Transcript Stellar Interiors

Stellar Interiors
Astronomy 315
Professor Lee Carkner
Lecture 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
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

T = 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
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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