Number 865, May 29, 2008 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 neutons 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 (firstname.lastname@example.org), 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)