Ch9Ch9

THE SUN AND THE SUN AND STARSSTARS

The star we see by day

Ch9Ch9

The Sun, Our Star

• The Sun is an average star.• From the Sun, we base our

understanding of all stars in the Universe.

• No solid surface.

Ch9Ch9

Vital Statistics

• Radius = 100 x Earth (696,000 km)• Mass = 300,000 x Earth (1.99 x 1030

kg)• Surface temp = 5,800 K• Core temp = 15,000,000 K• Luminosity = 4 x 1026 Watts• Solar “Day” =

– 24.9 Earth days (equator)– 29.8 Earth days (poles)

Ch9Ch9

In The Core

• Density = 20 x density of Iron

• Temperature = 15,000,000 K

• Hydrogen atoms fuse together.

• Create Helium atoms.

Ch9Ch9

Nuclear Fusion• 4H He• The mass of 4 H nuclei (4 protons):

4 x (1.6726 x10-27 kg) = 6.690 x 10-27 kg• The mass of He nuclei: = 6.643 x 10-27 kg• Where does the extra 4.7 x 10-29 kg go?• ENERGY! E = mc

• E = (4.7 x 10-29 kg ) x (3.0 x 108 m/s)2

• E = hc/ = 4.6 x 10-14 m (gamma rays)• So: 4H He + light!

Ch9Ch9

Solar Cross-Section

• Progressively smaller convection cells carry the energy towards surface.

• See tops of these cells as granules.

Ch9Ch9

H Sun

Photo by Big Bear Solar Observatory

Ch9Ch9

Prominences

Ch9Ch9

Ch9Ch9

Aurorae

• The solar windpasses outthrough theSolar System.

• Consists of electrons, protons and other charged particles stripped from the Sun’s surface.

• When charged particles and magnetic fields interact: light!

Ch9Ch9

Ch9Ch9

Solar Cycle

• Increase in solar wind activity- Coronal Mass Ejections

• Increase in Auroral displays on Earth• Increase in disruptions on and around

Earth.

Courtesy of SOHO/LASCO/EIT consortium.

Ch9Ch9

Magnetic fields and Sunspots

• At kinks, disruption in convection cells.• Sunspots form.

Ch9Ch9

Sunspots

• 11-year sunspot cycle.• Center – Umbra: 4500 K• Edge – Penumbra: 5500 K• Photosphere: 5800 K

Ch9Ch9

Magnetic fields and Sunspots

• Where magnetic fields “pop out” of Sun, form sunspots.

• Sunspots come in pairs.

Ch9Ch9

Ch9Ch9

Ch9Ch9

Ch9Ch9

Stars

Ch9Ch9

Ch9Ch9

Spectral Classificatio

ns

Ch9Ch9

Supergiants, Giants, and Dwarfs

Ch9Ch9

Stellar Radii

How big are stars?• We see stars have

different luminosities and different temperatures.

• Stars have different sizes.• If you know:

– Distance– Angular size

• Learn real size.

50 mas