Talk:Gliese 876
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[edit] Planet c in habitable zone?
In the article it says the apparent magnitude of the star is 10.16, which makes it 36.96 magnitudes dimmer than the sun, which translates to a factor of approx. 608*1012. If we correct for distance (multiplication with (1 AU / 15 ly)2 = approx. 1.11*10-12), we find that the sun is about 676 times as luminous as Gliese 876. The insolation in 0.13 AU distance (planet c) is 1/(0.132*676) = approx. 0.0876 times that at earth - it's as much as at 3.38 AU distance from our sun. I think this is outside the region where water is liquid. 193.171.121.30 19:55, 14 Jun 2005 (UTC)
Shouldn't we be talking about the absolute magnitude for calculations on Habitable Zones?? Stating its apparent magnitude is nice, but not helpful. Can you clarify this (aside from a not stated calculation to its absolute magnitude)? --JorisvS 12:50, 12 February 2006 (UTC)
If I would have taken the absolute magnitude instead of the apparent magnitude the distance correction would have had to be with the standard distance (10 parsec) and not with the actual distance. But I think the above explanation was too short, so here again in greater detail:
apparent magnitude of the sun: -26.8
apparent magnitude of Gliese 876: 10.18 (apparently changed from 10.16 in an earlier version of the article)
difference: 36.98
5 magnitudes difference mean a factor of 100, therefore 36.98 magnitudes mean a factor of 100(36.98/5) = 6.19 * 1014
This means as seen from earth, the sun looks 6.19 * 1014 times as bright as Gliese 876.
To make a better comparision, we've to put Gliese 876 at the same distance as the sun, i.e. at 1 AU. Since the brightness drops with the square of the distance, Gliese would be ((real distance of Gliese 876)/(1 AU))2 as bright as it is now if we put it at a distance of 1 AU. The real distance of Gliese 876 is 15.33 light years = 1.450 * 1017 m = 9.695 * 105 AU.
If we divide the brightness factor fom above by the squared distance measured in AU, we get a new factor of about 659, i. e. the sun is 659 times as bright as Gliese 876 when viewed at the same distance.
Now planet c is at a distance of 0.13 AU. To get the brightness of the star from there, we have again to take into the account the distance-squared law: The star would be 1/0.132 = 59.2 times as bright as at 1 AU, that is 59.2/659 = 0.0898 times as bright as the sun at 1 AU.
Now we want to know at which distance would the sun have this brightness? Again the distance-squared law, now in the other direction, i. e. we've got a brightness factor and want to know the distance; it should be clear that we have to take the inverse of the square root of the factor: 1/sqrt(0.0898) = 3.337 AU (not exactly the same numbers as above, maybe there was a different distance of the star in the article or I took less acurate numbers).
So far about the calculations, but I think the problem is that the star is a red dwarf and thus emitts a greater fraction of its power in the infrared, while the magnitude is only determined by the visible component.
The luminosity of the star as it is in the table is 0.0016 suns, although according to my calculations its 1/659 suns which is closer to 0.0015 suns (in the article about astronomical luminosity it is claimed that luminosity is directly related to magnitude, although it's also stated that "luminosity is the amount of energy a body radiates per unit time", which cannot both be true; of course my calculated value applies only in the case that it's directly related). With 0.0016 instead of 1/659 in the alculations above we get a bightness as at a distance of 3.249 AU in the solar system, which I think is still too far away for liquid water (but this is pretty much irrelevant now that we know about the infrared radiation). 193.171.121.30 06:48, 16 March 2006 (UTC)
[edit] Passed GA
This article matches the standard of other GAs on a similar topic, so I have no hesitation in passing it. Congratulations. It's good to see that Wikipedia is so well-served in terms of astronomy articles. MLilburne 08:04, 17 August 2006 (UTC)
[edit] temperature?
The temperature is given to ridiculous precision-- 4 significant figures? It appears the value 3223 K is based off the values for R, and L, although each of these is somewhat uncertain. It is far better policy to quote a value for temperature which is independently determined, such as from colors or from spectral fitting. Leggett 1996 seems to put T in the range 3100-3250
