By Judith E Braffman-Miller
When a massive star that weighs more than eight solar-masses reaches the end of that long and brilliant stellar road, it blasts itself to shreds in the incandescent rage of an explosive supernova death. The doomed star-that-was usually leaves behind a relic of its former existence--a very dense, weird stellar corpse termed a neutron star, or else a black hole of stellar mass. Supernovae can be so ferociously brilliant that they out-dazzle their entire host galaxy--at least for one brief shining moment. In January 2015, Harvard-Smithsonian Center for Astrophysics (CfA) and Dartmouth College astronomers announced that they have generated a new 3-D map of the interior of Cassiopeia A, or Cas A for short, which is one of the most well-studied supernova remnants in our entire Milky Way Galaxy. They found that the Cas A supernova remnant is made up of a collection of about a half-dozen cavities--or "bubbles"!
"Our three-dimensional map is a rare look at the insides of an exploded star," noted Dr. Dan Milisavijevic in a January 29, 2015 CfA Press Release. The new research, which has been likened to the "astronomical equivalent of a CAT scan" is published in the January 30, 2015 issue of the journal Science. The CfA is in Cambridge, Massachusetts.
About 340 years ago, in the constellation Cassiopeia, a massive, doomed star blew itself apart. As the star explosively met its fate, in a horrendous supernova conflagration, it rapidly cast searing-hot and radioactive matter outward from the dying star's core, furiously churning and mixing outer debris.
Alas, Cas A
The supernova remnant (SNR) Cas A is the most brilliant extrasolar radio source in the sky. The supernova blasted off about 11,000 light-years away from Earth, and the expanding cloud of material left in the explosion's wake now appears to be approximately 10 light-years across from Earth's point of view. The dazzling, multicolored SNR has been observed with amateur telescopes in wavelengths of visible light.
Astronomers think that the first light from this brilliant stellar blast reached our planet about 300 years ago. However, there are no historical records of any observations of the progenitor supernova, although there is some evidence that it was recorded as a sixth magnitude star 3 Cassiopeiae by the English astronomer John Flamsteed (1646-1719) on August 16, 1680. This unfortunate dearth of definite historical sightings is likely due to the presence of interstellar dust absorbing optical wavelength radiation before it reached our planet. Possible explanations suggest that the progenitor star was uncommonly massive and had earlier ejected a large percentage of its outer gaseous layers, prior to its going supernova. These outer layers would have veiled the dying star's death throes and reabsorbed much of the light emitted as the inner star collapsed.
The expansion shell has a temperature of approximately 50 million degrees Fahrenheit, and is expanding at about 4000-6000 kilometers per second. Cas A is the strongest radio source in the sky beyond our Solar System, and it was also among the first discrete sources to be detected in 1947. The optical component was spotted in 1950.
In 2013, it was reported that phosphorus had been detected in Cas A. This discovery confirmed that this element is produced in supernova blasts, with the Phosphorus-Iron ratio up to 100 times higher in material from the SNR than in the Milky Way in general.
Stars Do Not Live Forever
Stars do not live forever. Stars of all masses "live" out their entire hydrogen-fusing "lives" on the main-sequence by keeping a very precious and delicate balance between two highly antagonistic forces: gravity and radiation pressure. The radiation pressure possessed by a star is what keeps this huge ball of roiling, seething, glaring gas bouncy against the crushing force of its own gravity. The radiation pressure of a star pushes everything out, while gravity pulls everything in. Stellar radiation pressure is the result of nuclear fusion--the fusing of hydrogen, the lightest and most abundant atomic element in the Universe, into helium--which is the second lightest atomic element. This process of stellar nucleosynthesis fuses heavier atomic elements from lighter ones. All of the atomic elements heavier than helium (which are all termed metals by astronomers), were cooked up in the seething-hot nuclear-fusing hearts of the multitude of stellar sparklers inhabiting our immense Cosmos--or, alternatively, in the supernova explosions that ended the "lives" of the most massive stars.
When a heavy star, weighing at least eight solar-masses, has finally burned its necessary supply of hydrogen fuel, it can no longer keep itself bouncy--by way of the process of nuclear fusion--against the crushing squeeze of its own merciless gravity. In this way, gravity at last wins the final battle against the outward push of radiation pressure, and it pulls the material of the doomed star inward. Supernovae usually blast the dying star to shreds, violently tossing its gaseous layers off into interstellar space in a final, explosive tantrum. This occurs when the iron core of the massive star has reached 1.4 solar-masses. The most massive stars in the Universe collapse and blast themselves to smithereens, becoming the singularity that inhabits the secretive, hidden heart of a black hole of stellar mass. Massive stars--that are not quite that massive--also blast themselves to pieces in a final, furious, and fiery supernova tantrum, but they leave behind a dense remnant of their former stellar "lives" in the form of a neutron star.
Smaller stars, like our own Sun, perish more peacefully. When a small star like our Sun has finally burned up its necessary supply of hydrogen fuel, it first becomes a hideously swollen, crimson red giant star, that ultimately tosses off its multicolored, gaseous layers into interstellar space, leaving behind a small white dwarf--its relic core. White dwarfs are dense, small stellar corpses, but they are not as extreme as neutron stars.
Cas A's Bubbly Secret
It is difficult for scientists to model the complex physics behind supernova explosions, even with the most sophisticated, state-of-the-art simulations run on some of the most powerful supercomputers in the world. However, by carefully studying supernova remnants that are relatively youthful--such as Cas A--astronomers can try to understand which important processes drive these immense, brilliant, and violent stellar blasts.
"We're sort of like bomb squad investigators. We examine the debris to learn what blew up and how it blew up. Our study represents a major step forward in our understanding of how stars actually explode," explained Dr. Milisavljevic in the January 29, 2015 CfA Press Release.
Dr. Milisavljevic and his co-author Dr. Rob Fesen of Dartmouth College in Hanover, New Hampshire, examined Cas A in near-infrared wavelengths of light using the Mayall 4-meter telescope at Kitt Peak National Observatory near Tucson, Arizona, in order to create their 3-D map. Spectroscopy enabled them to measure expansion velocities of extremely dim material in Cas A's interior, which provided the all-important third dimension.
The two astronomers found that the large interior cavities appear to be connected to--and also very well explain--the previously observed large rings of debris that compose the bright and easily observed outer shell of Cas A. The two most well-defined cavities are 3 and 6 light-years in diameter, and the entire arrangement has been likened to the structure of Swiss cheese.
The bubble-like cavities were probably formed by plumes of radioactive nickel generated during the supernova blast. Because this nickel will decay to form iron, Dr. Milisavljevic and Dr. Fesen predict that Cas A's interior bubbly froth should be enriched with as much as a tenth of a solar-mass of iron. This iron-enriched interior debris has not yet been observed in previous studies, but next-generation telescopes may be required in order to detect the "missing" iron and validate the origin of the bubbles.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, journals, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.
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Keywords: Cassiopeia A,supernova remnant,supernova remnants,frothy interior,new 3-d map,supernovae,star death
