The Alan Sondheim Mail Archive

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Date: Wed, 28 May 2008 15:37:56 -0400
From: physnews@aip.org
To: sondheim@PANIX.COM
Subject: Physics News Update 865

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 865 May 28, 2008      www.aip.org/pnu
by Phillip F. Schewe and Jason S. Bardi

EXPLODING STAR CAUGHT ON TAPE
Call it fantastic timing. Early this year, a group of astronomers
led by Princeton University's Alicia Soderberg were using NASA's
Swift satellite to observe a new supernova-one of those spectacular
explosions that mark the end of a massive star's life.  This
supernova was in a galaxy some 100 million light years away. It was
relatively unremarkable, Soderberg admits. But then something
extraordinary happened. On January 9, in what some astronomers are
calling a remarkable stroke of good luck, another star in their
field of view went supernova. "We actually watched the star
explode," says Soderberg, who was in Michigan, talking to an
audience of fellow scientists about her research when the call about
the supernova came from her colleague. This set off a week of
scrambling to get astronomers across the globe to point telescopes
at the supernova to confirm and better study the phenomenon.

Astronomers have never before seen a star at the first moments of
its explosive death.  Usually, astronomers miss the earliest flash
of a supernova because the explosion is only visible to orbiting
x-ray detectors on platforms like Swift. In the 22 May 2008 issue of
Nature, Soderberg and her colleagues describe how the supernova's
initial burst lasted a few minutes and then faded away. Its power
was remarkable. In 10 minutes, the exploding star expelled the about
the same amount of energy as the sun puts out in 82,000 years.

"It's incredibly serendipitous," says Harvard astrophysics professor
Josh Grindlay, a supernova expert who was not involved in the
research. "This almost certainly provides a whole new way of
detecting supernovae." Though astronomers have known about
supernovas for hundreds of years, the events are rare, only seen
about once a century in any given galaxy. They are only visible to
the eye or to ordinary telescopes a few weeks after the initial
burst, when the supernova begins to shine brightly-sometimes
becoming one of the brightest objects in the evening sky.
Supernovae are remarkable events not only for such displays of power
but because they culminate a natural process of stellar renewal-sort
of like cosmological compost. As famed physicist Hans Bethe said in
1967, upon winning his Nobel Prize, “Stars have a life cycle much
like animals. They get born, they grow, they go through a definite
internal development, and finally they die, to give back the
material of which they are made so that new stars may live.”
What causes a supernova is that the star's core collapses into a
tiny, incredibly dense orb. But the rest of the material in the star
collapses as well, and when material from the outer layers of the
star falls upon this dense core, it bounces off. This forms a shock
wave that races out to the star's edge, and breaks out, creating the
enormous burst of X rays like the one that Soderberg and her
colleagues captured on tape.

The explosion also creates heavy elements and spreads these elements
throughout space. The heavy elements in the universe, including
those on Earth, originated long ago in supernova explosions. Some of
this matter is radioactive, and its decay over time creates the
brightly visible display we associate with supernovae.  The
accidental discovery of the new supernova in January is significant,
says Soderberg, because it demonstrates that the first light of
exploding stars are these x-ray bursts. They are like early warning
beacons heralding the sometimes luminous display that follows.
Bigger and better telescopes proposed for the future will be able to
scan the skies and detect these x-ray bursts routinely from all the
nearby galaxies. Grindlay, the Harvard astronomer, is the principle
investigator on a candidate future NASA mission  called EXIST that
will scan the entire heavens every few hours and look for nearby
black holes and distant gamma ray bursts. If built, the telescope
should be able to detect many supernovae in their first explosive
moments-perhaps hundreds a year.

NEW FORM OF ARTIFICIAL RADIOACTIVITY
The basic structure of matter has been known for almost a century,
and yet scientists keep learning new things by persistently poking
and ripping apart atoms.  An atom consists of a relatively heavy
part at the core, the nucleus, and a lighter part, a fleet of
electrons, orbiting the nucleus.  The electron part determines all
the important chemical, electrical, and optical properties of the
atom, but the nucleus is important too. It contains most of the
atom’s mass and energy, and the reactions among nuclei are
responsible for powering the sun.

Nature often plays tricks. Usually hydrogen atoms have a nucleus
with a lone proton, but sometimes that nucleus can possess a neutron
in addition.  This version, or isotope, of hydrogen is called H-2
since it has two nuclear units. Still another version of hydrogen,
H-3, has a nucleus consisting of one proton and two neutrons.
Similarly, the main form of helium, He-4, has four nuclear
particles, but can also get by with only three: the He-3 isotope
consists of two protons and one neutron. All the other elements also
have numerous isotopes, some of which are stable, which means they
can persist for millions of years, and some are unstable, which
means that they break apart after a certain typical period called a
half-life.

Radioactivity is the process by which unstable nuclei transform into
more stable nuclei. “Radio” refers not to the kind of radio waves
we
get from a station but to the castoffs---either in the form of
particles or electromagnetic waves---radiated by the parent nucleus.
Historically the main forms of radioactivity were identified as
alpha, beta, and gamma rays (these being the first three letters of
the Greek alphabet).  An alpha ray or alpha particle is none other
than a He-4 nucleus.  Beta rays are now known to be electrons.  And
gamma rays are really just high-energy waves, even more potent than
x rays.

The new kind of radioactivity, discovered in an experiment conducted
recently at the Istituto Nazionale di Fisica Nucleare, a nuclear
laboratory in Italy, consists of nuclear fragments made of two
protons. You can think of this as a new isotope of helium.  He-2, as
it would be called, is highly unstable and very quickly flies
apart.   Making the unexpected new nuclear species took some
ingenuity.   First a beam of neon-20 ions was crashed into a foil of
beryllium.  In this collision some of the neon nuclei suffered a
slight robbery: losing two protons they ended up as neon18 nuclei.
Next, these same flying nuclei encountered a foil of lead. This
second collision had the effect of exciting the Ne-18 nucleus into a
highly unstable condition.   The remedy for this instability was for
the Ne-18 nucleus to slough off a fragment. There are several ways
of doing this. Among the decay options, the Italian physicists
found, was a rare, never-before-demonstrated process in which the
Ne-18 nucleus turned itself into an oxygen-16 nucleus, plus that
He-2 fragment.

According to one of the researchers, Giovanni Raciti at the LNS-INFN
lab (raciti@lns.infn.it), the two-proton decay mode was predicted
about 50 years ago. A few experiments conducted before this showed
ambiguous evidence: two protons emerged from the decay but one
couldn’t tell that the protons had not been thrown out one at a time
or both at the same time randomly from the whole Ne-18 or from a
single lump.  The new experiment definitely shows that the two
protons come out together from the breakup of a He-2 cluster (see
figure at http://www.aip.org/png/2008/302.htm). The new form of
helium isn’t good for anything practical since it doesn’t survive
even for a billionth of a second.  Raciti believes, however, that
the observation of this slender isotope of helium will us understand
how are built very unstable nuclei with a number of protons
exceeding the one of neutrons and, conversely, how heavy
nuclei---the cores of the heavier atoms here on earth---are built up
in the interiors of stars. (Physical Review Letters, 16 May 2008)

***********
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