Lecture 21: Star Formation

Don't Panic

Sections

1. Introduction to Stellar Evolution
2. The Interstellar Medium and Molecular Clouds
3. Star Formation
4. Disks
5. The Evolution of Star Formation Regions

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Introduction to Stellar Evolution

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Stellar evolution is, to say the least, a shaggy dog story: it goes on and on with a wide variety of effects, parallel stories, and terminology. It's hard to keep everything straight---that is the instructor finds it hard to keep everything straight.

To mix metaphors, we will try to walk a dry path through the morass.

We might first say that the term stellar evolution is somewhat misleading in that stars don't evolve by replication and natural selection.

But there are generations of stars that show progressive change from generation to generation in the course of cosmic history, and so stars evolve---using the term evolve in its more general meaning.

Also individual stars have beginnings, courses of development, and final fates: i.e., birth, life, and death stories and we say they evolve too.

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We CANNOT directly observe the whole lifetime (i.e., all the evolution phases) of a star.

Question: Why can't we?
1. The whole lifetime of even the shortest-lived star is much LONGER than human history.
2. The whole lifetime of even the shortest-lived star is much SHORTER than human history.
3. We can't directly observe most stars the unaided eye.



Answer 1 is right.

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

A bit of a philosophical digression on ``observations'' is in order.

A practical definition is that an ``observation'' is an information input that relies on undisputed theory.

For example, we say we ``observe'' hydrogen Balmer lines in a star---but what the observer actually sees is a plot with lines generated by and dependent on a whole world of supporting theory including that of atomic spectra and the theory of our spectroscopic and computing technology.

But those theories are undisputed, and so we can concede that we ``observed'' hydrogen Balmer lines.

Naturally our ``observation'' could be wrong if we've blundered somehow: but it shouldn't be wrong if we've followed all the procedures faithfully.

Now things we infer about the star's atmosphere conditions based on the ``observations'' are dependent on a theoretical model.

The model may be too simplified to give the truth or somewhat wrong or even wildly wrong. The model is NOT indisputable.

So we call inferences from the model, model results and put them in a different category from ``observations.''

Of course, boundary between ``observations'' and ``model results'' is NOT clearly defined and is NOT constant in time as a model usually becomes more accurate with time and developments and at some point may achieve undisputed status.

We must, in fact, infer stellar evolution from theory (or theoretical modeling) and from observations of stars in different phases of evolution.

We do have ``snapshots'' of most kinds of stars in almost all phases and some rapid phases (e.g., supernova explosions) can be directly observed.

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Because the stellar evolution is a complex, long, and---let's admit it---difficult-to-remember story, we will start with a couple of cartoons showing the evolution of a low-mass star (the Sun) and a high-mass star (e.g., a 17.5 M_Sun B0 star [Cox-389]).

[Stars less massive than 8 M_Sun end their lives as exploding white dwarfs; those more massive that about 8 M_Sun end their lives by exploding supernovae.

Thus, 8 M_Sun is a usual dividing line between low and high mass stars.]

These cartoons just preview the whole long story.

Cartoon of the evolution of the Sun.

Cartoon of the evolution of a massvie star.

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The Interstellar Medium and Molecular Clouds

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Space is NOT empty. It is just very low density in matter: some of it is GAS and some of it is DUST.

Additionally, space is pervaded by electromagnetic radiation (EMR) in all wavelength bands: sometimes a very low amount.

Also there are magnetic fields which are a great complication---which we largely avoid talking about in this course.

Question: What is the blackbody temperature of the main component of the EMR that pervades space?
1. 5777 K.
2. 15.7*10**6 K.
3. 2.725 K.



Answer 3 is right (FK-640; HI-481).

This main component is the cosmic microwave background (CMB) which is leftover from the big bang: then it was hot, but it has cooled down since then.

We discuss the CMB in IAWL Lecture 31: Cosmology.

There are other components of EMR. The light from stars obviously, but 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 surrounded by accretion disks), and supernovae (giant exploding stars).

The space matter inside galaxies is called the interstellar medium (ISM).

Question: What is the gas between galaxies called?
1. The interstellar medium.
2. The intragalactic medium.
3. The intergalactic medium.



Answer 3 is right.

Intragalactic medium is the medium inside galaxies: i.e., it's an unused synonym for interstellar medium.

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 ISM is very complex.

Altogether the ISM in the Milky Way maybe about 10 % of the luminous matter (CK-299).

Most of the ISM is H and He gas: the H gas could be neutral or ionized atomic gas or molecular H_2 gas.

About 1 % of the ISM by mass is interstellar dust (HI-374) which we discuss below.

_____________________________________________________________________

   Comparison of Densities 
_____________________________________________________________________

  System          Order of Density    Comment 
                  (particle/cm**3)   
_____________________________________________________________________

  Room air        2*10**19            This mostly molecular nitrogen N_2 
                      (HI-334,370).

  Best Labora-    10**3                
   tory vacuum        (CM-289).

  Typical ISM     1                   This mostly H and He with
                                      temperatures of order 10--20 K
                      (HI-334).

  Molecular       10**4--10**9        This mostly H and He with temperatures
   clouds                             of order 10--100 K 
                      (HI-334;
                       FK-446).

_____________________________________________________________________

The component of interest of the ISM for star formation is the molecular cloud component.
[Clouds of all kinds in space are often called nebulae which is just Latin for clouds.

Objects that appear cloud-like were historically called nebulae too.

Other galaxies, for example, before being resolved into star were once called nebulae and still can be if one is speaking historically.]

Molecular clouds are irregular and turbulent with all kinds of motions and rotations. In some ways they are like sky clouds, but in other ways not, of course.

Molecular clouds are also cold and dark.

They are dark, because they have heavy concentrations of interstellar dust which makes them opaque in the visible and ultraviolet.

The dust is actually rather important:

1. The dust absorbs hot visible light and ultraviolet light and radiates it away as infrared light and this cools the cloud and lowers its pressure resistance to collapse to stars (HI-375; CM-119).

2. The dust is probably necessary for molecules to form in the molecular clouds.

   First, the cooling by the dust promotes molecule formation.

   Second, the dust may also act as a catalyst in molecule formation: atoms that stick to the dust may have an easier time combining into molecules (CM-298; Ze-337).

   The density of the gas has to be fairly high in any case for atoms to meet and bind as it is in clouds.

   Third, it is probable that the dust is necessary to shield the molecules from ultraviolet light which would break them up into their constituent atoms (CM-298).

Over 100 kinds of molecules have, in fact, been observed in interstellar space, mostly in molecular clouds??? (HI-373).

The fact that molecules are present in molecular clouds is probably unimportant in itself for star formation.

But organic molecules have been observed in molecular clouds and this is suggestive that such clouds may seed planetary systems with building blocks for life??? (HI-373).

There has even been speculation that molecular cloud could harbor life. Fred Hoyle (1915--2001), well known both as an astrophysicist and and science fiction writer, wrote a noted scifi book The Black Cloud about an intelligent molecular cloud entity: not very likely, I'd guess.

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Now the composition of molecular clouds should be approximately the mean cosmic composition which is approximately the same as that of the Sun.

Recall the solar composition is

  71 % hydrogen (H) by mass

  27   % 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.
                It can be as high a 4 % and as low as 0.1 % or 
                much lower???
               (HI-414).

         The leading metals in decreasing order of solar surface abundance
         by number are oxygen (O),  carbon (C), neon (Ne),
         nitrogen (N), magnesium (Mg), silicon (Si),
         iron (Fe), and sulfur (S)
         (Cox-28--29).

         These are just the ordinary elements that make up most
         of our terrestrial world.

Question: What is the main molecule in molecular clouds?
1. molecular hydrogen (H_2).
2. molecular helium (He_2).
3. carbon monoxide (CO).



Answer 1 is right.

If the most abundant species can form a molecule, it seems reasonable that it forms the dominant molecule.

By the way H_2 is the common terrestrial form of hydrogen and the one that one day you may be pumping into your hydrogen-powered car (Va-223).

Helium is a noble gas. It doesn't form any kind of molecule, except perhaps in some exceptional cases which we won't go into. Helium in in gaseous form always consists of free helium atoms.

Carbon monoxide is just a trace gas, but it is observationally important as we discuss just below.

Unfortunately, H_2 doesn't absorb or emit much light in the radio or microwave (Ze-331) where molecular clouds are strong emitters because of their low temperatures (of order 10 to 100 K).
[Molecular clouds are too dilute to radiate like blackbodies. They have emission line spectra in the radio and infrared (HI-335; CK-300).

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

Thus, H_2 is practically invisible in molecular clouds.

Helium gas is mostly invisible too.

But many other radio emitting molecules exist in minute amounts in molecular clouds.

CARBON MONOXIDE (CO) is readly observed and acts as a tracer of the density (CK-300).

The H_2 and He gases must be inferred from modeling and knowledge of the average cosmic composition.

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Now what about that darn interstellar dust?

Dust grains are believed to form in ejecta from stars and supernovae???.

This ejecta is rich in refractories (i.e., materials that condense at relatively high temperatures).

The refractories condence to form dust grain cores.

Surrounding the refractory grain cores, volatiles (i.e., materials that condense at relatively low temperatures) can condense to complete a grain consisting of a core and outer layer of different composition.

[The terms refractory and volatile 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.

See Se-418.]

We have no certain interstellar dust in hand to examine although certain solar system particles may resemble interstellar dust (HI-275).

We do have models of interstellar dust that result from a combination of remote observations of interstellar dust, laboratory experiments, knowledge about elemental abundances and the common elemental compounds, and theory (Ze2002-316).

Model of an interstellar dust grain: sizes down to 0.0003 microns can occur (Ze-337; HI-371,374). Ammonia is spelled wrong: at room temperatures a colorless, pungent gas that dissolves easily in water.

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

Question: What is the other form of pure carbon?
1. Diamond.
2. 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?

Although dust is highly opaque to visible and ultraviolet light, we can only see into and through molecular clouds in the infrared, microwave, and radio.

Let's look at dusty nebula.

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The Horsehead Nebula NOAO, Kitt Peak, Arizona.

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 atoms which then emit strongly in the hydrogen red line (i.e., the H alpha line).

[The excitation is usually accomplished by ionization of hydrogen followed by recombination to an excited level.]

Bright red dots at the base of the Horsehead are protostars. The Horsehead Nebula is also a star formation region.

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

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The largest molecular clouds are huge: these are giant molecular cloud:

      15  to 60 pc in size scale

     10**2 to 10**6 M_Sun 

 Reference:  Se-212,
 but note that numbers vary from book to book and depend on the
 definition of a
giant molecular cloud 
 that can vary too???.    
 

Many molecular clouds are much smaller than giant molecular clouds.

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Star Formation

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In molecular clouds there can be RUNAWAY GRAVITATIONAL COLLAPSES that lead eventually to star formation.

The time scale for formation 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 and modeling we can understand the time evolution of star formation to a degree.

Molecular clouds have a tendency to collapse under their own self-gravity. But that tendency is resisted by (HI-334--336; Se-221):

1. the clouds' gas pressure.

   The cooling effect of the interstellar dust does help to reduce this pressure (HI-375).

2. magnetic field pressure. A subject we will not go into.

3. rotational kinetic energy of the clouds. There is a ``sideways motion'' that causes objects to keep missing the central mass trying to pull them to it.

   In physics, this ``sideways motion'' is quantified as angular momenutm.

   Recall rotational kinetic energy keeps the planets from falling into the Sun. The planets have no means of dissipating their rotational kinetic energy, and so revolve quasi-perpetually.

Some TRIGGERING EVENT is needed it is thought to fragment the cloud and increase the density of a fragment to the point of a RUNAWAY GRAVITATIONAL COLLAPSE.

Four TRIGGERING EVENTS are:

1. Cloud-cloud collisions that great regions of high density.

2. Radiation pressure and stellar winds from hot young stars. Such stars have typically formed in an earlier generation of star formation in the same molecular cloud.

3. The shock pressure from supernova ejecta. Supernova progenitors are the hot young stars that typically formed in an earlier generation of star formation in the same molecular cloud. These stars go through their lives quickly within a few to ten million years and explode. This is TRIGGERING EVENT is closely related to TRIGGERING EVENT.

4. The compression of a spiral density wave which is the cause spiral arms in galaxies.

These TRIGGERING EVENTS are not isolated from each other: they probably often happen in proximity or even simultaneously.

A cartoon illustrates the TRIGGERING EVENTS.

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.

Inside the dense regions of runaway collapse, dense cores form that will become stars.

Collapse of a molecular cloud region to dense cores.

We CANNOT see the dense cores in the visible because they are hidden by the dust.

But the dense cores heat up and radiate EMR approximately like a blackbody in the radio to which the dust is relatively transparent.

A cartoon illustrates the energy transformations that occur in making a dense core.

Energy transformation in a molecular cloud fragment collapse.

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

When a dense core gets sufficiently hot to start radiating in the INFRARED (IR), it is usually called a protostar (Se-222).

The protostar phase ends when the star starts the nuclear burning of hydrogen to helium (Se-222).

[The term protostar actually has no universal definition. We are using one common way of speaking here.]

Like dense cores, the protostars are hidden by gas and dust from visible observations.

Even though rather cool protostars are very luminous because of their large radiating surface and large amount of heat energy that comes from the transformation of gravitational potential energy on collapse.

Protostars that will become low-mass main sequence stars are MORE luminous than those stars.

Protostars that will become high-mass main sequence stars are typically about as luminous than those stars.

The pre-main-sequence phase of stars like all phases decreases in time period as mass increases.

The following cartoon HR diagram illustrates the features just discussed. It is based on model calculations, of course.

[The solar system sequence omits the HR diagram.]

Cartoon of the pre-main-sequence evolution of stars on the HR diagram (not accurate) (HI-338).

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A protostar continues to grow in mass, contract in size, and heat up by accretion, but eventually it ``bites the hand that feeds its it.''

It develops a strong stellar wind probably owing partially to radiation pressure and partially to magnetic field effects (????), and blows away most of its cocoon of dust and gas (Se-223).

[Nearby hot young stars can also disperse cocoons turning off accretion early. This must lead to some difference in star behavior between those stars that got rid of their own cocoons and those that did not.]

The outward flow is often BIPOLAR due in part to a disk (see section Disks below) channelling flow out along the poles which are approximately perpendicular to the plane of the protostar.

The BIPOLAR FLOW is probably affected by magnetic effects (Ni-163).

The protostar still contracts and gets hotter and at some point, its core will start the nuclear burning hydrogen to helium---at which time one can stop calling it a protostar (Se-222).

After a bit of settling down evolution, the object becomes a main sequence star: i.e., a stable, relatively unchanging, hydrogen burning star.

The time for evolution from the initial stage of a protostar to main sequence star is strongly mass-dependent as the diagram above indicated and below:

       A 30 solar mass star takes about 30,000 years.

       A solar mass star takes about 30 million years.

       A 0.2 solar mass star takes about a gigayear.

       Reference Se-223.
       These numbers are subject to revision.

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Disks

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Protostars seem to be surrounded by disks almost always: protoplanetary disks or, as they now seem to be called, 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 dense/core protostar (FK-172,601--602; HI-271--272).

The disks formed by the relaxation process can be very thin compared to their diameters. Probably self-gravity helps to flatten them. Random turbulence keeps them from settling to perfectly flat disks which is an ideal limit.

By the way, the rotation frequency of the cloud on in-fall typically speeds up.

This is certainly partially due to the PARTIAL conservation of angular momentum as the distribution of the rotating material becomes more compacted. (It can't be completely conserved in these system I would say.)

Angular momentum depends on mass distribution and rotation frequency (cycles per unit time). If no outside forces act angular momentum is conserved.

In particular, if the mass distribution of a system becomes more compacted due to internal forces, then frequency must increase to compensate and convserve angular momentum.

We will not go into the detailed physics of conservation of angular momentum, but it's effects are pretty common if not too noticeable in everyday life. The speeding up a spinning skater when she/he pulls in arms is an example.
[The old rotating platform and hand masses demonstration is good here. Call for a foolhardy student volunteer.]

Magnetic field effects may also help speed up the rotation of disks???.

Disks are natural in infalling or tidally disrupted astrophysical contexts and their formation process is essentially the same as outlined above. For example:

1. In star formation as just discussed above.

2. In the collapse of the collapse of a large gas cloud to a spiral galaxy, where forms a disk which becomes the star and gas/dust disk of the spiral galaxy. Most of the stars form after collapse.

   Elliptical galaxies are different in that star formation happens too rapidly for the cloud of gas and dust to relax to a disk.

   Gravitational interactions among stars do not dissipate kinetic energy to heat much, and so do not allow for a relaxation to a disk.

   We discuss spiral galaxies and elliptical galaxies in IAWL Lecture 28: Galaxies.

3. Around black holes that accrete mass from a close binary companion as discussed in IAWL Lecture 25: Black Holes.

4. Around planets where they take the form of rings like Saturn's rings.

   In the case of planetary rings, the tidal force of the planet prevents the ring from coalescing under self-gravity into a moon.

   Planetary rings are discussed in IAWL Lecture 15: Gas Giants. But here's an image of Saturn.

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   Saturn over 1996--2000.

   Saturn's rings orbit Saturn's equator. Both rings and equator have a tilt of 26.73 degrees to the Saturn's orbital plane: the tilt is constant---over short times anyway.

   As viewed from the inner solar system, the rings will be seen edge-on twice per Saturnian year at the Saturnian equinoxes when Saturn's axis is perpendicular to the Sun-Saturn line.

   [The Earth is approximately at the location of the Sun relative to the outer solar system: i.e., we approximately see the outer solar system from the vantage point of the Sun.]

   Because the rings are very narrow---only about 100 m or less thick (HI-209)---the rings practically vanish at the equinoxes.

   The rings are most full at the two Saturnian solstices.

   Recall that the Saturnian year is 29.424 Julian years: a Julian year is exactly 365.25 standard days.

   Credit: NASA/HST.

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   The proplyds around potostars eventually can partially coalesce into planets, but that is the story of IAWL Lecture 10: Solar System Formation.

   They may NOT always do so, but we now know that planet formation is pretty common. See IAWL Lecture 18: Extrasolar Planets.

   Proplyds have been observed since the 1980s. Before that they were entirely theoretical, but they had seemed a reasonable idea since Immanuel Kant (1724--1804) first proposed, based on qualitative reasoning from Newtonian physics, that planets formed out of disks in 1755 (No-406).

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   Beta Pictoris and its famous disk.

   Beta Pictoris (Betapic) 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.

   This HST image (the star and clear region are artificially occulted) shows the disk.

   We see the disk almost exactly edge-on. It is of order 10 AU thick.

   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).

   See also SEDS on Betapic

   Credit: NASA.

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   The Evolution of Star Formation Regions

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   Star formation regions have complicated evolutions.
   1. Some relatively random history brings about a concentration of gas and dust. All of astrophysical history on the smaller scales has a large element of random in it. The concentration can become a molecular cloud.

   2. The molecular cloud can become a star formation region. Such regions are rather turbulent and chaotic They come in all kinds of shapes and sizes, and so no two will ever be exactly the same.

   3. Some massive, hot, bright, blue OB stars (which will just call hot young stars hereafter) 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 in the immediate vicinity of the hot young stars by increasing the pressure that prevents collapses.

      Also the direct pressure of the radiation and stellar wind??? from the hot young stars tends to blow away the local material that could go into forming stars.

      If there are a large number of hot young stars relatively close together, they could form an OB association.

      The UV light from the hot young stars causes hydrogen to become ionized and on recombination in an excited state, the hydrogen emits a hydrogen line spectrum which in the visible consists of the hydrogen Balmer lines. The light in the visible is often red or pink because the strongest hydrogen Balmer line is the red Halpha.

      Because ionized hydrogen is conventionally called H II in astronomy, such emission regions are called H II regions.

   4. Although in the immediate locality the pressure increase caused by hot young stars tends to turn off star formation, the pressure of radiation and winds from hot young stars can initiate star formation nearby.

   5. Hot young stars have short main-sequence lives---just like in Hollywood---and in under about 10 million years (Se-247??) they will blow up as supernovae which create expanding ejecta that can both disperse star forming clouds and compress them to start new star formation.

   6. The relatively non-local initiation of new star formation by successive short-lived generations of hot young stars (either in their lives or as supernovae) can cause self-propagation of star formation through a large molecular cloud.

   7. After probably of order tens to hundreds of millions of years or so of existence (HI-376), a star formation will end up being partially used up and partially dispersed. The fragments of the cloud can end up becoming parts of new star formation regions.

      (I believe the efficiency of star production is rather low: little cloud mass ends up in stars????. But I can't find reference now.)

   The complex feedback mechanisms of the hot young stars and supernovae make star formation an immensely complicated process. The history of a star formation region must be somewhat chaotic.

   As the above discussion indicates dark molecular clouds with star formation are often in proximity to bright emission clouds (H II regions) powered by hot young stars that formed out of the clouds at an earlier time. The result is a bit of a mess.

   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.

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   The Eagle Nebula from NOAO, Kitt Peak, Arizona.

   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).

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   The three pillars of dust-laden gas in the Eagle Nebula from HST.

   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.

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   Hot young stars will blow up as supernovae leaving a neutron star or black hole remnant if greater than about 8 M_Sun. If less than 8 M_Sun, their lives end as white dwarfs. In both cases, they all their nuclear burning lives (of order 10 million years or less) in or near the star formation region where they were born.

   The compact remnants (white dwarfs, neutron stars, and black holes if they exist) of the hot young stars will, of course, wander from away from their birth star formation regions which will disperse after tens to hundreds of millions of years or so (HI-376) as mentioned above.

   Less massive stars like the Sun can live billions of years. They will not explode as a supernovae and will wander far from their place of formation and often from their sibling stars. Their star formation regions are likely long dispersed by the time such stars are middle-aged.

   Thus we can know little about the individual histories of long-ago formed stars. Also their orbital evolution since formation has been somewhat chaotic.

   We can only have a general account of the average star formation history for such stars.

   This is, of course, even true of Sun and solar system about which we have so much information.

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   Coronal loops or the Earth in Hell.

   A false color, ultraviolet image of coronal loops with the Earth superimposed and to scale.

   Coronal loops resemble prominences: they are arcs consisting of gas spiraling around magnetic field lines.

   But coronal loops are much hotter: prominences are of order tens of thousands of degrees; coronal loops are of order milions of degrees.

   Because of their high temperature they are most visible in the ultraviolet.

   Coronal loops rise up and crash down at high speeds (of order 100 km/s) and last ????.

   The corona itself can be thought of as largely consisting of coronal loops.

   Most of the heating of the loops seems to occur near the base where they emerge from the photosphere.

   The heating is somehow affected by magnetic effects.

   Credit: NASA/GSFC, TRACE spacecraft; download site Views of the Solar System: Calvin J. Hamilton.

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   Question: We can, in fact, know a little bit about the Sun's formation history because:
   1. of clues the solar system has brought with it since formation.

   2. the detailed composition of the solar system, some possible surviving relics of primordial material, and asteroidal geology allow inferences and deductions about the Sun's formation environment.

   
   
   Answers 1 and 2 are both right.

   Answer 2 just gives some detail about what the clues are.

   The solar system composition ??? and the geology of some asteroids suggest that the solar system may have formed from material including the ejecta of a supernova that happened shortly before formation (???; Se-565).

   Given the above discussion of star formation regions, this theory seems not unlikely.