Naked Science Forum
General Science => Question of the Week => Topic started by: IzzieC on 09/08/2018 13:54:41
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Geoff asks:
"It’s often stated that there are more stars in the universe than there are grains of sand on all the beaches on Earth. My question is - who counted them?"
Any thoughts?
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Nobody. But we have counted enough galaxies, and (by assuming the universe is fairly isotropic) made sufficiently reasonable estimates of the average number of stars in a galaxy and the probable number of distant galaxies too faint to distinguish, to come up with an estimate of the number of stars in the observable universe.
Only human vanity assumes that the observable universe is all that there is. There is no reason to suppose that it ends at the Schwarzchild radius, and every reason to believe it doesn't. So the whole universe is likely to be a lot bigger, and possibly a lot weirder, than the bit we can see.
Estimating the number of grains of sand is a bit easier - if you have a definition of a grain.
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Nobody has counted all the stars (just as no one has counted all the grains of sand on the Earth). Various people have made estimates of the number of stars, and their educated guesses vary by a few orders of magnitude based on their definitions of "star," what assumptions they make, and what data they base their guesses on.
For instance, one could use a powerful telescope to look at a small fraction of the sky and actually count the number of stars that are visible there, and then (assuming that there is nothing special about that particular portion of the sky) divide that number by the fraction of the sky that it is (this will make the number bigger--for instance if I counted 200000 stars in 1/1000000000000 of the sky, then I would estimate that the observable universe contains 200000*1000000000000 stars).
Or one could use data collected on many stars to say that the average brightness of a star is X. Then one could measure the brightness of a galaxy that is a known distance away, and use that to estimate how many stars are in that galaxy. Then, assuming that that galaxy is representative, one can multiply by how many galaxies there are in the observable universe (likely using an approach to galaxy counting that is similar to the star counting described above).
Or one could do the same thing as the last way, but instead define how much a star weighs, and determine the mass of a galaxy...
These last two approaches both assume that galaxies are roughly the same (poor assumption, but easily corrected with a more complex model) and that most stars are in galaxies (a reasonable assumption, but still can cause significant errors).
Regarding definitions of what counts as a star... there is some ambiguity at the low end of the mass scale, on the spectrum of enormous planets < brown dwarves < red dwarves, and there is also some ambiguity on the large (or just dense) end: black holes, neutron stars, quasars, pulsars, quark stars?
EDIT: Oops, crossed with alan!
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This question has now been answered, and you can listen to it here - https://www.thenakedscientists.com/articles/questions/qotw-are-there-more-stars-universe-or-grains-sand-earth (https://www.thenakedscientists.com/articles/questions/qotw-are-there-more-stars-universe-or-grains-sand-earth).
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In short, it all boils down to extrapolation. We calculated a little part and assumed how the whole picture looks.