Sky & Telescope - October 2019 - 37

ROSS FA LCON

Puzzling Interior
While Winget and Montgomery test the physics of plasma
at white dwarf surfaces, Bailey's own work is to understand
speciﬁcally how radiation propagates through a star's material. His experiments replicate conditions matching those
deep inside the Sun, at the boundary where an inner region
in which energy travels as radiation meets the roiling outer
region, where convective cells of plasma carry energy to the

If you increase the total opacity
value of a star by, say, 10%,
that means the star is 10% less
luminous and therefore will live
10% longer.
surface. The stellar characteristic he's studying is something
called opacity, which describes how much radiation is blocked
or absorbed by intervening matter.
"High opacity," says astrophysicist Aldo Serenelli (Institute
of Space Sciences, Spain), "is like an insulating blanket." A
star whose material has high opacity traps the heat from the
star's furnace. A low opacity means a lot of that radiation can
pass through the material. (A glass window, for example, has
low opacity to visible light but high opacity to some ultraviolet wavelengths.)
Some chemical elements absorb or trap radiation more
than others, which means a star's chemical composition
affects how much light makes it through the stellar layers
and to the surface. That also means a star's luminosity - the
amount of energy it emits - relates to its ingredients' opacities. Therefore, says Serenelli, opacity affects how fast the star
can burn its fuel and how long it lives. If you increase the
total opacity value of a star by, say, 10%, that means the star
is 10% less luminous and therefore will live 10% longer. The
opacities are by far "the most sensitive aspect" in Serenelli's
simulations of stellar interiors.

1.0

Fraction of radiation transmitted

than a human hair. The current runs through those tungsten
wires and heats them up so quickly that they vaporize into
a hot, ionized gas - a plasma. That same current produces
a magnetic ﬁeld that forces the tungsten plasma to collapse toward the cylinder's central axis, the inner shaft of
the spool. There sits a 6-mm-diameter piece of hydrocarbon
foam. As the tungsten plasma falls toward the cylinder's axis,
it runs into the foam. The impact generates X-rays.
It's these X-rays that researchers use to heat their experiments. Surrounding the tungsten-threaded spool is a gold
cylinder, with several openings through which radiation
can escape. The scientists place their sample in front of one
of those openings. "What we want is for the X-rays to just
stream through our sample," says Bailey. The radiation heats
whatever the researchers set in its path. In some experiments,
to keep the material from expanding as its temperature rises,
it's tamped down, creating densities like those in stars.
UT's Center for Astrophysical Plasma Properties (CAPP)
runs four or ﬁve experiments during each shot. For Winget
and Montgomery's studies of white dwarf material, they set
up a travel-mug-size vessel of gas between 30 and 35 cm away
from the X-ray source. The X-rays stream into that gas cell and
slam into the cell's gold back wall, which in turn also radiates
X-rays, to bathe the sample fully in high-energy radiation. The
scientists collect data on the wavelengths the gas absorbs and
emits and what those spectral patterns look like.
They then compare the shapes of the spectral lines created in the lab to those from white dwarf stars. Interactions
among particles in the plasma affect the shapes of the line
widths, and so from the spectra they can learn how many
particles are in a given volume. As the densities increase to
stellar values, atoms are perturbed, and the width of the
material's spectral lines changes.
The widths they're ﬁnding in their experiments differ from
those seen in observations. "It's turning out that the theory is
wrong by a fair amount," says Montgomery.
The higher, lab-measured density correlates directly to
a higher measure of surface gravity, and thus mass, than
astronomers had expected. "It means the masses of the white
dwarfs are a little bigger," adds Winget - 10% to 20% bigger
than the old theories predicted, actually. More massive stars
cool more slowly, and temperatures are part of how observers
calculate a white dwarf's age. Not only will the revised theory
help astronomers better interpret the dwarfs they observe, he
says, but it will help physicists understand what happens to
matter at these extreme densities.

Hydrogen-beta
Absorption Line

0.8

0.6

0.4

1970s theory
2000s theory
New theory
Z machine data

0.2

0.0

470

480

490

500

Wavelength (nanometers)
p PLASMA SPECTRUM The UT Austin team found that the hydrogen
plasma they created with the Z machine (data points) did not absorb
radiation quite the same way as previous theories (black and blue lines)
had predicted it would. The discrepancy corresponds to a 10% to 20%
difference in white dwarf masses, which could in turn change estimates
of these stars' ages by up to a billion years.
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Sky & Telescope - October 2019

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