Regarding David’s column on Are the stars really all burned out?, he mentions tha if the Sun were to stop shining tomorrow at 5:00 AM, we wouldn’t know it for a while, since the light takes a small amount of time to reach us (about 8 minutes, in this case). There’s another effect, too, which is much more significant, depending on what you mean by “the sun burning out”. If the fusion powerhouse in the core of the sun were to suddenly stop for some reason, we actually wouldn’t know it for a couple of million years. Although the photons from the center of the sun would only take a couple of seconds to reach the surface, if they could do so in a straight line, what they actually do is a random walk, travelling only a few centimeters at a time before being absorbed and re-emitted in a random direction. Of course, if by “burn out”, you mean “stop shining entirely”, then David’s answer is correct, and for extremely distant stars, the time to reach us is much greater than the time to reach the surface
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe”
–A. Einstein
I think that, when he talks about the sun being “burnt out”, David meant that the sun gets tired of doing its job, gets depressed, swallows cartons of St Johns Wort, and has a midlife crisis, probably dumps his wife for some bimbo, and buys a Porsche. That sort of burn out.
Straight dope science board and Chronos dropped the ball a bit on this one. From Earth, we can only see stars that are relatively close to us. I’m not sure of the exact maximum distance, but it’s probably not more than a few thousand light years for most stars. Our galaxy is about 120,000 light years across and we can’t distinguish stars on the other side of it. Any really distant objects that we do see are galaxies, not stars. That being said, given a reasonable lifespan of around 10,000,000,000 years for a smallish star like our sun, it is very unlikely at any given time that a star close enough to be in our viewing area has ‘burned out’. It will happen, but so rarely that you are very unlikely to even once in your lifetime look at a star that is no longer there.
I agree with what you said, but some of the numbers you mentioned need some amendment.
While it’s true that a medium bright star like the sun has a lifespan of 10 billion years, most of the stars we see in the sky are much brighter and have much shorter lifespans. The very brightest (intrinsically) have lifespans of only half a million years. An A-type star like Sirius has a lifespan of about 2 billion years. Most of the stars we see are either A-type or brighter if they are main sequence, or else they are giants or supergiants, in which case they are in one of later stages of their evolution (nearer extinction than a main sequence).
But while this ups the probability that a naked-eye visible star has burnt out, it doesn’t raise it enough to change your conclusion.
“If the fusion powerhouse at the core of the sun were to stop suddenly”, the outer layers of the sun, having lost the themal pressure and support from the core, would fall inward, rapidly heat, and set off a supernova. After the 8 minutes or so travel time, we would know it for about half a nano-second before we would be reduced to component atoms.
The “thermal pressure” would not instantaneously, or even very rapidly, disappear. All the heat in the interior would still have to find its way to the surface and radiate away. This would take millenia. And until it did, the pressure generated by the heat would remain.
A supernova occurs because the fusion process in the star’s core suddenly accelerates, not because it suddenly stops.
“I’ll tell him but I don’t think he’ll be very keen. He’s already got one, you see!”
I don’t think so, Pluto. I agree that it normally takes a long time for a photon to work its way to the sun’s surface. But if the sun is in a state of equilibrium where the thermal pressure from fusion processes just balances the gravitational pressure, by turning off the furnace, it would seem that there would have to be a collapse.
My understanding of the supernova process is that the ultimate fusion process is the “burning” of silicon into iron. Any further fusion requires energy rather than giving it off. The iron at the star’s core is next squeezed into neutrons, precipitating a rapid if not instantaneous collapse of the core. The “floor” is pulled out from under the upper layers (which still contain H, He, etc) and they fall into the 3 billion degree Kelvin neutron core at a speed of up to .15c. The material bounces off the neutron core (you can imagine solid neutrons are very elastic). The shock wave goes back out through the star. The result is a supernova.
This requires a stellar mass of 25 times that of the sun. And since the core of our sun is only 15 million K, there would not be a supernova. But I still believe that, without continual fusion going on, there would be a collapse and 6000K surface material falling into 15 millionK core material should set off at least a so-sonova.
At the very least you would almost instantly detect the increased neutrinos, at the most you would see solar material blown out to the orbit of Mercury.
Of course, in order to really say what would happen if the Sun abruptly “turned off”, you’d really need to specify how it turned off. It’s not really meaningful to ask what would happen if fusion stopped over a short time period, because that can’t happen for a Sun-like star. What actually will happen, to the best of our knowledge, is that as fuel grows scarce, the Sun will gradually (over the course of maybe a billion years or so) cool off and expand into a red giant, puff off its outer layers in a planetary nebula, and leave a relatively quiet white dwarf at the core.
“There are only two things that are infinite: The Universe, and human stupidity-- and I’m not sure about the Universe”
–A. Einstein
I’m gonna have to stand up for my ironclad reasoning in this case, mipsman. The pressure resisting gravitational collapse is derived from the temperature of the interior. If all heat sources were suddenly turned off the temperature would not suddenly drop to zero. It would gradually be reduced and gravity would shrink the star to some small size. The crux of the matter is that this temperature reduction would take a long time since the heat would still have to escape the interior to be radiated away before the collapse could proceed.
Your contention, I think, is that the loss of heat sources would disrupt the equilibrium that exists between the gravitational and thermal pressures. So it would, but departure from equilibrium does not imply catastrophe. In fact, true equilibrium does not, in fact, exist, since the sun is slowly losing the anti-gravity battle through consumption of its resources. The balance of the forces would shift, certainly, but we still can’t suspend the laws of thermodynamics. The heat can’t instantaneously disappear even though we’re supposing the source of heat can.
Nuclear processes are not necessary to resist collapse. In the initial formation of the star the temperature of the accreting hydrogen rises due to conversion of the potential (gravitational) energy of the infalling matter. This non-nuclear heat is sufficient to slow (but not prevent) the formation of the star until the interior pressure and temperature rise to the fusion ignition temperature. But throughout all this there is only one way for the star to get rid of excess energy and that is through radiation.
Having said that I will concede a part of your argument. Photons aren’t the only by-product of nuclear reactions. There also exists an interior pressure due to the neutrinos generated by the fusion process. IIRC, this is much smaller than the thermal pressure, but perhaps it is not negligible. Assuming that the neutrino pressure is significant and that it suddenly ceased there could be some catastrophic effects.
The missing element in this hypothetical sudden-shutdown case that is present in the supernova case is that the nuclear reactions change the number of particles involved in the equilibrium. Every helium atom formed reduces the number of particles in the sun by three. As the reactions switch to the larger atomic numbers this reduction in particle count accelerates. Additionally the reactions themselves proceed at faster and faster rates. By the time carbon and oxygen are being consumed, they burn so rapidly that they lead to the catastrophic collapse you described. The bottom falls out, not because the interior got cooler (in fact, it’s hotter than ever), but because the number of particles available to sustain the pressure is greatly reduced. According to the ideal gas law: P = NRT/V. “N” is the quantity suddenly changing here.
I have a textbook describing the evolution of stars at my desk at work. (I’m at home at the moment.) I’ll take a look tomorrow to be sure I haven’t told any whoppers here. But deep in my heart I know I’m right!
Isn’t there a theory that the Sun actually HAS gone out? I seem to recall reading this in an article about the puzzling lack of neutrinos in that collector made out of a flooded mine. The theory proposed that the sun goes out, gradually descreasing pressure causes it to start to collapse, and the energy released and increase in pressure re-lights the fire. Thus you get a variable period of gravitational collapse causing fusion, which causes gravitational expansion.
Having made a quick search of Stellar Structure and Evolution, by R. Kippenham and A. Weigert (Springer-Verlag 1990), for information pertaining to this topic, I would just like to say that, according to this text, anybody who makes any generalizations whatsoever about stellar collapse is probably wrong. There are so many exceptions and counter-considerations to any statement they make that it’s just about impossible to draw any conclusions.
I did however come across this gem on page 339:
I’ll take that as confirmation of my previous speculations no matter what errors I made in details. (And, apparently, there were many of those.)
In particular my description of interior pressure loss due to fusion and subsequent reduction in the number of particles was just so much bunkum. As near as I can tell, the primary mechanism for the onset of the catastrophic collapse that leads to a supernova is a phenomenon known as “carbon flash”, where the carbon (or carbon-oxygen) shell created by previous fusion is consumed almost instantaneously. The actual process is beyond my understanding. The explanation given is rife with terms like “relativistic electron degeneracy”.
The shock wave created by this reaction compresses the core and blows off the outer layers of the star. It is only through this sort of compression that densities high enough to produce a neutron core are reached. Throughout all this other reactions are proceeding furiously, allowing the formation of elements of atomic numbers higher than iron, which would not normally occur because, as was pointed out above, these reactions are endothermic.
“We are stardust, we are golden,
We are three-billion year-old carbon.”
Little known fact, but in the Garden of Eden, there was an ON/OFF switch for the sun, to turn it on in the morning and off at night. That’s what the question meant, of course, none of these super-nova myths or nothin’ like that.
The eight-minute lag, every morning when the sun was turned on, and every night when the sun was turned off, built up during the period of life in the Garden… to exactly one full day. This was the day that was accounted for when Joshua later stopped the sun, so that the calendars would work out.