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

[flyingpig]

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

 

[SammyS]

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.