Expert: Ed - 1/11/2005

Question
Hi Ed, thanks for taking the time to read this...

I'm writing a short sci-fi story, and I wanted to get some accurate info into it rather than just guessing at distances and such. my question is this:

Around different kinds of stars what would be an approximate distance for the radiation/temperature levels to be condusive to humanoid life similar to us? Obviously I appreciate that there are a lot of other factors to take into consideration such as the type of atmosphere around a given planet, whether the planet has an internal heat source and at what point in it's life the star is at etc., but I'm after an approximate answer rather than anything exact. Primarily I'm interested in Blue/white/red supergiants, blue/white main sequence stars and white dwarves.

Many thanks,
John.

Answer
John,

Sounds like a cool story.  I am sorry I'm late getting back to you.  I actually should have been temporarily off the AllExperts site, because I was on vacation.  Probably I goofed somehow, and didn't put myself on vacation correctly, from my experts login site.  Dang.

Anyway -- Kasting, Whitmire, and Reynolds
http://www.matessa.org/~mike/habitable.html

have theorized that stars of spectral type F would have a habitable zone approximately 1.5 to 3.5 astronomical units from the star in question.  An astronomical unit, as you probably know, is equal to the distance from the Earth to our sun, which is generally given as roughly 93 million miles.  So, for blue or white supergiants, you would probably need to position your planets at LEAST 350 million miles from their stars.  Maybe even 400 or 500 million, depending upon mass, etc.

Blue-white main sequence stars could have habitable planets at similar distances.

Red supergiants are much cooler, as you know.  You could get away with having planets a heck of a lot closer to their parent stars, for there to be a chance of habitability.  The authorities I mentioned above seem to concur that you could plausibly hypothesize about habitable planets existing from (roughly) 10 million to 100 million miles from such stars, also depending upon variables such as atmospheric composition, etc.

White dwarfs (or "dwarves," using your pleasantly Tolkienesque spelling) are an interesting scenario.  I love this article.  Check it out:
http://www.space.com/scienceastronomy/astronomy/born_again_020207.html

Here is the most important part of that website, in case the link doesn't work:

"But life around a white dwarf would be excruciatingly difficult.

In addition to the low luminosity of a white dwarf, its output changes constantly over time as the dying star cools off. In fact, it might be one of the few places where advanced life stands a better chance of surviving than the microbial variety, suggests Zuckerman.

"Any habitable zone would have to be very close to the white dwarf and moving closer as time advanced," Zuckerman said. "I think the best chance for life would be little green men living in a giant spacecraft orbiting close to the white dwarf, and they move inward as the white dwarf cools. They can still gather raw materials, if need be, by traveling outward to objects -- planets, moons, whatever -- that were not destroyed when their white dwarf was a red giant."

Also...

You need to take some serious time and look through this awesome website.  It's perfect for you.
http://www.stellar-database.com/lwdtr.html

Before I forget, check out this Cornell website:
http://instruct1.cit.cornell.edu/courses/astro101/lec33.htm

Look near the bottom of that website.  There's lots of good stuff.  Maybe you could even e-mail the lecturer!

Beyond that -- you should try googling the phrase "habitable zone," or also "ecosphere."  You will get loads of good stuff that way.  I just picked the Cornell website because I thought it had some particularly useful information for you, but of course I don't even know what your story is about, so I could be wrong.

If you're feeling adventurous, here is the website of James Kasting, whose research I checked out in attempting to answer your question.  You could e-mail him, I suppose.  He might take issue with some of what I'm telling you here, but I guess debate is always a good thing!
http://www.geosc.psu.edu/~kasting/PersonalPage/Kasting.htm

I hope that this is useful for you.  Let me know how things go.  Keep Looking Up!
--Ed