Lecture 9: The Life of the Sun

Don't Panic

Introduction

SUN

Question:

The time scale for significant solar evolution is much LONGER than human history.
The time scale for significant solar evolution is much SHORTER than human history.
We can't directly observe the Sun with the unaided eye.

Answer 1 is right.

But, of course, one could wax philosophical and say that nothing is directly knowable, except that ``there is thinking'' as Bertrand Russell, probably among others, noted.

SUN

Some things about its life are fairly certain, but other things are uncertain: e.g., many quantitative details.

Some things we will never know.

For example, we will never know---I suppose---where exactly the Sun was formed: it has wandered a long way from there and the environment it was born in has probably been dispersed.

[Because of the many gravitational perturbations stars are subjected to, their orbits are chaotic over gigayears.]

Because stellar evolution is something of a shaggy dog story, we should look at a cartoon of the Sun's life which provides a simplified road map of what we'll discuss below.

Cartoon of the life of the Sun.

The Interstellar Medium and Molecular Clouds

NOT

GAS

DUST

Additionally, space is suffused by electromagnetic radiation (EMR) and magnetic fields.

Question:

5777 K.
15.7*10**6 K.
2.726 K.

Answer 3 is right (HI-481).

This main component is a cosmic microwave background (CMB) left over from the big bang and cooled down since then.

There are other components of EMR. The light from stars obviously, put also glowing gas in many different environments: e.g., from accretion disks around black holes (if they exist), quasars (which may be supermassive black holes and accretion disks), and supernovae (giant exploding stars).

The space matter inside galaxies is called the INTERSTELLAR MEDIUM (ISM).

The ISM is not homogeneous, but is divided into components which themselves vary widely in density, temperature, and ionization. The components also have varying shapes and behaviors. The system is very complex.

The component of interest for star formation is the MOLECULAR CLOUD COMPONENT.

MOLECULAR CLOUDS are cold and dense enough for molecules to form.

       T ranges over of order 10 to 50 K

       density ranges over of order 10**2 to 10**6 atoms/cm**3
           (ISM average density is about 1 atom/cm**3) 


          air density is about 2.5*10**19 Ni_2 molecules/cm**3
                (HI-370)

       most of the ISM probably has densities well below  10**2 atoms/cm**3

          Reference Ze-333,
          but note that numbers vary from book to book and depend on
          definitions of ISM components that vary too.

MOLECULAR CLOUDS are irregular and turbulent with all kinds of motions and rotations. In some ways not unlike sky clouds.

Now the universal composition is approximately the same as that of the Sun.


  73 % hydrogen (H) by mass

  25   % helium (He) by mass

 of order 2 % everything else (metals in astro-jargon), 
                but ``everything else'' 
                can vary widely, particularly downward in some 
                low metal stars.

                The leading other constituents are carbon (C),
                nitrogen (N), magnesium (Mg), silicon (Si), and
                iron (Fe).

The dominant material in molecular clouds is MOLECULAR HYDROGEN: H_2.

This is the common terrestrial form of hydrogen that one day you may be pumping into your hydrogen-powered car.

Unfortunately H_2 doesn't absorb or emit light in the radio or microwave (Ze-331). At the temperature of molecular clouds, H_2 is invisible.

Emission peak region of a line spectrum depends on temperature, but not in so simple a way as for a blackbody radiator.]

CARBON MONOXIDE

Se-212

Also there is DUST in molecular clouds---but not your ordinary, under-the-couch dust.

The DUST forms somehow in coldish environments with REFRACTORIES condensing to form a core first and VOLATILES condensing on the core later.

REFRACTORIES

VOLATILES are those elements or compounds that melt and evaporate at a low temperature.

The terms are relative, of course, but iron, silicates (silicon oxygen substances which make up rock), and carbon are ordinarily REFRACTORIES in our astrophysical contexts.

Hydrogen, helium gas, other noble elements, N_2, carbon dioxide, water (H_2O), methane (CH_4), ammonia (NH_3) are examples of VOLATILES in our astrophysical contexts.

References Se-418 , etc. ]

Model of an interstellar dust grain (Ze-337).

Note SILICATES are what most ordinary rock is made of. Graphite is the common form of pure carbon found in pencils.

Question:

Diamond.
Fullerenes or Buckyballs.

Well both answers are right.

By the way I've read (2004mar in New Scientist) that manufactured diamonds of gem quality and size have been produced. Does this mean diamonds are going to become cheap?

DUST

Ze2002-316

We don't have any samples for sure of typical interstellar DUST: we have some solar system dust samples????.

The DUST is probably only a small component: of order 1 % by mass only (Se-212).

But DUST is highly opaque to visible light. Thus we can only see into and through molecular clouds in the infrared, microwave, and radio.

DUST is also probably an important catalyst for molecule formation: unbound atoms stick onto grains and there bond more easily than in free space and then escape as molecules (Ze-337).

Let's look at DUSTY NEBULA.

./formation/noao_horsehead_nebula_003.jpg

The Horsehead Nebula because of its accidental resemblance to a horse's head is one of the most famous of all nebulae.

This is approximately a true color image. The Horsehead Nebula is a dark dusty nebula. The difference in star counts above and below the mid-image line illustrates that no or few stars are seen through the dark nebula: most of the stars in the lower image are probably foreground.

The pink color is an emission nebula. Ultraviolet light from hot young stars creates excited hydrogen which then emits strongly in its red line (i.e., the H alpha line).

Credit: N.A.Sharp/NOAO/AURA/NSF.

The largest molecular clouds are huge: these are GIANT MOLECULAR CLOUDS:


      45  to  180 light-years in size scale

     10**2 to 10**6 solar masses

 Reference Se-212,
 but note that numbers vary from book to book and depend on the
 definition of a GIANT MOLECULAR CLOUD that could vary too???.

Many molecular clouds are much smaller than GIANT MOLECULAR CLOUDS.

Star Formation

MOLECULAR CLOUDS

The time scale is tens of millions of years, and so we DO NOT observe star formation happening in time.

But there are many sites of STAR FORMATION in various stages, and so from these SNAPSHOTS we can understand the time evolution of STAR FORMATION to a degree.

Molecular clouds have a tendency to collapse under their own gravity. But that tendency is resisted by

     The clouds' gas pressure.

     Magnetic field pressure.

     Rotational kinetic energy of the clouds. 
         Recall rotational kinetic energy keeps the planets from
         falling into the Sun.
 
     Reference Se-221.

Some triggering event is needed it is thought to fragment the cloud and increase the density of the fragment to the point of a runaway gravitational collapse.

Triggering mechanisms for the collapse of a molecular cloud (Se-221).

In a runaway gravitational collapse,


    the mass compacts into a smaller region,

    thus its self-gravity is higher because gravity increases as
      distances decrease,

    this causes more compaction,

    this causes higher self-gravity

    and so on.

DENSE CORES

Collapse of a molecular cloud region to DENSE CORES.

We can't see the DENSE CORES in the visible because they are hidden by the opaque dust. But they can be observed in the radio.

The observed cores are of order a solar mass and extend over of order a third of a light-year (Se-220).

The cores heat up and radiate EMR something like a blackbody.

Energy transformation in a molecular cloud fragment collapse.

When the core starts radiating in the INFRARED (IR), it is usually called a PROTOSTAR (Se-222). The PROTOSTAR phase ends when the star starts nuclear burning (Se-222). (The term PROTOSTAR is often used rather loosely.)

The PROTOSTAR is still hidden by gas and dust from visible observations.

PROTOSTARS seem to be surrounded by disks almost always: protoplanetary disks: PROPLYDS.

PROPLYDS are a result of the initial rotational kinetic energy of the cloud or cloud fragment. There always is some.

Cartoon of disk formation around a protostar (FK-172).

DISKS are natural in collapsing astrophysical contexts: they also form around black holes and spiral galaxies are a form of disk with complicated spiral arms, of course.

The DISKS eventually can partially coalesce into planets, but that is the story of Intro-Astro Lecture 10: Solar System Formation.

PROPLYDS have been observed since the 1980s. Before that they were entirely theoretical.

Beta Pictoris is second brightest star in the visual in the southern constellation Pictor: see the Munich Astro Archive on constllation Pictor.

Betapic is about 50 lyr away.

Since 1983, Betapic has been known to have a disk of gas and dust and it has been suspected that planets are forming in or even have formed from the disk. The discovery of the disk was a crucial confirmation of the disk aspect of the star formation process.

The disk is visible in visual and infrared: it starts about 50 AU from the star and extends to at least 2000 AU. Inside 50 AU is a clear region. The disk is of order 10 AU thick. This HST image (the star and clear region are artificially occulted) shows the disk.

The image is from the visual and shows reflected light. The top view is just a simple image, I think.

The bottom view is in false color to show the intensity of the reflected emission.

The slight tilt near the inner edge of the disk from the mean disk plane (shown by a dashed line) suggests that planets have formed in clear region and they perturb the disk. (FMW-409).

Evolution of Star Formation Regions

STAR FORMATION REGIONS

They start probably usually as rather turbulent, chaotic MOLECULAR CLOUDS that come in all kinds of shapes and sizes, and so no two will have the same initial conditions.
Some BIGGEST, HOTTEST, BRIGHTEST STARS probably form first and they provide strong ultraviolet (UV) light that tends to heat up the gas of dark nebula and evaporate the dust. This tends to turn off further star formation by increasing the pressure that prevents collapse.
But on the other hand pressure from the heated gas caused by the HOT YOUNG STARS can initiate star formation nearby.
The very massive HOT YOUNG STARS have short main sequence lives---under about 10 million years (Se-247??)---and then blow up as SUPERNOVAE which create expanding ejecta that can both disperse star forming clouds and compress them to start new star formation.

STAR FORMATION

Often dark clouds with star formation are in proximity to bright emission clouds powered by HOT YOUNG STARS.

A famous image of this mess is the HST image of the Eagle Nebula with the THREE PILLARS. But first let's look at a wider field of the Eagle Nebula.

The Eagle nebula is star forming region about 7000 lyr away in the constellation Serpens.

The image is a mosaic in false color from emission line images: hydrogen-alpha (green), oxygen [O III] (blue) and sulfur [S II] (red). H alpha should be red and I imagine that [O III] should be green.

The famous THREE PILLARS from the HST image are in the center.

Note that one has bright emission gas clouds heated by newly formed hot young stars that are evaporating the dust in the dark dusty clouds where star formation continues.

Credit: T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and B.A.Wolpa (NOAO/AURA/NSF).

The Eagle nebula is star forming region about 7000 lyr away in the constellation Serpens.

The image is false color. Red shows emission from singly-ionized sulfur atoms. Green shows emission from hydrogen. Blue shows light emitted by doubly- ionized oxygen atoms.

The pillars protrude from a molecular cloud below them: the tallest is about 1 lyr long.

Above them and off the image are HOT YOUNG STARS that formed at an earlier epoch. Those young stars emit UV radiation that is evaporating the pillars.

The pillars contain small DENSE CORES that are being exposed: they stand out as dangling globules from the pillars. In some of these globules are new or forming stars that will appear when the dust and gas globules are evaporated.

Near the lower left of the center region there is a dangling globule that appears to have newborn star emerging.

Credit: NASA.

A massive star has a short lifetime as indicated above: only millions of years. It will usually explode as a SUPERNOVA near to where it was formed.

A star of middling mass like the SUN can live billions of years. It will not explode as a supernova. It will wander far from its place of formation and often from it's siblings. The SUN'S star formation region was probably long ago dispersed and no longer exists.

Thus, we can know very little about the individual early history of the SUN itself.

We can only understand how stars like the Sun form, not the particular history of the SUN.

The detailed composition of the solar system and some relics of primordial material might give some clues as to the SUN'S formation environment, but that requires a lot of deductions and inferences.

The Main Sequence Life of the Sun

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.

    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 Intro-Astro Lecture 11: The Earth.

The Fate of the Sun

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, 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 will suddenly start burning to CARBON and OXYGEN pretty much everywhere in the core within perhaps a few minutes (Ni-177; Ze2002-343).
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 copious 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, the remnant Sun will have lost most of its hydrogen and helium and perhaps about half its mass and shrunk to a size of order the size of the EARTH.
The Sun is then a WHITE DWARF.
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 if 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.

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.

But WHITE DWARFS have no energy sources.

They can only lose thermal 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

EARTH

SUN

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.''