Stars are born within cold, shapeless, swirling, giant molecular
clouds of dust and gas that are scattered throughout most galaxies. The
most widely recognized of astronomical objects, stars represent the most
fundamental building blocks of the galaxies, and the distribution,
composition, and age of a galaxy's sparkling stellar inhabitants trace
the history, evolution, and dynamics of that galaxy. In December 2014, a
team of astronomers announced that they may have solved an interstellar
mystery of why and how baby stars are born, thanks to the most
realistic supercomputer simulations of galaxies yet devised. Theoretical
astrophysicist Dr. Philip Hopkins of the California Institute of
Technology (Caltech) in Pasadena, California, led the research that
found that stellar activity, such as supernova explosions or merely the
surrounding sea of lovely starlight, play a major role in the birth of
other stellar sparklers and the growth of the galaxies themselves.
"Feedback
from stars, the collective effects from supernovae, radiation, heating,
pushing on gas, and stellar winds can regulate the growth of galaxies
and explain why galaxies have turned so little of the available supply
of gas that they have into stars," Dr. Hopkins explained in a December
10, 2014 University of Texas Press Release.
Supercomputer galaxy simulations were tested on the Stampede supercomputer at the University of Texas at Austin, which is part of the Texas Advanced Computing Center (TACC), an Extreme Science and Engineering Discovery Environment-allocated (XSEDE) resource funded by the National Science Foundation.
The first results of the study were published in the September 2014 issue of the Monthly Notices of the Royal Astronomical Society. Dr. Hopkins' work was funded by the National Science Foundation, the Gordon and Betty Moore Foundation, and a NASA Einstein Postdoctoral Fellowship.
The Mystery Begins
The
mystery begins in the space between stars where enormous, dark
molecular clouds billow and swirl phantom-like throughout most galaxies
in the Universe. These giant clouds are composed primarily of hydrogen,
which is the most abundant, as well as the lightest, of all atomic
elements. The ghostly clouds possess the mass of thousands or, perhaps,
millions of Suns, and they contain dense blobs that eventually condense
and give birth to new, fiery baby stars.
A familiar example of such a swirling, ghostly star-birthing cloud is the Orion Nebula.
Turbulence within these amorphous clouds gives rise to blobs with
sufficient mass for the dust and gas within them to begin to collapse
under the weight of their own gravitational attraction. As the blob
collapses, the material at the center heats up dramatically to become a
neonatal protostar. The seething-hot core of the protostar
is at the very heart of the collapsing cloud, and it will eventually
evolve into a mature star. Three-dimensional supercomputer models of
star birth predict that the whirling clouds of collapsing dust and gas
break up into two or three blobs. This would serve to explain why most
stars in our own barred-spiral Milky Way Galaxy dwell in either binary
or multiple star systems.
As the cloud collapses under the weight
of its own gravity, the seething hot core forms and starts to gather
dust and gas. Not all of this material winds up being part of the new
star--the remaining dust that circles around the neonatal star may
become planets, moons, comets, and asteroids--or it may merely linger
around as dust.
Within the billowing, swirling depths of molecular
clouds, baby stars set the surrounding darkness on fire with their
marvelous light, as they blast into stellar existence within these
starry nurseries that exist in galaxies. Deep within the billows of
these enormous clouds, delicate threads of star-making material
intertwine around each, and eventually merge, continuing to grow in size
for hundreds of thousands of years. The relentless squeeze of gravity
at long last becomes so crushing that the hydrogen atoms within these
dense blobs suddenly fuse. This lights the baby star's fires, that will continue to rage brilliantly for as long as the new star "lives".
Nuclear fusion is the process that ignites the new star. Searing-hot, seething, roiling protostars manage to stay "alive" by balancing two eternally battling forces in order to reach stellar maturity. Literally all main-sequence (hydrogen-burning) stars must spend their "lives" maintaining a critical balance between the two opposing forces of radiation pressure and gravity. While the relentless crushing squeeze of gravity pulls in the surrounding gas, radiation pressure
keeps the star bouncy by pushing everything out and away from the hot,
fiery star. The critical balance between these two battling forces keeps
the star blissfully "alive", and on the hydrogen-burning main-sequence.
Alas, stars inevitably grow old, and when an elderly star has finally
succeeded in burning up its entire necessary supply of hydrogen fuel,
its core collapses, and the star is ready to make its grand finale.
Relatively small stars, like our own Sun, go gently and beautifully
into that good night by puffing off their multicolored outer gaseous
layers into interstellar space. The sad relic core of a small star
evolves into a stellar corpse termed a white dwarf. However,
more massive stars do not go gently, and they meet their doom with a
magnificent fury. Massive stars, when they have finally reached the end
of the stellar road, blast themselves to smithereens in the raging
tantrum of a Type II (core-collapse) supernova.
The billowing, immense, frigid dark molecular clouds are the precursors of what are termed HII regions. HII regions
put on dazzling displays as they hurl their magnificent light into the
space between stars. The enormous molecular clouds can persist in a
stable condition for a very long time--however, collisions
between clouds, supernova explosions, and magnetic interactions can set
off collapse and, when this occurs, because of this collapse and
fragmentation, brilliant baby stellar sparklers may be born. An HII region
usually appears clumpy and irregular, and could easily be the cradle
for thousands of baby stars over the span of several million years. Some
of these fiery new stars can cause the HII region to glow, and also sculpt its shape. In fact, HII regions
can be observed sporting a variety of different shapes. This is because
the gas and stars within them are distributed irregularly.
Once the new stars inhabiting an HII region
have become toddlers, they are the source of ferocious, powerful winds
composed of fleeing particles that soar away from these massive stars.
These winds serve to both shape and blast away the ambient gases.
Solving A Stellar Mystery!
Astrophysicists
have been perplexed since the 1970s because their observations suggest
that only a small fraction of material composing molecular clouds is
ever transformed into stars.
"That's really what we were trying to
figure out and address, for the first time, by putting in the real
physics of what we know stars do to the gas around them," Dr. Hopkins
explained in the December 10, 2014 University of Texas Press Release.
A
collaboration formed between several universities to perform this
study. The members of this collaboration are from Caltech, the
University of California at Berkeley, the University of California at
San Diego, the University of California at Irvine, Northwestern
University, and the University of Toronto. The team produced an entirely
new set of supercomputer models dubbed Feedback in Realistic Environments (FIRE). FIRE focused the computing power on small scales of only a few light-years across.
"We
started by simulating just single stars in little patches of the
galaxy, where we trace every single explosion. That lets you build a
model that you can put into a simulation of a whole galaxy at a time.
And then you build that up into simulations of a chunk of the Universe
at a time," Dr. Hopkins continued to explain.
The Stampede supercomputer performed most of the computation. "Almost all of these simulations were run on XSEDE resources. In particular the Stampede supercomputer at TACC was the workhorse of these simulations... Stampede
was an ideal machine--it was fast, it had large shared memory nodes
with a lot of processors per node and good memory per processor. And
that let us run this on a much faster timescale than we had originally
anticipated. Combined with improvements we made to the parallelizationof
the problem, we were able to run this problem on thousands of CPUs at a
time, which is record-breaking for this type of problem," Dr. Hopkins
continued to note in the December 10, 2014 University of Texas Press Release.
Dr. Hopkins was surprised at how realistic the FIRE
galaxy supercomputer simulations came out. Earlier studies with
sub-grid models of how supernovae explode and how radiation interacts
with gas required manually tweaking the model after each run.
"My
real jaw-dropping moment was when we put the physics that we thought had
been missing from the previous models in without giving ourselves a
bunch of nobs to turn. We ran it and it actually looked like a real
galaxy. And it only had a few percent of material that turned into
stars, instead of all of it, as in the past," Dr. Hopkins added.
FIRE
has primarily simulated the more common and smaller galaxies, and Dr.
Hopkins wants to build on its earlier success. "We want to explore the
odd balls, the galaxies that we see that are of strange sizes or masses
or have unusual properties in some other way," Dr. Hopkins told the
press on December 10, 2014.
He also wants to model galaxies that
harbor supermassive black holes in their secretive hearts. Supermassive
black holes are thought to haunt the centers of perhaps all large
galaxies in the Cosmos, and they weigh-in at a staggering millions to
billions of solar-masses! Our own Milky Way harbors a supermassive black
hole in its heart--a relative light-weight, as supermassive black holes
go, weighing "merely" millions as opposed to billions of times more
than our Sun.
"In the process of falling in, before matter
actually gets trapped by the black hole and nothing can escape, it turns
out that for the most massive galaxies, this is even more energy than
released by all the stars in the galaxy. It's almost certainly
important. But it's at the edge, and we're just starting to think about
simulating those giant galaxies," Dr. Hopkins continued to explain.
Judith E. Braffman-Miller is a writer and astronomer whose
articles have been published since 1981 in various newspapers,
magazines, and journals. Although she has written on a variety of
topics, she particularly loves writing about astronomy because it gives
her the opportunity to communicate to others the many wonders of her
field. Her first book, "Wisps, Ashes, and Smoke," will be published
soon.
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