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parallaxicality
2008-Apr-09, 09:34 PM
I've never really been a fan of Anne McCaffrey; I find her style too earnest. But I've found myself in the middle of a most peculiar argument about her planet, Pern.

For those of you who don't know, Pern is a planet around the star Alpha Sagittarii. Within its system is another planet, the Red Star, that has a highly elliptical orbit that lasts for 250 "turns", or Pernian years. When it reaches perihelion, the Red Star sends a fungus called thread onto the surface of Pern which decimates everything it touches.

And here's where the argument comes in. Apparently McCaffrey classes the Red Star as a "Sedna class inner Oort cloud object". Whether it is or not depends, naturally, on how long a "turn" is. For the Red Star to have a year as long as Sedna's, a Pernian "turn" would have to be 40 years long. And thankfully, that's where I know I can count on you guys.

Alpha Sagittarii, or Rukbat, is 2.3 solar diameters across, 3.2 solar masses, and 112 solar luminosities. So, all I need you clever science types to do is figure out where the star's habitable zone is, and then we can figure out how long Pern's year is.

If you do this for me I'll be very very nice.

Jason
2008-Apr-09, 09:55 PM
Well the real problem is that the real Rukbat is a B8 star, not a G-type like it is in the books.

korjik
2008-Apr-10, 06:25 PM
That and unless the inhabitants have much longer lifespans than today, the turn is close to a year long, judging by the ages of the characters.

Basically, the red star has to be a planet on a very elliptical orbit at a resonance with pern so that the orbits would misalign and give the long intervals between threadfalls.

Noclevername
2008-Apr-11, 06:36 AM
Inner Oort cloud? Sheesh, can't someone throw the poor Kuiper belt a bone before skipping all the way to the Oort cloud?

Jason
2008-Apr-11, 03:04 PM
According to The Dragonlover's Guide to Pern, a Pernese year is 366 Earth days (362 Pernese days, as they are slightly longer than 24 hours).

eburacum45
2008-Apr-11, 05:01 PM
Using this site
http://www.geocities.com/area51/corridor/8611/hard-sf.htm
one can calculate that for a star 112 times as luminous as the Sun, a planet at 10.58 AU would receive the same amount of light as the Earth; its year would be 18.27 Earth years long.

But as Jason points out, Rukbat is a B-class star, putting out most of its light in the UV range. The UV would likely be so strong as to be fatal to plants and animals. It also means thaat visible light would be a smaller component of the luminosity, so the world would be noticably darker on the surface.

korjik
2008-Apr-12, 09:48 PM
Using this site
http://www.geocities.com/area51/corridor/8611/hard-sf.htm
one can calculate that for a star 112 times as luminous as the Sun, a planet at 10.58 AU would receive the same amount of light as the Earth; its year would be 18.27 Earth years long.

But as Jason points out, Rukbat is a B-class star, putting out most of its light in the UV range. The UV would likely be so strong as to be fatal to plants and animals. It also means thaat visible light would be a smaller component of the luminosity, so the world would be noticably darker on the surface.

If I remember right at any specific frequency, a higher temp blackbody curve has a higher luminosity. The planet would still be brighter but a much higher percentage of the light would be uv.

eburacum45
2008-Apr-13, 01:46 PM
Well, only if you consider invisible light as bright. To warm a planet up to an Earth-like temperature we have to consider the total, or bolometric luminosity of the star; for a B-class star most of that luminosity would be in the ultraviolet range; this means the visible component of the light would be less.

Similarly a red dwarf would give most of its light in the infrared, so the visible component would be dimmer. By a not-so-strange coincidence, the visible component of a star's light as seen from a planet in the habitable zone is brightest for a G-class star.

Under a B-class star any flourescent surfaces might glow noticably; I would certainly recommend shades and factor million suncream.