Stellar evolution is driven entirely by the never ending battle between pressure and gravity. As imbalances are reached, the star is driven to find a new energy source. Each stage in stellar evolution is marked by a different energy generation mechanism.

STELLAR EVOLUTION

HR Diagramat the end of this lecture, you'll not only understand this stellar evolution diagram, but will be able to make one of these yourself!

PV = nRTIDEAL GAS LAW

(pressure) x (volume) = (particle density) x (constant) x (temp)

How PV = nRT works!

Increasing the temperature increases the volume and decreases the pressure.

Decreasing the volume increases the pressure (and the increases the temperature)

Stars regulate their internal pressure and temperature via the ideal gas law PV = nRT. The rate at which atoms fuse together is a function of both pressure and temperature. So, if a star gets really hot and the pressure gets really high, the star will expand and cool down, thus lowering the fusion rate.

If the fusion rate

increases, the

temperature and pressure

go up...

...the star will expand, and in

the process lower its temperature and

pressure, and thus its fusion rate.

Eventually, Hydrostatic

Equilibrium will be

reached. The pressure

caused by the energy

generation rate will

balance the inward force of gravity

Why Doesn't a Star Burn all its Fuel Instantly?

What is Light?Light is a form of energy, called radiative energy. It is both a wave and a particle! It can be characterized by its wavelength and frequency:

c

speed of light in vacuum wavelength frequency

Color and Wavelength

The color of the light depends on its wavelength. Longer wavelengths correspond to “redder” light; shorter wavelengths correspond to “bluer” light.

Color and Temperature

The peak wavelength (or color) emitted by an object is a function of its temperature. Hotter objects emit more of their light at shorter wavelengths and are said to be “bluer”; cooler objects emit more of their light at longer wavelengths and are said to be “redder”. The relation between wavelength and temperature (in Kelvin) is given by Wein's Law,

Everything with a temperature emits light. Even as you sit there, you are emitting light in the infrared!

peak

T = 0.0029 meters

Wein's Law

peak

T = 0.0029 meters

Hotter objects emit more light at all wavelengths than cooler objects. Hotter objects also appear bluer than cooler objects.

Which Horseshoe Is the Hottest?

The Main Sequence

A star on the main sequence is one that is generating light and

heat by the conversion of hydrogen to helium by nuclear

fusion in its core.

hotter cooler

brighter

dimmer

<---------------- temperature ---------------->

<--

----

----

lum

inos

ity -

----

----

->

main sequence

Stage 1: Protostar

Star formation begins with a dense cloud of gas. A disturbance in the gas triggers a collapse, and the cloud begins to condense under its own gravity to form a protostar.

A protostar is a forming star that has not yet reached the point where sustained fusion can occur in its core.

The energy source for a protostar is gravitational contraction.

The star is cool, so its color is red, but it is very large so it has a high luminosity.

Sun's Age: 1-3 years old

Stage 2: Pre-Main Sequence

During the contraction the star's temperature stays about the same, but its luminosity decreases because of its shrinking size. Once nuclear reactions begin in the core, the star readjusts to account for this new energy source.

In the pre-main sequence star, both gravitational contraction and nuclear fusion provide energy.

Sun's Age: 10 million years old

Once the star is close to hydrostatic equilibrium, the contraction slows down. However, the star must continue to contract until the temperature in the core is high enough that nuclear fusion can begin and support the star!

Stage 3: Zero- Age Main Sequence

Finally, the rate of fusion becomes high enough to establish gravitational equilibrium. At this point, fusion becomes self-sustaining and the star settles into its hydrogen burning, main sequence life. The main sequence phase is the longest phase of a star's life, about 10 billion years for a star with one solar mass.

The main sequence is not a line, but a band in the H-R Diagram. The position of a star on the main sequence is determined by its mass and composition.

Sun's Age: 27 million years old

More massive stars have shorter lifetimes!

Chihuahuas live 14-15 years.

Great Danes only live 6-10 years.

Hummer: 32 Gallon Gas tank

13 miles per gallonso…416 miles per

tank

Camry: 18.5 Gallon Gas tank

28.5 miles per gallon

so…527 miles per tank

A small change in a star’s mass gives a big change in luminosity:

L = M3.5 where L = luminosity in solar luminosities

M = mass in solar masses

So, for a star that’s twice as massive as the sun:

L = 23.5 = 11.3

It’s ~11 times more luminous!!

A star’s lifetime (t, in solar lifetimes) can be given by:

t = Amount of fuel

Rate of fuel consumption

=M

L=

M

M3.5

So a star that’s twice as massive as the sun lives 2-2.5 = 0.17 times as long as the sun. That 0.17*1010 = 1.7

billion years

=1

M2.5

Stage 4: End of Main Sequence

A star ends its life on the main sequence when it has used up all the hydrogen in its core. Once the core hydrogen has been exhausted, a shell of hydrogen surrounding the core begins to burn, providing energy to the star. During its life on the main sequence, the size and luminosity of the star has changed very little.

Sun's Age: 10 billion years old

Stage 5: Post Main Sequence

Now that hydrogen is exhausted in the core, there is no energy to support the Helium core. Thus, the core contracts and energy is released. The hydrogen burning shell continues to provide energy to the outer layers of the star.

Sun's Age: 11 billion years

Stage 6: Red Giant – Helium Flash

As the helium core contracts, the temperature and pressure increases. This increase in temperature causes the rate of hydrogen fusion in the shell surrounding the core to go up. As a result, the star expands (by as much as 200 times!). The star is now very cool, but luminous – a Red Giant!

The contraction of the core causes the temperature and density to increase such that, by the time the temperature is high enough for Helium to fuse to form Carbon, the core of the star has reached a state of electron degeneracy.

Stage 7: Helium Burning Main Sequence

The pressure due to electron degeneracy is significantly different from the pressure produced by the Ideal Gas Law – it is independent of temperature!

In the core, the temperatures reach 200 million Kelvin and Helium can now fuse into Carbon, known as the Triple Alpha Process. This happens quite suddenly and is known as the Helium Flash.

This process produces only about 20% as much energy as hydrogen burning, so the lifetime on the Helium Burning Main Sequence is only about 2 billion years.

When the Helium is exhausted in the core of a star like the sun, no further reactions are possible.

Stage 8: Planetary Nebula

Helium and Hydrogen burning shells will continue outside the core for a while. During Helium Shell Burning, a final thermal pulse produces a giant "hiccough" causing the star to eject as much of 10% of its mass, the entire outer envelope, known as a Planetary Nebula.

The Planetary Nebula phase is relatively short lived, estimated to be about 25,000 years, and there are about 10,000 planetaries in the Milky Way.

Stage 9: White Dwarf

As the nebula disperses, the shell nuclear reactions die out leaving the stellar remnant, known as a White Dwarf, supported by electron degeneracy, to fade away as it cools down. The white dwarf is small, about the size of the Earth, with a density of order 1 million g/cm3, about equivalent to crushing a Volkswagen down to a cubic centimeter or a "ton per teaspoonful."

A white dwarf star will take billions of years to radiate away its store of thermal energy because of its small surface area. The white dwarf will slowly move down and to the right in the H-R Diagram as it cools until it fades from view as a "black dwarf"

Hertzsprung-Russell Diagram

Branches

Red Giant Branch stars have a hydrogen burning shell and their core is contracting.

Horizontal Branch stars have helium core-burning and hydrogen shell-burning.

Asymptotic Giant Branch stars have a helium burning shell and their core is contracting.