ASTRONEWS
WWW.ASTRONOMY.COM 11A supernova
fades away
Sagittarius A*, the supermassive black
hole that dwells in the center of the
Milky Way, is about 4 million times
the mass of our Sun. Astronomers
have long kept their eyes on a mys-
terious group of high-speed stars
that circle it. On May 19, one of those
stars, called S2, passed through the
black hole’s gravitational field.
The event was recorded in incredi-
ble detail, and it gave scientists a
prime opportunity to study whether
a phenomenon predicted by
Einstein’s theory of general relativity,
dubbed gravitational redshift, takes
place under the most extreme cases.
And, it turns out, it does.
Gravitational redshift happens as
light travels through an intense gravi-
tational field and loses some of its
energy, causing it to shift toward the
red, or less energetic, end of the elec-
tromagnetic spectrum. As the light
increasingly feels the pull of gravity,
it must work harder to keep traveling
at a constant speed. So rather than
oscillating at its original frequency,
its wavelength is stretched out,
becoming longer.
Scientists have long hoped to
observe how this phenomenon plays
out within the realm of a black hole,
but Sagittarius A* lies some 26,
light-years away, hidden behind large
clouds of dust and gas. Recently,
however, the European Southern
Observatory (ESO) equipped the Very
Large Telescope array (VLT) with a
new tool called GRAVITY. It combineslight harvested by VLT’s four tele-
scopes, yielding 15 times the resolv-
ing power and accuracy as just one
of the telescopes. Such resolution is
equivalent to being able to observe a
tennis ball on the Moon from Earth.
This setup let astronomers gather
hour-by-hour measurements as
S2 passed by Sagittarius A*. They
watched what happened as the star
came within 12 billion miles (19 bil-
lion kilometers) of the black hole.
During this close encounter, S
reached speeds of nearly 5,000 miles
(8,000 km) per second, nearly 3 per-
cent the speed of light.
The ESO team compared the new
data with previously collected data of
S2’s last such approach 16 years ago,
to understand how the star’s dipping
into the intense gravitational field
altered its light. They confirmed that
light emitted from the star became
less energetic — drained of energy, if
you will — due to the pull of the blackhole, shifting to a lower frequency.
“This is the second time that we
have observed the close passage of
S2 around the black hole in our galac-
tic center. But this time, because of
much-improved instrumentation, we
were able to observe the star with
unprecedented resolution,” said
Reinhard Genzel of the Max Planck
Institute for Extraterrestrial Physics,
and co-author of a paper on the
results published July 26 in
Astronomy & Astrophysics, in a state-
ment. “We have been preparing
intensely for this event over several
years, as we wanted to make the
most of this unique opportunity to
observe general relativistic effects.”
Moving forward, the researchers
plan to monitor how this encounter
may have shifted S2’s trajectory,
which will yield insights into the
distribution of gas and dust around
Sagittarius A*. — Michelle Hampson,
Jake ParksQUICK TAKES
WATER WORLDS
One-third of known super-
Earths may be up to 50 percent
water by mass. For
comparison, Earth is 0.
percent water by mass.- NEW SOURCES
Astronomers superimposed
multiple images of the sky to
create a new catalog of almost
72,000 X-ray sources.
SEARCHING SUBURBIA
The outskirts of galaxies are
likely prime spots to look for the
black hole collisions that
generate gravitational waves,
new research suggests.
SNAIL’S PACE
A rebounded shock wave from
a massive star’s supernova
explosion surprised
astronomers when it caused
effects lasting hours, instead
of just a few seconds.
TERRIBLE TWOS
Meteorite crystals older than
Earth show the infant Sun
bombarded the solar system
with loads of energetic
particles, as is seen in other
young stars.
ALF
An international team of
astronomers identified for the
first time one source of a
radioactive molecule in space:
26-aluminum fluoride (^26 AlF),
found in the variable star
CK Vulpeculae.
VERY COOL•
A space station experiment
produced ultra-cold atoms,
known as Bose-Einstein
condensates, with
temperatures just one
ten-millionth of a kelvin
above absolute zero.- SIT TIGHT
For the second time this year,
India has delayed its next
lunar spacecraft mission,
Chandrayaan-2. The new launch
window begins in January 2019.
FAR OUT
The new record-holder for most
distant radio galaxy ever
discovered is TGSS J1530+1049,
some 12 billion light-years away.
HOT STUFF
For the first time, astronomers
have detected titanium and
iron in the atmosphere of an
alien world that just so happens
to be the hottest exoplanet
ever discovered. — J.P.
DISRUPTIVE NEIGHBOR. New research suggests many strange features of the outer solar system, such as the orbits
of objects beyond Neptune, are well explained by a close encounter with another star while the Sun was still forming.Star’s black hole
encounter confirms
Einstein’s theory
NASA/JPL-CALTECH/IPAC
ZAP AND SNAP. The Very Large Telescope array uses a laser guide star and four
connected infrared telescopes to help astronomers pierce the turbulent core of our
galaxy and study the supermassive black hole that lurks there. G. HÜDEPOHL/ESOOLD AND NEW. NASA’s Spitzer
Space Telescope captured one of
the Milky Way’s oldest and largest
supernova remnants, dubbed
HBH 3, in this infrared image, which
combines data at 3.6 micrometers
(blue) and 4.5 micrometers (red).
The wispy streams of red at the
center are likely molecular gas
that became energized when
the star exploded and created a
shock wave, sometime between
80,000 and 1 million years ago. The
remnant lies 6,400 light-years from
Earth and is estimated to stretch
150 light-years in diameter. The
image also shows a cloudlike white
region (left), which represents
a portion of neighboring star-
forming regions known as W3,
W4, and W5. — A.J.