# Drop a star in a river [closed]

I (as I am sure you will be able to tell) know very little about nuclear physics, so please understand if something here seems dumb or obvious.

I am going to question some Hollywood physics here. At the end of Spiderman 2, Octavius tells Spiderman to drop what essentially amounts to a tiny star into the river in order to extinguish it and prevent it from destroying Manhattan. Ignoring the problems of properly containing the fusion reaction on earth, getting it to begin in the first place, and all the other problems with fusion as an energy source that scientists are currently trying to sort out, I am curious about only this ending.

As I understand it (and feel free to correct me if I am wrong), fusion works by, at very high temperatures, taking two hydrogen atoms and fusing the nuclei together, creating one helium atom. There are two ways I can think that submerging a star in water could "put out" the star:

1. cooling the star to the point that the fusion reactions no longer take place
2. cutting off the star's supply of fuel (hydrogen)

Since I don't know much about nuclear physics, I'm just curious whether either of these would "extinguish" the star, or whether there is another factor I am not considering.

As a follow up, I am also curious what the consequence of extinguishing the star might be. would there be a tiny supernova, would the star collapse, or would there be some other kind of death? You can get an idea of the size of the star from this video.

EDIT As Ali points out in the comments, a star that small probably could not exist at all. For the sake of discussion, I am amending the question as such:

Imagine the star in question is the mass and size of our Sun, and that somehow, somewhere, there is a body of water large enough to submerge the Sun completely.

• One should first ask, whether stars of that size will ever form? Just to give a intuitive hint, Jupiter(the big old planet) is low by $30\%$ mass to form the smallest known stars. So I'm not sure if the rivers Alpheus and Peneus would be sufficiently large for that goal. $:)$ – Ali Jul 24 '13 at 1:16
• @Ali Certainly a good point. This probably makes it difficult, or impossible, to know for sure what would happen. For the sake of the theoretical discussion, though, I am going to amend the question. – ewok Jul 24 '13 at 1:20
• Surface temperature of the sun is $\sim$6,000 K (and gets up towards $10^7$ K at the core; boiling point of water is $\sim$370 K. Pretty sure the star would evaporate all of the water before the star would be submerged. – Kyle Kanos Jul 24 '13 at 1:51
• – Nathaniel Jul 25 '13 at 2:09
• IMO this is off-topic because it is a question about fictional physics. It has too many counterfactual ingredients to allow a scientific answer. Voting to close as off topic. – user4552 Jul 25 '13 at 2:29

Just few more points.

• A blob of water few times larger than sun would already be a supergiant.(I'll ignore this in the rest of my answer)
• The water will not stay still; it will be attracted by star's gravitational field. This will cause the star's mass to increase.
• The water which is falling on the star, even if it's initially freezing cold, will heat up dramatically due to the gravitational potential.
• Even if the water lowers the temperature instantly to just few hundred kelvins, which may instantly stop the fusion, after the star begins collapsing the temperature will increase rapidly and fusion will start over again shortly.
• Depending on how fast the star enters the cold blob of water, I can consider two possibilities:
1. rapid sinking: The layers of water closer to the star will evaporate, while the rest of the water droplet! is still liquid. This phase transition and the high pressure following it, will shoot the rest of the water out.
2. adiabatic sinking: As I said earlier, the water will pour into the star increasing its mass. This will certainly change the star's evolution and death; shifting from white and black dwarfs towards neutron stars and blackholes.

To summarize, I don't think dropping a star in a river can halt its fusion; it will possibly cause either of the above possibilities.

Stars have multiple phases to their life cycles, but the typical phase one thinks of is known as the "main sequence."1 This is defined by the stage when nuclear burning of hydrogen is balancing the gravitational tendency to collapse. That is, the outer layers of the star would fall toward the center but for the fact that there is a pressure gradient, and that pressure gradient is due in large part to the fact that energy is being released in the center of the star.

This phase contains an excellent example of a negative feedback loop in nature. If the star contracts, the plasma will heat up (gravitational potential energy is converted into thermal energy). Nuclear reaction rates tend to be very temperature-sensitive, and in particular the processes by which hydrogen is converted to helium will proceed much faster at higher temperatures. With more power being released in the center, the outer layers of the star are pushed outward, countering the contraction. And so the star is in a stable equilibrium configuration.

If you were to remove most of the heat all at once, fusion would shut off and the star would begin to collapse. For a star the size and mass of the Sun, this collapse proceeds at a rate such that, if unchecked, the star would become a black hole in roughly half an hour. Some people find that figure surprisingly short, others surprisingly long.

But, at some point the collapsing matter would simply reheat itself. This is again a natural consequence of anything that compresses under its own gravity. Once the core attained $10{,}000{,}000\ \mathrm{K}$ or so, the collapse would reverse, and the star would settle back into its previous state.2

This all assumes your magic heat sink was one-time use. If it continuously kept the temperature low, the collapse would not be halted until the density gradient of the star could create a large enough pressure gradient, which for the Sun would be known as a white dwarf.

1 The name comes from the fact that on a scatter plot of color versus luminosity, one tends to see most of the points forming a curve - a one-parameter sequence, if you will - through the middle of the diagram.

2 Though this could very well take millions of years - one would have to do a slightly more detailed calculation to figure out the timescale.

• Thanks for the explanation. As I understand it, and you can correct me again if I am wrong, the answer is basically that the water MAY cool the star and prevent further fusion from taking place, at which point it would collapse. But under the collapse, the pressure increases, causing a proportional rise in temperature that simply restarts the fusion. Therefore the water fails to extinguish the star. – ewok Jul 24 '13 at 2:20
• Also, as this is a theoretical question about impossible circumstances, I'm hesitant to mark this as correct. At least right now. if it remains the best answer in a few days I will – ewok Jul 24 '13 at 2:22