The Sun as a White Dwarf Star - Universe Today Universe Today The Sun as a White Dwarf Star Article Updated: 24 Dec , 2015 by Ian O'Neill [/caption] What will happen to all the inner planets, dwarf planets, gas giants and asteroids in the Solar System when the Sun turns into a white dwarf? This question is currently being pondered by a NASA researcher who is building a model of how our Solar System might evolve as our Sun loses mass, violently turning into an electron-degenerate star. It turns out that Dr. John Debes work has some very interesting implications. As we use more precise techniques to observe existing white dwarf stars with the dusty remains of the rocky bodies that used to orbit them, the results of Debes’ model could be used as a comparison to see if any existing white dwarf stars resemble how our Sun might look in 4-5 billion years time… A comparison of the Sun in its yellow dwarf phase and red giant phase Today, our Sun is a healthy yellow dwarf star. If you want to be precise, it is a “G V star”.

This yellow dwarf will happily burn 600 million tonnes of hydrogen per second in its core for 10 billion years, generating the light that is required to make our planet habitable. The Sun is approximately half-way through this hydrogen burning phase, so it’s OK, things aren’t going to change (for the Sun at least) for a long time yet. But what happens then? What happens in 4-5 billion years when the supply of hydrogen runs out in the core? Although our Sun isn’t massive enough to entertain the thought of going out in a blaze of supernova glory, it will still go through an exciting, yet terrifying death. After evolving through the hydrogen-burning phase, the Sun will puff up into a huge red giant star as the hydrogen fuel becomes scarce, expanding 200 times the size it is now, probably swallowing the Earth. Helium, and then progressively heavier elements will be fused in and around the core. The Sun will never fuse carbon however, instead it will shed its outer layers forming a planetary nebula. Once things calm down, a small sparkling jewel of a white dwarf star will remain.

This tiny remnant will have a mass of around half that of our present Sun, but will be the size of the Earth. Needless to say, white dwarfs are very dense, intense gravitational pull countered not by fusion in the core (like all Main Sequence stars), but by electron degeneracy pressure . Relative sizes of IK Pegasi A (left), IK Pegasi B (lower center; a white dwarf) and the Sun (NASA) When the Solar System reaches this phase in its evolution, what will it look like? What will become of the asteroids, gas giants, moons and rocky planets? I was very fortunate to chat with astrophysicist Dr John Debes, from NASA’s Goddard Space Flight Center, at January’s American Astronomical Society (AAS) conference in Long Beach (California) who is developing an n-body code simulating an evolving Solar System. After the Sun has stopped hydrogen fusion in its core, it loses mass as it sheds its outer layers after the red giant phase and subsequent planetary nebula formation. It is estimated that the Sun will lose about 50% of its mass during this time, naturally affecting the Solar System as a whole.

As the Sun loses mass, the outer planets (such as Jupiter) will drift outwards, increasing their orbital radii. In the simulation, Debes is very careful to ensure there is a gradual reduction in solar mass to ensure stability in the simulation. What we are left with is an old Solar System, where little is left of the inner planets (it is likely that anything within the orbit of the Earth will have been swallowed by the Sun as it expanded through the red giant phase). Although the future white dwarf Solar System will seem very alien to present day, some things won’t change. Jupiter’s orbit might have receded with the drop in solar mass, it will remain a planetary heavyweight, causing disruption in asteroid orbits. Using known asteroid data, the motion of these chunks of rocks are allowed to evolve, and over millions of years, they may get thrown out of the Solar System, or more interestingly, pushed closer to the white dwarf.

Once the whole system has settled down, resonances in the asteroid belt will become amplified; Kirkwood Gaps (caused by gravitational resonance with Jupiter) will widen, and according to Debes’ simulations, the edges of these gaps will become perturbed even more, making more asteroids available to be tidally disrupted and shredded to dust. Artists concept of shredded asteroid around white dwarf (NASA/JPL-Caltech) The AAS conference was full of amazing research into white dwarf observations . The reason for this is that there are many white dwarf candidates out there with dusty metallic absorption lines. This means that there used to be rocky bodies orbiting these stars, but became pulverised (by tidal shear) for astronomers to analyse. These white dwarf systems can give us a clue as to what mechanisms could be supplying the white dwarfs with dusty material, even giving us a glimpse into the future of our Solar System. “We have a physical picture for the link between planetary systems and dusty white dwarfs,” Debes said when describing his model in relation to the mysterious dusty white dwarf observations. “Dusty white dwarfs are truly a mystery!

We think we know what might be going on, but we don’t have a smoking gun yet.” However, Debes is getting close to finding a possible smoking gun, he’s basing his model on some of the key characteristics of these ancient dusty remnants to see what the Solar System could look like in billions of years time. So, where does this dust come from? As the asteroid orbits are perturbed by Jupiter, they may get close enough to be tidally disrupted. Get too close and they will get shredded by the gravitational shear created by the steep tidal radius of the compact white dwarf. The asteroid dust then settles into the white dwarf. The presence of this dust has a very obvious signature in the absorption lines of spectroscopic data, allowing researchers to infer an accretion rate for metal-rich white dwarfs. In Debes’ model, he has set the upper limit to 1016 g/year and a lower limit to 1013 g/year, consistent with observed estimates. Spectra of G29-38. Could this resemble the spectra of the Sun after turning into a white dwarf?

(NASA/Spitzer) In his evolved Solar System model, Jupiter’s gravity controls this accretion rate, pushing asteroids toward the white dwarf and, by using a powerful supercomputer to track the perturbations and eventual shredding of known asteroids, there may be an opportunity to arrive at a profound conclusion. Debes is able to use his model to compare observations of known dusty white dwarfs with the simulated outcome of the Solar System. With reference to previous studies (in particularly Koester & Wilken, 2006 in the journal Astronomy & Astrophysics), Debes has found some similar white dwarf “Suns”. “For G29-38 , the canonical dusty white dwarf, they [Koester & Wilken] estimate a total mass of 0.55 solar masses–about what people believe the mass that our own sun will have remaining when it becomes a white dwarf,” Debes added. “But mass estimates are a bit uncertain–I’ve seen estimates ranging from 0.55-0.7 solar masses for this particular white dwarf.” The Sun's future?

The white dwarf G29-38 (NASA) “Another good candidate is a DAZ [a metal-rich white dwarf] called WD 1257+278, which does not show dust but is spot on with the mass expected for the Sun–0.54 MSun,” said Debes. “Its accretion rate is also consistent with my model predictions so far assuming an asteroid belt mass and characteristic perturbation timescale that I found in my simulations.” Debes is continuing to make his model more and more sophisticated, but already the results are promising. Most exciting is that we may already be observing white dwarfs, like G29-38 or WD 1257+278, giving us a tantalizing glimpse of what our Solar System will look like when the Sun becomes a white dwarf star, ripping apart any remaining asteroids and planets as they stray too close to the Sun’s tidal shear. However, it also raises the question: if white dwarfs like G29-38 are being fed by the remains of tidally-blended asteroids, are there massive planets shepherding asteroids in these white dwarf systems too? Share this: How Will Our Solar System (and our planet) Die?

How Will Our Solar System (and our planet) Die? Written By The sun. ( Image Source ) Just as the life on Earth is dependent upon the energy from the sun for sustenance, so does the fate of our solar system hinge on the sun’s survival. Our sun, which is classified as a yellow dwarf (a misnomer since the sun is neither small nor yellow), is a middle age star that’s approximately 5 billion years old. As a main-sequence star with a finite lifespan, it will eventually die. This end will occur following the depletion of the last of the hydrogen forged in its core. But what exactly will happen when it uses the last of its hydrogen? Well, when this occurs, the core of the sun will shrink under its own gravity and become so dense that the helium atoms will begin to collide to form carbon and oxygen atoms. The collisions of said elements will churn out more energy than what the sun currently does (a lot more). This is because the current amount of energy is dependent on the sun’s fusion of hydrogen into helium, and hydrogen and helium are the lightest elements.

The extra energy will prove to be the beginning of the end for home (no, I don’t mean your house; I mean the Earth…well, and also your house). For one, the core will increase in temperature, causing the sun to swell to hundreds of times its present size, changing its status from a yellow dwarf to a red giant , which will certainly be the end of the two innermost planets of our solar system, Mercury and Venus (both will be incinerated in the initial expansion). The fate of our home planet is still largely a guessing game. Many scientists speculate that our “blue marble” will become a black husk, and that it will be consumed by our sun. Some others claim that the Earth will be pushed out of orbit, away from the sun; however, regardless of whether Earth survives total incineration during the initial increase in the sun’s surface area, it will no longer be habitable to humans (or most other Earthlings, for that matter). The oceans will boil and evaporate, the atmosphere will be blown away – lost to space forever – while all of the lush vegetation – along with any surviving ecosystems – will be destroyed.

All that will be left is a barren wasteland not fit for humans…or any H2O dependent Earth-based life. Astronomy: Roen Kelly, after Klaus-Peter Schroeder and Robert Cannon Smith The outer planets, those that are located beyond the orbit of Mars, might have another fate altogether, especially some of the moons of Jupiter and Saturn. Particularly, Europa and Enceladus . Both are known to host frozen bodies of liquid water under their icy surfaces. Europa is believed to contain more water on it than there is water in all of the oceans, lakes, and streams on Earth. Thanks to the huge expansion of the sun, places in the outermost regions of our solar system will have a few million years to thaw out a bit. Think of it as a short springtime after a 10 billion year winter–one last golden era before the sun sets for the final time and perpetual darkness takes over. But, this could play out another way entirely. As stars lose mass, it’s typical for orbits within a planetary system to change . Any bodies orbiting closer in (the inner planets of the solar system) will experience drag due to the increased density of released gas from the dying sun.

Those farther out will sometimes be spared, but will have orbits that slowly expand as the mass interior to their orbit is shed. Planets at different radii will feel the combination of these effects in different ways — causing their orbits to change in ways unrelated to one another. So some stars will migrate and interact with other planets in the system in a way similar to past behaviors (generally, when the solar system was much younger). Any close interactions could potentially cause the planets and moons to collide, either sending them into looping elliptical orbits, or flinging them out of the system entirely ( like a rogue planet ). An approximation of Earth and a white-dwarf. (Credit: ESA and NASA) Meanwhile, on the sun, the helium-burning reaction will produce strong solar winds like sun has never done before. As the material drifts from the sun’s surface and into the surrounding region, it will carry off some of its remaining hydrogen in its outermost layers, forming a brilliant planetary nebula. The resulting nebula will be visible for thousands of years to any civilizations existing within a few hundred light-years of our former stellar neighborhood. Eventually, that too will end when there isn’t enough pressure at the core to keep the process of helium fusion going.

What’s left of our precious sun will inevitably contract under its own gravity and become a much more compact, dense star that radiates leftover heat (from the earlier nuclear fusion). At this point, our once life-giving sun will be a white dwarf …and the king of a dead solar system. A white dwarf will eventually cool over a few hundred million years, with the remaining mass being jammed into a sphere approximately the same size as Earth, but much more dense. Eventually, this energy too will dissipate, leaving behind a stellar corpse called a black dwarf; effectively ending the journey of life in our star system (unless something comes along to jump-start it, of course). Written By Sun Facts ☀️ - Interesting Facts about the Sun - Space Facts Sun size compared to Earth, Neptune, Uranus, Saturn and Jupiter Facts about the Sun At its centre the Sun reaches temperatures of 15 million °C. The Sun is all the colours mixed together, this appears white to our eyes. The Sun is mostly composed of hydrogen (70%) and Helium (28%). The Sun is a main-sequence G2V star (or Yellow Dwarf). The Sun is 4.6 billion years old.

The Sun is 109 times wider than the Earth and 330,000 times as massive. Detailed Sun Facts One million Earths could fit inside the Sun. If a hollow Sun was filled up with spherical Earths then around 960,000 would fit inside. On the other hand if these Earths were squished inside with no wasted space then around 1,300,000 would fit inside. The Sun’s surface area is 11,990 times that of the Earth’s. The Sun contains 99.86% of the mass in the Solar System. The mass of the Sun is approximately 330,000 times greater than that of Earth. It is almost three quarters Hydrogen, whilst most of the remaining mass is Helium. The Sun is an almost perfect sphere. There is only a 10 kilometre difference in its polar diameter compared to its equatorial diameter. Considering the vast expanse of the Sun, this means it is the closest thing to a perfect sphere that has been observed in nature. The temperature inside the Sun can reach 15 million degrees Celsius.

At the Sun’s core, energy is generated by nuclear fusion, as Hydrogen converts to Helium. Because hot objects generally expand, the Sun would explode like a giant bomb if it weren’t for its enormous gravitational force. The temperature on the surface of the Sun is closer to 5,600 degrees Celsius. Eventually, the Sun will consume the Earth. When all the Hydrogen has been burned, the Sun will continue for about 130 million more years, burning Helium, during which time it will expand to the point that it will engulf Mercury and Venus and the Earth. At this stage it will have become a red giant The Sun will one day be about the size of Earth. After its red giant phase, the Sun will collapse, retaining its enormous mass, but containing the approximate volume of our planet. When this happens, it will be called a white dwarf. Light from the Sun takes eight minutes to reach Earth. With a mean average distance of 150 million kilometres from Earth and with light travelling at 300,000 kilometres per second, dividing one by the other gives us an approximate time of 500 seconds, or eight minutes and 20 seconds.

Although this energy reaches Earth in a few minutes, it will already have taken millions of years to travel from the Sun’s core to its surface. The Sun travels at 220 kilometres per second. The Sun is 24,000-26,000 light years from the galactic centre and it takes the Sun 225-250 million years to complete an orbit of the centre of the Milky Way . The distance from the Sun to Earth changes throughout the year. Because the Earth travels on an elliptical orbit around the Sun, the distance between the two bodies varies from 147 to 152 million kilometres. The distance between the Earth and the Sun is called an Astronomical Unit (AU). The Sun is middle-aged. At around 4.5 billion years old, the Sun has already burned off about half of its store of Hydrogen. It has enough left to continue to burn Hydrogen for approximately another 5 billion years. The Sun is currently a type of star known as a Yellow Dwarf The Sun has a very strong magnetic field. Solar flares occur when magnetic energy is released by the Sun during magnetic storms, which we see as sunspots.

In sunspots, the magnetic lines are twisted and they spin, much like a tornado would on Earth. The Sun generates solar wind. This is a stream of charged particles, which travels through the Solar System at approximately 450 kilometres per second. Solar wind occurs where the magnetic field of the Sun extends into space instead of following its surface. Satellites How old is the Sun? how old is the Sun? how old is the Sun? How old is the Sun? Actually it's middle-aged. It was formed about 4,500,000,000 (four and a half billion) years ago and we expect it to carry on pretty much as it is now for a few billion years yet - so no surprises in our lifetimes! start exploring here . We can’t do your home work for you, but if you have a question about the Sun you can always Image Credit: Casey Reed/NASA A star’s life-cycle depends on its mass. Stars like the Sun spend most of their lives quietly eating up the hydrogen in their centre. That means their temperature and size stay very much the same for billions of years.

When the Sun is about 7 billion years old it will slowly start to change and will become bigger and cooler. By the time it is 10 billion years old it will have changed into a red giant and its atmosphere will stretch out to near where the Earth is today. Content on this page requires a newer version of Adobe Flash Player. The Sun will grow old in much the same way as humans do. In a few billion years time it will be unrecognisable as the Sun we know today- a bit like the difference between an 80-year old person and a photograph of them as a baby! Under-sea coral? Enchanted castles? Space serpents? These eerie, dark, pillar-like structures are actually columns of hydrogen gas and dust where new stars are being formed. This picture of the Eagle Nebula was taken with the Hubble Space Telescope. The nebula is 7000 light years from Earth and is huge - the largest pillar is about four light years from top to bottom! The Sun and stars don't seem to change much year after year so how can we know about their lifecycles? One method is to have computer programs to calculate how a star evolves as it uses up its energy resources.

Another way is to observe many stars. We see many different kinds of stars and we assume the reason for this is that we are seeing stars at different stages of their lifecycle. We can then begin to build up an explanation of how and when they change. It's a bit like observing all the people in a town centre. We assume that they are all basically the same kind of animal (human beings) but that the main differences are caused by them being at different stages of their lifecycle (age). Fortunately, we don't have the complication of male and female stars! When and how will the Sun die? The Sun will not die in the way we know plants and animals die. But in a few billion years it will have changed so much it will not be recognisable as the Sun we know today. Calculations show that, as the hydrogen fuel in the core is used up, the outer parts of the Sun will begin to expand. The Sun will turn from a 'yellow dwarf ' into a 'red giant'. And it really means 'giant'! It will grow so big that it will almost engulf the Earth in its orbit .

The outer atmosphere will eventually be puffed out in a gentle explosion and the Sun will be at the centre of a planetary nebula - just like this. , or maybe like the one below. Can you see the old star in the middle of that nebula? The Facts About our Sun(star type is Yellow Dwarf (G2V)) | kiranvoid The Facts About our Sun(star type is Yellow Dwarf (G2V)) Posted on by Kiran “The Sun is the closest star to Earth” The Sun or Sol, is the star at the centre of our solar system and is responsible for the Earth’s climate and weather. The Sun is an almost perfect sphere with a difference of just 10km in diameter between the poles and the equator. The average radius of the Sun is 695,508 km (109.2 x that of the Earth) of which 20–25% is the core. The Sun is one out of billions of stars. The Sun rotates once every 27 days. The Sun is now a middle-aged star, meaning it is at about the middle of its life. The Sun formed over four and a half billion years ago.

You may think the Sun will die soon, but it will keep shining for at least another five billion years. Star Profile Circumference at Equator: 4,370,005.6 km Mass: 1,989,100,000,000,000,000,000 billion kg (333,060 x Earth) Surface Temperature: 5500 °C Size Of The Sun: 1.989E30 kg sun, mass Facts About The Sun One million Earths could fit inside the Sun: If a hollow Sun was filled up with spherical Earths then around 960,000 would fit inside. On the other hand if these Earths were squished inside with no wasted space then around 1,300,000 would fit inside. The Suns surface area is 11,990 times that of the Earth’s.

Eventually, the Sun will consume the Earth: When all the Hydrogen has been burned, the Sun will continue for about 130 million more years, burning Helium, during which time it will expand to the point that it will engulf Mercury and Venus and the Earth. At this stage it will have become a red giant The Sun will one day be about the size of Earth: After its red giant phase, the Sun will collapse, retaining its enormous mass, but containing the approximate volume of our planet. When this happens, it will be called a white dwarf. The Sun contains 99.86% of the mass in the Solar System: The mass of the Sun is approximately 330,000 times greater than that of Earth. It is almost three quarters Hydrogen, whilst most of the remaining mass is Helium. The Sun is an almost perfect sphere: There is only a 10 kilometre difference in its polar diameter compared to its equatorial diameter. Considering the vast expanse of the Sun, this means it is the closest thing to a perfect sphere that has been observed in nature.

Light from the Sun takes eight minutes to reach Earth: With a mean average distance of 150 million kilometres from Earth and with light travelling at 300,000 kilometres per second, dividing one by the other gives us an approximate time of 500 seconds, or eight minutes and 20 seconds. Although this energy reaches Earth in a few minutes, it will already have taken millions of years to travel from the Sun’s core to its surface. The Sun travels at 220 kilometres per second: The Sun is 24,000-26,000 light years from the galactic centre and it takes the Sun 225-250 million years to complete an orbit of the centre of the Milky Way. The distance from the Sun to Earth changes throughout the year: Because the Earth travels on an elliptical orbit around the Sun, the distance between the two bodies varies from 147 to 152 million kilometres. The distance between the Earth and the Sun is called an Astronomical Unit (AU). The Sun is middle-aged: At around 4.5 billion years old, the Sun has already burned off about half of its store of Hydrogen.

It has enough left to continue to burn Hydrogen for approximately another 5 billion years. The Sun is currently a type of star known as a Yellow Dwarf The Sun has a very strong magnetic field: Solar flares occur when magnetic energy is released by the Sun during magnetic storms, which we see as sunspots. In sunspots, the magnetic lines are twisted and they spin, much like a tornado would on Earth. The temperature inside the Sun can reach 15 million degrees Celsius: At the Sun’s core, energy is generated by nuclear fusion, as Hydrogen converts to Helium. Because hot objects generally expand, the Sun would explode like a giant bomb if it weren’t for its enormous gravitational force. The Sun generates solar wind: This is a stream of charged particles, which travels through the Solar System at approximately 450 kilometres per second. Solar wind occurs where the magnetic field of the Sun extends into space instead of following its surface. How old is the Sun? How old is the Sun? Check out some of the answers from our scientists Do you know when the Sun is going to burn out, or will it burn out or not?

The sun has another 4 or 5 billion years before it uses up its nuclear fuel, at which point its outer layers will inflate turning it into a red giant, and then blow off altogether leaving a cooler, dense, white dwarf star. See the attached links! Solar Facts and The end of the Sun: Dr. SOHO Do you know when the SUN is going to burn out? We think that it will finish burning up its hydrogen in about 5 billion years. It won't just burn out in a simple way, though. You can read more about it at the following site: Terry Kucera When will burn out and what star taks it place? The sun runs out of hydrogen fuel in its core about five billion years from now; it will become a red giant star, expanding to fill the inner solar system. No other star will take its place. Sallie Baliunas Do you have a prediction when the sun is going to stop? The Sun is abut 4.5 billion years old now. It is about 300 degrees hotter and abput 6% greater in radius than when it was first born.

It will continue to increase,in temperature, luminosity and radius at about the same rate for about another 5 billion years. At that point the luminosity will be twice its present value and and will be 40% larger in radius. At this point there will be no more hydrogen in the core, forcing energy tot ake place in whatis called a hydrigen burning shell. The core wil starts to contract as the outer layes expand. Over the next 1.5 billion years the surface should enlarge to about 3.3 times its present size while the surface cools to 4,300 degrees Kelvin. Next the Sun will rapidly swell and become a red giant star. Within the next 250 million years it will expand to 100 times its present size and 500 times more luminous. When the core temparture reaches 100,000,000 degrees Kelvin, all the rest of the helium will fuse into carbon in one big bang, causing one third of the solar envelope to be thrown out into space.

After this the Sun will become brighter and the outer layers will be blown out into space in the form of a a dense solar wind known as a planetary nebula. At that point all that will be left is a white dwarf. for more info see What Kind of Star is the Sun? - Universe Today Universe Today What Kind of Star is the Sun? Article Updated: 24 Dec , 2015 by Fraser Cain Question: What kind of star is our Sun? Answer: As you probably know, our Sun is just a star . It’s our closest, most familiar star, but it’s still just a star. With a great big Universe out there, populated with countless stars, astronomers have been able to see examples of stars in all shapes, sizes, metal content and ages. According to their system of classification, the Sun is known as a yellow dwarf star. This group of stars are relatively small, containing between 80% and 100% the mass of the Sun. So the Sun is at the higher end of this group. The official designation is as a G V star.

Stars in the this classification have a surface temperature between 5,300 and 6,000 K, and fuse hydrogen into helium to generate their light. They generally last for 10 billion years. But there’s more to this question, because G V Stars can experience several different stages. Some are newly forming, others are in their middle ages, and others are nearing the end of their lives. Our Sun is right in the middle ages, in a time known as the main sequence. It has already lived for 4.3 billion years, and will likely last another 7 billion years or so. At that point, it will balloon into a red giant star, and eventually collapse down into a white dwarf. The Sun also belongs to the Population I group of stars, which contain relatively large amounts of heavier elements. The first ever stars, made from pure hydrogen and helium are Population III. These exploded as supernovae, producing fusing the lighter elements into heavier and heavier elements. Our Sun, then, contains the metal from previous generations of stars that went supernova. Some other examples of the yellow dwarf star group include Alpha Centauri, Tau Ceti and 51 Pegasi.

For the quick answer, the Sun is a Population I yellow dwarf star, in the main sequence. Why is the Sun yellow ? It’s actually because of the Earth’s atmosphere. If you saw it from space, it would actually look white. Reference: The End Of The Sun The End Of The Sun The Helium Flash The beginning of the end for a red giant the mass of our Sun occurs very suddenly. As the helium "ashes" continue to pile up at its center, a higher fraction of them turn electron-degenerate. It is an odd paradox: even as the outer layers of a red giant star are expanding into a huge but tenuous cloud, its inner core is contracting down to form a buried white dwarf. The temperature and pressure in the Sun's core will soar to 10 times their current values. And roughly 1.2 billion years after it leaves the main sequence, at the height of its glory as a red giant, the center of the helium core of the Sun will become sufficiently massive, dense, and hot that something amazing will happen: within a matter of minutes, it will ignite and burn.

When the temperature in the core reaches about 100 million degrees, the helium will begin to fuse into carbon by a reaction known as the triple-alpha process, because it converts three helium nuclei into one carbon atom. This generates a great deal of heat. However, unlike when the Sun was young and its core contained normal matter, adding more heat to the electron-degenerate helium does not cause it to expand and cool. As I noted when I was discussing quantum mechanics , electron-degenerate matter behaves more like a liquid than a gas when you heat it: its temperature swiftly rises, but it doesn't expand. In other words, the self-regulating mechanism that keeps main-sequence stars so stable (hydrostatic equilibrium) is turned off in electron-degenerate matter. If you add heat to a white dwarf, it just gets hotter. As it happens, the triple-alpha process is exceptionally highly temperature dependent: doubling the temperature of the reaction causes it to run roughly a trillion times faster! So, as the fusing helium heats the core, which cannot expand to cool down, the increased temperature causes the helium fusion to suddenly proceed millions of times faster, which very rapidly heats the core even more, which in turn causes the helium to fuse way, way faster . . .

In short, the center of the helium core explodes. About 6% of the electron-degenerate helium core, which by now weighs in at about 40% of a solar mass, is fused into carbon within a few minutes. (This corresponds to burning roughly ten Earth masses of helium per second, if you are keeping score.) For obvious reasons, astronomers call this the helium flash. In roughly the time it takes to toast a bagel, the flash releases as much energy as our current Sun generates in 200 million years. At the height of the flash, the Sun's core will very briefly equal the combined luminosity of all the stars in the Milky Way galaxy! One might imagine that a conflagration of this magnitude would have a dramatic impact on the red giant – and it does, in a way, but not nearly so suddenly or violently as you might think. This is because we tend to underestimate gravity. Compared to the intimidating power of nuclear weaponry, the energy generated by dropping a few rocks doesn't seem very impressive.

But in fact, the gravitational energy of extremely dense, extremely large masses is startling – it is only our human prejudice, arising from the fact that we live on a puny pebble that is neither massive nor dense, which makes us think otherwise. Navigation Menu After The Supernova Suppose we do take the Earth as an example of a large, dense object, even though it is about as dense as cotton candy when compared to a white dwarf. To inflate the Earth to twice its size – that is, to lift the mass of the Earth against its own gravity until its radius is doubled – would require all the solar energy striking the surface of the Earth (a mere 185,000,000,000 megawatts) for the next 13 million years! During the helium flash, a star's degenerate core is heated so intensely that it finally "vaporizes", so to speak. That is, individual nuclei begin moving so fast that they can "boil away" and escape it. The core reverts back into a (spectacularly dense) normal gas, and powerfully expands.

The enormous gravitational energy needed to expand 100,000 Earth masses out of degeneracy and up to several times their original volume is on a par with the energy release of the helium flash. Or in other words, almost all the energy of the flash is absorbed by the titanic weight-lifting necessary to lift the core out of its white-dwarf condition. Essentially none of the energy reaches the surface of the red giant, and indeed, if you were observing the red giant with your naked eye as its helium core flashed over, it is doubtful that you'd notice anything at all. So, by human standards, the helium flash is a disappointing dud to watch. By galactic standards, however, the red giant has been shot through the heart. The sudden expansion of the core results in cooling so severe that it is something like the onset of an Ice Age. The cooling immediately leads to much lower pressure in the hydrogen-burning shell that surrounds the core, and therefore to a calamitous drop in the energy output.

On a timescale which is almost instantaneous compared to the usual timescale that stars run on (perhaps as little as 10,000 years), the red giant's diameter and luminosity plummet to less than 2% of their former values. For stars the mass of our Sun, the result of the helium flash is a collapse into an orangeish-yellow star with perhaps ten times the current solar diameter and 40 times the luminosity. It is quite a comedown. The End Of The Sun The final 140 million years or so of the Sun's life will be very complicated. After its collapse, as illustrated in Figure 1, the Sun will reestablish itself as a star with a double energy source: it will have a dense (but not electron-degenerate) carbon-oxygen core surrounded by a shell where helium is burning into carbon, and outside of that it will have another shell where hydrogen is burning into helium. (The core oxygen is created by slow fusion between carbon and helium at the core's surface.

In heavier stars, the oxygen can in turn fuse with the helium to make neon.) Helium fusion produces only 9% as much energy per kilogram as hydrogen fusion, so energy-wise, the Sun continues to be mainly a hydrogen reactor. 90% of its luminosity still comes from burning hydrogen. However, it is the helium surrounding the core which now dictates how the Sun will evolve. The Sun more-or-less repeats what it did as a aging main-sequence star, except now with a carbon-helium mix in the core rather than a helium-hydrogen mix. For a time it achieves relative stability and maintains hydrostatic equilibrium in its new incarnation as an orangeish-yellow "subgiant" star. Thus, stars in this phase of their existence are sometimes said to be on the "helium main sequence". From the fleeting perspective of a human lifetime, subgiant stars seem calm enough: the well-known bright star Arcturus, whose light was used to open the 1933 Chicago World's Fair, is such a star. It has not changed in any measurable way since the invention of the telescope. But the high temperatures necessary to maintain helium burning mean that the Sun can only burn helium one way: very fast.

The hot core dictates rapid hydrogen burning as well. When it was on the normal main sequence, the Sun's luminosity held fairly close to 1.0 Lo for around nine billion years before brightening to about 2.7 Lo at the end. On the helium main sequence, the Sun's luminosity will hold at about 45 Lo before brightening to about 110 Lo at the end. Not so impressive as a red giant, but very bright nonetheless. To maintain its subgiant lifestyle the Sun must tear through the fuel in its helium core 100 times faster than it did with its original hydrogen core. After only a hundred million years on the helium main sequence, the Sun will once again begin to climb towards the realm of the red giants, and for the same reasons as it did before. But there is no "carbon flash" equivalent of the helium flash that stopped the Sun the first time. The temperature and pressure needed to ignite carbon-carbon fusion is too great for the Sun to achieve no matter how compressed its core becomes, so the carbon just accumulates and becomes ever denser.

The trend that the Sun showed on its first run as a red giant, when its core was crushed to white dwarf densities even as the outer layers billowed to tens of millions of kilometers in diameter, is unstoppable now. The Sun becomes a red giant again, this time with a peak luminosity above 3,000 Lo. Its outer layers blow further and further outward, beyond the orbit of Jupiter, even as its electron-degenerate core swiftly grows more massive and therefore smaller and more dense. And eventually the day comes when the two part company. The final days of a star are extremely complicated, because the helium-burning and hydrogen-burning shells don't burn at the same rate. The hotter, faster-burning helium shell tends to race outwards and overtake the hydrogen-burning shell, and when that happens there is no more helium left to burn, so the helium shell fizzles out. But the giant star quickly cooks up more helium, which then collects on the white-dwarf core until it suddenly flares up in a run-away helium ignition that is something like a baby version of a helium core flash. The helium flare-up disrupts (turns off) the hydrogen burning for a short time, and so it goes.

At the very end, the Sun will literally cough itself to death as multiple fuel ignitions and choked-off fusion extinguishments rip through its atmosphere. In four or five huge bursts, spaced roughly 100,000 years apart, the outer layers of the Sun will separate from the core and be completely blown away. They will form an enormous, expanding shell around the solar system, and move outward to rejoin the interstellar gas. Roughly 45% of the Sun's mass will escape in this way. The remaining 55% of the Sun's mass is soon compressed into the white-hot, ultra-dense core. To someone watching the Sun from far away, the Sun would appear to rapidly shift colors from red to white as the gaseous veil surrounding it is lifted. (By "rapidly", of course, I mean a time span only a few times longer than the age of the pyramids.) The exposed surface of the searing solar core will be so hot, at least 170,000 K°, that it will emit more x-rays than visible light.

(Post-red-giant stars are the hottest stars known, excepting neutron stars.) Its luminosity will be a brilliant 4,000 Lo. The Sun will have become a radiation source of truly galactic stature, its energy lighting up the escaping gas around it like a huge neon sign. Such clouds are called planetary nebula, a misleading name, because 18th-century astronomers could barely see them with the telescopes of the time and thought that they looked like planets. They are among the most beautiful sights in astronomy. The photograph at right, of the nebula known as NGC 6751, is of one of my favorites. The bright spot in the center is the post-red-giant parent star. Remarkably, there is a star right at the point of blowing off its outer layers which can be seen with the naked eye. This is Mira, the "Amazing One", so named by Arabian astronomers in the Middle Ages because Mira rather erratically varies over a span of roughly 330 days from being the brightest star in its constellation (Cetus, the Whale) to total invisibility. Mira is the only classically named star that you cannot see, much of the time.

Modern instruments reveal that Mira is a vastly over-extended bag of deep-red gas that is not even closely spherical and which, at 2,000 K°, is also one of the coolest stars known. Its atmosphere is undergoing complex undulations and oscillations as the nuclear burning below it sputters and gasps. Hence, its variability. In a paltry 500,000 years or less, Mira will be a planetary nebula. As for the Sun, without its outer layers to supply it with more hydrogen, it can only maintain the gorgeous display of its nebula for a few thousand years, hardly more than a snap of the fingers by galactic standards. The last dregs of fuel on the dense core will finally burn out, and for the first time in over twelve billion years the Sun will cease to produce energy. The nebula will disperse and fade. The Sun has become a white dwarf, little larger than the Earth but 200,000 times more massive, and for billions of years to come all it will do is slowly cool off.

Due to their immense density, the time it takes white dwarfs to cool off is so great that not even the oldest known (nearly 12 billion years) have had time to cool much below 5000 K°. These very old "white dwarfs" could perhaps more accurately be called "yellowish-white" dwarfs, but in any case, the Milky Way does not contain any "black dwarfs". All of the ten billion or so white dwarf stars that our galaxy has produced since the Big Bang are still shining, however dimly. What is a Yellow Dwarf – Types of Stars: G Type Main Sequence – PlanetFacts.org The Moon Yellow Dwarf A yellow dwarf is a star and is often referred to as a G-type main sequence star. A perfect example of a yellow dwarf would be the sun. A yellow dwarf has a mass almost like the mass of the sun. Its color ranges from white to a lighter yellow. Among the stars in the galaxy, yellow dwarf stars are bigger than most of the stars although giant stars are the biggest of all.

Being a yellow dwarf, the sun is 1,412,000,000,000,000,000 cubic kilometers big and has a size that is more or less 1,989,100,000,000,000,000,000,000,000,000 kilograms. The sun is more than a million times bigger than the Earth and is 332,900 times weightier. The sun is made of hydrogen, helium, oxygen, iron, and carbon. However, its main component is hydrogen at 75%. The life span of a yellow dwarf is said to be ten billion years and it glows brighter as years go by. Its hydrogen fuses into helium which makes its core hotter. Its energy eventually surmounts its gravity while it grows bigger and turns into a red giant. Ultimately, the yellow dwarf will turn into a white dwarf, a star that has collapsed and turned cooler. While the white dwarf may become a black hole, the yellow dwarf does not have a size that will make it into a black hole or a supernova.

At present, the earth’s sun is just in the middle of its lifespan. It will probably take a billion more years for it to become a white dwarf.

Question: The sun is currently a yellow dwarf. What will it become in about 5 billion years?
Answer: Red Giant