Preparing for a talk that will include Proxima Centauri, it occured to me that it might be of interest to some folks here. The European Southern Observatory has used their new VLT interferometer (VLTI), which went into operation last fall, to measure the angular diameters of a few red dwarf stars, including Proxima Centauri. See their online press release How Small are Small Stars Really?

Proxima Centauri has a mass of only 0.123 solar masses, where 0.08 solar masses is about as small as a star can be (where "star" means that its able to fuse hydrogen into helium on the main sequence). And its size is only 0.145 solar radii. But a comparison with Jupiter is interesting. Jupiter has only about 0.001 solar masses, so Proxima is 123 times more massive. But the radius of Jupiter is 0.102 solar radii, so Proxima is only about 1.4 times the radius of Jupiter, about 2.7 times the volume of Jupiter, despite being over 100 times more massive.

I figure it has to be the self gravity of that 123 Jupiter masses. If it weren't for the outward pressure from the heat of fusion in the core of Proxima, it would be even smaller than Jupiter just from its own weight (bigger mass, smaller volume, much bigger density).

The ESO site also has some images & data on the Alpha Centauri system. The A & B components are nearly sun-like, "A" being 1.1 solar masses, and "B" being 0.9 solar masses. But I didn't realize they were quite so close, they come within 11 AU of each other, a little bigger than Saturn's 9.5 AU orbit around the sun, well inside the sun - Uranus distance of about 19 AU. So there can't be stable orbits around either star, much beyond 2 AU from the star. Proxima hangs out about 13,000 AU from the pair, on an orbit with a period of 500,000 years, if it is in orbit at all. Could be, since it seems to share in the proper motion of Alpha A & B, but it's far enough out that it might not be strongly bound.

Proxima has an apparent magnitude of +11.05, and an absolute magnitude of +14.44. If you put it in place of the sun, it would have an apparent magnitude of -17.12, compared to -12.5 for the full moon or -26.77 for the sun. I figure the heat we would get from Proxima in that case, would be about the same amount of heat delivered by a 100 Watt light bulb at a distance of 30 feet. Definitely makes for a chilly planet. To be comfy around Proxima, we would need to orbit at 0.007 AU, which is only about 1.5 solar radii, or about 10 Proxima radii. But it's a flare star, as many M-dwarfs are (Proxima is class M5Ve), so at that distance we would have to contend with that too.

Cheers.

We would also be tidally locked, so we wouldn't actually be very cold.

The Alpha Centauri system (including Proxima), in the Internet Stellar Database:

http://www.stellar-database.com/Scri...e?Name=Proxima

I have it on good authority that the site's maintainer will be updating the diameter of Proxima to match that given by the ESO's recent experiments within the next day or so.

FWIW, Tim, I wrote an article for Astronomy magazine about a planet orbiting a red dwarf like Proxima. The numbers I got surprised me too. I found an Earthlike temperature at a distance of about 6 million kilometers, a bit larger than what you found. I used a luminosity of 0.00006 solar, and a diameter of 0.2 solar. I think I may have used an M0 dwarf, whereas Proxima is M5, which might make the difference. The numbers change rapidly at that end of the main sequence.

I wish the article could have been longer; given the extrapolationsI was using, I could have had 3000 words easy. I had to strip it down to 800.

I came up with a visual comfort zone distance for Proxima Centauri of only 1.1 million kilometers. But that's kinda cheating; the visual comfort zone distance is the distance at which the star would have the same apparent visual magnitude as the sun does as seen from the Earth (-26.72). What we really want is the temperature comfort zone distance, i.e. the distance at which a planet orbiting that star would receive the same amount of energy as the Earth does from the sun. The visual comfort zone is a fine-and-dandy approximation of the temperature comfort zone for a sunlike star, but the farther a star is away from good ol' spectral class G2 the farther off this is going to be.

I just wish there was an easy way to calculate the bolometric correction factor for a star, given, say, only that star's B-V/U-B/R-I color indices.

This New Scientist article discusses the possibility of life around red dwarfs.

Good artcle Crimson. BA, do you know where we can find your one?

__________________
Freedom For Fission A breath of fresh Iodine-131

From the above-linked New Scientist article:
They can do a hell of a lot worse than just increasing the mutation rate. The flares from these flare stars can emit thousands of times as much X-ray radiation as flares of the same size on our sun. If your planet is orbiting only a few million km away from one of these dim red dwarf flare stars so that it can catch enough light between the flares so that it won't freeze, these X-ray-intensive flares could do all sorts of terrible things to any life that happened to be out and exposed on the planet's surface.

Life forms living on such a planet would have to live either underground or in the ocean to avoid these X-ray onslaughts. Either that, or be able to burrow underground quickly whenever they saw a flare coming.

It would be interesting to see land animals evolve on the hypothetical planet to anticipate flares and run for cover. Maybe their skin could sense the radiation buildup that would indicate a solar particle event is about to occur. Isn't there a thing about some animals being able to detect incoming weather with greater accuracy than any of our instruments? Like cows sit down on their patch of grass just before rain. These alien animals could be the key to solar flare prediction.

I would think that a good Enterprise is enroute with this news, but there is about as much chance of the Creators making a good, creative episode based on this interesting idea as there is of the Dark Lord declaring his undying love for Jim Lovell.

__________________
Freedom For Fission A breath of fresh Iodine-131

I was trying to find out how much atmosphere you'd need to block the X-rays from such a flare. No luck, perhaps one of you know. The site i did find said that the atmosphere blocks X-rays as effectively as 5 meters of concrete. So, if the atmosphere was roughly terrestrial and there's still a problem, your critters are going to have to burrow maybe 10 meters underground to dodge the flare's effects. That's going to be nasty. Plants would need to keep their genetic material buried deep--only leaves on the surface. Leaves to be discarded and regrown after a flare.

Any idea as to the mean time between flares? If it's months, you could get by with animals with short lifespans that lay eggs in protected areas If it's days or less, you get mostly subterranean animals.

The flare events on Proxima Centauri are irregular, but a flare is bound to come along every couple of hours or so. Each flare lasts for only a few minutes. Occasionally, two flares can be active simultaneously.

Of course, as flare stars go, Proxima Centauri is peanuts compared with UV Ceti.

Did you notice these numbers?

Star A:
Comfort Zone: 0.00749 A.U.s
Orbital period in CZ: 16.4112 hours
Angular size of star in sky in CZ: 10.673077 degrees

Star B:
Comfort Zone: 0.00706 A.U.s
Orbital period in CZ: 16.4334 hours
Angular size of star in sky in CZ: 10.576152 degrees

YOW!!!

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Fortunately, those comfort zone distances are actually "cheats." They're based on the distance at which the star would appear as the same visual magnitude as the Sun does in the skies of Earth (-26.75). Red dwarfs emit a lot more of their energy at red and infrared wavelengths, so their visual magnitude doesn't accurately reflect all the energy they put out. (To get the proper figure we'd need the star's Bolometric magnitude, but there's no easy way to calculate that -- you can only make an educated guess based on the star's spectrum.)

The real comfort zone distances for those stars might be as much as six times the figures quoted above, in which case the orbital periods for planets at the real comfort zone distances would be on the order of a whole whopping 10 days instead of a paltry 16 hours.

Edit after reviewing a table of guesstimated Bolometric correction factors I scrounged up somewhere:

According to this here table, if I shoehorn Proxima Centauri into it, I get a bolometric V correction factor of about -2.5. This would be about what the two UV Ceti stars would use as well. If their bolometric magnitudes were 2.5 brighter than their V magnitudes, this would mean they'd put out 10 times as much energy as their V mag. alone would suggest. That means their "real" comfort zone distances would be about 3.2 times their visual comfort zone distances, not the "six times" figure I used above. That would give any planets at those distances an orbital period more like 4 days.

Thanks tracer. That makes more sense.

__________________
Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

The operator of the Internet Stellar Database is sorry for the inaccuracy of his Comfort Zone modelling, and to make sure this kind of misunderstanding occurs less often in the future, he has updated his star details display generators to say "Comfort Zone (visual):" instead of just "Comfort Zone:".

He also informs me that if he had a formula for converting B-V/U-B/R-I color indices into bolometric correction factors, he would integrate it into his database code so that he could calculate the real comfort zone distances. (Hint, hint.)

If your star was ten degrees in size, that would be cool.

__________________
Freedom For Fission A breath of fresh Iodine-131

Here's another red dwarf article, which recently appeared in Sky & Telescope.

Crimson just noticed that there is an error in the ESO Press Release. It gives the wrong year of Proxima's discovery. The correct year is 1915.

Reference: Innes, R., 1915. A Faint Star of Large Proper Motion. Union Observatory Circular No. 30.

Crimson is a proud possessor of said reference.

Also, the press release gives the wrong year of Innes' death. He died in 1933.

Yeah, but that was assuming the old "visual" comfort zone distance. If the "real" comfort zone distance was 3.2 times farther away, the UV Ceti stars would only be a paltry 3.3 degrees wide in the planet's sky.

Unfortunately, that article came out before the ESO's VLTI measurement of Proxima Centauri's diameter. The above-linked Sky & Telescope article says:

"the dimmest red dwarfs, such as Proxima Centauri, have masses close to the hydrogen-burning limit, 8 percent of the Sun"

... whereas the more recent ESO press release in the OP says:

"In the case of Proxima Centauri, both the mass and the diameter are about 1/7 of those of the Sun."

(A more detailed article on the ESO's VLTI measurements, at http://obswww.unige.ch/~segransa/vlm...us_referee.pdf, estimates Proxima Centauri's mass at 12.3% of the mass of the Sun.)

There's no contradiction. Compared to the maximum red-dwarf mass (around 60 percent solar), Proxima Centauri (if its mass is 12 percent solar) IS close to the hydrogen-burning limit of 8 percent.