Timeline for Earth after our sun begins to die?
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16-04-2016, 12:28 AM
RE: Timeline for Earth after our sun begins to die?
(15-04-2016 11:51 PM)Erikjust Wrote:  
(15-04-2016 10:35 PM)EvolutionKills Wrote:  It is time on a cosmic scale, so you're looking at billions of years, with margins of error in the millions of years. Nothing that can be easily quantifiable into a human lifespan. Plus, nobody on Earth would see anything as the sun burned the last of it's fuel, as half of the solar system (including Earth) would have already been destroyed by the prior red giant phase.

Unless i have once again miss understood the science our sun is currently using Hydrogen as fuel and as long as it has Hydrogen we´re fine.
When it runs out of it, that´s when it will slowly start to cook the earth and slowly start to increase in size.

But life on the surface of the earth will be long gone by the time it starts growing.
It´s that particular timeline i´m interested in from when the sun runs out of Hydrogen and starts heating up The Earth til there´s no more advanced life on planet Earth.

Did you see the chart Sensei posted? All of the times are listed in X amount of additional years from now.

500,000 years - We've most likely been hit by a 1km asteroid, which without intervention, is a global extinction level event. It's also when the next ice age should kick in.

10,000,000 years - A nearby star, T Pyxidis, should goes supernova and possibly bathes the Earth in gamma radiation.

50,000,000 years - Antarctica's ice is melted, raising sea levels by 75m.

60,000,000 years - Earth's orbit unpredictable, effects of gravity multiply over long periods, making orbits chaotic and impossible to predict.

600,000,000 years - End of C3 photosynthesis, 99% of all life goes extinct.

800,000,000 years - C4 photosynthesis ends, multi-cellular life goes extinct.

1,000,000,000 years - GAME OVER. Sun's luminosity increases 10%, Earth's surface temperature rises to 47C. The oceans evaporate, atmosphere becomes a moist greenhouse, small pockets of water may exist at the poles allowing for simple life.

1,300,000,000 years - CO2 starvation kills all cells with a nucleus (eukaryotes), only those without nuclei (prokaryotes) survive.

2,300,000,000 years - Earth's core freezes, and with that it's magnetic field collapses; without which the planet quickly looses it's atmosphere and surface water, much like Mars did.

2,800,000,000 years - Earth's surface temperate hits 147C, ending all life.

3,500,000,000 years - Earth is now Venus 2.0, quite literally hell on Earth.

4,000,000,000 years - Milky Way collides with Andromeda, forming a new galaxy.

5,400,000,000 years - The Sun finally exhausts it's supply of Hydrogen and starts transitioning into it's red giant phase.

7,900,000,000 years - Sun hits max size, 256 times current size. Mercury, Venus, and possibly the Earth are destroyed (remember, unpredictable orbits as per 60 million years).

8,000,000,000 years - White Dwarf - The sun starts burning carbon-oxygen and drops down to 54% of it's current mass.

14,400,000,000 years - Black Dwarf - Temperature and luminosity plummet, making it invisible to human eyes against the backdrop of space.

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16-04-2016, 04:39 AM (This post was last modified: 16-04-2016 04:48 AM by Deesse23.)
RE: Timeline for Earth after our sun begins to die?
By the way: the sun will wont ever run out of hydrogen at all! When the helium flash happens, the sun starts to burn helium instead of hydrogen and it will grow and get hotter, there will be plenty of hydrogen still left when this happens.

Its because, unlike red dwarfs, the sun (a yellow dwarf) is not fully convective (big stars work different again). The sun is not like a "hot pot" where the material (plasma) is stirred up constantly. This happens only in the outer layers. The central part of the sun consists of Plasma so hot and dense that it is in a degenerate form of plasma/matter and energy can only be transported by radiation and not convection. Thus, a lot of hydrogen will never be transported to the vore and be availiable for nuclear fusion.

So: There is no point in time at which the sun runs out of hydrogen. There is a time when it will start to burn helium instead of hydrogen, indicated by the helium flash. However it will be hard to impossible to even observe it from the outside (earth). So the starting point of wour whole scenario is (from todays point of view) not even able to be safely defined once it happens and anyone would be left to observe it. You wont be able to observe it at all.

So as i said earlier, your question is somewhat meaningless.

P.S.: The fact that red dwarfs are fully convective (= 100% of their hydrogen is availiable for nuclear fusion during their lifetime) and the very slow rate of fusion in those small stars makes up for their insane lifetimes given.

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16-04-2016, 04:23 PM
RE: Timeline for Earth after our sun begins to die?
OK, much of this is model dependant since we've never witnessed the entire sequence from start to finish in an individual star. It takes a billion years or so and we haven't been around that long. We've observed many different stages of the terminal stages of a star's life cycle in many different stars so we have a pretty good idea of how it will go but the exact details are still theoretical.

(1) Everything dies long before the Sun enters the red giant phase of its life. The sun is slowly getting hotter. It has increased in luminosity by about 30% over the last 4.5 billion years and is currently increasing in luminosity at about 1% per 100 million years. This happens because the Sun fuses 4 H into 1 He, and a single He takes up less space in the core. As it ages, the Sun's core becomes smaller, denser and hotter.

That's been offset by a very convenient negative feedback loop on Earth that sequesters carbon in geological reservoirs. As the temperature goes up geological weathering increases, trapping more carbon underground. As CO2 is removed from the atmosphere the greenhouse effects are diminished stabilizing global temperature.

Unfortunately, we're nearly out of CO2 to remove from the atmosphere. Unless some new negative feedback emerges to counter it, the planet's temperature will begin to rise as the sun continues to brighten. The growing instability in the carbon thermostat is likely one of the causes for the spate of ice ages we've had in the last few million years. Before the last of the CO2 is exhausted its concentration will drop low enough that plants won't be able to breath. All complex life dies.

As the temperature begins to rise, more water vapor will be pumped into the atmosphere and water vapor is a very effective greenhouse gas. Temperatures rise rapidly. The oceans boil, plate tectonics stop and Earth becomes Venus 2.0. The exact timing on this depends on whether you're a pessimist or optimist but some time in the next 100 million to billion years is likely.

(2) 4 to 5 billion years from now the Sun will begin to exhaust the last of the hydrogen in its core. This is a gradual process where the H-H collisions necessary for fusion become progressively less and less common as helium concentrations rise. Much like trying to burn progressively soggier paper. To maintain fusion, the core contracts under gravity, increasing in temperature. This increases the total number of collisions, maintaining the necessary number of H-H collisions to sustain core fusion.

Over a billion years the sun slowly brightens, expands and cools to a red giant. Nobody watching will notice any significant change because it happens on such a long time scale. The short-term cyclical variation in the Sun will overwhelm any observation made in an individual's lifetime. There's little change to the Sun's spectrum because the helium that's building up is trapped in the core. If your observers know something about stellar evolution they'll likely be able to figure out that the end is well and truly nigh. If by "nigh" you mean "in another 100 million years."

One of the fun things about giant star is that they lack the discrete boundary between photosphere and corona that dwarf stars have. Giant stars lack a distinct surface, having a much more gardational transition from the photsphere to the corona. That means that it isn't so much a s question of if Earth will be engulfed by the Sun as much as when and how much.

(3) The core exhausts the last of its hydrogen and fusion gutters out. Still nobody notices because hydrogen fusion is ongoing in a more H-fertile shell surrounding the core. The now inert core contracts, heating up as it does, until it finally becomes degenerate Helium, held up by quantum pressures rather than the more typical electromagnetic effects. Helium produced in the overlying fusion shell continues to accumulate adding more mass to the core and increasing the temperature further.

(4) Temperatures in the core reach a critical level around 100 million K and helium begins fusing into carbon. Because the core is in a degenerate state, thermal expansion doesn't compensate for the rise in temprature. With normal matter you'd get a stable adiabatic expansion that would drop the temperature but in degenerate matter P and T are ony weakly linked. The helium fusion begins a thermonuclear runaway in the Sun's core called a helium flash that burns roughly 6% of the core's mass in a matter of minutes.

Still nobody notices. The overwhelming majority of the energy liberated is confined to the core and inflates it from a degenerate state to a more normal state of matter. The core is now burning helium with a hydrogen fusing shell above that.

Unless your observer has a neutrion detector that is. In that case they just dumped a load in their drawers because they caught the massive blast of neutrinos produced by something like 8000 times the mass of the Earth worth of helium being fused to carbon.

(5) Helium fuses at much higher temperatures but releases less energy per fusion than hydrogen, so the Sun's core burns through its helium inventory in about 100 million years. One again the core heats up as the helium fusion reaction slows then stops. The core, now dominantly carbon nitrogen and oxygen, contracts to a degenerate state once again. Helium continues to fuse in a shell above the core with hydrogen still valiantly fusing in a shell above that. The sun get's hotter still, expanding and growing redder.

This time the end is well and truly nigh and people actually will notice. The Earth has long since been consumed but the civilizations now flourishing on Ganymede and Europa are fucked. Our Sun doesn't have the mass to bring the core temperature up high enough to fuse oxygen into silicon. Just as well since that ends in tears anyway. Fusion continues in the H and He burning shells above the core but the vast amount of energy being produced isn't sustainable.

Instabilities feed back on one another leading to a series of catastrophic thermal pulses that blow the outer half of the Sun clear of the core. The outer envelope of the Sun is blown into space, forming a planetary nebula that lasts for a few tens of thousands of years.

The Sun's core remains behind as a white dwarf stellar remnant, composed largely of degenerate CNO. Lacking fusion, it slowly cools and contracts, finally becoming too dim to radiate in the visible spectrum 100 billion years later.

The Earth, assuming it survived, continues to orbit as a scorched and then frozen cinder. Tidal forces and losses to gravitational radiation slowly rob it of momentum, spiralling ever closer to the Sun. Eventually the Earth's orbit spirals within the Roche limit and tidal forces tear the planet apart. The fragments rain down on the surface of the black dwarf that remains of the sun, briefly brightening what was once the solar system.

Flesh and blood of a dead star, slain in the apocalypse of supernova, resurrected by four billion years of continuous autocatalytic reaction and crowned with the emergent property of sentience in the dream that the universe might one day understand itself.
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