ASTRONEWS
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16 ASTRONOMY • APRIL 2019
In early December, research-
ers announced gravitational
wave detections from four
new black hole mergers.
These powerful ripples in
space-time, caused when
massive objects are dis-
turbed or disrupted, were
discovered by the National
Science Foundation’s Laser
Interferometer Gravitational-
wave Observatory (LIGO) and
Europe’s VIRGO gravitational
wave detectors. Among
them is the largest and most
distant merger ever found.
The detectors caught the
largest merger, GW170729, in
July 2017. Researchers think
the waves were caused by
two black holes, 9 billion
light-years from Earth, that
collided about 5 billion years
ago. They estimate this mas-
sive merger produced about
5 solar masses’ worth of
gravitational waves, and that
the resulting body is now 80
times the mass of our Sun.
So far, VIRGO and LIGO
have detected gravitational
waves from 10 stellar-mass
black hole mergers, plus
one neutron star merger.
Currently, the number of
binary black holes in our
universe remains unknown.
Their masses, along with how
fast they spin when merging,
are also shrouded in mystery.
But the gravitational waves
that they produce can shed
light on their curious nature.
And more detections are
soon to come. After VIRGO
and LIGO completed their
observing campaign in
August 2017, researchers
upgraded their instruments.
With their next campaign
starting this year, the two will
be able to identify gravita-
tional waves from farther
events than ever before. — A.J.
Four new black hole
mergers are detected
The season’s best and brightest
CHRISTMAS COMET. Comet hunters got an extra-special treat last holiday season.
On December 16, Comet 46P/Wirtanen passed just 7 million miles (11 million kilometers)
from Earth, giving observers a stellar look at the icy object. Because it appeared in
conjunction with the holiday season, many dubbed it the “Christmas Comet.” Taken on
December 4 by photographer Gerald Rhemann, this image shows the comet’s green hue
and dim tail. Comet 46P/Wirtanen has a relatively short orbital period, circling the Sun once
every 5.4 years. And in 2018, it celebrated being the brightest comet of the year. Even better,
the comet’s flyby lined up with December’s Geminid meteor shower, giving stargazers yet
another wintertime treat. — A.J.
25
The number of years (as
of December 2018) since
the Wide Field Planetary
Camera 2, often called
the “camera that saved Hubble,” was
installed alongside the optics that
corrected the observatory’s mirror flaw.
The camera was retired in 2009.
FINAL MOMENTS. This simulated image shows the last stages
of a black hole merger, as the two objects spiral closer before colliding.
Emitted during the merger are gravitational waves, which carry
information about this process. THE SXS (SIMULATING EXTREME SPACETIMES) PROJECT
GOTTA JET. New simulations show that tangled magnetic fields around supermassive black holes can generate
conditions that hurl cosmic rays through space at great energies and speeds.
GALAXY PERCENTAGES
STAR CITIES. During the
past century, astronomers
have learned a lot about
the Milky Way. Perhaps the
most important thing is
that our galaxy is not the
only one in the universe.
As scientists began to study
and categorize galaxies,
they discovered four main
types, with one of them —
spiral — divided into two
subgroups. — M.E.B.
Scientists now think
that the universe
contains 2 trillion
galaxies.
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