Sky & Telescope - December 2022 - 16

The Big Black Hole Next Door
Simulations normally assume gas falls in toward a black
hole from a fat surrounding donut called a torus, which
would feed the accretion disk in an orderly way. But this is
a solution of convenience, Ressler says - he doesn't know
of a torus ever forming in simulations with realistic initial
conditions. Winds, on the other hand, create a diffuse
flow with less turbulent motion, and thus less variability. In
fact, Ressler and two colleagues recently tested the wind-fed
scenario against other observations of Sgr A*'s behavior and
found that it matched how the brightness changed with time.
Navigating Oz
If EHT astronomers set aside Sgr A*'s variability - which
busts every model tested - then they are left with an interesting
subset of findings about the black hole. But there's no
yellow-brick road here: We're traveling in shadowy territory,
and any of these inferences might be wrong.
First, the spin. Astronomers define a black hole's spin as a
fraction of how fast the object can whirl. The few measurements
we have of supermassive black holes similar to Sgr A*'s
heft are gas gobblers that spin nearly as fast as they can, likely
spun up by a steady stream of gas.
If we rely on the subset of simulations that fit most of
Tipped
towards
viewer
Tipped
away from
viewer
INCLINATION ANGLE The spin axis of Sgr A* appears to partially
point toward us, angled at most 50° away from our line of sight. It's
unclear whether the tilt is toward or away from us (both options shown).
Additional data favor an angle of roughly 30° and an axis tipped away
from us.
what's seen, then Sgr A* also spins fast, at least 50% of its
max. But there's reason to doubt that number. Many theorists
think that black holes power jets with their spins, and Sgr
A* shows no definite sign of shooting jets. Some astronomers
have also suggested that if our beast spins quickly, then
it should drag spacetime around with itself so much that
it would disrupt the orbits of the stars closest to it, which
doesn't appear to be the case.
From a galactic-evolution standpoint, Sgr A* perhaps
shouldn't spin at all, says Angelo Ricarte (Center for Astrophysics,
Harvard & Smithsonian). The Milky Way hasn't
M87* VS.
SGR A* A black hole's
shadow is larger than
its event horizon:
The innermost stable
photon orbit lies just
outside the event
horizon, and the
black hole's gravity
lenses this light to
create an oversized
silhouette. If we had
perfect resolution,
the light would form
a narrow ring where
the dotted circles
are. Because the
black hole M87* is
1,500 more massive
than Sgr A*, its event
horizon is much
bigger - the whole
solar system would
easily fit inside.
M87*
Sgr A*
o
Sun's
diameter
Size of
Pluto's orbit
Distance of
Voyager 1
from Sun
M87*
Ring width: 42 microarcseconds
Black hole mass: 6.5 billion Suns
Distance: 55 million light-years
Jet? Yes
Spin axis's inclination to our line of sight: 17°
Sgr A*
Ring width: 52 microarcseconds
Black hole mass: 4 million Suns
Distance: 26,000 light-years
Jet? Unknown
Spin axis's inclination to our line of sight: <50°
16 DECEMBER 2022 * SKY & TELESCOPE
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INCLINATION ANGLE: GREGG DINDERMAN / S&T; BLACK HOLE COMPARISON: GREGG DINDERMAN /
S&T, IMAGES: EHT COLLABORATION (ACKNOWLEDGMENT: LIA MEDEIROS, XKCD)
Sky & Telescope - December 2022

Table of Contents for the Digital Edition of Sky & Telescope - December 2022
