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In 2010: Odyssey Two Jupiter is turned into a star, but what kind of star is it turning into?

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    An artificial one. Not one on the main scale. – Valorum Dec 22 '16 at 12:32
  • @Valorum Wouldn't it follow that other stars we currently observe also be artificial? So even though it's artificial it might still fit into categories of observed stars. – Todd Wilcox Dec 22 '16 at 13:55
  • @Toddwilcox - Possibly, but there's nothing in the films/books to suggest it. – Valorum Dec 22 '16 at 14:03
  • @ToddWilcox Not really, no - the definitions of stars are based primarily on their mass, and Jupiter isn't anywhere close to the mass of a star. – Werrf Dec 22 '16 at 14:11
  • The book would indicate that it's a really bloody weird one; scifi.stackexchange.com/questions/93646/… – Valorum Dec 22 '16 at 14:17
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While it's common, especially in sci-fi and popular science, to call Jupiter a "failed star", it's rather a misleading description. Let's look at some stars...

Brown Dwarfs

The smallest objects science considers "stars" are brown dwarfs. These are objects that are not massive enough to fuse ordinary hydrogen in their cores, but can fuse deuterium, an isotope of hydrogen that has a single neutron in its nucleus (ordinary hydrogen doesn't have any neutrons). Of all stellar bodies, these are the closest to Jupiter's mass - but even the smallest brown dwarf has AT LEAST 13 times the mass of Jupiter.

Red Dwarfs

Next up in the scale, and arguably the smallest 'true' stars, are red dwarfs (no, not the mining ship). These range from about 0.075 solar masses to around 0.5 solar masses, and they're the smallest stars that exist on the Main Sequence.

For comparison, Jupiter's mass is 0.001 solar masses.

Mass is the critical component here. It's the gravitational energy from the enormous mass of even the smallest stars that compresses them enough to sustain nuclear fusion. Jupiter's mass isn't even close to high enough for that

If we can't use mass, how about temperature?

Well, there's bad news there, too...

Europa's average orbit around Jupiter is around 670,000 km. That would change a bit with the reduction in Jupiter's own diameter when it became Lucifer, so let's round up to something like 700,000 km. How hot would Lucifer have to be for Europa to be in its habitable zone?

Here is a habitable zone calculator. It returns the size in AU of the habitable zone for a star with a given temperature and luminosity. That unit, AU, gives us the first clue of how much trouble we're in. 1 AU is equal to the distance from the Earth to the sun, around 149,600,000 km. Europa's orbit equals roughly 0.004 AU.

I played with the numbers in the calculator a little bit. To get a habitable zone of 0.003 AU, I reduced the star's temperature to 1,000 kelvin, and 0.00001 times the luminosity of the sun.

These numbers are just in the range for a very cool, very dim brown dwarf (though most measures of luminosity don't go anywhere near that low). That's probably the closest to an answer we'll see.

The trouble is, of course, that Lucifer was supposed to light the night side of Earth; a very dim brown dwarf would be little brighter than the planet Jupiter, and wouldn't have anything like that great an effect.

It seems that there is no good answer - a star that could light the night side of Earth from Jupiter's orbit could not also sustain life on a moon as close as Europa. While Arthur C. Clarke's writing was generally very scientifically literate, on this one he just didn't get the numbers right.

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    It was visible from Earth during the height of daytime. That doesn't suggest "Brown Dwarf" to me. – Valorum Dec 22 '16 at 14:19
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    @Valorum It also didn't immediately incinerate Europa; going any brighter would do that. Unlike a lot of Clarke's writing, this one just really isn't feasible. – Werrf Dec 22 '16 at 14:20
  • Another problem is the mass/temperature balance. Fusion occurs when a mass of hydrogen collapses under its own gravity. The fusion heats up the hydrogen and creates an expanding force. This creates (an ever-changing) equilibrium. Without extra mass (as stated in the book) there is an upper limit to the temperature Lucifer could have, and thus an upper limit to the luminosity. There is no possible way that a Jupiter-sized mass could continuously generate that much light. – user45485 Dec 22 '16 at 14:51
  • @Hans it doesn't. About 100 years later, the star goes out (see the epilogue of the book mentioned in the comment). – Mark Gardner Dec 22 '16 at 17:53
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    @Hans Firstly the life on Europa was already well-developed by the time of 2010, but it couldn't develop any further because of the ice. Secondly, the original book's epilogue was 20,001, which gives the Europans much more time. Thirdly, there was a Monolith on Europa from the start, accelerating their development. – Werrf Dec 22 '16 at 20:19

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