December in Antarctica: The Sun never sets.The images are 1 hour apart.

The abundances of the chemical elements in the Sun (the cosmic abundances), relative to106 Si atoms.

Light is a form of electromagnetic radiation and consists of alternating electric and magnetic fields

The various types of electromagnetic radiation, also referred to as the electromagnetic spectrum.

Note: The shorter the wavelength of electromagnetic radiation, the higher is its energy.

The spectrum of visible (“white”) light can be separated into its various wavelength components with the aid of a glass prism, into

the colors of the rainbow.

Definition: 1 Å (Ångstrom) = 10-8 cm.

Bunsen’s discovery of the characteristic spectral absorption lines of the chemical elements.

Characteristic spectra of the chemical elements Li, Fe, Ba, Ca and white light

Determination of the abundances of the chemical elements in the Sun through the measurement of their spectra with a spectroscope

mounted on a telescope

Spectrum of our Sun, showing the spectral lines of the multitude of chemical elements present in our star

Spectrum of our Sun, indicating the presence of the spectral lines of the chemical element Fe

Schematic diagram of the interior make-up of the Sun.

Listed are the gas density and the temperature as a function

of depth in the Sun.

Note: density of water = 1,of rock =~ 3,

and of iron metal = ~ 8.

Relationship between the 3 temperature scales.

In the sciences, the Kelvin scale is commonly used.

Sunspots appear dark, because they are cooler

(~ 4,000°K) than the

surrounding photosphere of

the Sun (~ 5,800°K).

Sunspots occur in pairs

Galileo, the discoverer of

sunspots

Observation of sunspots on successive days show the

rotation of the Sun around its axis.

These observations indicate that the Sun does not rotate like a

solid body, but has differential rotation:

Sunspots at Sun’s equator rotate once around the Sun in ~ 25

days, whereas those at 75° N and S rotate once around the Sun in

~ 33 days.

Origin of sunspots by compression of magnetic field lines due to differential rotation of the star.

Sunspots occur in pairs, representing the N and S poles of the magnetic field lines

The number of sunspots varies from year to year, with an 11 year sunspot cycle.

The 11 year cycle in the frequency of sunspots, observed continuously back to Galileo

“Granules” in the photosphere of the Sun, 300-1,000 km across. Granules are convection cells, with hotter gases (~ 200°K) rising in bright centers with velocities of ~ 2 km/sec, and sinking in dark areas.

Solar flare

~ 10 Trillion megawatts

Solar eclipse, showing “solar wind”.Photographed in 1973 in Kenya.

The “Northern Lights”, or Aurora Borealis