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Προτείνω τη δημοσίευση αποσπασμάτων και τον σχολιασμό από διευθνή και Ελληνικά άρθρα που αφορούν το μέλλον του πλανήτη Γη και του ηλιακού μας συστήματος.
Αρθρο: Sun's growing brightness a threat to Earth
The Sun is slowly getting brighter and warmer; in seven billion years it will engulf Earth - but much sooner, in 1 billion years, the Sun will grow 11 percent brighter, raising average terrestrial temperatures to around 50 °C, causing oceans to evaporate; the solution: move Earth away from the Sun!
John Maynard Keynes said that “In the long run we are all dead,” but this does not mean that we should not be concerned with the state of the world even when we are no longer around.
The Sun is slowly getting warmer as it burns the hydrogen in its core. In about five billion years the Sun will begin evolving into a bloated red giant. Scientists calculate that as the Sun’s outer gas shell swell up, it will, seven billion years from now, engulf the Earth. The troubles will begin much sooner: In 1 billion years the Sun will grow 11 percent brighter, raising average terrestrial temperatures to around 50 °C (120 °F). This will warm the oceans so much that they will evaporate without boiling, like a pan of water left on a sunny kitchen counter.
What is to be done? Scientists say that we cannot keep the Sun smaller and cooler, but we can increase the distance between Sun and Earth by moving Earth away!
Colin McInnes, a mechanical engineer at the University of Strathclyde, says we can do that by using a giant solar sail. Solar sails are thin, mirror-like films that are propelled by the weak pressure of the sunlight that falls on them. Hecht writes that McInnes’s idea is to put a free-floating solar sail at a point near the Earth where the pressure of solar radiation essentially balances the Earth’s gravitational pull. His analysis shows that the reflection of sunlight from the sail will pull the Earth outward along with the sail (to use physics terminology: there is a need to increase the Earth’s orbital energy and accelerating the center of mass of the system outward, away from the Sun).
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ΟΤΑΝ Ο ΗΛΙΟΣ ΓΙΝΕΙ ΕΡΥΘΡΟΣ ΓΙΓΑΝΤΑΣ
The long-anticipated and widely discussed doomsday will come, but it is going to happen in approximately five billion years. The core of the Sun will run out of hydrogen by this time, and our celestial body will turn into a red giant. The star will then gradually turn into a white dwarf - the core of the Sun.
The Sun is not eternal, the Universe is not stable at all, and any of its bodies, including stars, undergo evolutionary cycles that start with their birth and end with death.
Billions of years in the future, when our Sun bloats up into a red giant, it will expand to consume the Earth’s orbit. But wait, you say, the Earth travels the Earth’s orbit… what’s going to happen to our beloved planet? Will it be gobbled up like poor Mercury and Venus?
Astronomers have been puzzling this question for decades. When the sun becomes a red giant, the simple calculation would put its equator out past Mars. All of the inner planets would be consumed.
The explosion of the Sun is not going to look like an instant blast. It will become the result of complicated and long processes. It may happen in remote future, when mankind will most likely leave planet Earth. Humans will either become extinct or find other planets to live on.
Usually, a star begins its evolution in the form of a cold low-density cloud of interstellar gas. The cloud gets compressed under the influence of its own gravitation and takes a spherical shape. During the process of compression, the gravitational power transforms into heat and the body begins to warm up. As soon as the temperature inside the body reaches 15-20 million kelvins, thermonuclear reactions begin to take place inside the sphere. The compression process stops, and the object becomes a star.
A star of medium weight, like our Sun, remains in the state of the hydrogen-helium cycle throughout its life. When it burns the entire stock of hydrogen fuel, and hydrogen inside the core of the star turns into helium, the thermonuclear combustion of the hydrogen mantle continues.
The luminosity of the object continues to grow, its outer layers continue to expand, whereas the surface temperature decreases. The star grows 100 times its size, and becomes a red giant. The star remains in this phase for a much shorter period of time - several millions of years.
The helium core of the giant can no longer stand its own weight and begins to compress.
If the object is massive enough, the growing temperature may trigger the transformation of helium into heavier elements - carbon, oxygen, silicon and iron. This period may last for billions of years for the stars of medium weight.
As long as helium combustion reactions are very sensitive to temperature, the star may begin to pulsate vehemently and turn into planetary Nebula.
The bared core in the center of medium-weight stars (like our Sun) cools down and becomes a helium white dwarf, the diameter of which is comparable to that of the Earth. Since white dwarfs have no sources of energy, they get dark and invisible as they continue to cool down. More massive stars turn either into neutrons (pulsars) or black holes.
The evolution of stars ends with the birth of supernovas.
The gravitational power of the Sun will decrease as the celestial body slowly loses its weight. As a result, the objects of the solar system may descend from their orbits. Most likely, many planets, including Earth, will be burnt during the stage of the red giant.
As the Sun reaches this late stage in its stellar evolution, it loses a tremendous amount of mass through powerful stellar winds. As it grows, it loses mass, causing the planets to spiral outwards. So the question is, will the expanding Sun overtake the planets spiraling outwards, or will Earth (and maybe even Venus) escape its grasp?
K.P. Schroder and Robert Cannon Smith, are two researchers trying to get to the bottom of this question. They’ve run the calculations with the most current models of stellar evolution, and published a research paper entitled, Distant Future of the Sun and Earth Revisted.
According to Schroder and Smith, when the Sun becomes a red giant star 7.59 billion years, it will start to lose mass quickly. By the time it reaches its largest radius, 256 times its current size, it will be down to only 67% of its current mass.
When the Sun does begin to bloat up, it will go quickly, sweeping through the inner Solar System in just 5 million years. It will then enter its relatively brief (130 million year) helium-burning phase. It will expand past the orbit of Mercury, and then Venus. By the time it approaches the Earth, it will be losing 4.9 x 1020 tonnes of mass every year (8% the mass of the Earth).
But the habitable zone will be gone much sooner.
Astronomers estimate that will expand past the Earth’s orbit in just a billion years. The heating Sun will evaporate the Earth’s oceans away, and then solar radiation will blast away the hydrogen from the water. The Earth will never have oceans again. It will eventually become molten again.
One interesting side benefit for the Solar System. Even though the Earth, at a mere 1.5 astronomical units, will no longer be within the Sun’s habitable zone, much of the Solar System will be.
The new habitable zone will stretch from 49.4 AU to 71.4 AU, well into the Kuiper Belt. The formerly icy worlds will melt, and liquid water will be present beyond the orbit of Pluto. Perhaps Eris will be the new homeworld.
Back to the question… will the Earth survive?
According to Schroder and Smith, the answer is ΝΟ. Even though the Earth could expand to an orbit 50% larger than today’s orbit, it won’t get the chance. The expanding Sun will engulf the Earth just before it reaches the tip of the red giant phase. And the Sun would still have another 0.25 AU and 500,000 years to grow!
Once inside the Sun’s new atmosphere, the Earth will collide with particles of gas. Its orbit will decay, and it will spiral inward.
If the Earth were just a little further from the Sun, at 1.15 AU, it would be able to survive the expansion phase! Although it’s science fiction, the authors suggest that future technologies could be used to speed up the Earth’s spiraling outward from the Sun.
I’m not sure why, but thinking about this far future of the Earth gives an insight into human psychology. People are genuinely worried about a future billions of years away. Even though the Earth will be scorched much sooner, its oceans boiled away, and turned into a molten ball of rock, it’s this early destruction by the Sun that feels so sad.
Έχει επεξεργασθεί από τον/την Γεωτεχνική στις Κυρ Οκτ 28, 2012 1:05 pm, 2 φορές συνολικά
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|Θέμα: Απ: To τέλος Ηλιου & πλανήτη Γή Σαβ Νοε 26, 2011 5:57 pm|| |
Understanding Solar Evolution (Posted on 23 March 2011 by Chris Colose)
When considering the evolution of planetary atmospheres, a critical factor that inevitably comes into the discussion is the gradual brightening of the sun over geological time. Observations that Earth and even Mars likely had liquid water in their distant past, despite a less luminous sun, present a paradox (Sagan and Mullen, 1972): If the sun was so faint, how would the early planets not have been frozen over? Alternatively, if the planets were warm enough to support liquid water then, how are they not extremely hot now? Examining this question is critical to paleoclimate and habitability issues in general, including understanding arguments on why greenhouse gas levels have been higher in the past while still being compatible with relatively cold conditions.
Our own sun is a G2V type star, the ‘V’ also indicating a ‘dwarf,’ or in other words a luminosity classification that tells us the star resides on the “main sequence” (i.e., a locus of points in a plot of stellar luminosity vs. temperature, known to astronomers as a Hertzprung-Russell diagram, see: http://casswww.ucsd.edu/public/tutorial/images/hr_local.gif). It is on the main sequence that the sun will spend most of its lifetime in a stable configuration with nuclear fusion as a power source.
Stellar classifications (O, B, A, F, G, K, M), which have become even more finely tuned (for example with numbers, like A0, A1…A9, etc, as the field grows) are characterized according to their spectral properties.
Measurements of stellar energy distribution show many narrow wavelength bands with reduced fluxes (i.e., or spectral lines) due to absorption by atoms and ions in the surface layer of the star. The strength of lines and the elemental abundances are in turn affected by temperature. Hydrogen has the strongest lines for A, B, and F stars, while for ‘redder’ stars hydrogen lines are weaker, and heavy atoms may be seen. M-dwarfs are low mass stars (between ~0.08-0.5 Msun) with temperature of about 2,400-3,900 K (compared to 5,800 K for our sun) and luminosities of 0.02-6% of our own sun.
Giants and White dwarfs are distinguished from main-sequence stars above. Note that luminosity is a measure of the total power output (for example in Watts), so a star can still have a high luminosity even with a relatively low temperature if it is very big. Likewise, white dwarfs are very hot, but are rather small and so have a weak luminosity.
In this post, we are interested in the question of why the sun actually evolves in the way it does. The implications for planetary atmospheres will be discussed in a follow up Part 2.
We cannot directly observe the evolution of a single star, since they evolve on timescales far longer than humans observe them, although how the luminosity can increase with time on the main-sequence is generally well-accepted. This is through sound theoretical models, but also because we can observe clusters of stars elsewhere in the galaxy with hundreds of thousands of stars (and different masses) in order to build confidence in stellar evolution theory.
The key to understanding the evolution of stellar atmospheres relates directly to the principle of hydrostatic balance, the same fundamental force balance which allows Earth’s atmosphere to stand up as it does (rather than gravity collapsing all the air molecules into a thin layer near the surface). In a hydrostatically balanced atmosphere, the downward force due to gravity is also compensated for by a force upward that results from the pressure of the underlying fluid. In other words, hydrostatic equilibrium defines a balance between gravity and the vertical pressure gradient. If the constraints imposed by hydrostatic balance were not obeyed to high accuracy, than even for a body as large as the sun, noticeable fluctuations in its radius would occur over a characteristic dynamical timescale of just many minutes to an hour.
The next step toward understanding stellar evolution is nuclear fusion, which occurs in the solar core. It turns out that the easiest nuclear reaction is one between a proton and a deuteron. The timescale for two protons to form a deuteron however is rather slow, on the order of 1010 years, but this slow rate helps to set the timescale over which the sun evolves on the main-sequence. Once deuterium is formed, it can smash into another proton to make Helium-3, which can in turn react with He3 again, or for temperatures higher than about 1.4×107 K (achievable in the stellar interior), He3 prefers to react with He4 (various possible paths are shown here ). In any case, what we’ve ended up doing is converted 4 hydrogen atoms into a Helium nucleus.
The decrease in hydrogen abundance as it is converted to Helium is critical to the sun’s evolution. The important point for evolution on the main sequence is that this process leads to an increase in the mean molecular weight in regions where fusion is important. At still higher temperatures, other fusion chains like the CNO cycle or the triple-alpha process become important. The ideal gas law gives a relation:
Ρ = ρ / μ (RT)
where P is the pressure at the center of the star, R is a constant, ρ is the density of the gas, μ is the mean molecular weight, and T is the temperature. As μ changes, the temperature and pressure must also change to compensate, which in turns impacts the stellar luminosity. How does this work?
Stars become helium rich over time only in the interior, while hydrogen remains abundant in the outer envelope, since the core is where nuclear fusion is most efficient. In the core, there is a consequent reduction over time in the number of particles per unit mass. From the ideal gas law, the decreased pressure of this sphere is no longer sufficient to support the overlying envelope, so if you were to imagine a hypothetical sphere drawn out in the sun, the increase in helium makes it impossible for the sphere to stay at the same radius. Contraction occurs and as the core density goes up, gravitational potential energy is released and (through something called the virial theorem) half of energy is radiated away and half increases temperature of the gas.
The luminosity also increases, which is reflected in an increase in the solar irradiance striking a planet. Gough (1981) proposed the following general equation to describe the luminosity as a function of time:
L(t) = L(t0) = L(t0) [1+2/5(1-t/t0)]-1
where L(t0) is the luminosity at the current age of the sun, t0 ≈4.6 billion years, and L(t) is the luminosity at time t.
It follows for example that 3.5 billion years ago, the solar luminosity was at only ~76% of today's value, while during the Neoproterozoic ~700 mya near the last snowball episode, it was ~94% of today's value.
Since there is still hydrogen in the core of the sun, slow evolution on the main sequence will occur for a few billion years still. The central contraction will cause a hydrogen “shell” to get hotter and burn more strongly (note that eventually the CNO-cycle dominates, and energy generation becomes concentrated in a narrow region around a helium-rich core), and high interior temperatures and pressures are too high to be in equilibrium with gravitational forces.
The extra energy output results in the dramatic envelope expansion that causes the sun to evolve onto the red giant phase. This terminates the main-sequence phase of the star’s life, and by this time the surface of the sun will actually be somewhat cooler, but its radius will be extremely large (engulfing Mercury for example) and overall luminosity much higher. Considerable mass loss can occur on the Red Giant Branch, in which case Venus and Earth's orbit will be moved outward (inversely related to the mass), potentially being saved from being completely engulfed.
The duration over which other stars will evolve on the main-sequence, as well as the rate at which end phases of its evolution cause it to expand, contract, or change its surface gravity depend largely on its mass. The evolution of stars will therefore differ depending on initial characteristics; for high mass stars for example, the details outlined above are modified somewhat in that they have a convective core, so the newly formed helium actually becomes well-mixed in the stellar interior.
When considering stellar evolution and the prospect of life evolving, it is worth noting that the lifetime on the main sequence is inversely related to the mass, to a power generally between 3-4, a consequence of the efficiency at which they burn fuel. This means large O-type stars may only last a couple million years on the main-sequence, while no one will ever find remnants of former main-sequence M-types, since their lifetime is longer than the current age of the universe. M-types therefore are more stable than stars like our own with respect to luminosity variations.
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|Θέμα: Απ: To τέλος Ηλιου & πλανήτη Γή Κυρ Απρ 29, 2012 3:25 pm|| |
ΑΚΟΛΟΥΘΕΙ ΕΝΑ ΑΠΟΣΠΑΣΜΑ ΠΟΥ ΑΦΟΡΑ ΤΟΝ ΠΛΑΝΗΤΗ ΑΡΗ, ΠΟΣΟ ΠΡΟΣΙΤΟΣ ΕΙΝΑΙ ΓΙΑ ΤΟΝ ΑΝΘΡΩΠΟ?
Think of Mars as a massive fixer-upper. Sure, it's nowhere nearly as nice as our current planetary home, but perhaps with a little work we could live there. Given enough time and effort, can we one day terraform the red planet and turn it into a new Earth? Or is Mars nothing but a hopeless money pit in the sky?
First up, the terrible atmosphere. Humans are pretty picky when it comes to atmospheric conditions. If the pressure's too high or too low, we die. If we don't get enough oxygen, we die. After all, we've evolved to live within a very specific layer of Earth's gaseous outer layer.
Mars' atmosphere is very thin and incredibly cold. It lacks sufficient air pressure and contains way too much carbon dioxide.
Second, if you're looking to move into a new planet, make sure there's an intact electromagnetic field. Earth has one, generated by hydrodynamic convection between its liquid outer core and solid inner core. Without this shielding, we'd be exposed to a deadly stream of highly charged particles called the solar wind. For reasons we don't entirely understand yet, Mars lacks this protection and possesses only remnants of a magnetic field at its polar ice caps.
It gets worse. According to a 2010 study from the Swedish Institute of Space Physics and the University of Leicester, double solar radiation waves periodically strip away 30 percent of the sparse Martian atmosphere. These waves occur when one solar wave overtakes another to produce a single, more powerful wave. What little atmosphere remains is due to comet strikes and the occasional melting of polar ice.
So what would it take to fix Mars up to Earthling standards?
Scientists have made various proposals to induce a greenhouse effect on Mars through the use of mirrors, atmosphere factories or asteroid impacts. We could melt the polar ice caps to release trapped carbon dioxide or generate greenhouse gases in factories. In time, we could theoretically start using widespread atmosphere factories to turn carbon dioxide into oxygen in a manner similar to plants. All of this tinkering might thicken up the atmosphere and provide greater radioactive shielding, but Mars will continue to face atmosphere loss due to double solar radiation waves. There's no getting around the fact that Mars desperately lacks an electromagnetic field.
Scientists disagree on the makeup of Mars' modern core. It might be solid, liquid or some combination of the two. It all depends on which scientist you talk to and which study they choose to support.
Whatever the truth is, we know something isn't working down there. A 2008 University of Toronto study theorized that, more than 4 billion years ago, incoming asteroids applied a gravitational tug to liquids in Mars' core, producing enough of a dynamo effect to generate a temporary electromagnetic field. Could the introduction of a new, artificial moon give the core the kick-start it needs? Other proposals involve injecting Mars' core with radioactive waste to fire it up, while other scientists think artificial magnetic fields may be the answer.
Mars is quite the fixer-upper. Even the best-case scenarios entail centuries of renovating, and the technology to induce or recreate electromagnetic field protection may be centuries off as well.
Explore the links on the next page to learn even more about Mars.
NASA can build a vehicle that would take astronauts to Mars. In fact, that's what the Orion project aimed to eventually accomplish before it was drastically cut back by U.S. President Barack Obama in early 2010 [source: NASA]. So if NASA isn't planning to go to Mars anytime soon, where does that leave us? It seems unfathomable that a trip to Mars would ever be cost-efficient. And by virtue of Obama's decision to cut the Constellation program in early 2010, it appears any venture would have to be privately funded. [source: Marcus].
Estimates by some experts have put the price tag for a Mars trip in the hundreds of billions of dollars [source: Tyson].
The exorbitant costs can be attributed to many factors but a large one lies in the need to send multiple vehicles for a return trip to Earth. For instance, one vehicle would contain nothing but fuel and supplies needed for re-entry. Of course, one radical idea is to forgo a return trip entirely. Instead of returning, travelers to Mars would get a one-way ticket to stay and colonize, with periodic reinforcements of people and supplies.
One-way trips could cut costs considerably, perhaps as much as 10 times according to some experts [source: Tyson]. The time between supply runs would probably be two years as Mars' distance from Earth ranges between 34 million miles (55 million kilometers) at its closest to 249 million miles (401 million kilometers) when the two planets are at opposite ends of their orbits around the Sun [source: Cain].