The Sun

★ Basic characteristics

The Sun

★ Luminosity: the total amount of energy a star radiates per

second

★ The Sun's luminosity is 4×10²⁶ (400 trillion trillion)

Watts

★ At the distance of the Earth from the Sun, the Sun's light

is being spread out over 2.8x10²³ m²

○ Intensity: I = L/(4πR²) = 1400 W/m²

The Nature of Light

The Inverse Square Law

I = L/(4πR²)

The Sun

★ The Sun from the outside

has some notable features

○ Corona

○ Sun Spots

★ More complex than meets

the eye

The Sun

★ Core

★ Radiative Zone

★ Convective Zone

★ Photosphere

★ Corona

Top Hat Question

The Sun

★ Evidence of convective heat

transfer in the Sun

○ Granulation: hotter regions

(about the size of Texas) are

brighter

○ Cooler regions are darker

○ Following Stefan's Law: the

intensity of a fixed area of

ideal thermal emitter rises

quickly with temperature

The Sun

★ Why doesn’t heat transfer work the same way throughout the

Sun?

○ Farther from the core, the temperature goes down

○ As the temperature gets lower, the plasma (gas made of ions)

inside gets more opaque (not see through)

○ Far enough from the center of the Sun, energy can't get anywhere

radiatively

The Sun

★ Due to hydrostatic equilibrium,

if energy can't get out at a

certain place:

1) Temperature goes up

2) Pressure goes up

3) Gravity isn't enough to hold

material in place anymore and

pressure wins

The Sun

★ The temperature of the

photosphere - the outermost

opaque layer is 5800 K

★ Because it is opaque and an

excellent absorber, we can

treat the photosphere like an

ideal thermal emitter

The Sun

★ The photosphere can be

approximated as a blackbody

★ Cooler atoms in outer

layers absorb light at

particular wavelengths

★ This spectrum has

absorption from 67

different elements

The Sun

★ The photosphere can be

approximated as a blackbody

★ Cooler atoms in outer

layers absorb light at

particular wavelengths

★ This spectrum has

absorption from 67

different elements

The Sun

★ The photosphere can be

approximated as a blackbody

★ Cooler atoms in outer

layers absorb light at

particular wavelengths

★ This spectrum has

absorption from 67

different elements

The Sun

★ The spectrum of the Sun

is further modified by

absorption in our

atmosphere when we

observe it from the

surface of Earth

The Sun

★ The Corona

○ >3 million degrees, but

extremely low density

○ Extends to 10 million km

from the Sun

○ Visible during a solar

eclipse

○ Some material flows outward

("solar wind")

The Sun

★ The Corona

○ Traces out the Sun’s

magnetic field

The Sun

★ Density is 150x higher than water

at the Sun’s core

★ Density is 6000x lower than air

at the photosphere

★ This occurs because the Sun is in

Hydrostatic Equilibrium

The Sun

★ Hydrostatic equilibrium:

○ Inward gravitational force must

be balanced by outward pressure

○ Otherwise the Sun would

expand/explode contract/implode

○ Pressure depends on temperature

& density of gas

The Sun

★ All of the energy produced in

the core is eventually

transferred all the way to

the surface

★ The light in the radiative

zone is much higher energy

than the light radiated from

the photosphere

The Sun

★ Sun Spots

○ First discovered by Galileo

○ Dark spots come and go,

following the rotation of

the Sun

○ Evidence against the

classical picture of objects

beyond the Earth all being

perfect and unchanging

The Sun

★ The Sun has a powerful magnetic

field running through it

★ Magnetic fields resist being

bent or compressed and interact

with ionized gas

○ The gas will move with the

field, and the field will move

with the gas

The Sun

★ Sunspots come and go,

typically in a few days

★ They are linked in pairs

by the Sun's magnetic

field

The Sun

★ Sunspots appear dark because they

are slightly cooler than their

surroundings: 4500 K vs. 5800 K for

typical photosphere (remember

Stefan’s Law!)

★ Magnetic 'pressure' keeps them from

being squashed, even though they

are cooler than surroundings

The Sun

★ Sunspots originate when magnetic field lines are distorted

by Sun’s varying rotation rate

○ To limit twisting, field bursts out of surface

The Sun

★ We observe an 11-year sunspot cycle, during which sunspot

numbers rise, fall, and then rise again

○ We are near maximum now

★ Every 11 years the Sun's north and South magnetic poles swap

positions!

The Sun

★ Solar Flares

○ Large explosions on Sun’s surface lasting seconds or minutes

○ Associated with the Sun's magnetic field suddenly adjusting its

configuration

○ Sometimes associated with large outflows of particles (Coronal

Mass Ejections)

The Sun

★ Solar Wind

○ Particles escape the Sun where its magnetic field extends

outward

○ Particles fly out at 400-800 km/sec (900,000 miles per hour)

○ The Sun loses 2 million tons per second this way

■ Only ~.01% of its mass

mass over its lifetime

The Sun

★ Sun-Earth magnetic interactions

The Sun

★ Aurora Borealis

The Sun

★ The Sun is powered by nuclear fusion

○ This is when particles (often nuclei) collide

○ In general: nucleus 1 + nucleus 2 → nucleus 3 + energy

The Sun

★ Atoms and nuclei

○ Atom: a bound state of protons, neutrons and electrons

○ Nucleus: the center of an atom made up of protons and neutrons

○ Bohr model (1913): electrons orbit the nucleus of an atom

○ Modern models say that the electron doesn’t orbit but is in a

cloud of possible locations (quantum mechanics)

The Sun

★ Components of atoms

○ Electrons: light, negatively charged particles

○ Protons: ~2000x more massive than electrons, positively charged

○ Neutrons: about as massive as protons, no electric charge

★ Atoms have an equal # of protons and electrons so they are

neutral (not net charge)

★ Ions/plasma have electrons stripped away due to collisions

and therefore have some net charge

Next Time

★ Wrap up the Sun

○ More on nuclear fusion

○ Observations of the Sun