Lecture 9: The Life of the Sun

Don't Panic

Sections

Introduction

Therefore at this point we will just click over to IAWL Lecture 21: Star Formation for that history and return to this lecture to pick up the particular story of the Sun on the Main Sequence.

The Main Sequence Life of the Sun

The main sequence (i.e., core hydrogen burning) life of stars is their longest nuclear burning phase.

It seems about 90 % of their nuclear-burning life is on the main sequence (Se-230).

The more massive the star, the faster it lives.

Stars more massive than about 8 solar masses end as supernovae after only 20??? or less million years.

A 0.1 solar mass star is theoretically predicted to live 6000 Gyr (Gyr = gigayear = 10**9 years) on the main sequence (Se-265).

Question:

Not if their theoretical lifetimes are right.
Not if their theoretical lifetimes are wrong.
Yes, of course.

Answer 1 is right.

SUN

FK-493

On the main sequence, the Sun evolves very slowly.

Question:

It's carbon is being converted to diamond.
It's core hydrogen is being converted to helium.
It's surface hydrogen is being converted to helium.

Answer 2 is right.

The surface is much too cool and rarefied for hydrogen burning.

The main thing that is happening is that hydrogen in the core (i.e., the region out to about 0.2 solar radii) is slowly being transformed to helium.

The decrease in fuel in the core has the seemingly paradoxical result that the Sun will get brighter over its main sequence life.

    Pressure support depends on the number of particles.

    When hydrogen is fused to helium,
      4 particles are converted to 1.

   This causes a tendency to loss in pressure 
      which in turn causes a tendency to contraction 
        due the gravitational force on the Sun's own mass.

    But contraction increases the temperature 
      essentially due to gravitational potential 
       energy being converted into thermal energy.

    The higher the temperature, the higher the pressure 
      and thus collapse is resisted.

    But higher temperature and density increases the 
      collision rate of hydrogens 
        and thus the hydrogen burning rate.

    So there is a higher rate of energy production 
      and the Sun gets brighter to compensate and stay
      in a quasi-steady-state.

    Main sequence stars burn brighter as they exhaust their
      fuel.

    Reference Se-246;
              FK-467.

The Sun is now about 30 % brighter than when initially on the main sequence about 4.6 Gyr ago.

It will be about 30 % brighter than now in about 3.5 Gyr (WB-106; FK-493)

This gradual brightening of the Sun is entirely theoretical. But we think we understand main sequence stars pretty well in their main behavior.

So the brightening is about as certain a result as a purely theoretical result can be.

The brightening is a pretty modest change for the Sun.

But it probably means the doom of complex life on Earth within of order a gigayear or two by first eliminating the carbon dioxide from the atmosphere and then eliminating liquid water.

This unhappy fate is discussed in IAWL Lecture 11: The Earth.

The Fate of the Sun

When the hydrogen in the core of the Sun becomes exhausted, the main sequence life ends in about 6 Gyr (

FK-493

A hydrogen burning shell will still exist around the core.
The Sun's core continue contracting because of thermal energy loss, but it will get hotter because of the contraction.
The energy generation rate will actually increase because of the hotter core.
The Sun's will then expand into a RED GIANT with a radius of order 1 AU because of the extra heating (Se-249; FK-470).
The Sun will then be much brighter than it is now because of its much larger surface area, but its photosphere temperature will be lower eventually reaching about 3000 K (Ze2002-341).

By Wien's law, the peak wavelength of it's emission will then be


      wavelength_max = about 3000 micron-K / 3000 K  = 1 micron

      and thus in the infrared.

The Sun in the visible will be red: hence the name RED GIANT.
When the helium core's temperature reaches about 100*10**6 K, the HELIUM (He-4) will suddenly start burning to CARBON and OXYGEN pretty much everywhere in the core within perhaps a few minutes (Ni-177; Ze2002-343).
Because of its double proton positive charge and its stability???, He-4 requires higher temperatures and densities to burn than hydrogen.
This is the HELIUM FLASH. We never see this event because it is always buried in the core and lasts only seconds---but theoretically it must happens it seems (FK-473).
HELIUM FLASHES happen only to stars in the 0.4--2 or 3 solar mass range (Se-251; FK-472).
Post-helium flash, the Sun will then contract and become a bit less bright oddly enough. Essentially this is because there is again nuclear burning in the core, but now its helium burning.
At this stage the Sun is called a HORIZONTAL BRANCH STAR. (Shu-152;. Shu-252).
The Sun from Main Sequence to the Horizontal Branch.
But the helium burns much more quickly than hydrogen.
So the helium exhausts in the core and then there is an non-burning carbon-oxygen core, surrounded by a helium-burning shell surrounded by a hydrogen-burning shell.
The Sun then puffs up to be a RED GIANT again---but only for awhile.
The helium burning is unstable and of order every 100,000 years has a runaway burst of burning called a THERMAL PULSE (FK-493).
Each THERMAL PULSE causes a superwind that blows off much of the the Sun's envelope.
The escaping material forms a complex expanding nebula called a PLANETARY NEBULA---the name is a total misnomer since these nebulae have nothing to do with planets.
The Sun from the 2nd red giant phase to white dwarfdom.
Planetary nebula atoms are excited by UV radiation from the remnant star---which is very hot on the surface---and emit LINE SPECTRA.
Green lines from doubly ionized oxygen are often particularly noticeable (Se-268).
PLANETARY NEBULAE have huge range of behavior---which we won't go into here, but we will show some pictures.

The Ring Nebula from HST. The best known of all planetaries.
The Ring Nebula is about 2000 lyr away in constellation Lyra. The ring is about 1 lyr in diameter. The green color is due to green forbidden lines of O III (i.e., twice ionized oxygen). The blue is from helium and the red color is from ionized nitrogen.
Credit: NASA: Imagine the Universe.
Planetary nebula Cat's Eye from HST.
The Cat's Eye is about 3000 lyr away in constellation Draco. It is one of the most complex of planetary nebula and is estimated to be only about 1000 years old. The green color is due to green forbidden lines of O III (i.e., twice ionized oxygen).
Credit: NASA: Imagine the Universe.

The PLANETARY NEBULAE disperse in the interstellar medium after a few thousands to tens of thousands of years.
After some thermal pulses and several planetary nebula phases (say ???), the remnant Sun will have lost most of its hydrogen and helium and will NOT be able to burn what is left even though it is still very hot (FK-493--494).
As the Sun is losing mass, what is left shrinks and heats from contraction.
It will finally have about half its original mass and be of order the size of the EARTH.
The Sun is then a WHITE DWARF: a dense compact remnant.
The time from becoming a 2nd red giant (AGB star) to being a WHITE DWARF is about 0.7 Myr (FK-493).

SOLAR WHITE DWARF

CARBON

OXYGEN

Stars that start out more massive than about 4 solar masses can burn carbon and oxygen (FK-497), but they too end as white dwarfs if they arn't massive enough to become supernovae: i.e., have masses greater than about 8 solar masses.

But although the SOLAR WHITE DWARF will be mostly carbon and oxygen, the lighter helium and hydrogen atoms float on the top and form a skin, and so white dwarfs usually (always??) appear as hydrogen or helium objects: their interior composition is a modeling inference.

WHITE DWARFS are not held up by ordinary gas and EMR pressure, but by a quantum mechanical pressure of degenerate electrons that is NOT very temperature dependent.

WHITE DWARFS are initially very hot with a surface temperature of order 30,000 to 100,000 K (Ni-178).

Initially, a WHITE DWARF'S EMR peaks in the UV and in the visible they appear white: hence the ``white'' in WHITE DWARF.

But they arn't very bright because of their small size.

WHITE DWARFS have no nuclear energy sources. They can generate only a little heat by contraction.

Essentially, they lose heat energy and cool off forever.

Eventually they will drop to such low temperatures that they only radiate in the microwave or radio. They will then be called BLACK DWARFS.

But their cooling time is very long---many billions of years. Since the Big Bang, no WHITE DWARF has cooled off to become a BLACK DWARF it is thought.

The coldest existing WHITE DWARFS have surface temperatures of about 6000 K????.

Caveat: the post-main-sequence evolution of stars is fairly well understood in qualitatively from a combination of observations and theory.

But quantitative predictions are difficult. The three-dimensional hydrodynamic computations required test the limits of our modern computer codes.

We can only tell the best story we can given present understanding.

The Fate of the Earth

What happens to the EARTH when the SUN dies?

Well its a bit uncertain. It depends on how large Sun gets in its red giant phases.

If the Sun's radius is always less than 1 AU, the Earth might survive as a cold rock for many billions of years after the Sun becomes a white dwarf. It's orbit will have to change somehow as the Sun loses mass.

If the Sun exceeds 1 AU as some sources claim (Ze2002-344), then the Earth will be enveloped by the Sun.

Once inside the Sun's atmosphere, there will be drag force that will cause the Earth to spiral into deeper layers and there it will be evaporated. The time scale after envelopment is about 200 years (Ze2002-344).

`` So the glory of this world passes away: sic transit gloria mundi.''