The Properties of Our Sun

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Journey from the Sun’s core to the edge of its ‘atmosphere.’

See where its light originates.

See what the different regions of the Sun are like.

See how energy in the core makes it to the light we see on Earth.

Slide 11

In The Core

Density = 20 x density of Iron

Temperature = 15,000,000 K

Hydrogen atoms fuse together

Create Helium atoms.

Slide 12

Nuclear Fusion

4H  He

The mass of 4 H atoms:

4 x (1.674 x10-27 kg) = 6.694 x 10-27 kg

The mass of He atom: = 6.646 x 10-27 kg

Where does the extra 4.8 x 10-29 kg go?

ENERGY!  E = mc2

E = (4.8 x 10-29 kg ) x (3.0 x 108 m/s)2

E = hc/l  l = 4.6 x 10-14 m (gamma rays)

So: 4H  He + light!

Slide 13

The Radiation Zone

This region is transparent to light.

Why?

At the temperatures near the core all atoms are ionized.

Electrons float freely from nuclei

If light wave hits atom, no electron to absorb it.

So: Light and atoms don’t interact.

Energy is passed from core, through this region, and towards surface by radiation.

Slide 14

The Convection Zone

This region is totally opaque to light.

Why?

Closer to surface, the temperature is cooler.

Atoms are no longer ionized.

Electrons around nuclei can absorb light from below.

No light from core ever reaches the surface!

But where does the energy in the light go?

Energy instead makes it to the surface by convection.

Slide 15

Convection

A pot of boiling water:

Hot material rises.

Cooler material sinks.

The energy from the pot’s hot bottom is physically carried by the convection cells in the water to the surface.

Same for the Sun.

Slide 16

Solar Cross-Section

Progressively smaller convection cells carry the energy towards surface.

See tops of these cells as granules.

Slide 17

The Photosphere

This is the origin of the 5800 K blackbody radiation we see.

Why?

At the photosphere, the density is so low that the gas is again transparent to light.

The hot convection cell tops radiate energy as a function of their temperature (5800 K).

l = k/T = k/(5800 K)  l = 480 nm (visible light)

This is the light we see.

That’s why we see this as the surface.

Slide 18