Stellar Interiors

Astronomy 315Professor Lee

CarknerLecture 10

What is Inside?

Why does a given star have a given T, R, M and L? How are T, R, M and L related? Must be due to what is going on in the

star

The Insider

Can only see the very outer layers directly

Our best information comes from the Sun

Energy

However, the energy must somehow get to the surface and pass through the regions along the way

We want to determine 2 things:

How is energy transported?

Equations of Stellar Structure

Weight of each layer of a star is balanced by

the pressure of the layers beneath it

A star is a sphere and as you move out from the center you enclose more and more mass (no gaps)

Relationship between pressure, temperature

and density

Hydrostatic Equilibrium

Star wants to contract

Star wants to expand Star must be in a state where gravity

and thermal pressure balance

Basic Hydrostatic Equilibrium

Cross Section of H.E.

Central T and P We can use the equations to stellar structure to find

things like the pressure and temperature

We know mass from binary stars

PC = 3 X 109 atmospheres for Sun

TC = 1.5 X 107 K for Sun

Stellar Models

Requires large computing power

Temperature, pressure and density are strongest near the core and fall off toward the surface

Model of the Sun’s Interior

Why Does the Sun Shine?

What could power the Sun for this length of time? Chemical energy (burning) -- Kelvin-Helmholtz contraction

(gravitational energy) -- Nuclear Fusion Reactions --

Fusion Hydrogen fusion requires the atoms to be

moving fast (high T) and to be packed tightly together (high P)

Each reaction converts 4 H to 1 He and some mass to energy (E=mc2)

Rate of reactions depends on the temperature and pressure

Hydrogen Fusion

Scientific Notation The numbers is astronomy are often too large to

represent with words or even by writing out all digits

Example: number of seconds in a year

Move decimal point seven places to the left, so our exponent is 7

To put into calculator use “EE” key in place of “X10”

However, you should always write out the “X10”, don’t write “EE” in what you hand in “There are 3.1536 X 107 seconds in one year.”

The Main Sequence Explained

Why is there a main sequence?

The stronger the pressure the higher the temperature and the more reactions

High mass stars produce more energy in their cores and thus have a larger temperature and luminosity

Energy Transport In general energy is transported in 3

ways: Conduction -- Radiation -- Convection --

Star have no conduction

Radiation and convection both very important

Solar Granulation

Convection in Granules

Opacity Opacity determines whether energy is

transported by radiation or convection Opacity --

High opacity -- very little radiation can penetrate

Low opacity -- radiation penetrates easily

Ionization

Atoms can only absorb photons if they have electrons

High ionization means low opacity

At high temperatures it is easy for photons to move freely through a gas

M < 0.4 Msun

Completely Convective

Low temperatures mean atoms are not ionized and can absorb radiation better

0.4 Msun < M < 4 Msun Inner Radiative Zone

Free electrons and protons don’t absorb light very well so the primary means of energy transport is radiative

Outer Convective Zone

The atoms absorb the radiation and heat up

Interior of a 1 Msun Star

Energy Transport M > 4Msun Inner convective zone

Even though the opacity is low, there are so many photons that enough get absorbed to produce convection

Outer radiative zone

Energy Transport Down the Main Sequence

What is a Star?

A big sphere of gas (mostly hydrogen) Powered by fusion reactions in the core

Energy gets out via radiation or convection depending on the opacity

Next Time Read Chapter 16.4-16.5