Mixing and igniting gasoline and
stretched by turbulence. The increase of
oxygen in an engine generates turbu-
the fusion reaction rate wrought by tur-
lence. Turbulence, in turn, increases the
bulence leads to the disruption of the
surface area of the fl ames by wrinkling
white dwarf in just a few seconds. The
and stretching them. The rate of fuel
debris expands at about 10,000 kilome-
consumption, proportional to flame
ters a second, in fair agreement with
area, goes up. A star, too, is naturally
what the observations show.
turbulent. Because the gas moves vast CRAB NEBULA is the gaseous debris of a core-
That said, many open questions lin-
distances at high velocities, even a slight
collapse supernova observed in 1054. At the
ger. What initially ignites the white
center is a neutron star ( arrow) spewing
disturbance quickly turns a smooth
dwarf is not understood at all. Another
particles that cause the gas to glow ( blue).
fl ow into a turbulent one. In a superno-
problem is that defl agration should eject
The outer fi laments consist mostly of hydrogen
va, the rising hot bubbles should stir up
and helium from the disrupted massive star.
a large fraction of the white dwarf es-
the material, causing the nuclear burn-
sentially unchanged, whereas observa-
ing to spread so fast that the star has no
ever, astronomers detect a wide variety tions suggest that at most a small part of
time to compensate.
of elements in these explosions, includ-
the star remains unaltered. So the ex-
In a properly working internal-com-
ing silicon, sulfur and calcium. That pat-
plosion cannot be a pure defl agration;
bustion engine, the fl ame propagates at
tern suggests the nuclear burning propa-
some detonation must be involved. The-
a subsonic speed limited by the rate at gates, at least initially, as defl agration.
orists have yet to explain why both pro-
which heat diffuses through the materi-
In the past few years, thermonuclear
cesses would operate. Nor can they ac-
al—a process called defl agration. In an
defl agrations have fi nally been convinc-
count for the observed variety of explo-
engine suffering from knocking, the ingly modeled by our group as well as
sions. It may well be that accretion onto
fl ame propagates at a supersonic rate, teams at the University of California, a white dwarf is not the only way to set driven by a shock wave passing through
Santa Cruz, and the University of Chi-
off a type Ia supernova; a merger of two
the fuel-oxidizer mixture and compress-
cago. The computer code we have honed
white dwarfs might, too.
y
sitre
S
v
WOLFGANG HILLEBRANDT, HANS-THOMAS JANKA and EWALD MÜLLER are scientists at
Gravity’s Tomb
R
Uni
O
the Max Planck Institute for Astrophysics (MPA) in Garching, Germany, and teach at
t h e o t h e r m ajo r t y p e of super-
e
att
Munich Technical University. Hillebrandt is one of the three directors of the MPA. His main
nova, arising from the collapse of a stel-
TH
S
U
research areas are nuclear astrophysics, stellar evolution and supernovae explosions,
lar core, is even trickier to explain. Ob-
ona
A
which he says he got into because he was fascinated by “really big explosions.” He won
servationally, these events come in a
E
Ariz RE
the Physics Prize of the German Physical Society in 1982 for his work on rapid neutron-
wider variety than thermonuclear su-
T
TH
SE
capture nucleosynthesis. In winter he can often be found on the ski slopes and in sum-
pernovae do: some have hydrogen, some
HFF
mer on a sailboat. Janka is interested in neutrino astrophysics, neutron star evolution,
do not; some explode into a dense inter-
E JD
and supernovae and gamma-ray bursts. One month after he had started his Ph.D. project,
stellar environment, some into nearly
ANL
Supernova 1987A was detected, and his career (let alone the universe) was never the
empty space; some eject large amounts
LO L
same again. He spends his spare time painting, drawing and doing handicrafts. Müller is
of radioactive nickel, some do not. The
NO
IS
leader of a research group on numerical and relativistic astrophysics. Together with
range of explosive energy and expansion
LL
/A
Janka, he won the Heinz Billing Award for Scientifi c Computing in 1993. His fascination
velocity is enormous. The most powerful
A
with astrophysics was inspired by science-fi ction novels. He is still a big fan of sci-fi
produce not only the classic supernovae
/ESAS
movies and enjoys hiking in the Bavarian Alps.
blasts but also long-duration gamma-ray
NA
46 S CIENTIFIC A MERIC A N
O C T O B E R 2 0 0 6
COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC.
CORE-COLL APSE SUPERNOVA
The other class of supernova involves the implosion of a star at least eight times as massive as the sun. This class is designated type Ib, Ic or II, depending on its observed characteristics.
1 As the massive star
nears its end, it takes
2 million
on an onion-layer structure
kilometers
of chemical elements
2 Iron does not undergo nuclear fusion, so the core
becomes unable to generate heat. The gas pressure
drops, and overlying material suddenly rushes in
200 km
Hydrogen
3 Within a second,
the core collapses
Neutron
to form a neutron star.
star
Helium
Material rebounds off the
Silicon
Carbon
neutron star, setting up a
Shock
Oxygen
shock wave
Iron
)
4 Neutrinos pouring out of the