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Why Hydrogen doesn't get solidified in the outer space?

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Why Hydrogen doesn't get solidified in the outer space?

Because outer space is not cold and dense enough for hydrogen to solidify. It's true that a macroscopic thermometer in outer space would register a temperature of about 2.7 kelvins, but that's because the thermometer wouldn't be registering the temperature of the very sparse medium.

Much of outer space is quite hot at the molecular level. The temperature of the gas in the space between galaxies is very hot, in the millions to hundreds of millions of kelvins. Over 99% of the gas within a galaxy is warm to very hot, with temperatures ranging from hundreds of kelvins to tens of millions of kelvins.

A few interstellar gas clouds are quite cold, with temperatures in the tens of kelvins. But even in those cold interstellar gas clouds, it's still too hot for hydrogen to solidify. Hydrogen's triple point is about 14 kelvins at a pressure of about 0.07 atmospheres. The pressure in those interstellar gas clouds is much, much less than the triple point pressure. While the temperatures of those cold, dense interstellar gas clouds are about the same as hydrogen's triple point temperature, the pressures of 10-18 atmospheres or less are much, much smaller than is hydrogen's triple point pressure. Hydrogen's sublimation temperature at those pressures is less than one kelvin.

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Well, let's just consider this for a minute. Hydrogen turns into a liquid below 33 Kelvin, or around -252.87 degrees Celsius. But we're not looking for liquid hydrogen, we're looking for solid hydrogen. Hydrogen becomes solid at 14.01 Kelvin, or around -259.14 degrees Celsius.

The temperature of space on the other hand varies quite a bit. For instance, the coldest it gets (cosmic background radiation) is 2.7 Kelvin, or around -270.45 degrees Celsius. However, the solar wind and gas between stars can be as hot as tens of thousands of degrees Celsius. So obviously temperature varies quite a bit.

However, let's say you put a thermometer Earth's distance away from the sun, with half of it facing the sun and half facing away from the sun. That thermometer would measure about 7.2 degrees Celsius, which is obviously much warmer than it would need to be for hydrogen to turn solid.

Another thing to consider here is that the coldest parts of space are also the emptiest parts of space (in most cases, anyway). If it is cold enough for hydrogen to turn solid, it is very likely that there isn't any hydrogen to turn solid there.

Hope this helps!

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    $\begingroup$ Re Hydrogen turns into a liquid below 33 Kelvin -- That's the critical point. The boiling point at one atmosphere of pressure is 20K. The triple point is about 14K and a pressure of 0.07 atmospheres. The pressure in space is much, much less than 0.07 atmospheres. I think I need to make this comment into an answer. $\endgroup$ – David Hammen Nov 19 '16 at 20:07
  • $\begingroup$ @DavidHammen, you should; I hadn't even considered that aspect of it. Thank you for bringing it up! $\endgroup$ – heather Nov 19 '16 at 20:31

protected by Qmechanic Jul 1 '17 at 5:56

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