Sky & Telescope - December 2022 - 13

The first image of our galaxy's central black hole gives us a peek at a bizarre object.
E
nthroned in the Milky Way's heart sits the cowardly
lion of black holes. Known as Sagittarius A*, this object
holds the equivalent of 4 million Suns squashed into a
region less than 20 times as wide as our star. A diffuse tulle
of hot gas skirts the beast, fueling a glow about 100 times
brighter than the Sun - so feeble that, if it lay in another galaxy,
it would probably be undetectable.
Astronomers first discovered Sgr A*
(pronounced " Sadj-ae-star " ) in 1974 as a
" compact radio source, " just as the realization
was dawning that big black holes
might sit in most galaxies' cores. Over the
decades, observers have tiptoed closer to
its lair. By the late 1990s they'd realized
the radio signal had structure, but it would
take another decade before they confirmed
this structure was on the scale of the event
horizon, a black hole's infamous point of
no return.
Even with these advances, Sgr A* has
elliptical galaxy M87 (S&T: Sept. 2019, p. 18). With two
images in hand of very different beasts - one extraordinary,
one ordinary - astronomers can now say that M87*'s ring
of light was no fluke; we are indeed seeing extreme gravity at
work. But Sgr A*'s blurry visage also carries the beginnings
of deep physical insight into our black hole and others like it,
insight that already challenges some of our
expectations.
Sgr A*'s " shadow "
spans about
50 microarcseconds
on the sky, roughly the
size of a grapefruit
seen at the distance of
the Moon.
remained just out of reach, crouched 26,000 light-years
away. But on May 12, 2022, astronomers with the Event
Horizon Telescope Collaboration jumped the distance and
brought us face-to-face with our black hole. The EHT image,
reconstructed from data taken at radio telescopes across the
Western Hemisphere, shows a luminous ring encircling a
dark center: the black hole's silhouette, marking where light
from surrounding gas is either bent around the hole or comes
too close and is swallowed.
Three years ago, the EHT team gave us a similar image
of the much larger, plasma-jet-shooting black hole in the
HELLO This is the first image of the black hole at the center of the
Milky Way Galaxy. Scientists created it by averaging thousands of possible
reconstructions, built with data from eight radio observatories.
SMT,
KP
OBSERVATORIES
Atacama Large
Millimeter/
submillimeter
Array (ALMA)
Atacama
Pathfinder
Experiment (APEX)
IRAM 30-meter
Telescope (IRAM)
James Clerk
Maxwell Telescope
(JCMT)
Large Millimeter
Telescope (LMT)
South Pole
Telescope (SPT)
Submillimeter
Array (SMA)
Submillimeter
Telescope (SMT)
Greenland
Telescope (GLT)
Kitt Peak 12-meter
Telescope (KP)
Northern Extended
Millimeter Array
(NOEMA)
PLANET-SCALE DISH Eight stations participated in the EHT's 2017 observing
campaign. All eight successfully observed Sgr A* on April 7th, collecting data over
about 12 hours. In 2018 the Greenland Telescope joined the array, and in 2021
NOEMA and Kitt Peak did as well. (LMT sat the 2021 run out but returned in 2022.)
SMA,
JCMT
LMT
As the World Turns
Sgr A*'s " shadow " spans about 50 microarcseconds
on the sky, roughly the size of
a grapefruit seen at the distance of the
Moon. To espy such a minuscule sight,
astronomers need an Earth-size radio
telescope. They " build " this telescope by
observing simultaneously with dishes
spread across the planet, then flying datafilled
hard drives back to supercomputers
in Massachusetts and Germany, where
they carefully sync the observations to within trillionths of a
second. This technique is called very long baseline interferometry,
or VLBI.
VLBI combines data from pairs of telescopes; more telescopes
give you more combinations. Each pair probes a different
scale, with close-together dishes sensitive to large-scale
structure and dishes separated by thousands of kilometers
picking up small structure. As the planet turns, different stations
have different views, and their positions relative to one
another change as seen from the target's perspective, filling
in bits of the virtual telescope's dish.
GLT
NOEMA
IRAM
APEX,
ALMA
SPT
sk yand tele scope .o r g * DECEMBER 2022 13
SGR A* SHADOW: EHT COLLABORATION; TELESCOPE ARRAY: LEAH
TISCIONE / S&T, SOURCE: EHT COLLABORATION / ASTROPHYSICAL
JOURNAL LETTERS 930:L12 (2022)
https://skyandtelescope.org/
Sky & Telescope - December 2022

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