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The data you have so far show a straight line when you plot distance and redshift, which suggests that the universe is expanding. This is an amazing result, but remember that you have only looked at a few galaxies in one tiny part of the sky. Because you looked at so few galaxies, scientists might be skeptical of your conclusions. They might say that something strange was happening in that part of the sky, or that what you found was only a lucky positioning of galaxies.

In fact, Edwin Hubble also had difficulty convincing scientists of his discovery. After he announced it in 1929, he teamed up with astronomer Milton Humason and began looking at more galaxies. They measured the distances and redshifts of thousands of galaxies, trying to prove that all galaxies plotted into a straight line on a Hubble diagram. They succeeded: by 1937, the expanding universe picture was firmly established by these observations.

Try looking at a few more galaxies beyond the six you’ve already looked at.

Explore 7 (optional): Make a Hubble diagram for the following six galaxies. Use this SkyServer Excel workbook to keep track of your work.

 Object ID RA Dec 1237674649923616961 168.39631 -0.21128 1237648702972952667 198.14058 -1.09188 1237655746703393411 262.60276 62.72059 1237678617428492404 24.32162 1.23198 1237666408439349389 352.9224 0.24129 1237654949988270431 242.571112 53.248867

In Explore 7, you went through exactly the same process that you went through in Explore 4 and 5. But instead of getting a straight line in your Hubble diagram, you got a mess! The reason for this is that the six galaxies in Explore 4 and 5 were chosen to give you a clear Hubble diagram. The galaxies in Explore 7 were chosen more randomly, to give you a more accurate sampling of galaxies in the universe.