar wind

• Sun’s atmosphere produces a stream of charged particles (mainly protons and electrons)

  • the particles flow outward from the Sun into the solar system at a speed of about 400km/s

  • exists because the gases in the corona are so bot and moving so rapidly that they cannot be held back by solar gravity

  • Sun loses about 1-2 million tons of material each second through this wind

coronal holes

large dark regions of the corona that are relatively cool and quiet

• magnetic field lines stretch far out into space away from the Sun, rather than looping back to the surface

• solar wind comes predominantly from coronal holes, where gas can stream away from the Sun into space unhindered by magnetic fields

auroras

• charged particles accelerated by the solar wind can follow the field down into our atmosphere

• as the particles strike molecules of air, they case them to glow, producing curtains of light (auroras) (northern and southern lights)