Astronomy

ASTRONEWS

WWW.ASTRONOMY.COM 19

Antimatter is matter’s opposite. For each par-

ticle of “normal” matter, there is an antimatter

particle with an opposite charge; for example,

positrons are the antimatter equivalent of elec-

trons. When matter and antimatter meet, they

annihilate each other, giving off gamma rays in

the process.

Astronomers have observed such gamma

rays in the center of our galaxy since the 1970s.

Despite extensive follow-up, researchers today

are still looking for the exact source (or sources)

of this emission. It could run the gamut from

the mundane — natural processes in a star’s life

— to the exotic, such as dark matter (see p. 12).

In a paper published May 22 in Nature

Astronomy, lead author Roland M. Crocker of

the Research School of Astronomy and

Astrophysics at Australian National University

and his co-authors suggest that positrons

resulting from white dwarf mergers could

contribute significantly to the observed

gamma-ray signal.

White dwarfs are left behind after a Sun-like

star runs out of fuel. If two low-mass stars

(about 1.4 to twice the mass of our Sun) circle

each other closely in a binary system, they can

interact via mass transfer, a process in which

gas from the stars is exchanged. The end result

is two white dwarfs that may eventually merge.

That merger can produce radioactive isotopes

that decay into positrons.

The current resolution of instruments used

to study this emission is not high enough to

see individual sources, such as single super-

nova remnants. Thus, more precise measure-

ments and computer simulations will be

needed to determine the positron production

rates from such events. Such information will

also shed light on the processes that shaped

the young Milky Way. — A. K.

White dwarf mergers seen as antimatter source

MOON RIVER. River channels on Saturn’s moon Titan resemble those seen on Mars rather than Earth. Titan and Mars

both show stream formation without influence from plate tectonics.

MAKING POSITRONS. The merger of two white dwarfs, depicted here as they spiral toward each other, could

provide a major source of positrons in the Milky Way’s center. NASA/TOD STROHMAYER (GSFC)/DANA BERRY (CHANDRA X-RAY OBSERVATORY)