The Sun Will Eventually Engulf Earth--Maybe - Scientific American Space The Sun Will Eventually Engulf Earth--Maybe Researchers debate whether Earth will be swallowed by the sun as it expands into a red giant billions of years from now By David Appell on September 1, 2008 Credit: Lynette Cook Photo Researchers, Inc. Advertisement | Report Ad The future looks bright—maybe too bright. The sun is slowly expanding and brightening, and over the next few billion years it will eventually desiccate Earth, leaving it hot, brown and uninhabitable. About 7.6 billion years from now, the sun will reach its maximum size as a red giant: its surface will extend beyond Earth’s orbit today by 20 percent and will shine 3,000 times brighter. In its final stage, the sun will collapse into a white dwarf. Although scientists agree on the sun’s future, they disagree about what will happen to Earth. Since 1924, when British mathematician James Jeans first considered Earth’s fate during the sun’s red giant phase, a bevy of scientists have reached oscillating conclusions. In some scenarios, our planet escapes vaporization; in the latest analyses, however, it does not.

The answer is not straightforward, because although the sun will expand beyond Earth’s orbit, or one astronomical unit (AU), it will lose mass along the way. As a result, Earth should drift outward as the gravitational tug lessens over time. (At its maximum radius of 1.2 AU, the sun will have lost about one third of its mass, compared with its current heft.) In this way, Earth could escape solar envelopment. But other factors complicate the analysis. Drag on the planet from the sun’s outermost, tenuous layers will cause Earth to drift inward. Smaller forces from the other planets—all in turn reacting to the same reducing, expanding sun—are even more difficult to account for completely. Earlier this year two teams reported different kinds of calculations indicating that Earth will be swallowed up by the sun. In a calculation that would thrill any college junior studying classical mechanics, Lorenzo Iorio of Italy’s National Institute of Nuclear Physics used perturbation theory. It simplifies analyses by dropping relatively small factors, thereby making complex equations of motions that describe the interactions between the sun and Earth mathematically manageable.

Assuming that the sun’s yearly mass loss (currently about one part in 100 trillion) remains small for the duration of its evolution to the red giant phase, Iorio calculates that Earth will move outward at about three millimeters a year, or only 0.0002 AU by the sun’s red giant phase. But at that point the sun will balloon up, in only a million years, to 1.2 AU in radius, thus vaporizing Earth. Iorio’s paper, submitted to Astrophysics and Space Science, has not yet been peer-reviewed. Several scientists question whether quantities that Iorio assumes are small will indeed remain small throughout the sun’s evolution. Even if Iorio got his number crunching wrong, he may have the right answer. In an analysis published in the May Monthly Notices of the Royal Astronomical Society, Klaus-Peter Schröder of the University of Guanajuato in Mexico and Robert Smith of the University of Sussex in England also conclude that Earth is doomed, by using more exact solar models and by considering tidal interactions. As the sun loses mass and expands, its rotation rate must also slow down—physics students learn this relation as the conservation of angular momentum.

The slowed rotation causes a tidal bulge on the sun’s surface. The gravity exerted by this bulge pulls Earth inward. With such a consideration, the researchers find that any planet with a present-day orbital radius of less than 1.15 AU will ultimately perish. Could Earth be saved if someone is still left at home? In a bold piece of astronomical engineering, Don Korycansky of the University of California, Santa Cruz, and his colleagues have proposed nudging Earth with a large asteroid arranged to pass nearby periodically. It could take one billion years to move our planet out to somewhere safe, like the orbit of Mars. Our moon, though, might have to be left behind, and any miscalculation could mean extinction. Needless to say, more study is required. [break] How does a star become a supernova or a black hole? How does the star decide which one to turn into? ABOUT US Question How does a star become a supernova or a black hole? How does the star decide which one to turn into? Asked by: Mudita Upman Answer Stars are sustained by the nuclear fusion reactions taking place in their cores.

For stars on the main sequence, such as our own Sun, this mostly involves combing hydrogen to form helium. The energy that these reactions produce is enough to support their mass against its own gravity. As a star runs out of fuel it can expand and will begin to form heavier elements such as carbon and iron (most of the matter in our solar system comes from extra-solar sources). Once it finally exhausts all of its fuel it will begin to collapse. It is here that the stars begin to undergo different fates. Our own sun will collapse until it becomes a white dwarf, at this point the Pauli exclusion principle keeps the electrons in the star far enough apart to resist further collapse - this energy is called 'electron degeneracy'. Stars greater than 1.4 times the mass of the Sun (called the Chandrasekhar limit after the Indian physicist who discovered it on his way to England) will tend to explode in a supernova casting off much of their mass.

A small central core will remain and like smaller stars this will collapse only this time electron degeneracy will not be enough to support the star's mass against its gravitational collapse and it will continue to shrink until it becomes a tiny, but hugely massive, neutron star held together by neutron degeneracy. If neutron degeneracy is not enough to resist the star's collapse it will continue to shrink until the matter is all compressed into an infinitely small, infinitely dense point called a singularity. This is the centre of a black hole. Answered by: Edward Rayne, Physics Undergraduate Student, Cambridge UK The life span of a star and its final state are determined by the mass of the star. All stars, as far as we know, are under two basic forces, that of gravity and that of the internal pressure formed by the fusion of hydrogen into helium. Gravity acts as an energy source that creates tremendous heat and pressure inside a star and begins the fusion process that produces the massive amounts of heat and energy that make the star shine and tries to push the gases away from the star until the balance is found. When most of the hydrogen is fused into helium, fusion stops and and gravity again takes over. The star starts to collapse without the energy from fusion.

What happens next in the life of a star is dependent on it's mass. For a low mass star (smaller than our sun) gravity is not strong enough to start fusion again and the burnt-out cinder becomes a brown dwarf and eventually a cold, dead body in space. For stars near sun's mass, the gravitational force is great enough to squeeze the center and make it hot enough for the star to start fusing helium into other light elements. This renewal of energy causes the star to swell into a red-giant and shed some of the outer layers into space as a planetary nebula. When our sun turns into a red giant (in about 5 billion years), the orbit of Mars will be inside the sun. After the star finishes fusing everything that it can, gravity again takes over and starts squeezing the atoms tighter and tighter until the repulsive forces of the electron shells around the atoms balances the force of gravity and the star comes to equilibrium as a white dwarf with a great density (1 teaspoon would weigh several tons). This star will eventually cool down and drift through space as a cold, burned out cinder. Stars greater than two and a half times the mass of the sun, their fate is even more exotic.

The force of gravity is great enough to produce iron in the center of the star from fusion. Iron is the heaviest substance that a star can make in its life because heavier elements require more energy to fuse together than they release, so the star collapses. For massive stars, this collapse is so violent that it causes a huge, catastrophic explosion known as a supernova. It is in these explosions that all elements heavier than iron are produced. It has been said that we are made of stardust. Supernovas are so bright that close ones have been seen even during the day during the middle ages. The final fate of the star after a supernova depends on the amount of mass left in the core after the explosion. Some of the stars are still massive enough to overcome the electrical repulsion of the electron shells and smash the electrons into the nucleus of the atoms thereby cancelling out the positive charge of the protons until the force of gravity is balanced by the force of the neutrons pressing against each other and a neutron star is formed. The material from this star weighs even more than the material from a white dwarf.

If a star is even more massive than the one that forms a neutron star, it goes through the same process of creating a supernova, but the force of gravity is so great because of the amount of mass involved, that the neutrons cannot halt the collapse of the star which continues to be squeezed into a smaller and smaller space until the gravity of the star is able to trap light inside of what is known as the 'event horizon' which creates a black hole. To give you an idea of the density of a black hole, if we wanted to compress the earth enough to create a black hole, all of the earth's mass would have to fit in the palm of your hand, and would tear your body apart and draw you into it thereby adding more mass to the hole. Black holes are detected by finding the ones that are close to other stars and watching for the radiation (mostly x-rays) given off by matter falling into it before the event horizon is crossed. Check out the Chandra X-Ray Telescope site for x-ray pictures of possible black holes. The key to the puzzle is gravity which is determined by mass. There may be other mechanisms for forming a black hole, but this is the general stellar process for the formation of these objects.

Answered by: Matthew Allen, B.S., Physics/Calculus Teacher St. Scholastica Academy Chp. 8 & 9 (The Deaths of Stars / The Milky Way Galaxy) at Missouri Western State University - StudyBlue StudyBlue Chp. 8 & 9 (The Deaths of Stars / The Milky Way Galaxy) b b. What property does matter inside the event horizon of a black hole retain? mass, angular momentum, and electric charge Consider the following objects: soft gamma ray repeaters, hypernovae, x-ray bursts. List the events in order of increasing energy. List the events in order of increasing rarity - x-ray bursts, soft gamma ray repeaters, hypernovae - x-ray bursts, soft gamma ray repeaters, hypernovae Advertisement ) Based on what you learned about what happens to matter that falls into a neutron star or black hole, select all of the correct statements from the following list. -Soft gamma-ray repeaters are thought to come from magnetars -Although not completely understood, jets seemed to be produced by "twisting" magnetic fields.

-A magnetar is a pulsar with a much stronger than usual magnetic field -Both neutron stars and black holes produce jets of radiation If the sun were replaced by a 1 solar-mass black hole, how would Earth's orbit change? What would the radius of the sun be in this case? Earth's orbit would not change ; 3 km What observational evidence do we have that stellar-death black holes really exist? Some X-ray binaries have unseen objects with masses greater than 3 solar masses. White dwarfs and neutron stars are both end products of stellar evolution. White dwarfs are composed of mostly carbon, oxygen, and electrons, whereas neutron stars are composed of mostly neutrons. What happens to the protons in the atomic nuclei and the degenerate electrons that were inside the star that creates a neutron star? The electrons and protons combine to form neutrons and neutrinos. Which of the following statements accurately describe(s) some observed properties of supernovae type Ia and supernovae type II? Supernovae type II have hydrogen lines in their spectra and supernovae type Ia are more luminous Why do we expect that neutron stars spin rapidly? the law of conservation of angular momentum What prevents neutron stars from contracting to a smaller size?

degenerate neutrons How much longer will the sun last? about 5 billion years What changes would occur if the sun were replaced with a one-solar-mass black hole? Earth's orbit would not change and the planets would disappear from view What is the Schwarzschild radius? If the sun has a Schwarzschild radius, why isn't it a black hole? What is the Schwarzschild radius of an object? all of the above ; It is not massive enough ; It can be any size, depending on the mass of an object Which of the following is an accurate description of the Schwarzschild radius? It is the radius to which an object must shrink to become a black hole and is the radius of the event horizon surrounding a black hole singularity Which is not true about a type I supernova? Which is not true about a type II supernova? It involves a massive star; It occurs in a binary system Why does an expanding giant star become cooler? Energy is absorbed in expanding and lifting the gas Advertisement Why do supernova remnants emit x-rays? because they are filled with hot gas Why can't massive stars generate energy from iron fusion? Iron fusion consumes energy Which type of supernova leaves NO core remnant?

supernova type Ia How does the main-sequence lifetime of a star compare to its entire fusion lifetime? Stars spend about 90% of their fusion lifetimes on the main sequence. Which is true for the event horizon of a black hole? What evidence is there that black holes have event horizons? all of the above ; both of the above Consider the following shells of gas in the core of a massive star: carbon, helium, hydrogen, iron, neon, oxygen, silicon. - List these in the order that the shells would be found, starting with the inner-most. - Rank the same gases in the order in which they would appear at the innermost core of a massive star as it evolves Iron, silicon, oxygen, neon, carbon, helium, hydrogen Hydrogen, helium, carbon, neon, oxygen, silicon, iron - Which is true for the H-R diagram of a star cluster that has many hot, blue, luminous stars? - If a star cluster has many hot, blue, luminous stars which of the following is probably correct? - What is the reason for the answers to the previous questions?

It will have stars over most of the main sequence ; It is young ; Hot, blue, luminous stars do not live long lives What happens to white dwarfs as they age? their surface temperatures decrease and luminosity decreases What is one of the reasons astronomers don't use visible-wavelength telescopes to locate neutron stars? Neutron stars are very small Consider the following possible stages in the evolution of a star like our sun: giant, main-sequence, planetary nebula, supernova, white dwarf. Rank the stages in the order they occur. Leave out any stages that will not occur in the evolution of a star of similar mass to the sun. main sequence, giant, planetary nebula, white dwarf What event marks the end of every star's main-sequence life? the end of hydrogen fusion in the core How can you estimate the age of a star cluster? What is the turnoff point of a star cluster? by its brightest stars the location on the H-R diagram where stars are leaving the main sequence Which is true for X-ray bursters? How do X-ray bursters resemble a nova? all of the above What is the approximate upper limit of the mass of a neutron star? Why is the upper limit only approximate?

3 solar masses The exact strength of pure neutron material is not actually known How can a black hole emit X-rays? X-rays emitted just before material crosses the event horizon can escape the gravitational pull The luminosity increases and the surface temperature decreases How are the ages of star clusters related to their turnoff points? The age of a cluster is the life expectancy of stars at its turnoff point and the lower the turnoff point, the older the star cluster. What prevents gravity from shrinking a white dwarf to a smaller size? degenerate electrons Consider the following objects: blackhole, neutron star, white dwarf. Rank the objects in order of increasing mass Rank the objects in order of increasing size Rank the objects in order of decreasing density Rank the objects in order of increasing rotation rate -white dwarf, neutron star, black hole -black hole, neutron star, white dwarf -black hole, neutron star, white dwarf -white dwarf, neutron star, black hole What is required for carbon fusion to occur in the core of a star? Why is it that can some stars cannot fuse carbon or heavier elements? all of the above; If they are not massive enough, there is not enough pressure on the core to achieve the necessary temperatures You have learned how stars evolve after they exhaust their hydrogen.

Based on what you learned about post main-sequence evolution, select all of the correct statements from the following list. - More massive stars will fuse the helium in their cores into heavier elements - After leaving the main sequence, most stars move to the right on the H-R diagram. - After exhausting its hydrogen fuel, a main-sequence star will become a very large, low-density star - The cores of main-sequence stars begin to collapse when they exhaust their hydrogen. Type II Why do the millisecond pulsars spin so fast? Accretion of matter from a nearby binary companion spins them up Which is a reason that a neutron star may not be observed from Earth as a pulsar What is meant by "Every pulsar is a neutron star, but not every neutron star is a pulsar"? All of the above are reasons that a neutron star may not be observed as a pulsar. Some neutron stars do not emit "pulses" of radiation for various reasons What is the lower limit for the mass of neutron stars? about 1.4 solar masses There were no heavy elements available when the older stars were forming.

The disk is where we would expect to find dust much larger than what we see What main conclusion did the Herschels draw from their star counts? The Milky Way is a disk of stars with the sun near the center. At which wavelengths are stars around Sgr A* observed? What is the reason for the answer to the previous question? infrared ; here so many stars that they warm the dust How are star clusters distributed in the sky? Open clusters lie along the Milky Way and half of the globular clusters are in or near the constellation Sagittarius What must be measured to determine distance by the Cepheid variable star method? the apparent magnitude and the period of pulsation of the variable star What main conclusion did Shapley draw from his measurements of the distances to the globular clusters? The sun is far from the center of the Milky Way. With the 100 inch telescope, Harlow Shapley could not resolve variable stars in the more distant globular clusters of the Milky Way. What basic assumption did Shapley make about the far-away globular clusters that allowed their distances to be found? Far-away globular clusters have the same average size as nearby globular clusters Why do some clusters lose stars more slowly than others?

Which type of cluster is more likely to have stars "wander away"? How is the rate of star loss affected by the age of a cluster? What is the reason for the answer to the previous question? all of the above Younger clusters lose stars faster More massive stars will burn out early in the cluster's life What does the observed heavy element abundance tell us about a star? A low percentage of metals indicates that a star formed long ago. Which single wavelength band is best for mapping out the spiral arm structure of the Milky Way Galaxy? radio What do we observe at radio, infrared, and X-ray wavelengths near the center of the Milky Way Galaxy that leads us to conclude that a supermassive black hole is located there? all of the below What do astronomers believe is responsible for the somewhat flocculent, somewhat grand design spiral arms of the Milky Way Galaxy? all of the below - How does the orbital velocity of stars around the center of the galaxy vary with distance from the center? (more than one answer may be correct) - What does the behavior of stars from question (a) above suggest about the mass of our galaxy? - What do the answers to the previous two questions suggest about our galaxy?

- As a result of all this, what have astronomers theorized about the structure of our galaxy? - relatively constant at distances farther from the center and stars closer to the center actually go slower - That it is more massive than we would expect from the observed stars - that there is much more mass than is observed - That there is a galactic corona made largely of dark matter. What is the approximate diameter of the disk component of the Milky Way Galaxy? 80,000 ly What did the Herschels find when they counted stars in 683 regions around the Milky Way? About the same number of stars in each direction. Why is it difficult to determine the size of the galactic disk? Why is it difficult to determine the size of the galactic halo? both of the above are reasons because we are on the inside looking out Where in the galaxy are "high-velocity" stars found? Why are "high-velocity" stars named so? How do the metal abundances of high-velocity stars compare to those of the sun? What is the reason for the answer to the previous question?

In the halo When they cut through the orbits of stars in the disk, they appear to have very high velocities They are lower They formed before the sun What measurements are needed to determine the entire mass of the Milky Way Galaxy? the rotational velocity of a star near the Galaxy's outer edge and the distance to a star near the Galaxy's outer edge Which of the following are problems facing the traditional hypothesis of the formation of the Milky Way? all of the below Which of the following are good visible-light spiral arm tracers? O and B associations and HII regions figure b Which is not true about spiral arms? What is the reason for the answer to the previous question? They are physically connected structures All of the above are reasons What fundamental principle did Shapley use to calibrate the period-luminosity relationship for Cepheid variable stars? Objects with large proper motion tend to be closer than objects with small proper motion. Which is not explained by the traditional theory of the formation of our galaxy? Some of the stars in the central bulge are very old. How are population II stars different than the sun, a population I star? Population II stars are lower in metals than population I stars. Which is not an example of a trigger for self-sustaining star formation?

Why does self-sustaining star formation often result in clouds of stars similar to a segment of a spiral arm? - spiral density waves - Differential rotation drags the inner edge of the cloud ahead of the outer edge. Why didn't astronomers realize at first how large the galaxy is? Who was the first astronomer to get a reasonable idea of the size and shape of the galaxy? interstellar dust obscuring starlight Shapley Stars in the disk of the galaxy tend to have _____orbits than those in the halo Stars in the halo of the galaxy tend to have ______ orbits that those in the disk more circular ; more elliptical Which is not an example of a spiral tracer? What is true about the age of most spiral tracers? What is the reason for the answer to the previous question? - objects used to map spiral arms - all of the above are spiral tracers - They are young - Spiral arms are places of star formation Where are the youngest stars in the Milky Way Galaxy located? in the flattened disk At what wavelength band can we observe the center of our galaxy? radio, infrared, x-ray At which wavelength is the center of the galaxy not observed? Which is not observed at the galactic center?

visible ; all of the above are observed Who discovered that when viewed through a telescope, the Milky Way is resolved into thousands of individual stars? Galileo Galilei (S-7) The Energy of the Sun (S-7) The Energy of the Sun Q2. Atoms (and 6 more) -------------------------- S-9.Nuclear Weapons The Sun is the source of most of the energy on Earth--the power source for plants, the cause of flows of atmosphere and of water, the source of the warmth which makes life possible. None would exist without it. At the Earth's orbit, neglecting absorption by the atmosphere, each square meter of area facing the Sun receives about 1380 joules per second (nearly 2 horsepower). That quantity is known as the solar constant and sensors aboard NASA's satellites over the 1979-99 interval suggest it varied by only about 0.2%. See graph here . But what powers the Sun itself? How much longer will it shine, before its fuel runs out? For how long has it given out its energy?

The first to consider these questions seriously was the great German physicist Hermann von Helmholtz, who noted in 1854 that the Sun's own gravity could supply an appreciable amount of energy. If the Sun were gradually shrinking--if all its matter was gradually falling towards its center--enough energy could be released to keep it radiating for a fairly long time. He calculated that this source could provide the Sun's energy for times of the order of up to 20 million years. Then radioactivity was discovered, the decay of heavy elements into lighter ones through the emission of fast particles, containing a great deal of energy. As it turned out, it was this energy, from radioactive elements in rocks, that provided the internal heat of the Earth. Radioactivity also allowed new estimates of the age of the Earth, since the amount of accumulated decay products in ores indicated how long the process had been going on. This suggested the Earth was much older than Helmholtz'es estimate, perhaps billions of years old. Could perhaps the new source of internal energy also supply the Sun's needs for such a long time? here .] Electrons and nuclei were kept together by electric attraction (negative attracts positive).

Furthermore, electrons were sometimes shared by neighboring atoms or transferred to them (by processes of quantum physics), and this link between atoms gave our world its many chemical compounds. But something else was needed to hold nuclei together, since all protons carried positive charges and repelled each other. Electric forces are definitely not the glue that holds nuclei together, they act in the wrong direction! Besides, binding neutrons to nuclei clearly requires a non-electrical attraction. All that suggested a different kind of force, a nuclear force, was holding nuclei together. That force had to be stronger than the electric repulsion at short distances, but weaker far away, or else different nuclei might have tended to clump together, too. In other words, it had to be a short-range force, like the force between two small magnets--very hard to separate when stuck together, but once pulled a short distance apart, the force between them drops almost to zero (do not take this analogy too literally!). Actually two kinds of force are active in the nucleus, known simply as the "strong nuclear force" and the "weak nuclear force," or else the "strong interaction" and the "weak interaction" (because their main effect is in converting and creating particles).

The weak interaction also affects electrons and other particles, but in the nucleus its main role is to maintain a balance between protons and neutrons, which except for their electric charge are very similar particles (diferent kinds of "nucleons"). Nuclear structure (in light nuclei, at least) favors nuclei containing equal numbers of protons and neutrons, and although moderate inequalities can also exist (in "isotopes"), when they get too big, the weak interaction can convert nucleons of one kind to the other, emitting an electron (or a positron, its positive counterpart) in the process. That is known as beta radioactivity and will not be discussed any further. The strong nuclear force (the only nuclear force considered from here on) can bind protons and neutrons into bigger nuclei. Being positively charged, all these nuclei repel each other, and therefore, except in the presence of extreme temperatures and pressures--such as exist in the core of the Sun--two different nuclei are not likely to combine into one. Their electric repulsion does not allow them to get close enough for the nuclear force to take over. The Binding Energy of Nuclei Nature contains nuclei of many different sizes.

In hydrogen they contain just one proton, in heavy hydrogen ("deuterium") a proton and a neutron; in helium, two protons and two neutrons, and in carbon, nitrogen and oxygen--6, 7 and 8 of each particle, respectively. The masses of all these nuclei have been measured, and an interesting fact was noted: a helium nucleus is lighter than the sum of the weights of its components. The same held even more for carbon, nitrogen and oxygen--the carbon nucleus, for instance, was found to be slightly lighter than the combined mass of 6 protons and 6 neutrons. The reason for this "mass defect" has to do with Einstein's famous formula E=mc2, expressing the equivalence of energy and mass. By this formula, adding energy also increases mass (both weight and inertia), removing energy, decreases it. If a combination of particles contains extra energy--for instance, in a molecule of the explosive TNT--weighing it will reveal some extra mass (compared to its end products--an unmeasurably small difference, for TNT). If on the other time we need invest energy to separate it into its components, the weight will be less than that of the components.

The latter is the case with nuclei such as helium: to break them up into protons and neutrons, we would have to invest energy. On the other hand, if a process existed going in the opposite direction, by which hydrogen atoms could be combined to form helium, a lot of energy would be released-- namely, ΔE=Δm c2 per nucleus, where Δm is the difference between the mass of the helium nucleus and the mass of four protons (plus 2 electrons, absorbed to create the neutrons of helium). As we go on to elements heavier than oxygen, the energy which can be gained by assembling them from lighter elements decreases, up to iron [more precisely, a low-abundance isotope of nickel marks the peak]. For nuclei heavier than iron, one actually gains energy by breaking them up into 2 fragments. That, of course, is how energy is extracted by breaking up uranium nuclei in nuclear power reactors. The reason the trend reverses after iron is the growing positive charge of the nuclei. The electric force may be weaker than the nuclear force, but its range is greater: in an iron nucleus, each proton repels 25 other protons, while (one may argue) the nuclear force only binds close neighbors.

As nuclei grow bigger still, this disruptive effect becomes steadily more significant. By the time uranium is reached (92 protons), nuclei can no longer acommodate their large positive charge, but emit their excess protons in the process of alpha radioactivity--the emission of helium nuclei, each containing two protons and two neutrons. (Helium nuclei are an especially stable combination.) Still heavier nuclei are not found naturally on Earth . (For more about the above curve, see ) [Why are helium nuclei especially stable? Some time ago I received e-mail from a teacher asking for suggestions of ways of demonstrating fusion to her students, "e.g. using M&Ms." One can do so (although tennis balls are better, being visible even from the last row of the classroom!) by showing how 4 balls (or candy pieces) can be stacked in a pyramid, each touching the other 3. Considering that the nuclear forces has a short range, this "close packing" gives the tightest binding. Maybe that is why helium (4 nucleons) is such a stable combination, as shown by its peak on the curve of binding energy above.

No larger number can be stacked symmetrically with every single ball touching all the others!] The Sun's Energy Source It is believed that the Sun is about 5 billion years old, formed when gravity pulled together a vast cloud of gas and dust, from which the Earth and other planets also arose. The gravitational pull released energy and heated the early Sun, much in the way Helmholtz had proposed. My candle burns at both ends; It will not last the night; But ah, my foes, and oh, my friends It gives a lovely light! (Another take on this rhyme, below ) Small stars last longer and many are dim; but whatever a star's size, ultimately it runs out of hydrogen. It can still release energy by "burning" heavier nuclei and combining them into bigger ones, up to iron: theory suggests this does happen, but it provides much less energy and does not greatly extend the star's lifetime. When all the fuel is gone, gravity again becomes the dominant source of energy, and the star again begins collapsing inwards. The Earth keeps its size because its gravity is not strong enough to crush the minerals of which it consists. Not so with a star massive enough to sustain nuclear burning.

A small star may crush all its atoms together, creating a "white dwarf"--e.g. of half the mass of the Sun, but only as big as the Earth. Some energy release continues (hence "white") but ultimately, the star probably becomes a dark cinder. This may be the fate of our Sun, too. In the final transition strange changes occur--the star becomes a "red giant," diffuse and enormously large, and later much of the material is blown to space where it forms a "planetary" nebula, but there is no explosion. See "The Complexity of Stellar Death" by Yervant Terzian, "Science" vol. 256 p. 425-6, 15 October 1999. Supernovas Stars several times the size of our Sun have enough gravity to crush together not just atoms but even nuclei, compressing all their matter to a sphere perhaps 15 kilometers across. After their collapse they become "neutron stars" consisting only of neutrons (the protons all switching form), giant nuclei as dense as the ones in atoms.

A huge amount of energy is liberated in that final collapse which is quite rapid, blowing off the top layers of the collapsing star and also producing elements heavier than iron. [You may wonder why that collapse is so rapid, considering that Helmholtz and Kelvin--cited at the start of this section--found the gravitational energy of the Sun was sufficient to keep it hot for tens of millions of years. The answer was given by George Gamov and the Brazilian physicist Mario Schenberg in 1941: enormous energy is indeed generated, but the extreme temperature produces nuclear processes which generate neutrinos and these remove energy very, very quickly. The neutrino is an almost massless uncharged particle with very weak interactions, able to move unimpeded through thick layers of material--even the entire solid Earth, even the Sun. Neutrinos escaping through the outer layers of the supernova carry away its energy, in what Gamov facetiously named "the Urca process," comparing it to the rapid way in which a gambler's money disappeared at the roulette tables of the Casino da Urca in Rio de Janeiro.

The process was dramatically confirmed by the supernova of 1987 (picture above shows it some years later; a larger picture, with added links, here ) whose observation coincided with a burst of 11 neutrinos, detected by the sensitive Kamiokande underground observatory in Japan, and by 8 registered independently on a detector in Ohio] Added 20 October 1999: The new Chandra orbiting X-ray telescope has taken a high-resolution picture in X-rays of the central region of the Crab nebula. Before this, astrophysicists guessed the remnant star might be surrounded by orbiting debris, with high-energy particles shooting out along its magnetic axis, the one direction in which magnetic field lines do not confine them. The picture on the right suggests something like that might indeed be happening. For the image of a supernova remnant in Centaurus, as seen by Chandra, see here . Theory suggests that a star much more massive than the Sun will collapse even further and become a black hole. What happens then can only be guessed and calculated, not observed, for the star's gravity in the collapsed state is so strong that no light and no information can return from it to the outside world.

One therefore expects such objects to be completely black (Except for light from matter falling into them!); they are called "black holes" because the general theory of relativity suggests that the matter in such a star keeps falling indefinitely, as the star contracts to a point. Thus in theory such stars are like the proverbial bottomless pit, although no observation could ever confirm it. Although astronomers cannot see such objects, they have considerable evidence that they exist, at least in a number of locations. For some time now it was believed that very massive black hole existed at the center of our galaxy, and if so, probably also at the centers of other galaxies, helping hold them together. We now have some pretty definite proof, and also a good estimate of what the mass of that monstrous object may be. The story of that discovery is given in the following section, " The black hole at the center of our galaxy ". Another take on Edna St. Vincent Millay's rhyme: My ice cream cone drips at both ends I eat it in great haste But ah, my foes, and oh, my friends It has a lovely taste!

Questions from Users: Red Giant Stars: Facts, Definition & the Future of the Sun Red Giant Stars: Facts, Definition & the Future of the Sun By Nola Taylor Redd, Space.com Contributor | August 21, 2013 06:00pm ET MORE Expanding red giant stars will swallow too-close planets. In the solar system, the sun will engulf Mercury and Venus, and may devour Earth, as well. Credit: James Gitlin/STScI AVL A red giant star is a dying star in the last stages of stellar evolution. In only a few billion years, our own sun will turn into a red giant star, expand and engulf the inner planets, possibly even Earth. What does the future hold for the light of our solar system and others like it? Forming a giant Most of the stars in the universe are main sequence stars — those converting hydrogen into helium via nuclear fusion . A main sequence star may have a mass between a third to eight times that of the sun and eventually burn through the hydrogen in its core. Over its life, the outward pressure of fusion has balanced against the inward pressure of gravity. Once the fusion stops, gravity takes the lead and compresses the star smaller and tighter.

Temperatures increase with the contraction, eventually reaching levels where helium is able to fuse into carbon. Depending on the mass of the star, the helium burning might be gradual or might begin with an explosive flash. The energy produced by the helium fusion causes the star to expand outward to many times its original size. Red giant stars reach sizes of 100 million to 1 billion kilometers in diameter (62 million to 621 million miles), 100 to 1,000 times the size of the sun today. Because the energy is spread across a larger area, surface temperatures are actually cooler, reaching only 2,200 to 3,200 degrees Celsius (4,000 to 5,800 degrees Fahrenheit), a little over half as hot as the sun. This temperature change causes stars to shine in the redder part of the spectrum, leading to the name red giant, though they are often more orangish in appearance. This picture of the dramatic nebula around the bright red supergiant star Betelgeuse was created from images taken with the VISIR infrared camera on ESO’s Very Large Telescope (VLT).

This structure, resembling flames emanating from the star, forms because the behemoth is shedding its material into space. Credit: ESO/P. Kervella Stars spend approximately a few thousand to 1 billion years as a red giant. Eventually, the helium in the core runs out and fusion stops. The star shrinks again until a new helium shell reaches the core. When the helium ignites, the outer layers of the star are blown off in huge clouds of gas and dust known as planetary nebulae . The core continues to collapse in on itself. Smaller stars such as the sun end their lives as compact white dwarfs. The material of larger, more massive stars fall inward until the star eventually becomes a supernova , blowing off gas and dust in a dramatic fiery death. [ Amazing Photos of Supernova Explosions ] Though they look serene and silent from our vantage on Earth, stars are actually roiling balls of violent plasma. Test your stellar smarts with this quiz.

Question: What, ultimately, will the sun become?
Answer: A white dwarf