How Do We Measure the Size of the Galaxy with Limited Telescope Scans?

AI Thread Summary
The discussion centers on the measurement of the Milky Way's size, which is estimated to be about 100,000 light years across, despite detailed scans only covering approximately 100 light years. The reference to limited scanning comes from Michio Kaku's comments on the SETI project, which focuses on detecting radio signals within a 100 light year radius. However, this does not imply that telescopes have only explored this small area; the Hubble telescope has captured images of galaxies billions of light years away. The data used to estimate the galaxy's size comes from various observational methods, not solely from direct scans. Understanding the galaxy's dimensions involves a combination of techniques beyond just localized telescope scans.
flyingpig
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Homework Statement



It is said that telescopes have found that our galaxy is about a hundred thousand light years across. It is also said that our telescopes have only scanned a hundred light years of the galaxy.

I am just quoting from this video:

So my question is, how can we know that the galaxy is a hundred thousand light years across when we've only scanned a hundred light years
 
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Hello flyingpig.

As usual, details are important.

In that video, Michio Kaku is speaking of the SETI (Search for Extra-Terrestrial Intelligence) project. Early in the video he says that we have only scanned perhaps 100 light years from Earth in some detail. What he refers to is scanning with radio telescopes for the purpose of detecting radio signals from some form of intelligent life.

He does not say that telescopes have only scanned 100 light years from earth.

The Hubble telescope has recorded optical images from galaxies which are billions of light years away.
 
Thread 'Variable mass system : water sprayed into a moving container'

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Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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