Ch9 THE SUN AND STARS The star we see by day. Ch9 The Sun, Our Star The Sun is an average star. From the Sun, we base our understanding of all stars in.

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THE SUN AND THE SUN AND STARSSTARS

The star we see by day

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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.

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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)

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In The Core

• Density = 20 x density of Iron

• Temperature = 15,000,000 K

• Hydrogen atoms fuse together.

• Create Helium atoms.

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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!

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Solar Cross-Section

• Progressively smaller convection cells carry the energy towards surface.

• See tops of these cells as granules.

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H Sun

Photo by Big Bear Solar Observatory

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Prominences

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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!

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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.

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Magnetic fields and Sunspots

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

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Sunspots

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

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Magnetic fields and Sunspots

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

• Sunspots come in pairs.

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Stars

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Spectral Classificatio

ns

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Supergiants, Giants, and Dwarfs

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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.

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