Dear Astronomer,
I understand that a pervasive current theory of astronomy/cosmology
regarding the formation of star systems like our own solar system
involves
the coalescence of dust and gas from collapsed or supernovae previous
generation stars. However this does not seem plausible for several
reasons.
1) First if our sun which is approximately 5 billion yrs old
and only
halfway though its estimated life (10 billion years) is representative
of an
average life cycle for a star and the estimated life of the universe
from
the Big Bang is 12 billion years would mean that there would not
be adequate
time for a full life cycle of even one previous star system?
2) Also if stars are the product of the remnants of dust and
gas of these
prior stars given that when they collapse they eject their contents
out vast
distances in all directions (as can also be seen by great universal
nebulae)
how is it practical that these contents could coalesce under weak
gravitational forces over such great distances? If this were the
case than
it seems that the Kiper belt and Ort cloud would have coalesced
into our
inner solar system. Also how is it possible given the tremendous
distances
(lights years) between most stars that these greatly dispersed
remnants
could interact in any meaningful way to reform their contents
into another
star (i.e. - only small fragments of the prior stars would have
the remote
possibility of interacting)?
First, I'll mention that what is called the "Nebular Theory"
of star formation is not simply a pervasive current theory,
it is the current theory of star formation, and I'm not
aware of any professional astronomer who believes it has been
falsified, or seriously supports any other theory. All of the
available data supports the nebular theory, and moreover, the
processes of star formation have been directly observed. Indeed,
the famous "gaseous pillars" image from Hubble is an
image of exactly this process. See, for instance, http://rsd.gsfc.nasa.gov/rsd/images/EGGs.html,
http://www.jpl.nasa.gov/releases/2001/release_2001_243.html,
http://antwrp.gsfc.nasa.gov/apod/ap031226.html,
and
http://antwrp.gsfc.nasa.gov/apod/ap030816.html.
In fact, star formation regions have been known for over a century,
but to actually observe the star formation process itself has
required the relatively recent ability to use infrared space telescopes.
> 1) First if our sun which is approximately 5 billion yrs
old and only
> halfway though its estimated life (10 billion years) is representative
of an
> average life cycle for a star and the estimated life of the
universe from
> the Big Bang is 12 billion years would mean that there would
not be adequate
> time for a full life cycle of even one previous star system?
10 Billion years represents an average, but lifetimes of stars vary greatly. The larger a star, the shorter its lifespan, because larger stars burn their hydrogen much more rapidly than smaller stars like the sun. Small sun-like stars have long lifetimes, and when they do die, don't really enrich the interstellar medium (ISM) with heavier elements. (This is because their deaths are "nonviolent" and most of the newly-made material remains behind in the white dwarf remnant.) The stars that do enrich the ISM with heavy elements are those that die in supernovae, at least 3-5 times the mass of the Sun. These stars have lifetimes of only hundreds of millions (not tens of billions) of years, and so plenty of time has elapsed for many generations of these larger stars to have come and gone. Indeed, the largest stars, those of 50 solar masses or so, have lifetimes much shorter still, about 100,000 years.
> 2) Also if stars are the product of the remnants of dust
and gas of these
> prior stars given that when they collapse they eject their
contents out vast
> distances in all directions (as can also be seen by great
universal nebulae)
> how is it practical that these contents could coalesce under
weak
> gravitational forces over such great distances? If this were
the case than
> it seems that the Kiper belt and Ort cloud would have coalesced
into our
> inner solar system. Also how is it possible given the tremendous
distances
> (lights years) between most stars that these greatly dispersed
remnants
> could interact in any meaningful way to reform their contents
into another
> star (i.e. - only small fragments of the prior stars would
have the remote
> possibility of interacting)?
Light-years may seem like large distances, but in actuality, there is a great deal of gas in those spaces irrespective of supernova remnants. Generally, the supernova gas itself does not do the collapsing, but rather the cold ISM with which it interacts. When you look at a supernova remnant, what you're often seeing is the heat dissipated by the collision of the highly energetic supernova gas with the stable gas of the ISM. This collision of the two gaseous states creates shock waves (which can also be directly observed) which compress the ISM. If the ISM is compressed enough, the gravity takes over and collapse ensues.
How much is "enough?" Well, the only reason in the world that the cloud does not collapse on its own anyway is the pressure of the gas. Throughout most of the ISM in the galaxy, the gas pressure is (barely) sufficient to hold the gas up against gravity. It is a simple calculation (which all undergraduate astronomy majors perform) to determine how much compression is necessary for the gravity of the cloud to exceed the pressure. Once that has occurred, then the gravity continues to compress the cloud until some other source of pressure (such as fusion in the cores of stars) becomes available. Hence, a star. You may wish to do a search on the "Jeans Criterion," named after the astronomer who first performed the calculation.
The Oort Cloud/Kuiper belt have not collapsed into the Sun for the same reason that the planets did not; they're teeny tiny leftovers of the formation process, carrying large angular momenta which prevent them from collapsing further. The mass of the Oort cloud and Kuiper belt represent less than .0001% of the mass of the Solar system. The planets themselves represent about 0.1% of the mass of the solar system. The rest is the fully collapsed Sun.
Gas on Earth, of course, does not ever collapse under its own gravity, because it is far too hot and not nearly massive enough. But that doesn't mean that the process can never occur for large cold gas clouds--- and indeed, it does.
Answers provided by HSU Astronomy Professor David Kornreich.