Alien residents of any planets circling the bright, visible star Rho Cassiopeiae had better pack their bags. An international team of astronomers using the Utrecht Echelle Spectrograph on the William Herschel Telescope has identified Rho Cassiopeiae as the best candidate to undergo a supernova explosion in the near future. Observations between 1993 and 2002 show the star’s temperature to have dropped from 7000 degrees to only 4000 degrees in less than a decade. Similar but less precisely measured periods of coolings have been observed in 1893 and 1945 as well. The conclusion is inescapable – like a sputtering car chugging to a stop on the side of the road, Rho Cassiopeiae is running out of the (nuclear) fuel needed to keep its (fusion) engine going. The end could come at any time – but then what?
Stars are an exquisite, delicate balance between gravity (which pulls inward, trying to crush the star) and heat (which pushes outward, trying to explode the star). When a star finally runs out of hydrogen gas fuel, the heat suddenly stops but the gravity relentlessly continues. The exploding force of heat vanishes and the crushing force of gravity suddenly takes over as the sole master of a star’s fate. In a gut-wrenching plunge far more dizzying than any roller-coaster drop, the star implodes inward, becoming much smaller and much more dense. If the ashes from its billions of years of burning are just the right mix and amount, an amazing thing can happen. At a level of compression incomprehensible to mere humans, those ashes can themselves become a nuclear fuel in a new reaction far more powerful than even the normal stellar energy of hydrogen fusion. The star is said to become a supernova with such a titanic level of energy output that it becomes visible not only throughout our galaxy but in other galaxies halfway across the universe as well. And in this malestrom of nuclear fury a true miracle occurs. The ashes of the now-consumed star – light gases like helium – are joined together in new ways to create carbon, and oxygen, and nitrogen and all the other elements up to iron. There is no other known source in the universe where this process occurs. The very stuff of life and the metals for civilization are born in dying stars like the one Rho Cassiopeiae is destined to become at any time.
And after – or instead of – a supernova? That way lies the birth of neutron stars and black holes and even stranger things – another story for another day.
Ignore the last half of the original story, if not more. His description of supernovas is bogus.
Stars “burn” elements through fusion. For an average star, it can take billions of years to burn through the hydrogen, then hundreds of millions of years to burn through the helium, etc. The end stage is iron – by the time a star is producing iron, all remaining fuel in the core is burned very quickly. <u>Discover</u> magazine had a nice description on this a while back, I think that once iron is produced all remaining fuel (in the core) is burned within 3 seconds.
What happens then is pure gravity. The outer layers of the star start to fall in, and can reach speeds of about a third of the speed of light before “the big crunch.” Needless to say this much pass hitting this this hard causes incredible compression, and at the “big crunch” the stellar core is several times denser than a neutron star.
Once matter stops infalling, the neutron star matter “rebounds” and that’s the energy that drives the supernova explosion. It’s all gravitational energy, briefly stored in the compression of neutronium.
As a secondary effect the explosion has so much energy that it can drive nuclear fusion of elements beyond iron. But the fraction of energy put into nuclear fusion is a trivial fraction of the energy released in the ‘rebound’ event.
I agree some of my statements are misleading and upon retrospect just plain wrong. The nucleosynthesis of elements up to iron does indeed occur in stars that have not yet (or may/will never) turned supernova, and I agree that supernovas are the stage of a stars life where elements from iron to uranium on the periodic table are produced. I totally ignored the crucial concept of “rebound” in my description, and this is indeed a glaring oversight. The basic principles that gravitational collape is involved in shifting stars from use of one fusion fuel to the next, that stars are the sole source of element synthesis in the universe and that if certain ratios of materials and sizes are met the process ultimately results in supernovas are the key points to remember. My acknowlegement and apologies for not describing this process with as much truth and beauty as it deserves.
… that the supernova’s already happened? Like, around the time we all were developing cities and agriculture around the Mediterranean? Because it takes ten thousand years for the radiation from the supernova to reach us, I hope they packed their bags (traversed their wormholes?) a long time ago.
Relatively speaking, of course.
–wayf
Yep, this star may very well be gone right “now” and we just haven’t seen the explosion yet and won’t for thousands of years. Sort of like the tree falling in the forest with nobody listening. Supernovas can be very Zen.