Stars form from the collapse of giant gas clouds comprised
almost entirely of hydrogen and helium, with heavy elements (i.e. elements
heavier than hydrogen and helium) making up only a tiny percentage of the
material. In these clouds, dust grains contain a large fraction of the heavy
elements. Astrophysicists refer to elements heavier than hydrogen and helium as
metals, and the concentration of heavy elements in a star is referred to as the
star’s metallicity. For example, the Sun’s metallicity is 0.02, which means 2
percent of its mass is in the form of elements heavier than hydrogen and helium.
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Turbulence is a ubiquitous process in nature. A study by Philip
F. Hopkins (2014) suggests that given the right conditions in star-forming
clouds, dust grains can behave as aerodynamic particles and decouple from the
gaseous hydrogen and helium. The presence of turbulence can preferentially
concentrate the dust grains in specific regions such that the local abundance of
heavy elements can become so high that stars made almost entirely of metal can
form. Such stars would have metallicities approaching 1.00. In a star-forming
cloud with preferential concentration of dust grains, perhaps one in 10,000
stars could form as a “totally metal” star.
For the preferential concentration of heavy elements to
occur in star-forming gas clouds, the dust grains must be of a certain size
range such that they are large enough to decouple from the gas but also small
enough to “feel” the gas flow. Eddies created by presence of turbulence in
these gas clouds would drive these dust grains into regions of lower vorticity
(i.e. gaps between eddies). As a result, heavy elements become concentrated separately
from gaseous hydrogen and helium. Stars that form in these regions can acquire
abnormally high metallicities and might even be “totally metal”.
The process of how a core of dust grains might collapse to
form a “metal” star is not entirely clear. Nevertheless, it is reasonable to think
that as a core of dust grains collapses under its own gravity, the dust grains
would shatter into smaller grains and melt to form gas-phase metals. Such a
process would cool the collapsing core and drive a more rapid collapse until a “metal”
star is formed. Since planets seem to be ubiquitous around stars, planetary systems
should also form around “metal” stars. Given the unique high metallicity
conditions, the formation of truly exotic planets might be possible.
With an exceedingly high metallicity, a “metal” star would
have a remarkably long lifespan and would appear very unusual. This brings to
mind the concept of “frozen stars” proposed by Fred Adams and Gregory Laughlin
(1997) in a paper entitled “A Dying Universe: The Long Term Fate and Evolution
of Astrophysical Objects”. An exceptionally high metallicity low mass star could
sustain internal nuclear fusion at a very slow rate for a thousand trillion years
or so, outliving the dimmest red dwarf stars by a factor of a thousand. Such a
star can even be cool enough to have water-ice clouds, hence the term “frozen
stars”.
References:
- Philip F. Hopkins (2014), “Some Stars are Totally Metal: A
New Mechanism Driving Dust Across Star-Forming Clouds, and Consequences for
Planets, Stars, and Galaxies”, arXiv:1406.5509 [astro-ph.GA]
- Fred Adams and Gregory Laughlin (1997), “A Dying Universe:
The Long Term Fate and Evolution of Astrophysical Objects”,
arXiv:astro-ph/9701131