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49
50
January/February 2018^ DISCOVER^55
TOP: NASA, ESA, AND K. SAHU (STSCI). BOTTOM: GENENTECH
Weighing a White Dwarf
❯
THREE YEARS AGO, ASTRONOMERS put
a white dwarf on a scale and watched the
needle move. Not literally, says Kailash Sahu,
an astronomer at the Space Telescope Science
Institute, but their pioneering method of weighing
the star really is that straightforward. Their findings
appeared in Science in June.
When the dwarf, named Stein 2051 B, passed in
front of another star from Earth’s perspective, Sahu’s
team followed the position of the background star. As
general relativity predicts, light from the background
star bent around the white dwarf, distorted by its
gravitational field. Like the deflection of a scale’s
needle, the deflection of the background star’s light let
astronomers calculate the white dwarf ’s mass (roughly
67.5 percent the mass of our sun). The movement was
minute, but the results were stunning. “I almost fell off
my chair,” says Sahu.
The white dwarf ’s mass was exactly in line
with predictions made in a theory developed by
Subrahmanyan Chandrasekhar in 1930. Previous
attempts to confirm the theory had relied on shaky
assumptions, but Sahu’s group demonstrated
Chandrasekhar’s accuracy while proving their own
new method really works. SYLVIA MORROW
Drug Treats Aggressive Form of MS
❯
MORE THAN 400,000
AMERICANS are afflicted
with multiple sclerosis, an
autoimmune disease that disrupts
the brain’s neural signals to the
body. In March, the Food and
Drug Administration approved
the drug ocrelizumab to treat not
only the milder form of MS, but
also the primary-progressive form,
for which there was no treatment
until now.
In both types of MS, immune
system cells attack and strip
away myelin, the fatty protective
sheathing that insulates nerve cells.
This interferes with nerve signals,
causing muscle weakness, lack of
coordination, blurry vision, bowel
and bladder problems and foggy
thinking.
The new-to-market drug takes a
novel approach. Whereas traditional
MS medications target the immune
system’s T-cells, ocrelizumab focuses
on destroying the system’s B-cells,
which fuel the brain inflammation
that causes the disease to worsen.
During clinical trials, MRI scans
showed that ocrelizumab reduces
new brain inflammation in the
milder relapse-remitting form, and
slows deterioration in the progressive
and most aggressive form. Along
the way, the experiments have
resulted in important information
on how MS attacks the body, says
Stephen Hauser, a neurologist at
the University of California, San
Francisco, whose lab spent decades
determining the critical role B-cells
play in the disease. LINDA MARSA
Astronomers used the Hubble Space Telescope to learn a white
dwarf's mass by seeing how much it deflected another star's light.
Ocrelizumab is the first FDA-approved
treatment for primary-progressive MS.
White dwarf
Real star position Observed star position