Sky & Telescope - October 2020 - 25

gamma rays from blazars (which have bone-breaking gravitational ﬁelds near the black holes' event horizons) or pulsars
(which have magnetic ﬁelds billions to trillions of times that
of Earth). Yet we've now seen gamma rays from more mundane cosmic zoo denizens, such as novae, binary star systems,
supernova remnants, and star-forming regions, confounding
scientists and dramatically changing our understanding of
their energetics.

Fermi's Discoveries

CO MPTON TELESCOPE: N ASA / MSFC; M OON IN G A M M A
R AYS: N ASA / D OE / FER MI L AT COLL A BOR ATION

S COMPTON The Compton Gamma-ray Observatory seen from the
Space Shuttle Atlantis during deployment. The lower dome on the spacecraft housed the instrument EGRET, which used the pair-production
process to detect gamma rays. The other instruments utilized different
kinds of scintillation detectors.

from something that is glowing or shining. Rather, gamma
rays are created by the acceleration of charged particles,
called cosmic rays, that are moving at very high speeds, often
near the speed of light. When a charged particle accelerates
or decelerates, it emits light. Faster acceleration (or deceleration) will produce higher-energy photons, and the most rapid
changes produce gamma rays. The bright gamma-ray glow
that we see from our own Milky Way Galaxy is likely created
when cosmic rays crash into particles of interstellar gas and
dust that are concentrated along the plane.
Decaying radioactive elements also produce gamma rays,
as does a mechanism called inverse Compton scattering, in
which visible or ultraviolet photons smash into relativistic
electrons. The crash gives a million-fold energy boost to the
incoming photon and transforms it into a gamma ray.
For cosmic objects,
the extreme acceleration of charged particles
needed to create gamma
rays typically occurs in
ultra-strong magnetic
or gravitational ﬁelds.
Astronomers therefore
weren't surprised to detect
X MOON IN GAMMA RAYS
This image from Fermi shows
what the Moon looks like in
gamma rays, specifically at
energies above 31 million
electron volts. The glow comes
from cosmic rays hitting the
lunar surface.

NASA's Fermi mission, launched into Earth orbit on June 11,
2008, has given us our most detailed view of the gamma-ray
universe. Its two onboard instruments combine scintillating
crystals and pair-conversion detectors to measure gamma
rays over an extremely wide band: from 8 keV to more than
300 GeV. At the highest energies, Fermi has an angular resolution of about 0.1°. That would not be enough to resolve the
largest craters on the Moon, but in gamma-ray astronomy it's
as high-deﬁnition as we can currently get.
In 2010, Fermi detected gamma rays from V407 Cygni, a
symbiotic binary system comprised of a white dwarf star and
a red giant star about 500 times the size of the Sun. Astronomers have long observed visible-light outbursts from novae,
caused by thermonuclear explosions on the white dwarf's surface. But no one expected to see strong gamma-ray emission
that persisted for 15 days! Fermi's discovery of this emission
from several novae revealed that the explosion's tremendous
shock front accelerated particles trapped by magnetic ﬁelds
to near light speed. When these highly relativistic particles
smashed into the wind ﬂowing out of the red giant, they
created gamma rays. Seeing such strong gamma emission
provided the ﬁrst proof that magnetic ﬁelds in novae can
approach the strength of those in their far more energetic
supernova cousins.
Fermi also revealed gamma rays coming from star-forming
regions. We usually think of these regions as dominated
by nascent stars shining brightly at infrared wavelengths,
cocooned within thick dust clouds that absorb most of their

Size and position
of the Moon

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Sky & Telescope - October 2020

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