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chemistry tells us that when mixing hydrogen and oxygen the result is an explosion and water.

my question is: does this happen on astronomical scale? for example, did they ever observe planet-size hydrogen cloud collide with planet-size oxygen cloud to produce a huge explosion and planet made primary of water?

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    $\begingroup$ I'm going to take a wild guess and say that's never been observed. $\endgroup$ – userLTK Jun 21 '17 at 22:29
  • $\begingroup$ Oxygen rare in space. I doubt there has ever been a cloud of oxygen discovered. $\endgroup$ – jmh Jun 21 '17 at 22:39
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Hydrogen and oxygen only react when there is sufficient energy. For instance, the autoignition temperature of hydrogen at 1 atmosphere is 536 °C. This is why you can do that experiment with mixed hydrogen and oxygen in a balloon, that only explodes when you touch the balloon with a lit taper.

Space is cold. Molecular clouds have temperatures in the tens of degrees Kelvin. In the rare (if not impossible) case that a molecular cloud of pure oxygen did exist, and it encountered a molecular cloud of hydrogen, there wouldn't be enough energy for an explosion.

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  • $\begingroup$ Is there really such a thing as a molecular cloud of oxygen? I thought they were almost 100% H2. $\endgroup$ – Mike H Jun 22 '17 at 3:19
  • $\begingroup$ That was why I said it would be rare if not impossible, because they probably don't exist. I didn't get that across very clearly though, sorry. $\endgroup$ – hartacus Jun 22 '17 at 4:25
  • $\begingroup$ Edited for clarity $\endgroup$ – hartacus Jun 22 '17 at 4:27
  • $\begingroup$ Temperature is not the issue. While molecular clouds have temperatures of tens of Kelvin, HI and HII clouds have hundreds, and tens of thousands of Kelvin. You just don't have a cloud of oxygen. $\endgroup$ – pela Jun 22 '17 at 7:40
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    $\begingroup$ Heavy elements, or "metals", occur in a mix of gas, usually in a mass ratio of roughly 1:100 wrt. hydrogen. Of this 1% only a fraction is oxygen. In cold clouds, oxygen may form oxygen molecules, just as other molecules will form, but it will still be mixed with a much larger fraction of hydrogen and helium. So, when you see an image of some interstellar cloud showing, e.g., oxygen, it's not because it's a "cloud of oxygen", but because the cloud contains oxygen that happens to emit light due to some physical process that makes oxygen emit light, e.g. collisions or ionization/recombination. $\endgroup$ – pela Jun 22 '17 at 11:35
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While an oxygen ball of gas isn't possible, an Oxygen rich atmosphere on a planet, through photosynthesis is possible. Earth for example. A hydrogen rich planet (Jupiter or Saturn) are also possible, as is a methane rich atmosphere (Titan).

If two bodies like that were to collide, you probably would see an enormous flame by the oxygen and fuel combustion process. That said, the gravitational impact would generate far more energy than any chemical combustion. If you were to collide two planets with atmospheres that together could combust, I'm not sure it would make much difference than two planets combining with atmospheres that wouldn't combust. The impact is what would stand out.

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