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Comment by junkblocker | Hacker News Reader

Reminds me of this BBC infographic on hottest temperatures in the universe, http://www.bbc.co.uk/bbc.com/future/bespoke/20131218-tempera... , very enlightening.

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Great infographic, but unfortunately it misses my favorite juxtaposition of temperatures:

Surface temperature of a red dwarf star (e.g. Wolf 359) 2500 C

Melting point of tungsten: 3400 C

I find the idea of making balloon-like objects out of tungsten and gas, with a density less than that of the star's photosphere, and floating them around on the surface of a star to be intriguing. It would be a great location to put a heat engine. A totally sci-fi idea, I know, but still interesting to think about.

Umm, actually, a "heat engine" needs both the source of heat and a place to dump the waste heat into, so the surface of a star would be a bad place to build one.

The only way of dumping heat would be to radiate it out to space, but "radiation" is a very inefficient way of losing heat. Unless I'm missing some really clever trick, soon your radiator will become about as hot as the surrounding gas, at which point the efficiency (= (T_hot - T_cold) / T_hot) drops to near zero.

* Not a physicist, so I might be wrong. :P

You are correct that a heat pump needs both a heat source and heat sink. However, radiation is quite effective at transmitting heat at high temperatures. Black-body radiators emit as the fourth power of the temperature, although real objects never emit quite as efficiently as ideal black-bodies. It works pretty well for stars, though, since radiation is how they lose the vast majority of their energy.

If the photosphere above the heat engine was opaque, then the heat engine would not work. So it makes sense to keep the heat engine near or above the top of the photosphere, without going high enough to overheat near the top of the chromosphere.

It's mainly just a fun idea. I have no plans of trying to build one in the near future. :)

* I am a physicist, but that is no protection against being wrong. ;)

FYI David Brin explored this idea in the novel 'Sundiver' (1980)[1]. In the novel a laser is used to radiate heat out into space. This is the first book of the Uplift Trilogy. Incidentally, the second book, 'Startide Rising' is really great imo, and won a bunch of awards.

[1] https://en.wikipedia.org/wiki/Sundiver

Aye, actually Sundiver is my favorite of David's books. I think his writing is better in later books, but I enjoy the audacious technical ideas and the classic 'closed room mystery' plot.

I met David earlier this year at the NASA NIAC symposium, and spent a nice afternoon hanging out with him and Joe Haldeman and his wife. Very nice people! We toured the Swampworks and launch sites at KSC, and talked about practical methods for moving planets. It was a very enjoyable day.

Very cool. You seem to be familiar with him and his work, so I'll ask this; I seem to recall that him and Vernor Vinge have sidelines doing 'scenario planning' for government agencies, is that something you've ever heard of? I've always wondered what that consisted of. Maybe I'm just searching for a reason to explain Vinge lack of productivety and imagined that?

I'm sorry. but I don't know much about that. However, you could ask them about it. Most sci-fi authors I have spoken with are very open to answering questions, as long as they are addressed in a respectful manner. I don't know about Vernor Vinge, but I know that David Brin has a website that you could email him at.

The outer atmosphere of a star can be much hotter than the surface, so you'd have some trouble getting it in place.

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