Lecture 11: The Sun

Question 1

Sun’s Atmosphere: the photosphere

Answer 1

• The apparent surface, what we see, is called the photosphere
• this is where most of the photons that we see come from
• lowest and densest region of the Sun’s atmosphere
• features include sunspots and granulation, as well as magnetic fields
• Sun is brighter at centre
• not smooth
• sunspots are regions of very strong magnetic fields, trapped gas cools, also migrate in latitude
• hotter gas rises, cooler gas falls
• Sun rotates more rapidly at the equator than at the poles
• ionized equatorial gas drags magnetic field lines into twisted configuration
• sunspots form where twisted lines break the surface

Question 2

Solar Chromosphere

Answer 2

• Lies above the photosphere
• visible during an eclipse
• fainter than the photosphere because it is less dense
• reddish colour
• active region with prominences, spicules

Question 3

Solar Corona

Answer 3

• Invisible except under special conditions
• very low density
• outermost part of the sun’s atmosphere, very hot
• outer parts of corona escape the sun as solar wind

Question 4

Sun’s Atmosphere: Chromosphere and Corona

Answer 4

• The Chromosphere is a transition region between lower photosphere and outer corona
• Less dense and less bright than photosphere
• Features include flares, prominences, loops
• T increases from 6000K to 1,000,000 K from inner to outer boundary
• Corona is a low density faint, nearly a vacuum
• spectral lines of highly ionized elements are hotter than 1,000,000 K

Question 5

Magnetic field Lines

Answer 5

• Pairs of sunspots are connected by tightly wound magnetic field lines
• also trap ionized gas above the photosphere
• Solar prominences are regions of trapped gas that can extend way above the photosphere
  -Solar flares occur when twisted magnetic field lines snap

Question 6

The Sun’s interior

Answer 6

• Source of energy is nuclear fusion (Hydrogen converted to Helium)
• By-product of reaction are neutrinos
• Solar atmosphere is the visible disc, but a tiny % of Sun’s mass
• Sun’s Luminosity and temperature have changed very little over time
• Internal pressure balances gravity

Question 7

Nuclear Fusion

Answer 7

• The centre of the Sun is so hot that all of the atoms have had their electrons removed, making ionized gas, plasma
• most gas is hydrogen, meaning just singular protons
• Although they highly repel each other, they can sometimes bind together because of the extremely high temperature
• ## Binding continues until helium is formed