Science questions not worth a thread I: I'm a moron!
- Thread starter The Imp
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There is an old adage that a lot of the stars we see in the night sky burned out long ago, but their light is just now reaching earth. How true is that? I mean, most of the stars we can see are presumably fairly close - I don't know, within 100 light years or so - and thus only stars within the last 100 years of their life cycle (which ranges from tens of thousands of years on up) would qualify. Has anyone ever tried to figure out what percentage of stars this is? Can't be more than a few.
GoodGame
Red, White, & Blue, baby!
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To estimate, for one you'd need to know the mean distance of the solar system from all the other stars in the milky way.
Since we're in one arm of the spiral, that probably increases that mean distance and therefore increases the chances of the adage being correct.
Also isn't the rate of new star formation decreasing? If yes, then that would further support the addage.
Also would have to have to consider the percentage of slow-burning dwarf stars (e.g. red dwarf, Wikipedia says they are the vast majority) since they are younger than the universe and are likely to continue to burn much longer.
Another problem is Technology. You have to specify if you mean "naked eye", "stars we can see with technology", and "stars we can't even see due to lack of technology".
Just playing devil's advocate.
Let's go with "naked eye" . . .
Naked eye is around 2,000 stars iirc. Now all of these are in our galaxy or an immediate neighbor. So no more than 200,000 light years for the very furthest. Now these stars will mostly be very bright ones, not dwarfs so will have far below average life spans. But if we assume average life spans of 100 million years, and average distance of 100,000 light years (very generous numbers think) 2 x 10^3 *1x10^5 / 1x10^8 which gives us two stars already burned out. If we reduce the life span to 10 million years we get 20 already burned out. Still not most by a long shot even with a dramatically low-balled life span.
In reality I think the average distance is probably 10,000 light years or less, and the average life span is probably over 100 million years so I'm guessing the probability that any of the stars you see are already dead is quite low.
Possibly a better attack would be to look at how many visible supernovae there have been recently. Wikipedia lists 9 recorded supernovas which have been visible to the naked eye since at least the year 185. which gives a death rate of potentially visible stars of 1/(1825) years. if we use our (grossly over sized) estimate of 100,000 light years we get just over 50 supernovas which we haven't yet seen. With a more accurate estimate of 10,000 light years we get just over 5. However we are still looking at visible supernovas most of which are probably from stars which themselves were not visible before going supernova.
Based on this I conjecture that the probability we are seeing any stars which are already dead is probably 10% or less, though that's I guess. However I can very confidently say we don't see more than 10, and I'm completely sure that more than half of them are not already dead.
ParadigmShifter
Random Nonsense Generator
I thought the Earth was 4 billion years old which makes the Sun even older so I think the lifetime estimate is waaaaayyy to low.
Well, no; our sun is longer-lived than larger, but more visible, stars may be.
The median star has a lifespan of much longer than our suns 10 billion years. However we are looking at a Bayesian question here. Given that a star is visible, what is the life span and distance likely to be?
Since brightness correlates with visibility and inversely correlates with lifespan, visible stars will have shorter than average lifespans.
trader/warrior
Deity
Why is this wrong?
Confusion of calories and kilocalories.
1 calorie (cal) is indeed the energy needed to heat 1 gram of water by 1 Kelvin. However, when talking about food, "calorie" is often used as shorthand for kilocalorie (kcal), or 1000 calories.
The example you quoted ignores that and thus uses the wrong value for the nutritional energy. If you correct that mistake the calculation looks like this (in less stupid units):
1l of beer is roughly 1kg of water, so to heat it by one 1K you need 1000cal or 1kcal. Let's say it is heated by 20K, so that gives us 20kcal. Now despite people talking about 400 "calories" per liter, the nutritional energy of beer is about 400kcal/l.
So 400kcal - 20kcal = 380kcal. The energy needed to get the beer to body temperature is thus only about 5% of the energy you get by digesting the beer.
Additionally the calculation neglects, that in typical situations, when you want to drink cold beer, the body often already generates excess heat and is already trying to cool down. So unless you drink so much cold beer, that you're getting cold and the body starts to heat by burning calories, the beer will just take some of the excess heat.
IdiotsOpposite
Boom, headshot.
http://www.wolframalpha.com/input/?i=primes+from+2+to+10^15
If you keep clicking "more", you'll get more and more primes. Eventually, you'll get all of them. It will take a while, but not nearly as long as if you tried maple or creating your own library.
ParadigmShifter
Random Nonsense Generator
There must be a way to script that - Till would probably know how to do it.
IdiotsOpposite
Boom, headshot.
A way to script what... automatic clicking of "more" until no longer possible?
ParadigmShifter
Random Nonsense Generator
Yeah a way to automate the extraction of data by simulating sending click commands to the site.
Till is your man for that try PMing him to see if he knows how to write scripts for Wolfram Alpha.
Till is your man for that try PMing him to see if he knows how to write scripts for Wolfram Alpha.
IdiotsOpposite
Boom, headshot.
Well, obviously I don't need the script. Besides, I'm more interested in calculating infinite series using the Wolfram-Alpha engine... some of the results it pulls out are absolutely amazing. AND it shows you the partial sum every time it shows the closed-form sum of a series.
Wolfram-Alpha is wonderful.
ParadigmShifter
Random Nonsense Generator
I agree.
Expermenting with this it appears inexact. Wolfram Alpha gives 2.999631778708223*^13 as the trillion prime, however since that is a decimal not an integer, which suggests to me that Wolfram Alpha is estimating somehow and losing precision.
At the moment I'm leaning towards checking my numbers for psuedo primes, that should have more reasonable compute times.
ParadigmShifter
Random Nonsense Generator
There's more than 13 decimal places in that number so huh???
It uses mathematica as it's underlying engine which has full precision calculations though.
It uses mathematica as it's underlying engine which has full precision calculations though.
NKVD
Cossack
Questions after reading a book on Newton.
Just like Newton's law which proved as a verified theory for a long time. Is there anything Newton's law couldnt explain or predict ?
Which laws proved true or was verified in the past (meaning it could explain or predicts everything observed) but was proven wrong years later. I know some of you will answer every theory. But is there any complete theory on earth ? i think yes.. but we know most of todays theories are not complete.
Just like Newton's law which proved as a verified theory for a long time. Is there anything Newton's law couldnt explain or predict ?
Which laws proved true or was verified in the past (meaning it could explain or predicts everything observed) but was proven wrong years later. I know some of you will answer every theory. But is there any complete theory on earth ? i think yes.. but we know most of todays theories are not complete.
IdiotsOpposite
Boom, headshot.
Well, Newton's Laws are correct, or at least a very good approximation. If they were completely wrong, we wouldn't teach them, after all!
However, on very large or very small scales (think galaxies or atoms), Newton's laws begin to be a lot worse. This is where we get the current theories of relativity and quantum physics.
As to your last paragraph, I think you're looking for the Theory of Everything, and while it doesn't exist yet, we're trying. We're trying hard.
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