Today's selection -- from Things That Go Bump in the Universe by C. Renée James. The violent death of a star:
“Because of its mass, the Sun's fate might not be particularly exciting, but plenty of stars do explode. To create the sort of supernova that Zwicky and Baade envisioned, you need to start with a star whose mass is between 9 and 25 times that of the Sun. Those stars are not easy to come by. Less than one in 100,000 stars are born with such heft, and those that are die in a flash. If the Sun's entire 10-billion-year life were compressed into a day, a star with 10 times its mass would be gone in about three minutes. A star with 25 times its mass would be gone in less than a minute. Of the millions of stars within 1,000 light-years of Earth, there is only one monstrous 25-solar-mass cosmic mayfly—Zeta Puppis, also known as Naos—and even it is likely a hair farther than 1,000 light-years.
“In the simplest explanation, the life of a star is dictated by how rapidly it uses up its own fuel stores, and this pace is determined by the unforgiving laws of physics. The nuclei of four hydrogen atoms can fuse into the nucleus of a single helium atom while converting some of the original mass to energy only in environments of extreme temperatures and pressures. The most-massive stars have such extreme environments in spades, and as a consequence they burn through their hydrogen at a rate tens of thousands times that of the Sun. If the Sun swaddles a billion Krakatoas each second in its core, these stars cradle tens of trillions. The end result is the same, though. Eventually both gluttons and dainty eaters will consume all the hydrogen on their plate (in their core), and this is where a star's mass makes all the difference.
“There is a poster in nearly every Astronomy 101 classroom that illustrates the seemingly unremarkable track that the Sun and its ilk will take from hydrogen fusion to giant to planetary nebula to white dwarf. The same poster reveals the slightly more exciting fate of the one-in-a-million stars with significantly higher masses. The extreme environment that allowed for hydrogen fusion shrinks, forcing helium nuclei to join to make carbon, oxygen, neon, magnesium, sulfur, and ever heavier atomic nuclei. Each new fusion channel is shorter and shorter in duration as the star's core desperately tries to squeeze another bit of life from the nuclear mass. All the while, the dying star's outer layers are swelling, and the star morphs into a supergiant.
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| The Cat's Eye Nebula, a planetary nebula formed by the death of a star with about the same mass as the Sun |
“When the core fuses its contents into iron, the star is done. Unable to produce further energy, but equally unable to efficiently shed the energy it has created, the heart of this seething monster hits temperatures of several billion degrees Celsius and densities billions of times that of water. Although the star has spent its entire life working to create its iron core, the high-energy light trapped within now destroys it, ripping apart the iron nuclei.
“It might not be immediately obvious why this should be a problem for the star, but pulling so much light energy out of the core to disintegrate the iron nuclei is like pulling out the first of many support blocks. The balance of light and particles and gravity was already a precarious one, with gravity held at bay largely by the outward push of electrons in the core. That balance is tipped ever so slightly by the removal of light energy and the rearrangement of the core's particles. The core begins to collapse, and as it collapses, it becomes hotter and denser. Soon, protons and electrons, typically holding each other at arm's length by the rules of subatomic particles, join to become neutrons. Taking all that like-charge repulsion out of the picture is like removing the last support block. The core has nothing left to hold itself up until the nuclear forces between the neutrons put a halt to the madness.
“All of this plays out in less than a second. In the time between the tick and the tock, the core has compressed almost to the point of vanishing, becoming even more intensely hot and dense in the process. Now 100 billion degrees Celsius and 100 trillion times as dense as water (about 100 million times as dense as a white dwarf), this least extreme forge of a massive star crafts a newly minted neutron star. The energy generated in this final dramatic act of the stellar core blows the rest of the star to kingdom come in a heartbeat.
“And that's what a not-so-well-behaved star can do.”
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| author: C. Renée James | |||
| title: Things That Go Bump in the Universe: How Astronomers Decode Cosmic Chaos | |||
| publisher: John's Hopkins University Press |
