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Hubble's Law

© Copyright 1998, Jim Loy

The universe is expanding. Clusters of galaxies are moving away from each other. Hubble's Law (named after the astronomer, not the telescope) says that, in general, the speed at which a cluster of galaxies is moving away from us depends on its distance from us. Hubble's Constant is a measure of just how fast these clusters are moving.

Here is a table of the brightest galaxies (brightest first):

The types are S for spiral, E for elliptical, I for irregular. The other letters and number, after the main type, denote differences in shape. The Magnitude indicates the brightness; a lower magnitude number means a brighter object.

The velocities are positive when the motion is away from us, and negative when they are moving toward us. I did not notice the unit of velocity. I will try to correct that oversight, soon. As you can see, the velocities vary tremendously. And they do not vary in a straightforward manner. But, we do see that only six galaxies are moving toward us, and 35 are moving away from us.

We begin to see the expansion of the universe, in this small sample. To the left is a graph of that data. As you can see, most of the data is at the edge of the graph. We need a lot more points.

We would have difficulty trying to deduce the Hubble Constant from this data. Since these are the brightest galaxies, as seen from earth, these are mostly the closest galaxies, representing only two or three clusters of galaxies. A much better sample would be a larger sample, maybe one with galaxies of the same type (and probably similar actual brightnesses). We wouldn't need a sample of galaxies in all directions; just a small part of the sky would be fine.

We are using brightness here, as a rough indicator as to distance. For, these closer galaxies, astronomers can measure the distance with a much more accurate yard-stick, the study of Cepheid Variables in these galaxies. For the very distant galaxies, they must rely on estimated brightness of the various galaxy types.

Addendum:

Here is a rough plot of Hubble's 1929 data for 24 spiral galaxies. I think that many of the distances were determined using Cepheid Variables. The graph makes a fairly decent match with a diagonal line. By 1931, with much more data, on much more distant galaxies, Hubble's new graph looked to be almost exactly a straight diagonal line.

Note: One big problem with my data and graph is that magnitude is not a linear measure of brightness (it is logarithmic). That is one reason that many galaxies clustered at the right side of my graph.

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