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Space Forum / Astronomy / July 2008Nearby stars-why do they start at 4LY?
Wiki et al show tables of the first 20 or 50 starsnearest the earth. The distances for the first few are .. .. .. 4.3 6.0 7.7 8.4 8.6 9.4 10.4 LY 1.7 1.7 .7 .2 1.2 1 the differences Why are there no stars from 0 to 4.3 LY? From the difference list there should be at least 2 closer than 4.3. These are probably the few that can be measured using the parallactic effect. It just doesn't seem likely that such a void exists. John Polasek >The distances for the first few are >.. .. .. 4.3 6.0 7.7 8.4 8.6 9.4 10.4 LY > 1.7 1.7 .7 .2 1.2 1 the differences >Why are there no stars from 0 to 4.3 LY? From the difference list >there should be at least 2 closer than 4.3. There's no "should" about it. You can't deduce anything from such a small set of data. But even supposing they were uniformly distributed, the volume with 10 LY of earth is 8 times the volume within 5. The differences in distances would be expected to tend to zero. -- Richard  SignaturePlease remember to mention me / in tapes you leave behind. | Wiki et al show tables of the first 20 or 50 starsnearest the earth. | The distances for the first few are [quoted text clipped - 5 lines] | effect. It just doesn't seem likely that such a void exists. | John Polasek Why are you a cretin? There should be at least 55 morons writing to sci.astro ahead of you and you the closest. The next poster is at least an hour away from you, it just doesn't seem likely that such a void exists. Look, imbecile, that's the way it IS, there is no "why" to it. > | Wiki et al show tables of the first 20 or 50 starsnearest the earth. > | The distances for the first few are [quoted text clipped - 11 lines] > likely that such a void exists. > Look, imbecile, that's the way it IS, there is no "why" to it. And why are you such an arsehole? The poster was simply asking an innocent question. If you can't be constructive, why don't you just shut the f.ck up? > | Wiki et al show tables of the first 20 or 50 starsnearest the earth. > | The distances for the first few are [quoted text clipped - 11 lines] > likely that such a void exists. > Look, imbecile, that's the way it IS, there is no "why" to it. But there is a simple "why" to it. The number of stars of stars less than x light year ways away is proportional to x^3, and the number between x and x+1 light years away is proportional to 3x^2. So the number close to the earth falls away with the square of the distance; its not a linear relationship; and the difference is most dramatic for small distances (x =1, 2, 3). Look, imbecile, that is the way it IS, the "why" is a geometrical argument. | > | Wiki et al show tables of the first 20 or 50 starsnearest the earth. | > | The distances for the first few are [quoted text clipped - 21 lines] | | Look, imbecile, that is the way it IS, the "why" is a geometrical argument. Look, moron: http://antwrp.gsfc.nasa.gov/apod/ap030323.html There is just one star within one AU of the EARTH (your chosen reference point), and four within 5 light years, namely Sol, Proxima Centauri, Alpha Centauri A and Alpha Centauri B. Then of course there is Barnard's star, moving rapidly. http://www.solstation.com/stars/barnards.htm At 8 light years you can add Sirius A and Sirius B. How many pips in an orange and why are they not at the rind? Because they are not homogeneous. What "geometrical argument", sh.t-for-brains? > Wiki et al show tables of the first 20 or 50 starsnearest the earth. > The distances for the first few are [quoted text clipped - 4 lines] > These are probably the few that can be measured using the parallactic > effect. It just doesn't seem likely that such a void exists. If there are 7 stars within a radius of 10 light-years, then the density is .00168 stars per cubic light year. Assuming a uniform density (no voids), the cumulative number should grow as R^3. On average, the first star will appear at a radius between 4-6 light years, which is a reflection that the inner 5 light years contains only about 1/8th of the total volume out to 10 light years. On average, the next stars will be separated by 1.8, 1.1, 0.8, 0.7, 0.6, 0.5 light years respectively. Seems like a reasonable approximation of reality to me. CM >> Wiki et al show tables of the first 20 or 50 starsnearest the earth. >> The distances for the first few are [quoted text clipped - 17 lines] > >CM I see. I agree, after a rough start, the sequence smooths out. Easier if you use 7stars/10^3 or .007 (in place of .00168 ) 'really really cubic' light years, bypassing the 4/3pi. Mystery solved. John Polasek > Wiki et al show tables of the first 20 or 50 starsnearest the earth. > The distances for the first few are [quoted text clipped - 5 lines] > effect. It just doesn't seem likely that such a void exists. > John Polasek If you actually do the arithmetic on star density in our Galaxy, 4 LY is just about the diameter of a sphere containing one solar mass. So there is no mystery about why the nearest star is 4.3 LY away. Very roughly: galaxy disk diameter 100,000 LY disk thickness 1,000 LY Disk volume pi R^2 x T = 3.14 (50,000)^2 x 1000 ~ 10^13 cubic LY (roughly) Mass of the galactic disk is of order 10^11 solar masses (possibly less, as the halo seems to have most of the mass). Volume per solar mass ~100 cubic LY. Cube root of 100 CLY is 4.64 LY which is average edge of a "box" with one solar mass inside. If stars were evenly spaced they would therefore be about 4-5 LY apart. In fact, the average star is smaller than the Sun but even this would not change the result by more than a factor of 2. So it is not surprising that the nearest star is currently about 4 LY away. Space is big. SignatureMike Dworetsky (Remove pants sp*mbl*ck to reply) >> Wiki et al show tables of the first 20 or 50 starsnearest the earth. >> The distances for the first few are [quoted text clipped - 28 lines] >So it is not surprising that the nearest star is currently about 4 LY away. >Space is big. Your 1 solar mass/100CLY or 5.6e-21 kg/mmm is about 3.7e5 times the density 1.5e-26 of the universe, m/l. Thank you. John Polasek |
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