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A computer
simulation of the environment around a Population III star. Image courtesy of
Ralf Kaehler and Tom Abel.
One hundred million years after the Big Bang, giant
primordial stars heated, ionized, and pushed the gas around them to form
present-day stars and galaxies. And now, for the first time, we can see it
happening—in a 3-D simulation.
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"The stunning thing about the simulation is its resemblance to
star-forming regions in our own galaxy, as seen from the Hubble
telescope," says SLAC's Tom Abel, whose team at the Kavli Institute of
Particle Astrophysics and Cosmology (KIPAC) developed the mathematical model
for the simulation.
The dazzling images from the simulation take us back to a time when the first
stars—massive twinklers millions of times brighter than the sun—lit up the
universe. Scientists have long believed that these primordial
"Population III" stars gave birth to galaxies, but until now they
have lacked the tools to understand how it might have happened. Since these
celestial events occurred far back in time, very little evidence is
observable with telescopes.
Computer simulation is a promising substitute, but modeling the dynamic
environment of a Population III star is a complex problem. The star would not
only have heated and pushed the gas around it, but its radiation would have
ionized the gas molecules, knocking off electrons and turning the molecules
into charged ions. To model the complex interplay of these phenomena, the
simulation has to solve a six-dimensional differential equation at hundreds
of millions of points for each instant in time.
"Naďve methods will fail," Abel said.
Previous simulation methods made several simplifying assumptions—for
instance, they assumed constant gas density around the star, or restricted
themselves to a one-dimensional problem. While such simplifications made the
problem tractable, they provided only limited insight into the underlying
cosmic phenomena.
Abel's elegant alternative is to use a computer graphics technique, called
adaptive raycasting, to model the outward spread of ionization from a star.
Previous methods were computationally overwhelmed by trying to calculate
everything on a uniform grid in space. Raycasting, in contrast, allows the
computations to be tied to gas density—denser regions are analyzed with a
finer grid, optimizing the calculations at any point in space. By combining
adaptive raycasting with gas dynamics, Abel's algorithm captures the dynamic
complexity of the cosmic churning that precedes star and galaxy formation.
Abel and colleagues, including KIPAC's John Wise and Columbia University's
Greg Bryan, have started to use this technique to study the earliest events
in the formation of galaxies.
"My long term goal is to build a galaxy, one star at a time," Abel
says.
Source: Stanford Linear Accelerator Center, by Chandra Shekhar
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