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?