Sky & Telescope - USA (2019-09)

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skyandtelescope.com • SEPTEMBER 2019 19

qMAKING THE SHADOW Photons traveling close to a black hole have their paths bent by gravity. Some will plunge in, but others are redirected

or even caught in circular orbits just outside the event horizon. These orbiting photons then escape and travel toward the observer (right), tracing

out a slightly magnifi ed picture of a light ring around a dark center.

radio. Very close to the event horizon, these photons can

be diverted or even temporarily trapped by gravity, looping

around and around the black hole in what’s called a photon

ring, explains EHT astronomer Feryal Özel (University of

Arizona). But if the light continues inward, it will plunge

past the event horizon and never reach us. These effects

combine to create what’s called the black hole’s shadow: a

dark circle surrounded by a bright halo. The halo’s inner

edge is the photon ring.

Einstein’s description of gravity, the general theory of

relativity (GR), predicts what this shadow should look like: a

nearly perfect circle approximately fi ve times wider than the

black hole’s horizon. The size and shape depend primarily

on the way the black hole bends light around itself, not the

behavior of the gas, radiation, or magnetic fi elds nearby.

However, the shadow only exists because of that nearby

matter. The bright ring outlining the darkness is made of

synchrotron radiation, radio photons spat out by electrons

corkscrewing along magnetic fi eld lines. (Magnetic fi elds

thread the giant tutu of gas that encircles the black hole.)

The closer observers look to the black hole, the hotter the

radiation. To peer as close to the black hole as possible,

where gas temperatures reach some 100 billion degrees ,

astronomers need to observe at short radio wavelengths of

about 1 millimeter.

But they also need large targets. The black holes that

form binaries with stars are only as wide as cities; to detect

shadows, astronomers must observe supermassive black

holes, which squash millions to billions of Suns’ worth of

mass into a region the size of a planetary system. The black

holes also have to be bright

at the right wavelengths, and

not all are.

Scientists thus settled on

two optimal targets: Sagit-

tarius A*, which sits in the

center of the Milky Way, and

M87*, which lies about 55

million light-years away in

the constellation Virgo (S&T:

Feb. 2012, p. 20). Each of

these black holes should have

a shadow roughly 50 micro-

arcseconds across, or the

apparent size of an orange

at the distance of the Moon.

They’re also very different

black holes: Sgr A* is a quiet beast, constantly fl ickering as

it snacks on gas, whereas M87* rages, spewing a plasma jet

toward us that stretches thousands of light-years into space.

Even so, M87* is a steadier source, varying on daylong time

scales instead of minutes as Sgr A* does.

But how do you take a picture of something the size of

an orange on the Moon?

Making the Invisible Visible

Working at a wavelength of 1.3 mm, astronomers need a

telescope approximately 10,000 km in diameter to resolve

the shadows of these black holes. That’s roughly the size of

Earth itself. No agency will fund that.

pHOME GALAXY The elliptical

galaxy M87 contains roughly

10 times more stars than the

Milky Way and dominates over

the Virgo Cluster’s some 2,000

galaxies. A powerful jet streams

from its central black hole,

stretching across several thou-

sand light-years.