12 ASTRONOMY • JUNE 2020
QUANT UM GR AVIT Y
One of the most curious conse-
quences of Einstein’s theory of
relativity is frame-dragging: When a
massive object rotates, it drags space-
time with it like a skirt swirling around
a spinning dancer. Now, an international
team of astronomers has used this effect
to take measurements previously impos-
sible. By watching how frame-dragging
has affected a neutron star circling a
spinning white dwarf, researchers have
measured the rotation speed of a white
dwarf and confirmed previous clues
about its unique history. Their work
appeared January 31 in Science.
The white dwarf (the remnant of a
Sun-like star) is part of a binary system
called PSR J1141-6545, which also
contains a neutron star (a remnant left
when a star more than eight times as
massive as the Sun goes supernova).
Astronomers believe the white dwarf
formed first and the pulsar later. And
between those two events, mass from
the star that would eventually explode to
create the pulsar f lowed onto the whitedwarf, causing it to “spin up” and rotate
more quickly than otherwise possible.
But measuring how fast the white
dwarf is rotating to confirm this theory
is difficult, the researchers said, because
the tiny remnant is too faint to use
traditional methods for measuring a
star’s rotation.
Fortunately, the neutron star compan-
ion is also a pulsar, beaming radiation
from its poles as it spins. Based on when
each pulse arrives at Earth, researchers
can determine how fast the pulsar is
spinning and how it is moving in space.
And the pulsar’s movement through
the twisted space-time caused by the
twirling white dwarf allowed researchers
to work backward and determine how
fast the white dwarf is spinning.
Using atomic clocks to precisely timethe pulsar’s blips, they determined the
pulsar’s orbit has precessed by about
93 miles (150 kilometers) in the 20
years since the system was discovered.
(Precession is a bit like spinning a hula
hoop around your arm — the place
where the hoop is nearest and farthest
from your arm can change, even if the
shape and size of the hoop do not.) Based
on how fast the white dwarf must spin for
frame-dragging to cause that change, the
team calculated the rotation of the white
dwarf: roughly once every 100 seconds.
That speed confirms the white dwarf
pulled mass off its companion star and
spun up before the pulsar’s creation.
And that gives researchers even more
information about the system and how
it formed, also shedding light on other
systems like it. — A.K.Swirling space-time
reveals white dwarf’s past
The percentage of molecules in Jupiter’s equatorial
atmosphere that are water. This is nearly three
times the amount of water present in the Sun.TWISTED UP. In this artist’s concept, a white
dwarf (the larger object) distorts and drags
space-time around it, affecting the orbit of its
neutron star companion. As the neutron star’s
radio beams sweep over Earth, astronomers can
time their arrival to determine conditions near
the pair. ESO/L. CALÇADA