Lois & Clark Forums Forums General Discussion Off Topic Oh my, we're doomed!

So, did you hear that within the next million years or so, we're doomed?

There has been a spate of astronomical news just lately (I guess there's been some sort of astronomy conference going on somewhere) and this was one of the tidbits that got out of this astro-conference and made headlines:

Yes, well, we're talking about a star which, according to astronomers, is going to blow its top as a blazing supernova a million years or so from now. The subsequent frying of the Earth is going to be much worse than the shaking, rattling and battering of the Earth that followed the asteroid impact that took place about 600 million years ago and which killed off the dinosaurs. As you can see in the quote above, astronomers expect this upcoming supernova to actually kill off life on Earth altogether, or at least that's how I read their announcement.

So what do you know about supernovae? Have you heard about Betelgeuse?



Betelgeuse is, if I may use that word, a ”sexy” star. It's big and bright, it is prominently situated in an eye-catching constellation, Orion, and it sports a nice and notable orangish hue. For a long time, astronomers clung to the idea that the big bright reddish stars were the ones that were on the verge on going supernova. However, recent observations have cast doubt on that theory. Of all the stars that have been observed going supernovae in recent years, only two have been photographed before they went off, and neither was red. The most famous of these is the star Sanduleak -69° 202a, a blue supergiant in the Large Magellanic Cloud, which went supernova in 1987:



The supernova is on the left, and the blue progenitor star is on the right.

Certainly it is not the color that made this star go supernova, but rather its mass. A sufficiently massive star, so says the theory, is destined to go supernova sooner or later. But to understand why, you need to understand a little of how and why stars shine in the first place.

All stars are massive. If they aren't sufficiently massive, they can't get their “stellar engine” going. I think that about 99% of all the mass in our solar system is locked up inside the Sun, and that is precisely the reason why there is not one more sun in our solar system, in spite of the fact that so many stars in the universe have stellar companions. But in the case of our own solar system, all the mass that is left outside the Sun is not enough to form another star. If you packed all the remaining mass inside a single ball of gas, it would not be massive enough to start the thermonuclear processes that are so characteristic of “real” stars.

Stars shine by fusing lightweight elements into more massive elements. Stars like the Sun are made up mostly of hydrogen, the lightest element in existence, and no other element is as good at fusion as hydrogen is!

This is the kind of fusion that goes on inside the Sun:



One ion of deuterium (”heavy” hydrogen, which consists of one proton and one neutron) merges with one ion of tritium (”double-heavy” hydrogen, which consists of one proton and two neutrons). As the deuterium and tritium merge (or “fuse”), the end product is an ion of helium and a “loose neutron”. But that is not all. The ion of helium plus the loose neutron contain less energy than the two ions of deuterium and tritium. The extra energy is released as an energetic photon. Every second, trillions or quadrillions of ions of deuterium and tritium are converted into helium deep inside the interior of the Sun. All the extra energy that is created inside the Sun as it converts, or fuses, hydrogen into helium, radiates away from the Sun as light and heat.



A solar eclipse. The full Moon covers the disk of the Sun. You can see the streamers of the solar corona radiating away from the Sun. All this radiation is caused by the fusion of hydrogen into helium deep inside the Sun.

A few billion years from now, the Sun will have used up all the hydrogen in its center and converted it all into helium. When that happens, the activity at the center of the Sun will cease, the center will shrink, and at the same time the outer layers of the Sun will expand. This is when the Sun will become a red giant.

However, as the center of the Sun shrinks, the temperature at the center will rise. Finally the temperature will be high enough for helium to fuse into carbon.

But when all the helium at the center of the Sun has been converted into carbon, the Sun will be able to do no better. The reason is that the Sun is not massive enough. Carbon still contains a lot of energy, and you can still squeeze energy out of it by fusing it into heavier elements. But the Sun will be unable to do that. Doing that takes gravity, the kind of relentless gravity that the Sun doesn't possess.

Let's repeat how nuclear fusion works. Nuclei repel each other, and they really don't want to meet! But inside the center of a sun the pressure is so great, due to the gravity of that sun, that the temperature rises to millions of degrees. At those temperatures, the nuclei move so fast that they don't always have time to “dodge” one another. And when the nuclei collide and fuse, they suddenly hold on to each other with an iron grip. The force that glues the nuclei together is incredibly strong.

But it takes mass and gravity to make the nuclei fuse. Only a body that is sufficiently massive can make its own center hot enough to make the nuclei collide and force them to merge.

The fusing of hydrogen into helium is the “first step” on the “fusion ladder”. It takes more heat and higher temperatures, and therefore more mass and more gravity, to fuse helium into carbon. A star that is a lot more massive than the Sun, however, possesses the kind of gravity that keeps on squeezing its own center so that it becomes smaller and smaller and hotter and hotter. Inside such a star, several fusion processes will go on at once. In an outer layer, hydrogen will fuse into helium. Deeper down, helium will be converted into carbon. On the next level, carbon will fuse into oxygen. Oxygen will be converted into magnesium, magnesium into neon and neon into silicon.



And guess what? Now the star is in trouble!

You can squeeze a bit of energy out of silicon by fusing it into iron. You don't get a lot of energy out of that fusion process, however. And once silicon has been converted into iron, you have reached the end of the line. Iron is a sort of equilibrium product. It contains no extra energy that can be extracted out of it by fusing it. Once the star has an iron core, it can produce no more energy in its center. No matter how much you compress its center, you will get no more fusion processes going there.

For the star, this means catastrophe. The star is enormously heavy and massive, and its center is enormously compressed. To hold itself up and avoid imploding, the center needs to generate energy of some sort. But it can't do that. The core has exhausted every last bit of energy it contained. All that is left is the relentless force of gravity, forcing the center of the star to collapse on itself. And when that happens, you have a supernova.

So as a massive star goes supernova, what actually happens is that the star's dead and inert center collapses in on itself. This releases a lot of energy, of course, mostly potential energy, and it is the fact that this energy is released asymmetrically that actually causes the star to blow itself apart.

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A massive star goes supernova.

So this is what will one day happen to Betelgeuse, and also to several of the bright blue stars in the constellation of Orion. They will all blow themselves apart in the end because they are too massive. In the long run, the dead centers of these massive stars will be unable to support the prodigious weight of these stellar behemoths.

But like I said, when a massive star goes supernove we are talking more of an implosion than an explosion. Also, about 90% of the energy that is released is released in the form of neutrinos. (Or so I think, because a quick Google search did not turn up how much of the energy of a supernova explosion of a massive star is actually released as neutrinos. Well, I'm sure I've read, several times, that most of the energy is released as neutrinos, and it could well be as much as 90%.)

Neutrinos are singularly harmless particles. They interact with almost nothing at all. Every second, the Sun produces trillions of neutrinos, and every second, huge numbers of neutrinos pass right through the bodies of every living being on the Earth. We don't notice them at all, because they don't interact with us. But this means, too, that if a supernova is producing a lot of neutrinos, the neutrinos are not going to harm us.

Okay. So much for the massive stars that go supernova. Astronomers are not too scared of the flamboyant supergiant stars of the universe. They look impressive, but they are punier than you would think. They have exhausted themselves before they go “boom”. Their bark is worse than their bite. Astronomers are confident that Betelgeuse, for example, will not be able to harm us when it collapses under its own weight. Betelgeuse is more than 600 light years away, and that is a more than sufficient “safety distance” between the Earth and a massive star that self-destructs.

So all is well, then? Not quite. Take a look at this image:



This is galaxy NGC 2770. Three supernovae have recently exploded in this galaxy. One of them, SN1999eh, exploded in 1999 and has faded away since then. But you can see two other supernovae in the galaxy, SN2007uy and SN2008D /XRF080109. You can tell from their names that one of them exploded in 2007 and the other in 2008. The one that exploded in 2008 is close to its peak brightness, and it is a “Betelgeuse”, that is, it is a massive star that collapsed under its own weight. The other supernova, however, the one from 2007, has been fading for a longer time, and yet it is brighter than the supernova from 2008. How can that be? Well, you see, the supernova from 2007 was not a massive star that exploded. It was a so-called white dwarf, not much more massive than the Sun, that suddenly crossed its “Chandrasekhar limit” and blew itself apart in a most violent fashion.

A white dwarf is a star whose core consists of elements that could still release energy by fusion, but the star is not massive enough to make its core hot enough to make this kind of fusion possible. Another way of putting it is that its core consists of “unburnt fuel”, which the star is unable to ignite. Or, correction! It's not just the star's core that consists of unburnt fuel, but the entire star consists of the same fuel, namely massively packed carbon and oxygen. In order to ignite this fuel the star needs to cross its Chandrasekhar limit by accreting a critical amount of mass. This can only happen if the white dwarf has a companion star orbiting very close to itself, so that it can siphon off the other star's atmosphere. When it has swallowed a sufficiently large helping of the other star's atmosphere, the white dwarf will have gained just the right amount of mass and gravity to compress its own center until it becomes hot enough to start fusing the elements there. This will lead to a run-away fusion process. Actually you can compare this star to a super-duper massive bomb suddenly going off. Imagine that the entire Earth was one single bomb, and imagine that you could make this bomb go off and explode violently. Ah, but a white dwarf needs to contain about 400,000 times as much mass as the Earth in order to cross its Chandrasekhar limit and explode as a supernova. But that means that we are talking about one single live bomb containing as much live explosives as the mass of 400,000 Earths. No wonder that these supernovae, the “white dwarf supernovae” also called the “type Ia supernovae”, are among the most prodigious explosions in the universe.

So how about the star that will blow us all to smithereens in the future? Unsurprisingly, it looks puny and unimpressive. It shines feebly at sixteenth magnitude, and believe me, that's faint! Betelgeuse, for comparison, is a first magnitude star, so it is fifteen magnitudes brighter! The little white guy is called T Pyxidis. Have you heard of it? Didn't think so.



T Pyxidis, not much of a looker. Here are more facts about it, including an explanation for why it is surrounded by a ring of matter (it's because it has sucked matter from it's companion before and blown it off in a series of smaller explosions in preparation for the really big one).

This little stellar critter is much farther away than Betelgeuse, about 3,000 lightyears away, compared with only about 600 lightyears away for Betelgeuse. Small consolation: it is the little white dwarf that will kill us, not the bright and grandstanding supergiant.

Would you like to find T Pyxidis before it finds us and does away with us? Here is a chart.

So we are doomed after all, and it will be T Pyxidis that kills us? Well. Maybe, maybe not. Remember that astronomers have not identified a single progenitor to a “white dwarf supernova”. That means that they don't have any data about even a single white dwarf star that later blew up as a “white dwarf supernova”. Can we really be so sure that astronomers know what they are talking about, now that they call T Pyxidis the doomsday machine that will destroy life on Earth a million years from now? Personally I'm doubtful. Anyway, I think it is statistically unlikely that a white dwarf supernova will explode so close to the Earth in such a short time from now, considering that modern astronomy has not seen a single “type Ia” supernova go off anywhere in our own galaxy since the invention of the modern telescope, and considering that life on Earth has survived for billions of years already, in spite of all the stellar events that have taken place all around us as we have orbited the center of the Milky Way.

I'm not too scared of T Pyxidis, but hey – I thought this whole thing was kind of cool anyway!

Ann

P.S. So I wonder... when will there be a macho-action movie where Bruce Willis or someone flies off to T Pyxidis to stop it from exploding? Could Superman do the trick? Maybe he could push either T Pyxidis, or its companion, or both, into a black hole or a worm hole?

Well, since I can't do much if the sun decides to go nova, I'll just continue with my daily life.

But the post was extremely interesting, and I love the pictures and graphics! I learned a lot.

I have something to add. (Don't I always?) Well, I'd like to say a few more things about what a white dwarf is.

A white dwarf is the hot but cooling core of a star that was as massive as the Sun, or a bit more massive. A white dwarf has blown off its outer layers, revealing its "naked core". During a fleeting stage of a Sun-like star's life, it will be a so-called "planetary nebula". The term was coined by astronomers who looked at planetary nebulae through early telescopes and thought that these nebulae looked like planet Uranus in the telescope. Unlike stars they had an obvious "disk" and were slightly greenish in color. Here are some pictures of planetary nebulae:



Abell 39.



Soap Bubble Nebula.



Ring Nebula.



Cat's Eye Nebula.

The "disks" you can see here are the cast-off atmospheres of former Sun-like stars. When a star like the Sun reaches the end of its life, its core will grow hotter, which will cause the outer atmosphere of the stars to expand. Later the core will start "hiccuping", which will blow the outer atmosphere even further away from the core. At the same time the newly exposed very hot core will make the cast-off atmosphere glow. The green color of many planetary nebulae is caused by the irradiation of oxygen ions in the cast-off atmosphere by the very hot core at the center. Take a look at the first planetary nebula posted here, Abell 39. You can see the green color of the cast-off atmosphere, but also the striking blue color of the exposed glowing hot core at the center of it. Strangely enough, the blue color means that the core is very hot.

All these formerly Sun-like stars have reached the end of their "nuclear fusion"-line. Nuclear fusion has ceased in their cores, which now consist of a mixture of helium, carbon and oxygen. They can't go on synthesizing heavier elements, because they aren't massive enough to make their cores hot enough, but they also won't collapse in on themselves like the dead cores of the really massive stars, and the reason for this is again that they aren't massive enough. If I remember correctly, astronomers talk about the "Pauli principle" (beware that I haven't even googled it), which says that when electrons get packed so closely that they basically "touch", they will resist further packing extremely strongly. In the core of a white dwarf star, the electrons are packed that closely. The gravity of a white dwarf star is not enough to overcome the outward pressure of the electrons inside it, so the white dwarf will not collapse under its own weight.


Our Sun will become a white dwarf one day, but it will have no chance whatsoever to become a white dwarf supernova. The first reason is that it is not massive enough. A white dwarf is just the dead core of a star, but in order to become a supernova the core needs to weigh 1.4 times as much as the Sun. Our Sun weighs, unsurprisingly, exactly one solar mass , but when it has cast off its outer atmosphere to become a white dwarf it will weigh less. Also a white dwarf has to have a closely orbiting stellar companion in order to ever go supernova, because it has to suck matter from the other star in order to gain enough mass to explode.

Here you can see the relative sizes of a normal star like the Sun, compared with the size of a red giant (a Sun-like star which has ceased nuclear fusion in its core and puffed up its atmosphere) and a white dwarf (a former red giant which has blown off its atmosphere). Note that a white dwarf is about the same size as the Earth, but it contains about as much mass as the Sun.



Ann

Ann, I always enjoy your posts on astronomy. I don’t always understand them easily but they are fascinating. I loved the picture of the eclipse. It looked like a delicate flower. You mentioned that stars had atmospheres. I had no idea that they did.

I tell ya’, that ring nebula and cat’s eye nebula kind of look like monsters!

So, is a nebula always a star? Or did I totally get that wrong?

So Sol can’t supernova. But can it nova?

Thanks for your kind words, Nancy!

A nebula is a gas cloud, not a star. In a planetary nebula, the nebula is the cast-off atmosphere. The burnt-out star is situated in the middle of a planetary nebula.

This is a famous part of the sky containing many different nebulae:



This is the famous Horsehead Nebula in Orion, plus a lot of surrounding nebulosity. Surely you can see the dark little horse poking its head out of some dark "murk", with what looks like rolling red hills in the background? And to the left and lower left of the horsehead, you can see a number of bluish patches of light. Also to the left of the horsehead is a bright yellow-orange patch of light, bisected by dark lanes. The yellow-orange nebula is called the Flame Nebula.

What you see here is a "massive star forming region", and the fantastic colors and shapes are caused by the interplay between stars and the surrounding gas clouds. Let's begin with the most important star here, Alnitak, which is the brightest blue object here, directly above and slightly to the right of the Flame Nebula. Alnitak is a blisteringly hot star, and it floods the whole region with searing ultraviolet light. This ultraviolet light is what causes the rolling red hills behind the horsehead nebula. When an energetic photon of ultraviolet light hits an atom of hydrogen, it "kicks" the electron of that atom into "a higher orbit". When the electron "falls down" to its old orbit again, it emits a characteristic photon of red light. In our galaxy there are very many clouds of hydrogen gas which are situated close to very hot stars. All these hydrogen clouds glow with a characteristic red color. Here are two other red nebulae, also called emission nebulae:



NGC 281, also known as the Pacman nebula. Note the hot bright star at the center of the nebula. To make a gas cloud glow red, a star needs to be as hot as 20,000 degrees Celcius or more.



The Rosette Nebula, with a whole cluster of hot blue stars in the middle. Isn't it lovely?

Do you remember the bluish patches of nebulosity in the "horsehead picture"? Blue nebulosity is also called "reflection nebulae", since they glow blue because they are illuminated by a more-or-less blue star, and it scatters the light from this star, so that large patches of the cloud is lit up. Gas clouds are usually particularly efficient at scattering blue light, which is why these nebulae so often are blue. The most famous of all blue nebulae is the reflection nebulae surrounding the lovely star cluster, the Pleiades:



The tempereature of the blue stars of the Pleiades is "only" about 12,000 degrees Celcius, so they can't make this gas cloud glow red. Instead the blue light they emit is scattered throughout the gas cloud.

A very interesting nebula is the Trifid Nebula, which is a combination of emission and reflection nebulosity. You can see the glowing red hydrogen cloud and the compact little cluster of hot stars at the center of it, which makes it glow. However, the red nebulosity is itself surrounded by a larger blue reflection nebula. Interestingly, the blue nebula is at its brightest near a bright yellowish star.



Could a yellow star make a gas cloud glow blue? It is at least possible. After all, the best-known of all reflection nebulae is in fact the Earth's blue sky, which is blue because the Earth's "gas atmosphere" scatters the blue light from the Sun! (But it should be pointed out that the Sun is not as yellow as we think it is. On the other hand, it does look more yellow in the sky precisely because much of the blue light it emits has been scattered all over the sky!)



By the way, why are the clouds in this picture white? According to an answer that I came across once, the white color of clouds, as well as the white color of snow, reflects the true color of the Sun. So the Sun is white. That also would explain why daylight on an overcast day is neutral-colored, not yellowish or beige, as it would have to be if the Sun was truly yellow.

It should be noted that if a star is too reddish, so that it basically contains no blue light at all, it can't give rise to a blue reflection nebula. A very interesting example of a non-blue reflection nebula is the bright yellow nebula surrounding a famous red supergiant, Antares.

I'm not sure I'm allowed to post this picture as an image, so I'm posting it as a link. You can see the striking yellow reflection nebula surrounding the red supergiant, as well as blue reflection nebulae surrounding bluer stars. You can also see two red emission nebula surrounding two hot blue star, whose ultraviolet light makes the nebulae glow red.

Colorful nebulae, including a yellow reflection nebula.

Let's return to the horsehead nebula. The horsehead itself is a so-called "dark nebula", an unilluminated gas cloud that is seen in silhoutte against a brighter background sky. You can see some really striking dark nebulae in the Antares picture, too. Dark nebulae are in fact extremely common.



The Snake nebula, seen against a background of a myriad of stars.

Finally, what is the Flame Nebula (at top in the horsehead picture) and why is it yellow-orange? Well, the Flame Nebula is actually the birthplace of new stars. The nebula itself has been "compressed" by light from the hot bright star Alnitak, and as a result stars are born inside it. The newborn stars have not yet blown away their gaseous cocoon, but the light they produce lights the Flame nebula up from within. However, the "dust" (fine interstellar particles) that is scattered through the gas clouds "reddens" the light from the newborn stars. It is actually the same processes that are at work when reflection nebulae look blue and when blue light is unable to penetrate gas clouds, because the blue light is scattered away. If you take a look at the picture of the Snake nebula, you can see how the stars peeking through the outskirts of that nebula have their light reddened by passing through the dust there.

Well, so much for the nebulae!

Ann

No. Astronomers are sure that novae happen when a white dwarf accretes matter from a companion star, and the white dwarf "hiccups" and blows off the newly accreted gas. A nova is a much smaller explosion than a supernova, because when a star goes nova it only blows off its surface, but when it goes supernova it burns itself out completely and scatters its innards all over its galactic neighbourhood!

Astronomers now believe that T Pyxidis is destined to go supernova in the future because it is currently accreting gas from its companion, and in the past it has gone nova several times, blowing off its recently-acquired gas. But now T Pyxidis has not gone nova for a couple of decades, and, to compare it with a woman who discovers that she is going to have a child, T Pyxidis is "late"! It is "pregnant" with a super-duper-nova explosion, and we just don't know how long the gestation period will be...

The Sun is never going to go nova, because it doesn't have a stellar companion, so it can't accrete gas from its companion and blow it off. The Sun is single and only moderately massive, and therefore it is destined to have a rather uneventful life!

Ann

Thanks for posting all this and also for the great pictures!

That pic is great. It might be the Horsehead Nebula, but I was instantly struck with another likeness. It looks like The Flash when he went just a little too fast. Come on... Tell me you can see it, too! (Costume flying off his head, eyes bugging out, and teeth bared...)


I think it’s neat when the sun looks white. What color would the skies be under another color sun? A red sun for instance. Would the skies still be blue? Or maybe pink or orange? Well, I see you noted the possibility of a yellow sky. Would there ever be the possibility of a purple (indigo) sky? It seems unlikely to me because that is the faintest color when colors come out of a prism and is the faintest color in a rainbow. Could that be different elsewhere?


Well, you know, maybe in a million years man can “fix” this. Kind of like in Doctor Who where future humans held off the annihilation of earth to make it a “side show”.

Hmmm... you mean the Flame Nebula would be the Flash's teeth? Yes, maybe...

If the Sun was yellower than it is, the sky would be less blue. If the Sun was as red (or rather, as yellow-orange) as Antares is, then the sky would be as yellow as the Antares reflection nebula. More specifially, the color (hue) of the sky would be yellow, but the general appearance of the sky would probably be a murky yellowish-brown color. Since blue light scatters much more efficiently than yellow light, it is probable that a yellow daylight sky would be considerably darker than the Earth's blue sky, which is quite bright, as we all know. Orange light scatters even less efficiently than yellow light, so an orange sky would be even darker. It could be that that an orange or yellow sky would be bright close to the sun that illuminated it, but farther away from the Sun the sky would be very noticably darker.

A yellow sky. If the Sun was much yellower than it is, this might be the normal color of the sky. But it would probably be darker than here.

Bear in mind that indigo is a faint (and seemingly dark) color only because our eyes don't respond strongly to it. I don't see why you couldn't have a purple or inidgo sky. It would take a very hot star, which produces a lot of ultraviolet light, to make a purple sky. However, you must bear in mind that the Earth's atmosphere protects us from the ultraviolet light of the Sun by actually blocking it, so that it can't reach the ground! Purple light is only a little different from ultraviolet light, so our atmosphere blocks some of the Sun's purple light, too. So the sky couldn't be purple unless the Earth's atmosphere was different and actually let much more of the purple light through. Also, of course, the Sun would have to be much hotter, so that it produced a lot more purple light in the first place.

However, purple light scatters very efficiently, so if you had a lot of it the sky would certainly be both bright and purple!

A purple sunset on a world with two suns. I don't know how realistic this might be....

Ah, but wait! I just remembered! If the sun produces a lot of ultraviolet light, then that light will "ionize" the atmosphere and make it glow red! So I guess you wouldn't have a purple sky after all. And in any case, a star that flooded the Earth with a lot of ultraviolet light would actually kill us...

Ann

Okay, ah... second correction...

Nebulae glow red because the hydrogen they are made up of has been ionized by ultraviolet light. But it so happens that there is no "free hydrogen" at all in the Earth's atmosphere! So if it takes hydrogen to make the Earth's atmosphere glow really red, I guess we won't ever see a truly "Flash-supersuit" red sky here!



The sky won't be that color!

Ah, but... if the Sun emitted a lot of ultraviolet light, it might "break apart" the water molecules in the Earth's atmosphere into hydrogen and oxygen. Then there would indeed be hydrogen in our atmosphere, and the sky might glow red.

Except it probably wouldn't look red to us. The red light emitted by ionized hydrogen is deep into the red part of the spectrum, every bit as far into the red part of the spectrum as purple is "far into the blue part of the spectrum". This means that the red light emitted by ionized hydrogen is very hard for the human eye to see. In fact, I've read somewhere that not a single one of the glorious red nebulae that show up so so splendidly in photographs can be seen as red at all by human observers, when they watch these nebulae through a telescope!

But there is another color of nebulae, which is far easier for humans to see. Not blue, because blue is also somewhere in the "outskirts of human vision". No, I'm talking about green. Do you remember that I said that planetary nebulae often look green? That's because their cast-off atmospheres contain quite a lot of oxygen, and oxygen can also be ionized by ultraviolet light. So called "doubly ionized oxygen" glows with a characteristic green color, which is moderately easy for humans to see. And there is certainly a lot of oxygen in the Earth's atmosphere. And when the Sun becomes a white dwarf one day, as it must, it will be blisteringly hot and emit lots and lots of ultraviolet light. Then that ultraviolet light will ionize the Earth's oxygen atmosphere, provided the oxygen atmosphere is still there after the Sun has swollen to gigantic proportions when it became a red giant. And then, if there are still humans on the Earth after the Sun has become a white dwarf (no, I really don't think so) , we could all see the sky above us glow green! Fancy that!



A blue Sun in a green sky, the future of the Earth!

Ann