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Astronomers Say Dying Sun Will Engulf Earth
iamlucky13 writes "A minor academic debate among astronomers is the final fate of the earth. As the sun ages and enters the red giant stage of its life, it will heat up, making the earth inhospitable. It will also expand, driven by helium fusion so that its outer layers reach past the earth's current orbit. Previously it had been believed that the sun would lose enough mass to allow earth to escape to a more distant orbit, lifeless but intact. However, new calculations, which take into account tidal forces and drag from mass shed by the sun, suggest that the earth will have sufficiently slowed in that time to be dragged down to its utter destruction in 7.6 billion years. "
This is the way I was taught it would happen on astronomy shows from the 1980s. I don't get the big deal.
I don't know why your karma would suffer from this... there are some interesting parallels between theories concerning the technological singularity and the Biblical book of Revelation (at least in some peoples' opinions). Why not add another metaphorical spin to things?
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And some of the academic references are actually a decade old: http://www.astronomy.ohio-state.edu/~pogge/Lectures/vistas97.html
During the hydrogen burning phase, inert helium gradually builds up in the core and hydrogen becomes less common. This means the core has to contract and become hotter in order to produce enough energy to support itself and the surrounding envelope. The fusion rate depends on the square of the hydrogen density (since you need the hydrogen atoms to collide with each other), so if the hydrogen density goes down, the core has to become hotter and more generally dense in order to maintain the same energy production rate. (This is why stars gradually become more luminous over their main sequence lifetime, as the core actually has to produce more energy in order to support itself in its more compact configuration.)
As a star finishes exhausting its hydrogen, this actually reaches a very extreme configuration where the core becomes much more compact (and much hotter) trying to squeeze out the required energy with very little hydrogen remaining. The total energy being produced by the core (in order to keep itself from collapsing) increases very rapidly at this point, and the larger luminosity will then push the envelope outward, puffing it up. This is why stars expand into red giants, and this is the stage where the Earth will probably be engulfed.
For trivia purposes, the central core eventually runs entirely out of hydrogen and sits there as an inert clump while the upper edges of the core burn hydrogen. When the hydrogen is exhausted for a large enough fraction of the core, the center eventually becomes hot and dense enough to fuse helium into carbon. At this point, the overall luminosity drops again (because the star doesn't need to keep frantically burning just hydrogen to support itself) and the star contracts a bit. The process then starts over again, with a shell of helium fusion surround an inert carbon core that (for stars more massive than the Sun) eventually ignites to fuse into neon, oxygen, etc.
Microsoft delenda est!
When speaking of planetary catastrophe the death of our Sun is but a distant worry. It has already been mentioned that in about 3 billion years the galaxy Andromeda will collide with our own Milky Way galaxy. That of course poses several dangers to Earth in itself, though none particularly likely due to the vast distances between stars within galaxies, the potential for a stellar marauder to interfere with our solar system and cause chaos for Earth does exist. More worrisome, though, is the fact that around the same time (3 billion years from now) the Earth's core will finally cool and it's magnetic field generating dynamo will shut down, causing the Earth's shielding from the solar wind to collapse and the atmosphere to be stripped away eventually leaving the planet as dry and barren as Mars. Well before that ever happens, Earth will have to deal with the solar system's bobbing and weaving in and out of the galactic plane, possibly exposing the planet to deadly cosmic rays. Even nearer to our future is the fact that a conveniently aimed gamma ray burst from an exploding star (Betelgeuse is ready to go any day now) could "sterilize" the planet. Then of course, there is the ever present threat of an Earth shattering asteroid impact, which happens every 100 million years or so on average... in which case you could consider Earth overdue for another one. So yeah... the Sun engulfing the Earth (or what's left of it) 7 billion years from now... I wouldn't sweat that one.
Back in the 60's or so they figured out the whole red giant phase of stellar evolution and realized the sun would expand to about the diameter of the earth's present orbit when it reached this point. It was a fascinating bit of trivia for Carl Sagan and the common folk to pass around that the sun would engulf the earth, but further investigation showed the sun would likely lose something like 30% of it's mass as heat from helium fusion blew away the outer layers (a process that looks really freaking cool from a distance). This would cause the earth, due to conservation of its orbital energy, to assume a much larger orbit...about as far out as Mars is today.
Therefore the popular notion was thought by many astronomers to be wrong. But in fact, nobody had ever done a really detailed model of the process until the subject of this article. It turns out, the professionals were wrong, and the common folk were correct, if only because we were a couple decades behind the times academically.
If you don't believe me, here's the archived wikipedia page for earth from last Friday. It's since been updated.
Space is big. There are only supposed to be about six stellar collisions, and most of those in the core region. Out here in the boondocks? We're probably fine.
I am a science fantasy fan
Unless there are some revisions in the laws of physics, the nuclear processes throughout stellar evolution are well known based on computer models.
When helium "ash" accumulates in the core, helium fusion is not the next thing that happens. The core starts contracting and heating up, but that lights off H2 fusion in the shell surrounding the core. That phenomenon changes the luminosity and heat transfer rates of the star, causing the outer atmosphere to swell up into the red giant stage.
When shell burning runs its course, again the core contracts and heats up some more, resulting in the helium flash. Based on computer models, the helium flash is a major disruptive event caused by the sudden onset of helium fusion, it does not cause the star to go nova or anything, but it causes the star to change modes as it were, becoming somewhat bluer and smaller, but still more luminous than Main Sequence. From the computer models, it is believed that the upper-righthand HR diagram stars, red giants, are H2 shell burners while the horizontal branch above the Main Sequence represents He core burners.
For a massive enough star, exhaustion of core He will initiate shell He ignition, sending the star back into the red giant range, perhaps as a red supergiant for a massive star.
The red giant phase is only one phase of an evolved star. Everyone just kind of assumed that a star that goes supernova would be a red giant, but it seems like the star that popped off in Supernova 1987a in the LMS was blue.