Not The Star Of Bethlehem, But This Supernova Yields Answers To Rutgers-Led Astronomy Team

The team's findings appear online in the Astrophysical JournalLetters at <http://www.journals.uchicago.edu/ApJ/journal/rapid.html>,and are slated for print publication Jan. 10, 2000. Hughes, Rutgersgraduate student Cara Rakowski, David Burrows of Pennsylvania StateUniversity and Patrick Slane of the Harvard-Smithsonian Center forAstrophysics are authors of the paper "Nucleosynthesis and Mixing inCassiopeia A."

The Chandra X-ray Observatory is NASA's newest space telescopeand is the world's most powerful X-ray telescope. Chandra has eighttimes greater resolution and can detect sources more than twenty timesfainter than any previous X-ray telescope. Chandra was launched by theSpace Shuttle Columbia on July 23, 1999 with an orbit 200 times higherthan the Hubble Space Telescope. It detects images from X-ray sourcesthat are billions of light years away.

According to Hughes, one of the most profound conclusions of20th century astronomy is that nearly all elements other than hydrogenand helium were created in the interior of stars. "During their lives,stars are factories that take the simplest element, hydrogen, andconvert it into heavier ones," he said. "After consuming all thehydrogen in its core, a star begins to collapse and its central partsform a black hole while the rest of the star is blown apart in atremendous supernova explosion."

Supernovae are rare, occurring only once every 50 years or so ina galaxy like our own. Hughes and his collaborators used Chandra tostudy the exploded remains of a 300-year old supernova called Cas A, inthe area of the constellation Cassiopeia, learning more about thegenesis of these elements.

"When I first looked at the Chandra image of Cas A, I was amazedby the clarity and definition," said Hughes. "The image was much sharperthan any previous one and I could immediately see lots of new details."

In addition to the image clarity, the Chandra data held greatpotential for measuring the composition of stellar materialconcentrations visible in Cas A. Not only could the astronomersdetermine the composition of these many knots and filaments, but theycould also infer where in the exploding star these features hadoriginated.

For example, the most compact and brightest knots were composedmostly of silicon and sulfur, lighter elements with little or no iron. This pointed to an origin deep within the star's interior, where thetemperatures had reached 3 billion degrees during the collapse andresulting supernova explosion. Elsewhere, they found fainter featuresthat contained significant amounts of iron, a heavier element, alongwith some silicon and sulfur. This material was produced even deeperwithin the star, where the temperatures during the explosion had reachedhigher values of 4 billion to 5 billion degrees.

The astronomy team also discovered that the iron-rich featuresfrom deepest within the star were closest to the edge of the supernovaremnant. This meant that they had been flung the farthest by theexplosion that created Cas A. Even now this material appears to bestreaming away from the site of the explosion with greater speed thanthe rest of the remnant.

By studying the Cas A Chandra data further, astronomers hope toidentify which of the processes that have been proposed by theoreticalstudies is likely to explain supernova explosions, both in terms of theexplosion dynamics and elements produced.

"In addition to understanding how iron and the other elementsare produced in stars, we also want to learn how they get out of starsand into the interstellar medium. This is why the study of supernovaeand supernova remnants is so important," said Hughes. "Once releasedfrom stars, newly created elements can then participate in the formationof new stars and planets in a great cycle that has gone on numeroustimes already. It is remarkable to realize that our planet Earth, andindeed even humanity itself, is part of this vast cosmic cycle.

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EDITOR'S NOTE: For more information, contact John P. Hughes who iscurrently on-leave in France but can be reached by telephone:33-1-69-08-43-84 or e-mail at jph@physics.rutgers.edu. For specificinformation on Chandra and to access high-resolution digital versions ofthe X-ray images, visit http://chandra.harvard.edu,http://chandra.harvard.edu/photo/casajph/ andhttp://chandra.nasa.gov.

An online version of the newsrelease, with links to images, can be found on NASA's Marshall Space Flight Center Web site at http://www1.msfc.nasa.gov/NEWSROOM/news/releases/1999/99-302.html.